src/librustc_typeck/astconv.rs

   1 // ignore-tidy-filelength FIXME(#67418) Split up this file.
   2 //! Conversion from AST representation of types to the `ty.rs` representation.
   3 //! The main routine here is `ast_ty_to_ty()`; each use is parameterized by an
   4 //! instance of `AstConv`.
   5
   6 // ignore-tidy-filelength
   7
   8 use crate::collect::PlaceholderHirTyCollector;
   9 use crate::middle::resolve_lifetime as rl;
  10 use crate::require_c_abi_if_c_variadic;
  11 use rustc_ast::{ast::ParamKindOrd, util::lev_distance::find_best_match_for_name};
  12 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  13 use rustc_errors::ErrorReported;
  14 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticId, FatalError};
  15 use rustc_hir as hir;
  16 use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
  17 use rustc_hir::def_id::{DefId, LocalDefId};
  18 use rustc_hir::intravisit::{walk_generics, Visitor as _};
  19 use rustc_hir::lang_items::SizedTraitLangItem;
  20 use rustc_hir::{Constness, GenericArg, GenericArgs};
  21 use rustc_middle::ty::subst::{self, InternalSubsts, Subst, SubstsRef};
  22 use rustc_middle::ty::{
  23     self, Const, DefIdTree, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
  24 };
  25 use rustc_middle::ty::{GenericParamDef, GenericParamDefKind};
  26 use rustc_session::lint::builtin::{AMBIGUOUS_ASSOCIATED_ITEMS, LATE_BOUND_LIFETIME_ARGUMENTS};
  27 use rustc_session::parse::feature_err;
  28 use rustc_session::Session;
  29 use rustc_span::symbol::{kw, sym, Ident, Symbol};
  30 use rustc_span::{MultiSpan, Span, DUMMY_SP};
  31 use rustc_target::spec::abi;
  32 use rustc_trait_selection::traits;
  33 use rustc_trait_selection::traits::astconv_object_safety_violations;
  34 use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
  35 use rustc_trait_selection::traits::wf::object_region_bounds;
  36
  37 use smallvec::SmallVec;
  38 use std::collections::BTreeSet;
  39 use std::iter;
  40 use std::slice;
  41
  42 #[derive(Debug)]
  43 pub struct PathSeg(pub DefId, pub usize);
  44
  45 pub trait AstConv<'tcx> {
  46     fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
  47
  48     fn item_def_id(&self) -> Option<DefId>;
  49
  50     fn default_constness_for_trait_bounds(&self) -> Constness;
  51
  52     /// Returns predicates in scope of the form `X: Foo`, where `X` is
  53     /// a type parameter `X` with the given id `def_id`. This is a
  54     /// subset of the full set of predicates.
  55     ///
  56     /// This is used for one specific purpose: resolving "short-hand"
  57     /// associated type references like `T::Item`. In principle, we
  58     /// would do that by first getting the full set of predicates in
  59     /// scope and then filtering down to find those that apply to `T`,
  60     /// but this can lead to cycle errors. The problem is that we have
  61     /// to do this resolution *in order to create the predicates in
  62     /// the first place*. Hence, we have this "special pass".
  63     fn get_type_parameter_bounds(&self, span: Span, def_id: DefId) -> ty::GenericPredicates<'tcx>;
  64
  65     /// Returns the lifetime to use when a lifetime is omitted (and not elided).
  66     fn re_infer(&self, param: Option<&ty::GenericParamDef>, span: Span)
  67     -> Option<ty::Region<'tcx>>;
  68
  69     /// Returns the type to use when a type is omitted.
  70     fn ty_infer(&self, param: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx>;
  71
  72     /// Returns `true` if `_` is allowed in type signatures in the current context.
  73     fn allow_ty_infer(&self) -> bool;
  74
  75     /// Returns the const to use when a const is omitted.
  76     fn ct_infer(
  77         &self,
  78         ty: Ty<'tcx>,
  79         param: Option<&ty::GenericParamDef>,
  80         span: Span,
  81     ) -> &'tcx Const<'tcx>;
  82
  83     /// Projecting an associated type from a (potentially)
  84     /// higher-ranked trait reference is more complicated, because of
  85     /// the possibility of late-bound regions appearing in the
  86     /// associated type binding. This is not legal in function
  87     /// signatures for that reason. In a function body, we can always
  88     /// handle it because we can use inference variables to remove the
  89     /// late-bound regions.
  90     fn projected_ty_from_poly_trait_ref(
  91         &self,
  92         span: Span,
  93         item_def_id: DefId,
  94         item_segment: &hir::PathSegment<'_>,
  95         poly_trait_ref: ty::PolyTraitRef<'tcx>,
  96     ) -> Ty<'tcx>;
  97
  98     /// Normalize an associated type coming from the user.
  99     fn normalize_ty(&self, span: Span, ty: Ty<'tcx>) -> Ty<'tcx>;
 100
 101     /// Invoked when we encounter an error from some prior pass
 102     /// (e.g., resolve) that is translated into a ty-error. This is
 103     /// used to help suppress derived errors typeck might otherwise
 104     /// report.
 105     fn set_tainted_by_errors(&self);
 106
 107     fn record_ty(&self, hir_id: hir::HirId, ty: Ty<'tcx>, span: Span);
 108 }
 109
 110 pub enum SizedByDefault {
 111     Yes,
 112     No,
 113 }
 114
 115 struct ConvertedBinding<'a, 'tcx> {
 116     item_name: Ident,
 117     kind: ConvertedBindingKind<'a, 'tcx>,
 118     span: Span,
 119 }
 120
 121 enum ConvertedBindingKind<'a, 'tcx> {
 122     Equality(Ty<'tcx>),
 123     Constraint(&'a [hir::GenericBound<'a>]),
 124 }
 125
 126 /// New-typed boolean indicating whether explicit late-bound lifetimes
 127 /// are present in a set of generic arguments.
 128 ///
 129 /// For example if we have some method `fn f<'a>(&'a self)` implemented
 130 /// for some type `T`, although `f` is generic in the lifetime `'a`, `'a`
 131 /// is late-bound so should not be provided explicitly. Thus, if `f` is
 132 /// instantiated with some generic arguments providing `'a` explicitly,
 133 /// we taint those arguments with `ExplicitLateBound::Yes` so that we
 134 /// can provide an appropriate diagnostic later.
 135 #[derive(Copy, Clone, PartialEq)]
 136 pub enum ExplicitLateBound {
 137     Yes,
 138     No,
 139 }
 140
 141 #[derive(Copy, Clone, PartialEq)]
 142 enum GenericArgPosition {
 143     Type,
 144     Value, // e.g., functions
 145     MethodCall,
 146 }
 147
 148 /// A marker denoting that the generic arguments that were
 149 /// provided did not match the respective generic parameters.
 150 #[derive(Clone, Default)]
 151 pub struct GenericArgCountMismatch {
 152     /// Indicates whether a fatal error was reported (`Some`), or just a lint (`None`).
 153     pub reported: Option<ErrorReported>,
 154     /// A list of spans of arguments provided that were not valid.
 155     pub invalid_args: Vec<Span>,
 156 }
 157
 158 /// Decorates the result of a generic argument count mismatch
 159 /// check with whether explicit late bounds were provided.
 160 #[derive(Clone)]
 161 pub struct GenericArgCountResult {
 162     pub explicit_late_bound: ExplicitLateBound,
 163     pub correct: Result<(), GenericArgCountMismatch>,
 164 }
 165
 166 impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 167     pub fn ast_region_to_region(
 168         &self,
 169         lifetime: &hir::Lifetime,
 170         def: Option<&ty::GenericParamDef>,
 171     ) -> ty::Region<'tcx> {
 172         let tcx = self.tcx();
 173         let lifetime_name = |def_id| tcx.hir().name(tcx.hir().as_local_hir_id(def_id));
 174
 175         let r = match tcx.named_region(lifetime.hir_id) {
 176             Some(rl::Region::Static) => tcx.lifetimes.re_static,
 177
 178             Some(rl::Region::LateBound(debruijn, id, _)) => {
 179                 let name = lifetime_name(id.expect_local());
 180                 tcx.mk_region(ty::ReLateBound(debruijn, ty::BrNamed(id, name)))
 181             }
 182
 183             Some(rl::Region::LateBoundAnon(debruijn, index)) => {
 184                 tcx.mk_region(ty::ReLateBound(debruijn, ty::BrAnon(index)))
 185             }
 186
 187             Some(rl::Region::EarlyBound(index, id, _)) => {
 188                 let name = lifetime_name(id.expect_local());
 189                 tcx.mk_region(ty::ReEarlyBound(ty::EarlyBoundRegion { def_id: id, index, name }))
 190             }
 191
 192             Some(rl::Region::Free(scope, id)) => {
 193                 let name = lifetime_name(id.expect_local());
 194                 tcx.mk_region(ty::ReFree(ty::FreeRegion {
 195                     scope,
 196                     bound_region: ty::BrNamed(id, name),
 197                 }))
 198
 199                 // (*) -- not late-bound, won't change
 200             }
 201
 202             None => {
 203                 self.re_infer(def, lifetime.span).unwrap_or_else(|| {
 204                     // This indicates an illegal lifetime
 205                     // elision. `resolve_lifetime` should have
 206                     // reported an error in this case -- but if
 207                     // not, let's error out.
 208                     tcx.sess.delay_span_bug(lifetime.span, "unelided lifetime in signature");
 209
 210                     // Supply some dummy value. We don't have an
 211                     // `re_error`, annoyingly, so use `'static`.
 212                     tcx.lifetimes.re_static
 213                 })
 214             }
 215         };
 216
 217         debug!("ast_region_to_region(lifetime={:?}) yields {:?}", lifetime, r);
 218
 219         r
 220     }
 221
 222     /// Given a path `path` that refers to an item `I` with the declared generics `decl_generics`,
 223     /// returns an appropriate set of substitutions for this particular reference to `I`.
 224     pub fn ast_path_substs_for_ty(
 225         &self,
 226         span: Span,
 227         def_id: DefId,
 228         item_segment: &hir::PathSegment<'_>,
 229     ) -> SubstsRef<'tcx> {
 230         let (substs, assoc_bindings, _) = self.create_substs_for_ast_path(
 231             span,
 232             def_id,
 233             &[],
 234             item_segment.generic_args(),
 235             item_segment.infer_args,
 236             None,
 237         );
 238
 239         if let Some(b) = assoc_bindings.first() {
 240             Self::prohibit_assoc_ty_binding(self.tcx(), b.span);
 241         }
 242
 243         substs
 244     }
 245
 246     /// Report error if there is an explicit type parameter when using `impl Trait`.
 247     fn check_impl_trait(
 248         tcx: TyCtxt<'_>,
 249         seg: &hir::PathSegment<'_>,
 250         generics: &ty::Generics,
 251     ) -> bool {
 252         let explicit = !seg.infer_args;
 253         let impl_trait = generics.params.iter().any(|param| match param.kind {
 254             ty::GenericParamDefKind::Type {
 255                 synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
 256                 ..
 257             } => true,
 258             _ => false,
 259         });
 260
 261         if explicit && impl_trait {
 262             let spans = seg
 263                 .generic_args()
 264                 .args
 265                 .iter()
 266                 .filter_map(|arg| match arg {
 267                     GenericArg::Type(_) => Some(arg.span()),
 268                     _ => None,
 269                 })
 270                 .collect::<Vec<_>>();
 271
 272             let mut err = struct_span_err! {
 273                 tcx.sess,
 274                 spans.clone(),
 275                 E0632,
 276                 "cannot provide explicit generic arguments when `impl Trait` is \
 277                 used in argument position"
 278             };
 279
 280             for span in spans {
 281                 err.span_label(span, "explicit generic argument not allowed");
 282             }
 283
 284             err.emit();
 285         }
 286
 287         impl_trait
 288     }
 289
 290     /// Checks that the correct number of generic arguments have been provided.
 291     /// Used specifically for function calls.
 292     pub fn check_generic_arg_count_for_call(
 293         tcx: TyCtxt<'_>,
 294         span: Span,
 295         def: &ty::Generics,
 296         seg: &hir::PathSegment<'_>,
 297         is_method_call: bool,
 298     ) -> GenericArgCountResult {
 299         let empty_args = hir::GenericArgs::none();
 300         let suppress_mismatch = Self::check_impl_trait(tcx, seg, &def);
 301         Self::check_generic_arg_count(
 302             tcx,
 303             span,
 304             def,
 305             if let Some(ref args) = seg.args { args } else { &empty_args },
 306             if is_method_call { GenericArgPosition::MethodCall } else { GenericArgPosition::Value },
 307             def.parent.is_none() && def.has_self, // `has_self`
 308             seg.infer_args || suppress_mismatch,  // `infer_args`
 309         )
 310     }
 311
 312     /// Checks that the correct number of generic arguments have been provided.
 313     /// This is used both for datatypes and function calls.
 314     fn check_generic_arg_count(
 315         tcx: TyCtxt<'_>,
 316         span: Span,
 317         def: &ty::Generics,
 318         args: &hir::GenericArgs<'_>,
 319         position: GenericArgPosition,
 320         has_self: bool,
 321         infer_args: bool,
 322     ) -> GenericArgCountResult {
 323         // At this stage we are guaranteed that the generic arguments are in the correct order, e.g.
 324         // that lifetimes will proceed types. So it suffices to check the number of each generic
 325         // arguments in order to validate them with respect to the generic parameters.
 326         let param_counts = def.own_counts();
 327         let arg_counts = args.own_counts();
 328         let infer_lifetimes = position != GenericArgPosition::Type && arg_counts.lifetimes == 0;
 329
 330         let mut defaults: ty::GenericParamCount = Default::default();
 331         for param in &def.params {
 332             match param.kind {
 333                 GenericParamDefKind::Lifetime => {}
 334                 GenericParamDefKind::Type { has_default, .. } => {
 335                     defaults.types += has_default as usize
 336                 }
 337                 GenericParamDefKind::Const => {
 338                     // FIXME(const_generics:defaults)
 339                 }
 340             };
 341         }
 342
 343         if position != GenericArgPosition::Type && !args.bindings.is_empty() {
 344             AstConv::prohibit_assoc_ty_binding(tcx, args.bindings[0].span);
 345         }
 346
 347         let explicit_late_bound =
 348             Self::prohibit_explicit_late_bound_lifetimes(tcx, def, args, position);
 349
 350         let check_kind_count = |kind,
 351                                 required,
 352                                 permitted,
 353                                 provided,
 354                                 offset,
 355                                 unexpected_spans: &mut Vec<Span>,
 356                                 silent| {
 357             debug!(
 358                 "check_kind_count: kind: {} required: {} permitted: {} provided: {} offset: {}",
 359                 kind, required, permitted, provided, offset
 360             );
 361             // We enforce the following: `required` <= `provided` <= `permitted`.
 362             // For kinds without defaults (e.g.., lifetimes), `required == permitted`.
 363             // For other kinds (i.e., types), `permitted` may be greater than `required`.
 364             if required <= provided && provided <= permitted {
 365                 return Ok(());
 366             }
 367
 368             if silent {
 369                 return Err(true);
 370             }
 371
 372             // Unfortunately lifetime and type parameter mismatches are typically styled
 373             // differently in diagnostics, which means we have a few cases to consider here.
 374             let (bound, quantifier) = if required != permitted {
 375                 if provided < required {
 376                     (required, "at least ")
 377                 } else {
 378                     // provided > permitted
 379                     (permitted, "at most ")
 380                 }
 381             } else {
 382                 (required, "")
 383             };
 384
 385             let (spans, label) = if required == permitted && provided > permitted {
 386                 // In the case when the user has provided too many arguments,
 387                 // we want to point to the unexpected arguments.
 388                 let spans: Vec<Span> = args.args[offset + permitted..offset + provided]
 389                     .iter()
 390                     .map(|arg| arg.span())
 391                     .collect();
 392                 unexpected_spans.extend(spans.clone());
 393                 (spans, format!("unexpected {} argument", kind))
 394             } else {
 395                 (
 396                     vec![span],
 397                     format!(
 398                         "expected {}{} {} argument{}",
 399                         quantifier,
 400                         bound,
 401                         kind,
 402                         pluralize!(bound),
 403                     ),
 404                 )
 405             };
 406
 407             let mut err = tcx.sess.struct_span_err_with_code(
 408                 spans.clone(),
 409                 &format!(
 410                     "wrong number of {} arguments: expected {}{}, found {}",
 411                     kind, quantifier, bound, provided,
 412                 ),
 413                 DiagnosticId::Error("E0107".into()),
 414             );
 415             for span in spans {
 416                 err.span_label(span, label.as_str());
 417             }
 418             err.emit();
 419
 420             Err(true)
 421         };
 422
 423         let mut arg_count_correct = Ok(());
 424         let mut unexpected_spans = vec![];
 425
 426         if !infer_lifetimes || arg_counts.lifetimes > param_counts.lifetimes {
 427             arg_count_correct = check_kind_count(
 428                 "lifetime",
 429                 param_counts.lifetimes,
 430                 param_counts.lifetimes,
 431                 arg_counts.lifetimes,
 432                 0,
 433                 &mut unexpected_spans,
 434                 explicit_late_bound == ExplicitLateBound::Yes,
 435             )
 436             .and(arg_count_correct);
 437         }
 438         // FIXME(const_generics:defaults)
 439         if !infer_args || arg_counts.consts > param_counts.consts {
 440             arg_count_correct = check_kind_count(
 441                 "const",
 442                 param_counts.consts,
 443                 param_counts.consts,
 444                 arg_counts.consts,
 445                 arg_counts.lifetimes + arg_counts.types,
 446                 &mut unexpected_spans,
 447                 false,
 448             )
 449             .and(arg_count_correct);
 450         }
 451         // Note that type errors are currently be emitted *after* const errors.
 452         if !infer_args || arg_counts.types > param_counts.types - defaults.types - has_self as usize
 453         {
 454             arg_count_correct = check_kind_count(
 455                 "type",
 456                 param_counts.types - defaults.types - has_self as usize,
 457                 param_counts.types - has_self as usize,
 458                 arg_counts.types,
 459                 arg_counts.lifetimes,
 460                 &mut unexpected_spans,
 461                 false,
 462             )
 463             .and(arg_count_correct);
 464         }
 465
 466         GenericArgCountResult {
 467             explicit_late_bound,
 468             correct: arg_count_correct.map_err(|reported_err| GenericArgCountMismatch {
 469                 reported: if reported_err { Some(ErrorReported) } else { None },
 470                 invalid_args: unexpected_spans,
 471             }),
 472         }
 473     }
 474
 475     /// Report an error that a generic argument did not match the generic parameter that was
 476     /// expected.
 477     fn generic_arg_mismatch_err(
 478         sess: &Session,
 479         arg: &GenericArg<'_>,
 480         kind: &'static str,
 481         help: Option<&str>,
 482     ) {
 483         let mut err = struct_span_err!(
 484             sess,
 485             arg.span(),
 486             E0747,
 487             "{} provided when a {} was expected",
 488             arg.descr(),
 489             kind,
 490         );
 491
 492         let kind_ord = match kind {
 493             "lifetime" => ParamKindOrd::Lifetime,
 494             "type" => ParamKindOrd::Type,
 495             "constant" => ParamKindOrd::Const,
 496             // It's more concise to match on the string representation, though it means
 497             // the match is non-exhaustive.
 498             _ => bug!("invalid generic parameter kind {}", kind),
 499         };
 500         let arg_ord = match arg {
 501             GenericArg::Lifetime(_) => ParamKindOrd::Lifetime,
 502             GenericArg::Type(_) => ParamKindOrd::Type,
 503             GenericArg::Const(_) => ParamKindOrd::Const,
 504         };
 505
 506         // This note will be true as long as generic parameters are strictly ordered by their kind.
 507         let (first, last) =
 508             if kind_ord < arg_ord { (kind, arg.descr()) } else { (arg.descr(), kind) };
 509         err.note(&format!("{} arguments must be provided before {} arguments", first, last));
 510
 511         if let Some(help) = help {
 512             err.help(help);
 513         }
 514         err.emit();
 515     }
 516
 517     /// Creates the relevant generic argument substitutions
 518     /// corresponding to a set of generic parameters. This is a
 519     /// rather complex function. Let us try to explain the role
 520     /// of each of its parameters:
 521     ///
 522     /// To start, we are given the `def_id` of the thing we are
 523     /// creating the substitutions for, and a partial set of
 524     /// substitutions `parent_substs`. In general, the substitutions
 525     /// for an item begin with substitutions for all the "parents" of
 526     /// that item -- e.g., for a method it might include the
 527     /// parameters from the impl.
 528     ///
 529     /// Therefore, the method begins by walking down these parents,
 530     /// starting with the outermost parent and proceed inwards until
 531     /// it reaches `def_id`. For each parent `P`, it will check `parent_substs`
 532     /// first to see if the parent's substitutions are listed in there. If so,
 533     /// we can append those and move on. Otherwise, it invokes the
 534     /// three callback functions:
 535     ///
 536     /// - `args_for_def_id`: given the `DefId` `P`, supplies back the
 537     ///   generic arguments that were given to that parent from within
 538     ///   the path; so e.g., if you have `<T as Foo>::Bar`, the `DefId`
 539     ///   might refer to the trait `Foo`, and the arguments might be
 540     ///   `[T]`. The boolean value indicates whether to infer values
 541     ///   for arguments whose values were not explicitly provided.
 542     /// - `provided_kind`: given the generic parameter and the value from `args_for_def_id`,
 543     ///   instantiate a `GenericArg`.
 544     /// - `inferred_kind`: if no parameter was provided, and inference is enabled, then
 545     ///   creates a suitable inference variable.
 546     pub fn create_substs_for_generic_args<'b>(
 547         tcx: TyCtxt<'tcx>,
 548         def_id: DefId,
 549         parent_substs: &[subst::GenericArg<'tcx>],
 550         has_self: bool,
 551         self_ty: Option<Ty<'tcx>>,
 552         arg_count: GenericArgCountResult,
 553         args_for_def_id: impl Fn(DefId) -> (Option<&'b GenericArgs<'b>>, bool),
 554         mut provided_kind: impl FnMut(&GenericParamDef, &GenericArg<'_>) -> subst::GenericArg<'tcx>,
 555         mut inferred_kind: impl FnMut(
 556             Option<&[subst::GenericArg<'tcx>]>,
 557             &GenericParamDef,
 558             bool,
 559         ) -> subst::GenericArg<'tcx>,
 560     ) -> SubstsRef<'tcx> {
 561         // Collect the segments of the path; we need to substitute arguments
 562         // for parameters throughout the entire path (wherever there are
 563         // generic parameters).
 564         let mut parent_defs = tcx.generics_of(def_id);
 565         let count = parent_defs.count();
 566         let mut stack = vec![(def_id, parent_defs)];
 567         while let Some(def_id) = parent_defs.parent {
 568             parent_defs = tcx.generics_of(def_id);
 569             stack.push((def_id, parent_defs));
 570         }
 571
 572         // We manually build up the substitution, rather than using convenience
 573         // methods in `subst.rs`, so that we can iterate over the arguments and
 574         // parameters in lock-step linearly, instead of trying to match each pair.
 575         let mut substs: SmallVec<[subst::GenericArg<'tcx>; 8]> = SmallVec::with_capacity(count);
 576         // Iterate over each segment of the path.
 577         while let Some((def_id, defs)) = stack.pop() {
 578             let mut params = defs.params.iter().peekable();
 579
 580             // If we have already computed substitutions for parents, we can use those directly.
 581             while let Some(&param) = params.peek() {
 582                 if let Some(&kind) = parent_substs.get(param.index as usize) {
 583                     substs.push(kind);
 584                     params.next();
 585                 } else {
 586                     break;
 587                 }
 588             }
 589
 590             // `Self` is handled first, unless it's been handled in `parent_substs`.
 591             if has_self {
 592                 if let Some(&param) = params.peek() {
 593                     if param.index == 0 {
 594                         if let GenericParamDefKind::Type { .. } = param.kind {
 595                             substs.push(
 596                                 self_ty
 597                                     .map(|ty| ty.into())
 598                                     .unwrap_or_else(|| inferred_kind(None, param, true)),
 599                             );
 600                             params.next();
 601                         }
 602                     }
 603                 }
 604             }
 605
 606             // Check whether this segment takes generic arguments and the user has provided any.
 607             let (generic_args, infer_args) = args_for_def_id(def_id);
 608
 609             let mut args =
 610                 generic_args.iter().flat_map(|generic_args| generic_args.args.iter()).peekable();
 611
 612             // If we encounter a type or const when we expect a lifetime, we infer the lifetimes.
 613             // If we later encounter a lifetime, we know that the arguments were provided in the
 614             // wrong order. `force_infer_lt` records the type or const that forced lifetimes to be
 615             // inferred, so we can use it for diagnostics later.
 616             let mut force_infer_lt = None;
 617
 618             loop {
 619                 // We're going to iterate through the generic arguments that the user
 620                 // provided, matching them with the generic parameters we expect.
 621                 // Mismatches can occur as a result of elided lifetimes, or for malformed
 622                 // input. We try to handle both sensibly.
 623                 match (args.peek(), params.peek()) {
 624                     (Some(&arg), Some(&param)) => {
 625                         match (arg, &param.kind, arg_count.explicit_late_bound) {
 626                             (GenericArg::Lifetime(_), GenericParamDefKind::Lifetime, _)
 627                             | (GenericArg::Type(_), GenericParamDefKind::Type { .. }, _)
 628                             | (GenericArg::Const(_), GenericParamDefKind::Const, _) => {
 629                                 substs.push(provided_kind(param, arg));
 630                                 args.next();
 631                                 params.next();
 632                             }
 633                             (
 634                                 GenericArg::Type(_) | GenericArg::Const(_),
 635                                 GenericParamDefKind::Lifetime,
 636                                 _,
 637                             ) => {
 638                                 // We expected a lifetime argument, but got a type or const
 639                                 // argument. That means we're inferring the lifetimes.
 640                                 substs.push(inferred_kind(None, param, infer_args));
 641                                 force_infer_lt = Some(arg);
 642                                 params.next();
 643                             }
 644                             (GenericArg::Lifetime(_), _, ExplicitLateBound::Yes) => {
 645                                 // We've come across a lifetime when we expected something else in
 646                                 // the presence of explicit late bounds. This is most likely
 647                                 // due to the presence of the explicit bound so we're just going to
 648                                 // ignore it.
 649                                 args.next();
 650                             }
 651                             (_, kind, _) => {
 652                                 // We expected one kind of parameter, but the user provided
 653                                 // another. This is an error. However, if we already know that
 654                                 // the arguments don't match up with the parameters, we won't issue
 655                                 // an additional error, as the user already knows what's wrong.
 656                                 if arg_count.correct.is_ok()
 657                                     && arg_count.explicit_late_bound == ExplicitLateBound::No
 658                                 {
 659                                     // We're going to iterate over the parameters to sort them out, and
 660                                     // show that order to the user as a possible order for the parameters
 661                                     let mut param_types_present = defs
 662                                         .params
 663                                         .clone()
 664                                         .into_iter()
 665                                         .map(|param| {
 666                                             (
 667                                                 match param.kind {
 668                                                     GenericParamDefKind::Lifetime => {
 669                                                         ParamKindOrd::Lifetime
 670                                                     }
 671                                                     GenericParamDefKind::Type { .. } => {
 672                                                         ParamKindOrd::Type
 673                                                     }
 674                                                     GenericParamDefKind::Const => {
 675                                                         ParamKindOrd::Const
 676                                                     }
 677                                                 },
 678                                                 param,
 679                                             )
 680                                         })
 681                                         .collect::<Vec<(ParamKindOrd, GenericParamDef)>>();
 682                                     param_types_present.sort_by_key(|(ord, _)| *ord);
 683                                     let (mut param_types_present, ordered_params): (
 684                                         Vec<ParamKindOrd>,
 685                                         Vec<GenericParamDef>,
 686                                     ) = param_types_present.into_iter().unzip();
 687                                     param_types_present.dedup();
 688
 689                                     Self::generic_arg_mismatch_err(
 690                                         tcx.sess,
 691                                         arg,
 692                                         kind.descr(),
 693                                         Some(&format!(
 694                                             "reorder the arguments: {}: `<{}>`",
 695                                             param_types_present
 696                                                 .into_iter()
 697                                                 .map(|ord| format!("{}s", ord.to_string()))
 698                                                 .collect::<Vec<String>>()
 699                                                 .join(", then "),
 700                                             ordered_params
 701                                                 .into_iter()
 702                                                 .filter_map(|param| {
 703                                                     if param.name == kw::SelfUpper {
 704                                                         None
 705                                                     } else {
 706                                                         Some(param.name.to_string())
 707                                                     }
 708                                                 })
 709                                                 .collect::<Vec<String>>()
 710                                                 .join(", ")
 711                                         )),
 712                                     );
 713                                 }
 714
 715                                 // We've reported the error, but we want to make sure that this
 716                                 // problem doesn't bubble down and create additional, irrelevant
 717                                 // errors. In this case, we're simply going to ignore the argument
 718                                 // and any following arguments. The rest of the parameters will be
 719                                 // inferred.
 720                                 while args.next().is_some() {}
 721                             }
 722                         }
 723                     }
 724
 725                     (Some(&arg), None) => {
 726                         // We should never be able to reach this point with well-formed input.
 727                         // There are three situations in which we can encounter this issue.
 728                         //
 729                         //  1.  The number of arguments is incorrect. In this case, an error
 730                         //      will already have been emitted, and we can ignore it.
 731                         //  2.  There are late-bound lifetime parameters present, yet the
 732                         //      lifetime arguments have also been explicitly specified by the
 733                         //      user.
 734                         //  3.  We've inferred some lifetimes, which have been provided later (i.e.
 735                         //      after a type or const). We want to throw an error in this case.
 736
 737                         if arg_count.correct.is_ok()
 738                             && arg_count.explicit_late_bound == ExplicitLateBound::No
 739                         {
 740                             let kind = arg.descr();
 741                             assert_eq!(kind, "lifetime");
 742                             let provided =
 743                                 force_infer_lt.expect("lifetimes ought to have been inferred");
 744                             Self::generic_arg_mismatch_err(tcx.sess, provided, kind, None);
 745                         }
 746
 747                         break;
 748                     }
 749
 750                     (None, Some(&param)) => {
 751                         // If there are fewer arguments than parameters, it means
 752                         // we're inferring the remaining arguments.
 753                         substs.push(inferred_kind(Some(&substs), param, infer_args));
 754                         params.next();
 755                     }
 756
 757                     (None, None) => break,
 758                 }
 759             }
 760         }
 761
 762         tcx.intern_substs(&substs)
 763     }
 764
 765     /// Given the type/lifetime/const arguments provided to some path (along with
 766     /// an implicit `Self`, if this is a trait reference), returns the complete
 767     /// set of substitutions. This may involve applying defaulted type parameters.
 768     /// Also returns back constraints on associated types.
 769     ///
 770     /// Example:
 771     ///
 772     /// ```
 773     /// T: std::ops::Index<usize, Output = u32>
 774     /// ^1 ^^^^^^^^^^^^^^2 ^^^^3  ^^^^^^^^^^^4
 775     /// ```
 776     ///
 777     /// 1. The `self_ty` here would refer to the type `T`.
 778     /// 2. The path in question is the path to the trait `std::ops::Index`,
 779     ///    which will have been resolved to a `def_id`
 780     /// 3. The `generic_args` contains info on the `<...>` contents. The `usize` type
 781     ///    parameters are returned in the `SubstsRef`, the associated type bindings like
 782     ///    `Output = u32` are returned in the `Vec<ConvertedBinding...>` result.
 783     ///
 784     /// Note that the type listing given here is *exactly* what the user provided.
 785     ///
 786     /// For (generic) associated types
 787     ///
 788     /// ```
 789     /// <Vec<u8> as Iterable<u8>>::Iter::<'a>
 790     /// ```
 791     ///
 792     /// We have the parent substs are the substs for the parent trait:
 793     /// `[Vec<u8>, u8]` and `generic_args` are the arguments for the associated
 794     /// type itself: `['a]`. The returned `SubstsRef` concatenates these two
 795     /// lists: `[Vec<u8>, u8, 'a]`.
 796     fn create_substs_for_ast_path<'a>(
 797         &self,
 798         span: Span,
 799         def_id: DefId,
 800         parent_substs: &[subst::GenericArg<'tcx>],
 801         generic_args: &'a hir::GenericArgs<'_>,
 802         infer_args: bool,
 803         self_ty: Option<Ty<'tcx>>,
 804     ) -> (SubstsRef<'tcx>, Vec<ConvertedBinding<'a, 'tcx>>, GenericArgCountResult) {
 805         // If the type is parameterized by this region, then replace this
 806         // region with the current anon region binding (in other words,
 807         // whatever & would get replaced with).
 808         debug!(
 809             "create_substs_for_ast_path(def_id={:?}, self_ty={:?}, \
 810                 generic_args={:?})",
 811             def_id, self_ty, generic_args
 812         );
 813
 814         let tcx = self.tcx();
 815         let generic_params = tcx.generics_of(def_id);
 816
 817         if generic_params.has_self {
 818             if generic_params.parent.is_some() {
 819                 // The parent is a trait so it should have at least one subst
 820                 // for the `Self` type.
 821                 assert!(!parent_substs.is_empty())
 822             } else {
 823                 // This item (presumably a trait) needs a self-type.
 824                 assert!(self_ty.is_some());
 825             }
 826         } else {
 827             assert!(self_ty.is_none() && parent_substs.is_empty());
 828         }
 829
 830         let arg_count = Self::check_generic_arg_count(
 831             tcx,
 832             span,
 833             &generic_params,
 834             &generic_args,
 835             GenericArgPosition::Type,
 836             self_ty.is_some(),
 837             infer_args,
 838         );
 839
 840         let is_object = self_ty.map_or(false, |ty| ty == self.tcx().types.trait_object_dummy_self);
 841         let default_needs_object_self = |param: &ty::GenericParamDef| {
 842             if let GenericParamDefKind::Type { has_default, .. } = param.kind {
 843                 if is_object && has_default {
 844                     let default_ty = tcx.at(span).type_of(param.def_id);
 845                     let self_param = tcx.types.self_param;
 846                     if default_ty.walk().any(|arg| arg == self_param.into()) {
 847                         // There is no suitable inference default for a type parameter
 848                         // that references self, in an object type.
 849                         return true;
 850                     }
 851                 }
 852             }
 853
 854             false
 855         };
 856
 857         let mut missing_type_params = vec![];
 858         let mut inferred_params = vec![];
 859         let substs = Self::create_substs_for_generic_args(
 860             tcx,
 861             def_id,
 862             parent_substs,
 863             self_ty.is_some(),
 864             self_ty,
 865             arg_count.clone(),
 866             // Provide the generic args, and whether types should be inferred.
 867             |did| {
 868                 if did == def_id {
 869                     (Some(generic_args), infer_args)
 870                 } else {
 871                     // The last component of this tuple is unimportant.
 872                     (None, false)
 873                 }
 874             },
 875             // Provide substitutions for parameters for which (valid) arguments have been provided.
 876             |param, arg| match (&param.kind, arg) {
 877                 (GenericParamDefKind::Lifetime, GenericArg::Lifetime(lt)) => {
 878                     self.ast_region_to_region(&lt, Some(param)).into()
 879                 }
 880                 (GenericParamDefKind::Type { .. }, GenericArg::Type(ty)) => {
 881                     if let (hir::TyKind::Infer, false) = (&ty.kind, self.allow_ty_infer()) {
 882                         inferred_params.push(ty.span);
 883                         tcx.ty_error().into()
 884                     } else {
 885                         self.ast_ty_to_ty(&ty).into()
 886                     }
 887                 }
 888                 (GenericParamDefKind::Const, GenericArg::Const(ct)) => {
 889                     ty::Const::from_opt_const_arg_anon_const(
 890                         tcx,
 891                         ty::WithOptConstParam {
 892                             did: tcx.hir().local_def_id(ct.value.hir_id),
 893                             const_param_did: Some(param.def_id),
 894                         },
 895                     )
 896                     .into()
 897                 }
 898                 _ => unreachable!(),
 899             },
 900             // Provide substitutions for parameters for which arguments are inferred.
 901             |substs, param, infer_args| {
 902                 match param.kind {
 903                     GenericParamDefKind::Lifetime => tcx.lifetimes.re_static.into(),
 904                     GenericParamDefKind::Type { has_default, .. } => {
 905                         if !infer_args && has_default {
 906                             // No type parameter provided, but a default exists.
 907
 908                             // If we are converting an object type, then the
 909                             // `Self` parameter is unknown. However, some of the
 910                             // other type parameters may reference `Self` in their
 911                             // defaults. This will lead to an ICE if we are not
 912                             // careful!
 913                             if default_needs_object_self(param) {
 914                                 missing_type_params.push(param.name.to_string());
 915                                 tcx.ty_error().into()
 916                             } else {
 917                                 // This is a default type parameter.
 918                                 self.normalize_ty(
 919                                     span,
 920                                     tcx.at(span).type_of(param.def_id).subst_spanned(
 921                                         tcx,
 922                                         substs.unwrap(),
 923                                         Some(span),
 924                                     ),
 925                                 )
 926                                 .into()
 927                             }
 928                         } else if infer_args {
 929                             // No type parameters were provided, we can infer all.
 930                             let param =
 931                                 if !default_needs_object_self(param) { Some(param) } else { None };
 932                             self.ty_infer(param, span).into()
 933                         } else {
 934                             // We've already errored above about the mismatch.
 935                             tcx.ty_error().into()
 936                         }
 937                     }
 938                     GenericParamDefKind::Const => {
 939                         let ty = tcx.at(span).type_of(param.def_id);
 940                         // FIXME(const_generics:defaults)
 941                         if infer_args {
 942                             // No const parameters were provided, we can infer all.
 943                             self.ct_infer(ty, Some(param), span).into()
 944                         } else {
 945                             // We've already errored above about the mismatch.
 946                             tcx.const_error(ty).into()
 947                         }
 948                     }
 949                 }
 950             },
 951         );
 952
 953         self.complain_about_missing_type_params(
 954             missing_type_params,
 955             def_id,
 956             span,
 957             generic_args.args.is_empty(),
 958         );
 959
 960         // Convert associated-type bindings or constraints into a separate vector.
 961         // Example: Given this:
 962         //
 963         //     T: Iterator<Item = u32>
 964         //
 965         // The `T` is passed in as a self-type; the `Item = u32` is
 966         // not a "type parameter" of the `Iterator` trait, but rather
 967         // a restriction on `<T as Iterator>::Item`, so it is passed
 968         // back separately.
 969         let assoc_bindings = generic_args
 970             .bindings
 971             .iter()
 972             .map(|binding| {
 973                 let kind = match binding.kind {
 974                     hir::TypeBindingKind::Equality { ref ty } => {
 975                         ConvertedBindingKind::Equality(self.ast_ty_to_ty(ty))
 976                     }
 977                     hir::TypeBindingKind::Constraint { ref bounds } => {
 978                         ConvertedBindingKind::Constraint(bounds)
 979                     }
 980                 };
 981                 ConvertedBinding { item_name: binding.ident, kind, span: binding.span }
 982             })
 983             .collect();
 984
 985         debug!(
 986             "create_substs_for_ast_path(generic_params={:?}, self_ty={:?}) -> {:?}",
 987             generic_params, self_ty, substs
 988         );
 989
 990         (substs, assoc_bindings, arg_count)
 991     }
 992
 993     crate fn create_substs_for_associated_item(
 994         &self,
 995         tcx: TyCtxt<'tcx>,
 996         span: Span,
 997         item_def_id: DefId,
 998         item_segment: &hir::PathSegment<'_>,
 999         parent_substs: SubstsRef<'tcx>,
1000     ) -> SubstsRef<'tcx> {
1001         if tcx.generics_of(item_def_id).params.is_empty() {
1002             self.prohibit_generics(slice::from_ref(item_segment));
1003
1004             parent_substs
1005         } else {
1006             self.create_substs_for_ast_path(
1007                 span,
1008                 item_def_id,
1009                 parent_substs,
1010                 item_segment.generic_args(),
1011                 item_segment.infer_args,
1012                 None,
1013             )
1014             .0
1015         }
1016     }
1017
1018     /// On missing type parameters, emit an E0393 error and provide a structured suggestion using
1019     /// the type parameter's name as a placeholder.
1020     fn complain_about_missing_type_params(
1021         &self,
1022         missing_type_params: Vec<String>,
1023         def_id: DefId,
1024         span: Span,
1025         empty_generic_args: bool,
1026     ) {
1027         if missing_type_params.is_empty() {
1028             return;
1029         }
1030         let display =
1031             missing_type_params.iter().map(|n| format!("`{}`", n)).collect::<Vec<_>>().join(", ");
1032         let mut err = struct_span_err!(
1033             self.tcx().sess,
1034             span,
1035             E0393,
1036             "the type parameter{} {} must be explicitly specified",
1037             pluralize!(missing_type_params.len()),
1038             display,
1039         );
1040         err.span_label(
1041             self.tcx().def_span(def_id),
1042             &format!(
1043                 "type parameter{} {} must be specified for this",
1044                 pluralize!(missing_type_params.len()),
1045                 display,
1046             ),
1047         );
1048         let mut suggested = false;
1049         if let (Ok(snippet), true) = (
1050             self.tcx().sess.source_map().span_to_snippet(span),
1051             // Don't suggest setting the type params if there are some already: the order is
1052             // tricky to get right and the user will already know what the syntax is.
1053             empty_generic_args,
1054         ) {
1055             if snippet.ends_with('>') {
1056                 // The user wrote `Trait<'a, T>` or similar. To provide an accurate suggestion
1057                 // we would have to preserve the right order. For now, as clearly the user is
1058                 // aware of the syntax, we do nothing.
1059             } else {
1060                 // The user wrote `Iterator`, so we don't have a type we can suggest, but at
1061                 // least we can clue them to the correct syntax `Iterator<Type>`.
1062                 err.span_suggestion(
1063                     span,
1064                     &format!(
1065                         "set the type parameter{plural} to the desired type{plural}",
1066                         plural = pluralize!(missing_type_params.len()),
1067                     ),
1068                     format!("{}<{}>", snippet, missing_type_params.join(", ")),
1069                     Applicability::HasPlaceholders,
1070                 );
1071                 suggested = true;
1072             }
1073         }
1074         if !suggested {
1075             err.span_label(
1076                 span,
1077                 format!(
1078                     "missing reference{} to {}",
1079                     pluralize!(missing_type_params.len()),
1080                     display,
1081                 ),
1082             );
1083         }
1084         err.note(
1085             "because of the default `Self` reference, type parameters must be \
1086                   specified on object types",
1087         );
1088         err.emit();
1089     }
1090
1091     /// Instantiates the path for the given trait reference, assuming that it's
1092     /// bound to a valid trait type. Returns the `DefId` of the defining trait.
1093     /// The type _cannot_ be a type other than a trait type.
1094     ///
1095     /// If the `projections` argument is `None`, then assoc type bindings like `Foo<T = X>`
1096     /// are disallowed. Otherwise, they are pushed onto the vector given.
1097     pub fn instantiate_mono_trait_ref(
1098         &self,
1099         trait_ref: &hir::TraitRef<'_>,
1100         self_ty: Ty<'tcx>,
1101     ) -> ty::TraitRef<'tcx> {
1102         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1);
1103
1104         self.ast_path_to_mono_trait_ref(
1105             trait_ref.path.span,
1106             trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
1107             self_ty,
1108             trait_ref.path.segments.last().unwrap(),
1109         )
1110     }
1111
1112     /// The given trait-ref must actually be a trait.
1113     pub(super) fn instantiate_poly_trait_ref_inner(
1114         &self,
1115         trait_ref: &hir::TraitRef<'_>,
1116         span: Span,
1117         constness: Constness,
1118         self_ty: Ty<'tcx>,
1119         bounds: &mut Bounds<'tcx>,
1120         speculative: bool,
1121     ) -> GenericArgCountResult {
1122         let trait_def_id = trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise());
1123
1124         debug!("instantiate_poly_trait_ref({:?}, def_id={:?})", trait_ref, trait_def_id);
1125
1126         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1);
1127
1128         let (substs, assoc_bindings, arg_count) = self.create_substs_for_ast_trait_ref(
1129             trait_ref.path.span,
1130             trait_def_id,
1131             self_ty,
1132             trait_ref.path.segments.last().unwrap(),
1133         );
1134         let poly_trait_ref = ty::Binder::bind(ty::TraitRef::new(trait_def_id, substs));
1135
1136         bounds.trait_bounds.push((poly_trait_ref, span, constness));
1137
1138         let mut dup_bindings = FxHashMap::default();
1139         for binding in &assoc_bindings {
1140             // Specify type to assert that error was already reported in `Err` case.
1141             let _: Result<_, ErrorReported> = self.add_predicates_for_ast_type_binding(
1142                 trait_ref.hir_ref_id,
1143                 poly_trait_ref,
1144                 binding,
1145                 bounds,
1146                 speculative,
1147                 &mut dup_bindings,
1148                 binding.span,
1149             );
1150             // Okay to ignore `Err` because of `ErrorReported` (see above).
1151         }
1152
1153         debug!(
1154             "instantiate_poly_trait_ref({:?}, bounds={:?}) -> {:?}",
1155             trait_ref, bounds, poly_trait_ref
1156         );
1157
1158         arg_count
1159     }
1160
1161     /// Given a trait bound like `Debug`, applies that trait bound the given self-type to construct
1162     /// a full trait reference. The resulting trait reference is returned. This may also generate
1163     /// auxiliary bounds, which are added to `bounds`.
1164     ///
1165     /// Example:
1166     ///
1167     /// ```
1168     /// poly_trait_ref = Iterator<Item = u32>
1169     /// self_ty = Foo
1170     /// ```
1171     ///
1172     /// this would return `Foo: Iterator` and add `<Foo as Iterator>::Item = u32` into `bounds`.
1173     ///
1174     /// **A note on binders:** against our usual convention, there is an implied bounder around
1175     /// the `self_ty` and `poly_trait_ref` parameters here. So they may reference bound regions.
1176     /// If for example you had `for<'a> Foo<'a>: Bar<'a>`, then the `self_ty` would be `Foo<'a>`
1177     /// where `'a` is a bound region at depth 0. Similarly, the `poly_trait_ref` would be
1178     /// `Bar<'a>`. The returned poly-trait-ref will have this binder instantiated explicitly,
1179     /// however.
1180     pub fn instantiate_poly_trait_ref(
1181         &self,
1182         poly_trait_ref: &hir::PolyTraitRef<'_>,
1183         constness: Constness,
1184         self_ty: Ty<'tcx>,
1185         bounds: &mut Bounds<'tcx>,
1186     ) -> GenericArgCountResult {
1187         self.instantiate_poly_trait_ref_inner(
1188             &poly_trait_ref.trait_ref,
1189             poly_trait_ref.span,
1190             constness,
1191             self_ty,
1192             bounds,
1193             false,
1194         )
1195     }
1196
1197     fn ast_path_to_mono_trait_ref(
1198         &self,
1199         span: Span,
1200         trait_def_id: DefId,
1201         self_ty: Ty<'tcx>,
1202         trait_segment: &hir::PathSegment<'_>,
1203     ) -> ty::TraitRef<'tcx> {
1204         let (substs, assoc_bindings, _) =
1205             self.create_substs_for_ast_trait_ref(span, trait_def_id, self_ty, trait_segment);
1206         if let Some(b) = assoc_bindings.first() {
1207             AstConv::prohibit_assoc_ty_binding(self.tcx(), b.span);
1208         }
1209         ty::TraitRef::new(trait_def_id, substs)
1210     }
1211
1212     /// When the code is using the `Fn` traits directly, instead of the `Fn(A) -> B` syntax, emit
1213     /// an error and attempt to build a reasonable structured suggestion.
1214     fn complain_about_internal_fn_trait(
1215         &self,
1216         span: Span,
1217         trait_def_id: DefId,
1218         trait_segment: &'a hir::PathSegment<'a>,
1219     ) {
1220         let trait_def = self.tcx().trait_def(trait_def_id);
1221
1222         if !self.tcx().features().unboxed_closures
1223             && trait_segment.generic_args().parenthesized != trait_def.paren_sugar
1224         {
1225             let sess = &self.tcx().sess.parse_sess;
1226             // For now, require that parenthetical notation be used only with `Fn()` etc.
1227             let (msg, sugg) = if trait_def.paren_sugar {
1228                 (
1229                     "the precise format of `Fn`-family traits' type parameters is subject to \
1230                      change",
1231                     Some(format!(
1232                         "{}{} -> {}",
1233                         trait_segment.ident,
1234                         trait_segment
1235                             .args
1236                             .as_ref()
1237                             .and_then(|args| args.args.get(0))
1238                             .and_then(|arg| match arg {
1239                                 hir::GenericArg::Type(ty) => match ty.kind {
1240                                     hir::TyKind::Tup(t) => t
1241                                         .iter()
1242                                         .map(|e| sess.source_map().span_to_snippet(e.span))
1243                                         .collect::<Result<Vec<_>, _>>()
1244                                         .map(|a| a.join(", ")),
1245                                     _ => sess.source_map().span_to_snippet(ty.span),
1246                                 }
1247                                 .map(|s| format!("({})", s))
1248                                 .ok(),
1249                                 _ => None,
1250                             })
1251                             .unwrap_or_else(|| "()".to_string()),
1252                         trait_segment
1253                             .generic_args()
1254                             .bindings
1255                             .iter()
1256                             .find_map(|b| match (b.ident.name == sym::Output, &b.kind) {
1257                                 (true, hir::TypeBindingKind::Equality { ty }) => {
1258                                     sess.source_map().span_to_snippet(ty.span).ok()
1259                                 }
1260                                 _ => None,
1261                             })
1262                             .unwrap_or_else(|| "()".to_string()),
1263                     )),
1264                 )
1265             } else {
1266                 ("parenthetical notation is only stable when used with `Fn`-family traits", None)
1267             };
1268             let mut err = feature_err(sess, sym::unboxed_closures, span, msg);
1269             if let Some(sugg) = sugg {
1270                 let msg = "use parenthetical notation instead";
1271                 err.span_suggestion(span, msg, sugg, Applicability::MaybeIncorrect);
1272             }
1273             err.emit();
1274         }
1275     }
1276
1277     fn create_substs_for_ast_trait_ref<'a>(
1278         &self,
1279         span: Span,
1280         trait_def_id: DefId,
1281         self_ty: Ty<'tcx>,
1282         trait_segment: &'a hir::PathSegment<'a>,
1283     ) -> (SubstsRef<'tcx>, Vec<ConvertedBinding<'a, 'tcx>>, GenericArgCountResult) {
1284         debug!("create_substs_for_ast_trait_ref(trait_segment={:?})", trait_segment);
1285
1286         self.complain_about_internal_fn_trait(span, trait_def_id, trait_segment);
1287
1288         self.create_substs_for_ast_path(
1289             span,
1290             trait_def_id,
1291             &[],
1292             trait_segment.generic_args(),
1293             trait_segment.infer_args,
1294             Some(self_ty),
1295         )
1296     }
1297
1298     fn trait_defines_associated_type_named(&self, trait_def_id: DefId, assoc_name: Ident) -> bool {
1299         self.tcx()
1300             .associated_items(trait_def_id)
1301             .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, trait_def_id)
1302             .is_some()
1303     }
1304
1305     // Returns `true` if a bounds list includes `?Sized`.
1306     pub fn is_unsized(&self, ast_bounds: &[hir::GenericBound<'_>], span: Span) -> bool {
1307         let tcx = self.tcx();
1308
1309         // Try to find an unbound in bounds.
1310         let mut unbound = None;
1311         for ab in ast_bounds {
1312             if let &hir::GenericBound::Trait(ref ptr, hir::TraitBoundModifier::Maybe) = ab {
1313                 if unbound.is_none() {
1314                     unbound = Some(&ptr.trait_ref);
1315                 } else {
1316                     struct_span_err!(
1317                         tcx.sess,
1318                         span,
1319                         E0203,
1320                         "type parameter has more than one relaxed default \
1321                         bound, only one is supported"
1322                     )
1323                     .emit();
1324                 }
1325             }
1326         }
1327
1328         let kind_id = tcx.lang_items().require(SizedTraitLangItem);
1329         match unbound {
1330             Some(tpb) => {
1331                 // FIXME(#8559) currently requires the unbound to be built-in.
1332                 if let Ok(kind_id) = kind_id {
1333                     if tpb.path.res != Res::Def(DefKind::Trait, kind_id) {
1334                         tcx.sess.span_warn(
1335                             span,
1336                             "default bound relaxed for a type parameter, but \
1337                              this does nothing because the given bound is not \
1338                              a default; only `?Sized` is supported",
1339                         );
1340                     }
1341                 }
1342             }
1343             _ if kind_id.is_ok() => {
1344                 return false;
1345             }
1346             // No lang item for `Sized`, so we can't add it as a bound.
1347             None => {}
1348         }
1349
1350         true
1351     }
1352
1353     /// This helper takes a *converted* parameter type (`param_ty`)
1354     /// and an *unconverted* list of bounds:
1355     ///
1356     /// ```text
1357     /// fn foo<T: Debug>
1358     ///        ^  ^^^^^ `ast_bounds` parameter, in HIR form
1359     ///        |
1360     ///        `param_ty`, in ty form
1361     /// ```
1362     ///
1363     /// It adds these `ast_bounds` into the `bounds` structure.
1364     ///
1365     /// **A note on binders:** there is an implied binder around
1366     /// `param_ty` and `ast_bounds`. See `instantiate_poly_trait_ref`
1367     /// for more details.
1368     fn add_bounds(
1369         &self,
1370         param_ty: Ty<'tcx>,
1371         ast_bounds: &[hir::GenericBound<'_>],
1372         bounds: &mut Bounds<'tcx>,
1373     ) {
1374         let mut trait_bounds = Vec::new();
1375         let mut region_bounds = Vec::new();
1376
1377         let constness = self.default_constness_for_trait_bounds();
1378         for ast_bound in ast_bounds {
1379             match *ast_bound {
1380                 hir::GenericBound::Trait(ref b, hir::TraitBoundModifier::None) => {
1381                     trait_bounds.push((b, constness))
1382                 }
1383                 hir::GenericBound::Trait(ref b, hir::TraitBoundModifier::MaybeConst) => {
1384                     trait_bounds.push((b, Constness::NotConst))
1385                 }
1386                 hir::GenericBound::Trait(_, hir::TraitBoundModifier::Maybe) => {}
1387                 hir::GenericBound::Outlives(ref l) => region_bounds.push(l),
1388             }
1389         }
1390
1391         for (bound, constness) in trait_bounds {
1392             let _ = self.instantiate_poly_trait_ref(bound, constness, param_ty, bounds);
1393         }
1394
1395         bounds.region_bounds.extend(
1396             region_bounds.into_iter().map(|r| (self.ast_region_to_region(r, None), r.span)),
1397         );
1398     }
1399
1400     /// Translates a list of bounds from the HIR into the `Bounds` data structure.
1401     /// The self-type for the bounds is given by `param_ty`.
1402     ///
1403     /// Example:
1404     ///
1405     /// ```
1406     /// fn foo<T: Bar + Baz>() { }
1407     ///        ^  ^^^^^^^^^ ast_bounds
1408     ///        param_ty
1409     /// ```
1410     ///
1411     /// The `sized_by_default` parameter indicates if, in this context, the `param_ty` should be
1412     /// considered `Sized` unless there is an explicit `?Sized` bound.  This would be true in the
1413     /// example above, but is not true in supertrait listings like `trait Foo: Bar + Baz`.
1414     ///
1415     /// `span` should be the declaration size of the parameter.
1416     pub fn compute_bounds(
1417         &self,
1418         param_ty: Ty<'tcx>,
1419         ast_bounds: &[hir::GenericBound<'_>],
1420         sized_by_default: SizedByDefault,
1421         span: Span,
1422     ) -> Bounds<'tcx> {
1423         let mut bounds = Bounds::default();
1424
1425         self.add_bounds(param_ty, ast_bounds, &mut bounds);
1426         bounds.trait_bounds.sort_by_key(|(t, _, _)| t.def_id());
1427
1428         bounds.implicitly_sized = if let SizedByDefault::Yes = sized_by_default {
1429             if !self.is_unsized(ast_bounds, span) { Some(span) } else { None }
1430         } else {
1431             None
1432         };
1433
1434         bounds
1435     }
1436
1437     /// Given an HIR binding like `Item = Foo` or `Item: Foo`, pushes the corresponding predicates
1438     /// onto `bounds`.
1439     ///
1440     /// **A note on binders:** given something like `T: for<'a> Iterator<Item = &'a u32>`, the
1441     /// `trait_ref` here will be `for<'a> T: Iterator`. The `binding` data however is from *inside*
1442     /// the binder (e.g., `&'a u32`) and hence may reference bound regions.
1443     fn add_predicates_for_ast_type_binding(
1444         &self,
1445         hir_ref_id: hir::HirId,
1446         trait_ref: ty::PolyTraitRef<'tcx>,
1447         binding: &ConvertedBinding<'_, 'tcx>,
1448         bounds: &mut Bounds<'tcx>,
1449         speculative: bool,
1450         dup_bindings: &mut FxHashMap<DefId, Span>,
1451         path_span: Span,
1452     ) -> Result<(), ErrorReported> {
1453         let tcx = self.tcx();
1454
1455         if !speculative {
1456             // Given something like `U: SomeTrait<T = X>`, we want to produce a
1457             // predicate like `<U as SomeTrait>::T = X`. This is somewhat
1458             // subtle in the event that `T` is defined in a supertrait of
1459             // `SomeTrait`, because in that case we need to upcast.
1460             //
1461             // That is, consider this case:
1462             //
1463             // ```
1464             // trait SubTrait: SuperTrait<i32> { }
1465             // trait SuperTrait<A> { type T; }
1466             //
1467             // ... B: SubTrait<T = foo> ...
1468             // ```
1469             //
1470             // We want to produce `<B as SuperTrait<i32>>::T == foo`.
1471
1472             // Find any late-bound regions declared in `ty` that are not
1473             // declared in the trait-ref. These are not well-formed.
1474             //
1475             // Example:
1476             //
1477             //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
1478             //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
1479             if let ConvertedBindingKind::Equality(ty) = binding.kind {
1480                 let late_bound_in_trait_ref =
1481                     tcx.collect_constrained_late_bound_regions(&trait_ref);
1482                 let late_bound_in_ty =
1483                     tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(ty));
1484                 debug!("late_bound_in_trait_ref = {:?}", late_bound_in_trait_ref);
1485                 debug!("late_bound_in_ty = {:?}", late_bound_in_ty);
1486                 for br in late_bound_in_ty.difference(&late_bound_in_trait_ref) {
1487                     let br_name = match *br {
1488                         ty::BrNamed(_, name) => name,
1489                         _ => {
1490                             span_bug!(
1491                                 binding.span,
1492                                 "anonymous bound region {:?} in binding but not trait ref",
1493                                 br
1494                             );
1495                         }
1496                     };
1497                     // FIXME: point at the type params that don't have appropriate lifetimes:
1498                     // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
1499                     //                         ----  ----     ^^^^^^^
1500                     struct_span_err!(
1501                         tcx.sess,
1502                         binding.span,
1503                         E0582,
1504                         "binding for associated type `{}` references lifetime `{}`, \
1505                          which does not appear in the trait input types",
1506                         binding.item_name,
1507                         br_name
1508                     )
1509                     .emit();
1510                 }
1511             }
1512         }
1513
1514         let candidate =
1515             if self.trait_defines_associated_type_named(trait_ref.def_id(), binding.item_name) {
1516                 // Simple case: X is defined in the current trait.
1517                 trait_ref
1518             } else {
1519                 // Otherwise, we have to walk through the supertraits to find
1520                 // those that do.
1521                 self.one_bound_for_assoc_type(
1522                     || traits::supertraits(tcx, trait_ref),
1523                     || trait_ref.print_only_trait_path().to_string(),
1524                     binding.item_name,
1525                     path_span,
1526                     || match binding.kind {
1527                         ConvertedBindingKind::Equality(ty) => Some(ty.to_string()),
1528                         _ => None,
1529                     },
1530                 )?
1531             };
1532
1533         let (assoc_ident, def_scope) =
1534             tcx.adjust_ident_and_get_scope(binding.item_name, candidate.def_id(), hir_ref_id);
1535
1536         // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
1537         // of calling `filter_by_name_and_kind`.
1538         let assoc_ty = tcx
1539             .associated_items(candidate.def_id())
1540             .filter_by_name_unhygienic(assoc_ident.name)
1541             .find(|i| {
1542                 i.kind == ty::AssocKind::Type && i.ident.normalize_to_macros_2_0() == assoc_ident
1543             })
1544             .expect("missing associated type");
1545
1546         if !assoc_ty.vis.is_accessible_from(def_scope, tcx) {
1547             tcx.sess
1548                 .struct_span_err(
1549                     binding.span,
1550                     &format!("associated type `{}` is private", binding.item_name),
1551                 )
1552                 .span_label(binding.span, "private associated type")
1553                 .emit();
1554         }
1555         tcx.check_stability(assoc_ty.def_id, Some(hir_ref_id), binding.span);
1556
1557         if !speculative {
1558             dup_bindings
1559                 .entry(assoc_ty.def_id)
1560                 .and_modify(|prev_span| {
1561                     struct_span_err!(
1562                         self.tcx().sess,
1563                         binding.span,
1564                         E0719,
1565                         "the value of the associated type `{}` (from trait `{}`) \
1566                          is already specified",
1567                         binding.item_name,
1568                         tcx.def_path_str(assoc_ty.container.id())
1569                     )
1570                     .span_label(binding.span, "re-bound here")
1571                     .span_label(*prev_span, format!("`{}` bound here first", binding.item_name))
1572                     .emit();
1573                 })
1574                 .or_insert(binding.span);
1575         }
1576
1577         match binding.kind {
1578             ConvertedBindingKind::Equality(ref ty) => {
1579                 // "Desugar" a constraint like `T: Iterator<Item = u32>` this to
1580                 // the "projection predicate" for:
1581                 //
1582                 // `<T as Iterator>::Item = u32`
1583                 bounds.projection_bounds.push((
1584                     candidate.map_bound(|trait_ref| ty::ProjectionPredicate {
1585                         projection_ty: ty::ProjectionTy::from_ref_and_name(
1586                             tcx,
1587                             trait_ref,
1588                             binding.item_name,
1589                         ),
1590                         ty,
1591                     }),
1592                     binding.span,
1593                 ));
1594             }
1595             ConvertedBindingKind::Constraint(ast_bounds) => {
1596                 // "Desugar" a constraint like `T: Iterator<Item: Debug>` to
1597                 //
1598                 // `<T as Iterator>::Item: Debug`
1599                 //
1600                 // Calling `skip_binder` is okay, because `add_bounds` expects the `param_ty`
1601                 // parameter to have a skipped binder.
1602                 let param_ty = tcx.mk_projection(assoc_ty.def_id, candidate.skip_binder().substs);
1603                 self.add_bounds(param_ty, ast_bounds, bounds);
1604             }
1605         }
1606         Ok(())
1607     }
1608
1609     fn ast_path_to_ty(
1610         &self,
1611         span: Span,
1612         did: DefId,
1613         item_segment: &hir::PathSegment<'_>,
1614     ) -> Ty<'tcx> {
1615         let substs = self.ast_path_substs_for_ty(span, did, item_segment);
1616         self.normalize_ty(span, self.tcx().at(span).type_of(did).subst(self.tcx(), substs))
1617     }
1618
1619     fn conv_object_ty_poly_trait_ref(
1620         &self,
1621         span: Span,
1622         trait_bounds: &[hir::PolyTraitRef<'_>],
1623         lifetime: &hir::Lifetime,
1624     ) -> Ty<'tcx> {
1625         let tcx = self.tcx();
1626
1627         let mut bounds = Bounds::default();
1628         let mut potential_assoc_types = Vec::new();
1629         let dummy_self = self.tcx().types.trait_object_dummy_self;
1630         for trait_bound in trait_bounds.iter().rev() {
1631             if let GenericArgCountResult {
1632                 correct:
1633                     Err(GenericArgCountMismatch { invalid_args: cur_potential_assoc_types, .. }),
1634                 ..
1635             } = self.instantiate_poly_trait_ref(
1636                 trait_bound,
1637                 Constness::NotConst,
1638                 dummy_self,
1639                 &mut bounds,
1640             ) {
1641                 potential_assoc_types.extend(cur_potential_assoc_types.into_iter());
1642             }
1643         }
1644
1645         // Expand trait aliases recursively and check that only one regular (non-auto) trait
1646         // is used and no 'maybe' bounds are used.
1647         let expanded_traits =
1648             traits::expand_trait_aliases(tcx, bounds.trait_bounds.iter().map(|&(a, b, _)| (a, b)));
1649         let (mut auto_traits, regular_traits): (Vec<_>, Vec<_>) =
1650             expanded_traits.partition(|i| tcx.trait_is_auto(i.trait_ref().def_id()));
1651         if regular_traits.len() > 1 {
1652             let first_trait = &regular_traits[0];
1653             let additional_trait = &regular_traits[1];
1654             let mut err = struct_span_err!(
1655                 tcx.sess,
1656                 additional_trait.bottom().1,
1657                 E0225,
1658                 "only auto traits can be used as additional traits in a trait object"
1659             );
1660             additional_trait.label_with_exp_info(
1661                 &mut err,
1662                 "additional non-auto trait",
1663                 "additional use",
1664             );
1665             first_trait.label_with_exp_info(&mut err, "first non-auto trait", "first use");
1666             err.emit();
1667         }
1668
1669         if regular_traits.is_empty() && auto_traits.is_empty() {
1670             struct_span_err!(
1671                 tcx.sess,
1672                 span,
1673                 E0224,
1674                 "at least one trait is required for an object type"
1675             )
1676             .emit();
1677             return tcx.ty_error();
1678         }
1679
1680         // Check that there are no gross object safety violations;
1681         // most importantly, that the supertraits don't contain `Self`,
1682         // to avoid ICEs.
1683         for item in &regular_traits {
1684             let object_safety_violations =
1685                 astconv_object_safety_violations(tcx, item.trait_ref().def_id());
1686             if !object_safety_violations.is_empty() {
1687                 report_object_safety_error(
1688                     tcx,
1689                     span,
1690                     item.trait_ref().def_id(),
1691                     &object_safety_violations[..],
1692                 )
1693                 .emit();
1694                 return tcx.ty_error();
1695             }
1696         }
1697
1698         // Use a `BTreeSet` to keep output in a more consistent order.
1699         let mut associated_types: FxHashMap<Span, BTreeSet<DefId>> = FxHashMap::default();
1700
1701         let regular_traits_refs_spans = bounds
1702             .trait_bounds
1703             .into_iter()
1704             .filter(|(trait_ref, _, _)| !tcx.trait_is_auto(trait_ref.def_id()));
1705
1706         for (base_trait_ref, span, constness) in regular_traits_refs_spans {
1707             assert_eq!(constness, Constness::NotConst);
1708
1709             for obligation in traits::elaborate_trait_ref(tcx, base_trait_ref) {
1710                 debug!(
1711                     "conv_object_ty_poly_trait_ref: observing object predicate `{:?}`",
1712                     obligation.predicate
1713                 );
1714                 match obligation.predicate.kind() {
1715                     ty::PredicateKind::Trait(pred, _) => {
1716                         associated_types.entry(span).or_default().extend(
1717                             tcx.associated_items(pred.def_id())
1718                                 .in_definition_order()
1719                                 .filter(|item| item.kind == ty::AssocKind::Type)
1720                                 .map(|item| item.def_id),
1721                         );
1722                     }
1723                     &ty::PredicateKind::Projection(pred) => {
1724                         // A `Self` within the original bound will be substituted with a
1725                         // `trait_object_dummy_self`, so check for that.
1726                         let references_self =
1727                             pred.skip_binder().ty.walk().any(|arg| arg == dummy_self.into());
1728
1729                         // If the projection output contains `Self`, force the user to
1730                         // elaborate it explicitly to avoid a lot of complexity.
1731                         //
1732                         // The "classicaly useful" case is the following:
1733                         // ```
1734                         //     trait MyTrait: FnMut() -> <Self as MyTrait>::MyOutput {
1735                         //         type MyOutput;
1736                         //     }
1737                         // ```
1738                         //
1739                         // Here, the user could theoretically write `dyn MyTrait<Output = X>`,
1740                         // but actually supporting that would "expand" to an infinitely-long type
1741                         // `fix $ τ → dyn MyTrait<MyOutput = X, Output = <τ as MyTrait>::MyOutput`.
1742                         //
1743                         // Instead, we force the user to write
1744                         // `dyn MyTrait<MyOutput = X, Output = X>`, which is uglier but works. See
1745                         // the discussion in #56288 for alternatives.
1746                         if !references_self {
1747                             // Include projections defined on supertraits.
1748                             bounds.projection_bounds.push((pred, span));
1749                         }
1750                     }
1751                     _ => (),
1752                 }
1753             }
1754         }
1755
1756         for (projection_bound, _) in &bounds.projection_bounds {
1757             for def_ids in associated_types.values_mut() {
1758                 def_ids.remove(&projection_bound.projection_def_id());
1759             }
1760         }
1761
1762         self.complain_about_missing_associated_types(
1763             associated_types,
1764             potential_assoc_types,
1765             trait_bounds,
1766         );
1767
1768         // De-duplicate auto traits so that, e.g., `dyn Trait + Send + Send` is the same as
1769         // `dyn Trait + Send`.
1770         auto_traits.sort_by_key(|i| i.trait_ref().def_id());
1771         auto_traits.dedup_by_key(|i| i.trait_ref().def_id());
1772         debug!("regular_traits: {:?}", regular_traits);
1773         debug!("auto_traits: {:?}", auto_traits);
1774
1775         // Transform a `PolyTraitRef` into a `PolyExistentialTraitRef` by
1776         // removing the dummy `Self` type (`trait_object_dummy_self`).
1777         let trait_ref_to_existential = |trait_ref: ty::TraitRef<'tcx>| {
1778             if trait_ref.self_ty() != dummy_self {
1779                 // FIXME: There appears to be a missing filter on top of `expand_trait_aliases`,
1780                 // which picks up non-supertraits where clauses - but also, the object safety
1781                 // completely ignores trait aliases, which could be object safety hazards. We
1782                 // `delay_span_bug` here to avoid an ICE in stable even when the feature is
1783                 // disabled. (#66420)
1784                 tcx.sess.delay_span_bug(
1785                     DUMMY_SP,
1786                     &format!(
1787                         "trait_ref_to_existential called on {:?} with non-dummy Self",
1788                         trait_ref,
1789                     ),
1790                 );
1791             }
1792             ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
1793         };
1794
1795         // Erase the `dummy_self` (`trait_object_dummy_self`) used above.
1796         let existential_trait_refs =
1797             regular_traits.iter().map(|i| i.trait_ref().map_bound(trait_ref_to_existential));
1798         let existential_projections = bounds.projection_bounds.iter().map(|(bound, _)| {
1799             bound.map_bound(|b| {
1800                 let trait_ref = trait_ref_to_existential(b.projection_ty.trait_ref(tcx));
1801                 ty::ExistentialProjection {
1802                     ty: b.ty,
1803                     item_def_id: b.projection_ty.item_def_id,
1804                     substs: trait_ref.substs,
1805                 }
1806             })
1807         });
1808
1809         // Calling `skip_binder` is okay because the predicates are re-bound.
1810         let regular_trait_predicates = existential_trait_refs
1811             .map(|trait_ref| ty::ExistentialPredicate::Trait(trait_ref.skip_binder()));
1812         let auto_trait_predicates = auto_traits
1813             .into_iter()
1814             .map(|trait_ref| ty::ExistentialPredicate::AutoTrait(trait_ref.trait_ref().def_id()));
1815         let mut v = regular_trait_predicates
1816             .chain(auto_trait_predicates)
1817             .chain(
1818                 existential_projections
1819                     .map(|x| ty::ExistentialPredicate::Projection(x.skip_binder())),
1820             )
1821             .collect::<SmallVec<[_; 8]>>();
1822         v.sort_by(|a, b| a.stable_cmp(tcx, b));
1823         v.dedup();
1824         let existential_predicates = ty::Binder::bind(tcx.mk_existential_predicates(v.into_iter()));
1825
1826         // Use explicitly-specified region bound.
1827         let region_bound = if !lifetime.is_elided() {
1828             self.ast_region_to_region(lifetime, None)
1829         } else {
1830             self.compute_object_lifetime_bound(span, existential_predicates).unwrap_or_else(|| {
1831                 if tcx.named_region(lifetime.hir_id).is_some() {
1832                     self.ast_region_to_region(lifetime, None)
1833                 } else {
1834                     self.re_infer(None, span).unwrap_or_else(|| {
1835                         // FIXME: these can be redundant with E0106, but not always.
1836                         struct_span_err!(
1837                             tcx.sess,
1838                             span,
1839                             E0228,
1840                             "the lifetime bound for this object type cannot be deduced \
1841                              from context; please supply an explicit bound"
1842                         )
1843                         .emit();
1844                         tcx.lifetimes.re_static
1845                     })
1846                 }
1847             })
1848         };
1849         debug!("region_bound: {:?}", region_bound);
1850
1851         let ty = tcx.mk_dynamic(existential_predicates, region_bound);
1852         debug!("trait_object_type: {:?}", ty);
1853         ty
1854     }
1855
1856     /// When there are any missing associated types, emit an E0191 error and attempt to supply a
1857     /// reasonable suggestion on how to write it. For the case of multiple associated types in the
1858     /// same trait bound have the same name (as they come from different super-traits), we instead
1859     /// emit a generic note suggesting using a `where` clause to constraint instead.
1860     fn complain_about_missing_associated_types(
1861         &self,
1862         associated_types: FxHashMap<Span, BTreeSet<DefId>>,
1863         potential_assoc_types: Vec<Span>,
1864         trait_bounds: &[hir::PolyTraitRef<'_>],
1865     ) {
1866         if associated_types.values().all(|v| v.is_empty()) {
1867             return;
1868         }
1869         let tcx = self.tcx();
1870         // FIXME: Marked `mut` so that we can replace the spans further below with a more
1871         // appropriate one, but this should be handled earlier in the span assignment.
1872         let mut associated_types: FxHashMap<Span, Vec<_>> = associated_types
1873             .into_iter()
1874             .map(|(span, def_ids)| {
1875                 (span, def_ids.into_iter().map(|did| tcx.associated_item(did)).collect())
1876             })
1877             .collect();
1878         let mut names = vec![];
1879
1880         // Account for things like `dyn Foo + 'a`, like in tests `issue-22434.rs` and
1881         // `issue-22560.rs`.
1882         let mut trait_bound_spans: Vec<Span> = vec![];
1883         for (span, items) in &associated_types {
1884             if !items.is_empty() {
1885                 trait_bound_spans.push(*span);
1886             }
1887             for assoc_item in items {
1888                 let trait_def_id = assoc_item.container.id();
1889                 names.push(format!(
1890                     "`{}` (from trait `{}`)",
1891                     assoc_item.ident,
1892                     tcx.def_path_str(trait_def_id),
1893                 ));
1894             }
1895         }
1896         if let ([], [bound]) = (&potential_assoc_types[..], &trait_bounds) {
1897             match &bound.trait_ref.path.segments[..] {
1898                 // FIXME: `trait_ref.path.span` can point to a full path with multiple
1899                 // segments, even though `trait_ref.path.segments` is of length `1`. Work
1900                 // around that bug here, even though it should be fixed elsewhere.
1901                 // This would otherwise cause an invalid suggestion. For an example, look at
1902                 // `src/test/ui/issues/issue-28344.rs` where instead of the following:
1903                 //
1904                 //   error[E0191]: the value of the associated type `Output`
1905                 //                 (from trait `std::ops::BitXor`) must be specified
1906                 //   --> $DIR/issue-28344.rs:4:17
1907                 //    |
1908                 // LL |     let x: u8 = BitXor::bitor(0 as u8, 0 as u8);
1909                 //    |                 ^^^^^^ help: specify the associated type:
1910                 //    |                              `BitXor<Output = Type>`
1911                 //
1912                 // we would output:
1913                 //
1914                 //   error[E0191]: the value of the associated type `Output`
1915                 //                 (from trait `std::ops::BitXor`) must be specified
1916                 //   --> $DIR/issue-28344.rs:4:17
1917                 //    |
1918                 // LL |     let x: u8 = BitXor::bitor(0 as u8, 0 as u8);
1919                 //    |                 ^^^^^^^^^^^^^ help: specify the associated type:
1920                 //    |                                     `BitXor::bitor<Output = Type>`
1921                 [segment] if segment.args.is_none() => {
1922                     trait_bound_spans = vec![segment.ident.span];
1923                     associated_types = associated_types
1924                         .into_iter()
1925                         .map(|(_, items)| (segment.ident.span, items))
1926                         .collect();
1927                 }
1928                 _ => {}
1929             }
1930         }
1931         names.sort();
1932         trait_bound_spans.sort();
1933         let mut err = struct_span_err!(
1934             tcx.sess,
1935             trait_bound_spans,
1936             E0191,
1937             "the value of the associated type{} {} must be specified",
1938             pluralize!(names.len()),
1939             names.join(", "),
1940         );
1941         let mut suggestions = vec![];
1942         let mut types_count = 0;
1943         let mut where_constraints = vec![];
1944         for (span, assoc_items) in &associated_types {
1945             let mut names: FxHashMap<_, usize> = FxHashMap::default();
1946             for item in assoc_items {
1947                 types_count += 1;
1948                 *names.entry(item.ident.name).or_insert(0) += 1;
1949             }
1950             let mut dupes = false;
1951             for item in assoc_items {
1952                 let prefix = if names[&item.ident.name] > 1 {
1953                     let trait_def_id = item.container.id();
1954                     dupes = true;
1955                     format!("{}::", tcx.def_path_str(trait_def_id))
1956                 } else {
1957                     String::new()
1958                 };
1959                 if let Some(sp) = tcx.hir().span_if_local(item.def_id) {
1960                     err.span_label(sp, format!("`{}{}` defined here", prefix, item.ident));
1961                 }
1962             }
1963             if potential_assoc_types.len() == assoc_items.len() {
1964                 // Only suggest when the amount of missing associated types equals the number of
1965                 // extra type arguments present, as that gives us a relatively high confidence
1966                 // that the user forgot to give the associtated type's name. The canonical
1967                 // example would be trying to use `Iterator<isize>` instead of
1968                 // `Iterator<Item = isize>`.
1969                 for (potential, item) in potential_assoc_types.iter().zip(assoc_items.iter()) {
1970                     if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(*potential) {
1971                         suggestions.push((*potential, format!("{} = {}", item.ident, snippet)));
1972                     }
1973                 }
1974             } else if let (Ok(snippet), false) =
1975                 (tcx.sess.source_map().span_to_snippet(*span), dupes)
1976             {
1977                 let types: Vec<_> =
1978                     assoc_items.iter().map(|item| format!("{} = Type", item.ident)).collect();
1979                 let code = if snippet.ends_with('>') {
1980                     // The user wrote `Trait<'a>` or similar and we don't have a type we can
1981                     // suggest, but at least we can clue them to the correct syntax
1982                     // `Trait<'a, Item = Type>` while accounting for the `<'a>` in the
1983                     // suggestion.
1984                     format!("{}, {}>", &snippet[..snippet.len() - 1], types.join(", "))
1985                 } else {
1986                     // The user wrote `Iterator`, so we don't have a type we can suggest, but at
1987                     // least we can clue them to the correct syntax `Iterator<Item = Type>`.
1988                     format!("{}<{}>", snippet, types.join(", "))
1989                 };
1990                 suggestions.push((*span, code));
1991             } else if dupes {
1992                 where_constraints.push(*span);
1993             }
1994         }
1995         let where_msg = "consider introducing a new type parameter, adding `where` constraints \
1996                          using the fully-qualified path to the associated types";
1997         if !where_constraints.is_empty() && suggestions.is_empty() {
1998             // If there are duplicates associated type names and a single trait bound do not
1999             // use structured suggestion, it means that there are multiple super-traits with
2000             // the same associated type name.
2001             err.help(where_msg);
2002         }
2003         if suggestions.len() != 1 {
2004             // We don't need this label if there's an inline suggestion, show otherwise.
2005             for (span, assoc_items) in &associated_types {
2006                 let mut names: FxHashMap<_, usize> = FxHashMap::default();
2007                 for item in assoc_items {
2008                     types_count += 1;
2009                     *names.entry(item.ident.name).or_insert(0) += 1;
2010                 }
2011                 let mut label = vec![];
2012                 for item in assoc_items {
2013                     let postfix = if names[&item.ident.name] > 1 {
2014                         let trait_def_id = item.container.id();
2015                         format!(" (from trait `{}`)", tcx.def_path_str(trait_def_id))
2016                     } else {
2017                         String::new()
2018                     };
2019                     label.push(format!("`{}`{}", item.ident, postfix));
2020                 }
2021                 if !label.is_empty() {
2022                     err.span_label(
2023                         *span,
2024                         format!(
2025                             "associated type{} {} must be specified",
2026                             pluralize!(label.len()),
2027                             label.join(", "),
2028                         ),
2029                     );
2030                 }
2031             }
2032         }
2033         if !suggestions.is_empty() {
2034             err.multipart_suggestion(
2035                 &format!("specify the associated type{}", pluralize!(types_count)),
2036                 suggestions,
2037                 Applicability::HasPlaceholders,
2038             );
2039             if !where_constraints.is_empty() {
2040                 err.span_help(where_constraints, where_msg);
2041             }
2042         }
2043         err.emit();
2044     }
2045
2046     fn report_ambiguous_associated_type(
2047         &self,
2048         span: Span,
2049         type_str: &str,
2050         trait_str: &str,
2051         name: Symbol,
2052     ) {
2053         let mut err = struct_span_err!(self.tcx().sess, span, E0223, "ambiguous associated type");
2054         if let (Some(_), Ok(snippet)) = (
2055             self.tcx().sess.confused_type_with_std_module.borrow().get(&span),
2056             self.tcx().sess.source_map().span_to_snippet(span),
2057         ) {
2058             err.span_suggestion(
2059                 span,
2060                 "you are looking for the module in `std`, not the primitive type",
2061                 format!("std::{}", snippet),
2062                 Applicability::MachineApplicable,
2063             );
2064         } else {
2065             err.span_suggestion(
2066                 span,
2067                 "use fully-qualified syntax",
2068                 format!("<{} as {}>::{}", type_str, trait_str, name),
2069                 Applicability::HasPlaceholders,
2070             );
2071         }
2072         err.emit();
2073     }
2074
2075     // Search for a bound on a type parameter which includes the associated item
2076     // given by `assoc_name`. `ty_param_def_id` is the `DefId` of the type parameter
2077     // This function will fail if there are no suitable bounds or there is
2078     // any ambiguity.
2079     fn find_bound_for_assoc_item(
2080         &self,
2081         ty_param_def_id: LocalDefId,
2082         assoc_name: Ident,
2083         span: Span,
2084     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorReported> {
2085         let tcx = self.tcx();
2086
2087         debug!(
2088             "find_bound_for_assoc_item(ty_param_def_id={:?}, assoc_name={:?}, span={:?})",
2089             ty_param_def_id, assoc_name, span,
2090         );
2091
2092         let predicates =
2093             &self.get_type_parameter_bounds(span, ty_param_def_id.to_def_id()).predicates;
2094
2095         debug!("find_bound_for_assoc_item: predicates={:#?}", predicates);
2096
2097         let param_hir_id = tcx.hir().as_local_hir_id(ty_param_def_id);
2098         let param_name = tcx.hir().ty_param_name(param_hir_id);
2099         self.one_bound_for_assoc_type(
2100             || {
2101                 traits::transitive_bounds(
2102                     tcx,
2103                     predicates.iter().filter_map(|(p, _)| p.to_opt_poly_trait_ref()),
2104                 )
2105             },
2106             || param_name.to_string(),
2107             assoc_name,
2108             span,
2109             || None,
2110         )
2111     }
2112
2113     // Checks that `bounds` contains exactly one element and reports appropriate
2114     // errors otherwise.
2115     fn one_bound_for_assoc_type<I>(
2116         &self,
2117         all_candidates: impl Fn() -> I,
2118         ty_param_name: impl Fn() -> String,
2119         assoc_name: Ident,
2120         span: Span,
2121         is_equality: impl Fn() -> Option<String>,
2122     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorReported>
2123     where
2124         I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
2125     {
2126         let mut matching_candidates = all_candidates()
2127             .filter(|r| self.trait_defines_associated_type_named(r.def_id(), assoc_name));
2128
2129         let bound = match matching_candidates.next() {
2130             Some(bound) => bound,
2131             None => {
2132                 self.complain_about_assoc_type_not_found(
2133                     all_candidates,
2134                     &ty_param_name(),
2135                     assoc_name,
2136                     span,
2137                 );
2138                 return Err(ErrorReported);
2139             }
2140         };
2141
2142         debug!("one_bound_for_assoc_type: bound = {:?}", bound);
2143
2144         if let Some(bound2) = matching_candidates.next() {
2145             debug!("one_bound_for_assoc_type: bound2 = {:?}", bound2);
2146
2147             let is_equality = is_equality();
2148             let bounds = iter::once(bound).chain(iter::once(bound2)).chain(matching_candidates);
2149             let mut err = if is_equality.is_some() {
2150                 // More specific Error Index entry.
2151                 struct_span_err!(
2152                     self.tcx().sess,
2153                     span,
2154                     E0222,
2155                     "ambiguous associated type `{}` in bounds of `{}`",
2156                     assoc_name,
2157                     ty_param_name()
2158                 )
2159             } else {
2160                 struct_span_err!(
2161                     self.tcx().sess,
2162                     span,
2163                     E0221,
2164                     "ambiguous associated type `{}` in bounds of `{}`",
2165                     assoc_name,
2166                     ty_param_name()
2167                 )
2168             };
2169             err.span_label(span, format!("ambiguous associated type `{}`", assoc_name));
2170
2171             let mut where_bounds = vec![];
2172             for bound in bounds {
2173                 let bound_id = bound.def_id();
2174                 let bound_span = self
2175                     .tcx()
2176                     .associated_items(bound_id)
2177                     .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, bound_id)
2178                     .and_then(|item| self.tcx().hir().span_if_local(item.def_id));
2179
2180                 if let Some(bound_span) = bound_span {
2181                     err.span_label(
2182                         bound_span,
2183                         format!(
2184                             "ambiguous `{}` from `{}`",
2185                             assoc_name,
2186                             bound.print_only_trait_path(),
2187                         ),
2188                     );
2189                     if let Some(constraint) = &is_equality {
2190                         where_bounds.push(format!(
2191                             "        T: {trait}::{assoc} = {constraint}",
2192                             trait=bound.print_only_trait_path(),
2193                             assoc=assoc_name,
2194                             constraint=constraint,
2195                         ));
2196                     } else {
2197                         err.span_suggestion(
2198                             span,
2199                             "use fully qualified syntax to disambiguate",
2200                             format!(
2201                                 "<{} as {}>::{}",
2202                                 ty_param_name(),
2203                                 bound.print_only_trait_path(),
2204                                 assoc_name,
2205                             ),
2206                             Applicability::MaybeIncorrect,
2207                         );
2208                     }
2209                 } else {
2210                     err.note(&format!(
2211                         "associated type `{}` could derive from `{}`",
2212                         ty_param_name(),
2213                         bound.print_only_trait_path(),
2214                     ));
2215                 }
2216             }
2217             if !where_bounds.is_empty() {
2218                 err.help(&format!(
2219                     "consider introducing a new type parameter `T` and adding `where` constraints:\
2220                      \n    where\n        T: {},\n{}",
2221                     ty_param_name(),
2222                     where_bounds.join(",\n"),
2223                 ));
2224             }
2225             err.emit();
2226             if !where_bounds.is_empty() {
2227                 return Err(ErrorReported);
2228             }
2229         }
2230         Ok(bound)
2231     }
2232
2233     fn complain_about_assoc_type_not_found<I>(
2234         &self,
2235         all_candidates: impl Fn() -> I,
2236         ty_param_name: &str,
2237         assoc_name: Ident,
2238         span: Span,
2239     ) where
2240         I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
2241     {
2242         // The fallback span is needed because `assoc_name` might be an `Fn()`'s `Output` without a
2243         // valid span, so we point at the whole path segment instead.
2244         let span = if assoc_name.span != DUMMY_SP { assoc_name.span } else { span };
2245         let mut err = struct_span_err!(
2246             self.tcx().sess,
2247             span,
2248             E0220,
2249             "associated type `{}` not found for `{}`",
2250             assoc_name,
2251             ty_param_name
2252         );
2253
2254         let all_candidate_names: Vec<_> = all_candidates()
2255             .map(|r| self.tcx().associated_items(r.def_id()).in_definition_order())
2256             .flatten()
2257             .filter_map(
2258                 |item| if item.kind == ty::AssocKind::Type { Some(item.ident.name) } else { None },
2259             )
2260             .collect();
2261
2262         if let (Some(suggested_name), true) = (
2263             find_best_match_for_name(all_candidate_names.iter(), assoc_name.name, None),
2264             assoc_name.span != DUMMY_SP,
2265         ) {
2266             err.span_suggestion(
2267                 assoc_name.span,
2268                 "there is an associated type with a similar name",
2269                 suggested_name.to_string(),
2270                 Applicability::MaybeIncorrect,
2271             );
2272         } else {
2273             err.span_label(span, format!("associated type `{}` not found", assoc_name));
2274         }
2275
2276         err.emit();
2277     }
2278
2279     // Create a type from a path to an associated type.
2280     // For a path `A::B::C::D`, `qself_ty` and `qself_def` are the type and def for `A::B::C`
2281     // and item_segment is the path segment for `D`. We return a type and a def for
2282     // the whole path.
2283     // Will fail except for `T::A` and `Self::A`; i.e., if `qself_ty`/`qself_def` are not a type
2284     // parameter or `Self`.
2285     pub fn associated_path_to_ty(
2286         &self,
2287         hir_ref_id: hir::HirId,
2288         span: Span,
2289         qself_ty: Ty<'tcx>,
2290         qself_res: Res,
2291         assoc_segment: &hir::PathSegment<'_>,
2292         permit_variants: bool,
2293     ) -> Result<(Ty<'tcx>, DefKind, DefId), ErrorReported> {
2294         let tcx = self.tcx();
2295         let assoc_ident = assoc_segment.ident;
2296
2297         debug!("associated_path_to_ty: {:?}::{}", qself_ty, assoc_ident);
2298
2299         // Check if we have an enum variant.
2300         let mut variant_resolution = None;
2301         if let ty::Adt(adt_def, _) = qself_ty.kind {
2302             if adt_def.is_enum() {
2303                 let variant_def = adt_def
2304                     .variants
2305                     .iter()
2306                     .find(|vd| tcx.hygienic_eq(assoc_ident, vd.ident, adt_def.did));
2307                 if let Some(variant_def) = variant_def {
2308                     if permit_variants {
2309                         tcx.check_stability(variant_def.def_id, Some(hir_ref_id), span);
2310                         self.prohibit_generics(slice::from_ref(assoc_segment));
2311                         return Ok((qself_ty, DefKind::Variant, variant_def.def_id));
2312                     } else {
2313                         variant_resolution = Some(variant_def.def_id);
2314                     }
2315                 }
2316             }
2317         }
2318
2319         // Find the type of the associated item, and the trait where the associated
2320         // item is declared.
2321         let bound = match (&qself_ty.kind, qself_res) {
2322             (_, Res::SelfTy(Some(_), Some(impl_def_id))) => {
2323                 // `Self` in an impl of a trait -- we have a concrete self type and a
2324                 // trait reference.
2325                 let trait_ref = match tcx.impl_trait_ref(impl_def_id) {
2326                     Some(trait_ref) => trait_ref,
2327                     None => {
2328                         // A cycle error occurred, most likely.
2329                         return Err(ErrorReported);
2330                     }
2331                 };
2332
2333                 self.one_bound_for_assoc_type(
2334                     || traits::supertraits(tcx, ty::Binder::bind(trait_ref)),
2335                     || "Self".to_string(),
2336                     assoc_ident,
2337                     span,
2338                     || None,
2339                 )?
2340             }
2341             (
2342                 &ty::Param(_),
2343                 Res::SelfTy(Some(param_did), None) | Res::Def(DefKind::TyParam, param_did),
2344             ) => self.find_bound_for_assoc_item(param_did.expect_local(), assoc_ident, span)?,
2345             _ => {
2346                 if variant_resolution.is_some() {
2347                     // Variant in type position
2348                     let msg = format!("expected type, found variant `{}`", assoc_ident);
2349                     tcx.sess.span_err(span, &msg);
2350                 } else if qself_ty.is_enum() {
2351                     let mut err = struct_span_err!(
2352                         tcx.sess,
2353                         assoc_ident.span,
2354                         E0599,
2355                         "no variant named `{}` found for enum `{}`",
2356                         assoc_ident,
2357                         qself_ty,
2358                     );
2359
2360                     let adt_def = qself_ty.ty_adt_def().expect("enum is not an ADT");
2361                     if let Some(suggested_name) = find_best_match_for_name(
2362                         adt_def.variants.iter().map(|variant| &variant.ident.name),
2363                         assoc_ident.name,
2364                         None,
2365                     ) {
2366                         err.span_suggestion(
2367                             assoc_ident.span,
2368                             "there is a variant with a similar name",
2369                             suggested_name.to_string(),
2370                             Applicability::MaybeIncorrect,
2371                         );
2372                     } else {
2373                         err.span_label(
2374                             assoc_ident.span,
2375                             format!("variant not found in `{}`", qself_ty),
2376                         );
2377                     }
2378
2379                     if let Some(sp) = tcx.hir().span_if_local(adt_def.did) {
2380                         let sp = tcx.sess.source_map().guess_head_span(sp);
2381                         err.span_label(sp, format!("variant `{}` not found here", assoc_ident));
2382                     }
2383
2384                     err.emit();
2385                 } else if !qself_ty.references_error() {
2386                     // Don't print `TyErr` to the user.
2387                     self.report_ambiguous_associated_type(
2388                         span,
2389                         &qself_ty.to_string(),
2390                         "Trait",
2391                         assoc_ident.name,
2392                     );
2393                 }
2394                 return Err(ErrorReported);
2395             }
2396         };
2397
2398         let trait_did = bound.def_id();
2399         let (assoc_ident, def_scope) =
2400             tcx.adjust_ident_and_get_scope(assoc_ident, trait_did, hir_ref_id);
2401
2402         // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
2403         // of calling `filter_by_name_and_kind`.
2404         let item = tcx
2405             .associated_items(trait_did)
2406             .in_definition_order()
2407             .find(|i| {
2408                 i.kind.namespace() == Namespace::TypeNS
2409                     && i.ident.normalize_to_macros_2_0() == assoc_ident
2410             })
2411             .expect("missing associated type");
2412
2413         let ty = self.projected_ty_from_poly_trait_ref(span, item.def_id, assoc_segment, bound);
2414         let ty = self.normalize_ty(span, ty);
2415
2416         let kind = DefKind::AssocTy;
2417         if !item.vis.is_accessible_from(def_scope, tcx) {
2418             let kind = kind.descr(item.def_id);
2419             let msg = format!("{} `{}` is private", kind, assoc_ident);
2420             tcx.sess
2421                 .struct_span_err(span, &msg)
2422                 .span_label(span, &format!("private {}", kind))
2423                 .emit();
2424         }
2425         tcx.check_stability(item.def_id, Some(hir_ref_id), span);
2426
2427         if let Some(variant_def_id) = variant_resolution {
2428             tcx.struct_span_lint_hir(AMBIGUOUS_ASSOCIATED_ITEMS, hir_ref_id, span, |lint| {
2429                 let mut err = lint.build("ambiguous associated item");
2430                 let mut could_refer_to = |kind: DefKind, def_id, also| {
2431                     let note_msg = format!(
2432                         "`{}` could{} refer to the {} defined here",
2433                         assoc_ident,
2434                         also,
2435                         kind.descr(def_id)
2436                     );
2437                     err.span_note(tcx.def_span(def_id), &note_msg);
2438                 };
2439
2440                 could_refer_to(DefKind::Variant, variant_def_id, "");
2441                 could_refer_to(kind, item.def_id, " also");
2442
2443                 err.span_suggestion(
2444                     span,
2445                     "use fully-qualified syntax",
2446                     format!("<{} as {}>::{}", qself_ty, tcx.item_name(trait_did), assoc_ident),
2447                     Applicability::MachineApplicable,
2448                 );
2449
2450                 err.emit();
2451             });
2452         }
2453         Ok((ty, kind, item.def_id))
2454     }
2455
2456     fn qpath_to_ty(
2457         &self,
2458         span: Span,
2459         opt_self_ty: Option<Ty<'tcx>>,
2460         item_def_id: DefId,
2461         trait_segment: &hir::PathSegment<'_>,
2462         item_segment: &hir::PathSegment<'_>,
2463     ) -> Ty<'tcx> {
2464         let tcx = self.tcx();
2465
2466         let trait_def_id = tcx.parent(item_def_id).unwrap();
2467
2468         debug!("qpath_to_ty: trait_def_id={:?}", trait_def_id);
2469
2470         let self_ty = if let Some(ty) = opt_self_ty {
2471             ty
2472         } else {
2473             let path_str = tcx.def_path_str(trait_def_id);
2474
2475             let def_id = self.item_def_id();
2476
2477             debug!("qpath_to_ty: self.item_def_id()={:?}", def_id);
2478
2479             let parent_def_id = def_id
2480                 .and_then(|def_id| {
2481                     def_id.as_local().map(|def_id| tcx.hir().as_local_hir_id(def_id))
2482                 })
2483                 .map(|hir_id| tcx.hir().get_parent_did(hir_id).to_def_id());
2484
2485             debug!("qpath_to_ty: parent_def_id={:?}", parent_def_id);
2486
2487             // If the trait in segment is the same as the trait defining the item,
2488             // use the `<Self as ..>` syntax in the error.
2489             let is_part_of_self_trait_constraints = def_id == Some(trait_def_id);
2490             let is_part_of_fn_in_self_trait = parent_def_id == Some(trait_def_id);
2491
2492             let type_name = if is_part_of_self_trait_constraints || is_part_of_fn_in_self_trait {
2493                 "Self"
2494             } else {
2495                 "Type"
2496             };
2497
2498             self.report_ambiguous_associated_type(
2499                 span,
2500                 type_name,
2501                 &path_str,
2502                 item_segment.ident.name,
2503             );
2504             return tcx.ty_error();
2505         };
2506
2507         debug!("qpath_to_ty: self_type={:?}", self_ty);
2508
2509         let trait_ref = self.ast_path_to_mono_trait_ref(span, trait_def_id, self_ty, trait_segment);
2510
2511         let item_substs = self.create_substs_for_associated_item(
2512             tcx,
2513             span,
2514             item_def_id,
2515             item_segment,
2516             trait_ref.substs,
2517         );
2518
2519         debug!("qpath_to_ty: trait_ref={:?}", trait_ref);
2520
2521         self.normalize_ty(span, tcx.mk_projection(item_def_id, item_substs))
2522     }
2523
2524     pub fn prohibit_generics<'a, T: IntoIterator<Item = &'a hir::PathSegment<'a>>>(
2525         &self,
2526         segments: T,
2527     ) -> bool {
2528         let mut has_err = false;
2529         for segment in segments {
2530             let (mut err_for_lt, mut err_for_ty, mut err_for_ct) = (false, false, false);
2531             for arg in segment.generic_args().args {
2532                 let (span, kind) = match arg {
2533                     hir::GenericArg::Lifetime(lt) => {
2534                         if err_for_lt {
2535                             continue;
2536                         }
2537                         err_for_lt = true;
2538                         has_err = true;
2539                         (lt.span, "lifetime")
2540                     }
2541                     hir::GenericArg::Type(ty) => {
2542                         if err_for_ty {
2543                             continue;
2544                         }
2545                         err_for_ty = true;
2546                         has_err = true;
2547                         (ty.span, "type")
2548                     }
2549                     hir::GenericArg::Const(ct) => {
2550                         if err_for_ct {
2551                             continue;
2552                         }
2553                         err_for_ct = true;
2554                         has_err = true;
2555                         (ct.span, "const")
2556                     }
2557                 };
2558                 let mut err = struct_span_err!(
2559                     self.tcx().sess,
2560                     span,
2561                     E0109,
2562                     "{} arguments are not allowed for this type",
2563                     kind,
2564                 );
2565                 err.span_label(span, format!("{} argument not allowed", kind));
2566                 err.emit();
2567                 if err_for_lt && err_for_ty && err_for_ct {
2568                     break;
2569                 }
2570             }
2571
2572             // Only emit the first error to avoid overloading the user with error messages.
2573             if let [binding, ..] = segment.generic_args().bindings {
2574                 has_err = true;
2575                 Self::prohibit_assoc_ty_binding(self.tcx(), binding.span);
2576             }
2577         }
2578         has_err
2579     }
2580
2581     pub fn prohibit_assoc_ty_binding(tcx: TyCtxt<'_>, span: Span) {
2582         let mut err = struct_span_err!(
2583             tcx.sess,
2584             span,
2585             E0229,
2586             "associated type bindings are not allowed here"
2587         );
2588         err.span_label(span, "associated type not allowed here").emit();
2589     }
2590
2591     /// Prohibits explicit lifetime arguments if late-bound lifetime parameters
2592     /// are present. This is used both for datatypes and function calls.
2593     fn prohibit_explicit_late_bound_lifetimes(
2594         tcx: TyCtxt<'_>,
2595         def: &ty::Generics,
2596         args: &hir::GenericArgs<'_>,
2597         position: GenericArgPosition,
2598     ) -> ExplicitLateBound {
2599         let param_counts = def.own_counts();
2600         let arg_counts = args.own_counts();
2601         let infer_lifetimes = position != GenericArgPosition::Type && arg_counts.lifetimes == 0;
2602
2603         if infer_lifetimes {
2604             ExplicitLateBound::No
2605         } else if let Some(span_late) = def.has_late_bound_regions {
2606             let msg = "cannot specify lifetime arguments explicitly \
2607                        if late bound lifetime parameters are present";
2608             let note = "the late bound lifetime parameter is introduced here";
2609             let span = args.args[0].span();
2610             if position == GenericArgPosition::Value
2611                 && arg_counts.lifetimes != param_counts.lifetimes
2612             {
2613                 let mut err = tcx.sess.struct_span_err(span, msg);
2614                 err.span_note(span_late, note);
2615                 err.emit();
2616             } else {
2617                 let mut multispan = MultiSpan::from_span(span);
2618                 multispan.push_span_label(span_late, note.to_string());
2619                 tcx.struct_span_lint_hir(
2620                     LATE_BOUND_LIFETIME_ARGUMENTS,
2621                     args.args[0].id(),
2622                     multispan,
2623                     |lint| lint.build(msg).emit(),
2624                 );
2625             }
2626             ExplicitLateBound::Yes
2627         } else {
2628             ExplicitLateBound::No
2629         }
2630     }
2631
2632     // FIXME(eddyb, varkor) handle type paths here too, not just value ones.
2633     pub fn def_ids_for_value_path_segments(
2634         &self,
2635         segments: &[hir::PathSegment<'_>],
2636         self_ty: Option<Ty<'tcx>>,
2637         kind: DefKind,
2638         def_id: DefId,
2639     ) -> Vec<PathSeg> {
2640         // We need to extract the type parameters supplied by the user in
2641         // the path `path`. Due to the current setup, this is a bit of a
2642         // tricky-process; the problem is that resolve only tells us the
2643         // end-point of the path resolution, and not the intermediate steps.
2644         // Luckily, we can (at least for now) deduce the intermediate steps
2645         // just from the end-point.
2646         //
2647         // There are basically five cases to consider:
2648         //
2649         // 1. Reference to a constructor of a struct:
2650         //
2651         //        struct Foo<T>(...)
2652         //
2653         //    In this case, the parameters are declared in the type space.
2654         //
2655         // 2. Reference to a constructor of an enum variant:
2656         //
2657         //        enum E<T> { Foo(...) }
2658         //
2659         //    In this case, the parameters are defined in the type space,
2660         //    but may be specified either on the type or the variant.
2661         //
2662         // 3. Reference to a fn item or a free constant:
2663         //
2664         //        fn foo<T>() { }
2665         //
2666         //    In this case, the path will again always have the form
2667         //    `a::b::foo::<T>` where only the final segment should have
2668         //    type parameters. However, in this case, those parameters are
2669         //    declared on a value, and hence are in the `FnSpace`.
2670         //
2671         // 4. Reference to a method or an associated constant:
2672         //
2673         //        impl<A> SomeStruct<A> {
2674         //            fn foo<B>(...)
2675         //        }
2676         //
2677         //    Here we can have a path like
2678         //    `a::b::SomeStruct::<A>::foo::<B>`, in which case parameters
2679         //    may appear in two places. The penultimate segment,
2680         //    `SomeStruct::<A>`, contains parameters in TypeSpace, and the
2681         //    final segment, `foo::<B>` contains parameters in fn space.
2682         //
2683         // The first step then is to categorize the segments appropriately.
2684
2685         let tcx = self.tcx();
2686
2687         assert!(!segments.is_empty());
2688         let last = segments.len() - 1;
2689
2690         let mut path_segs = vec![];
2691
2692         match kind {
2693             // Case 1. Reference to a struct constructor.
2694             DefKind::Ctor(CtorOf::Struct, ..) => {
2695                 // Everything but the final segment should have no
2696                 // parameters at all.
2697                 let generics = tcx.generics_of(def_id);
2698                 // Variant and struct constructors use the
2699                 // generics of their parent type definition.
2700                 let generics_def_id = generics.parent.unwrap_or(def_id);
2701                 path_segs.push(PathSeg(generics_def_id, last));
2702             }
2703
2704             // Case 2. Reference to a variant constructor.
2705             DefKind::Ctor(CtorOf::Variant, ..) | DefKind::Variant => {
2706                 let adt_def = self_ty.map(|t| t.ty_adt_def().unwrap());
2707                 let (generics_def_id, index) = if let Some(adt_def) = adt_def {
2708                     debug_assert!(adt_def.is_enum());
2709                     (adt_def.did, last)
2710                 } else if last >= 1 && segments[last - 1].args.is_some() {
2711                     // Everything but the penultimate segment should have no
2712                     // parameters at all.
2713                     let mut def_id = def_id;
2714
2715                     // `DefKind::Ctor` -> `DefKind::Variant`
2716                     if let DefKind::Ctor(..) = kind {
2717                         def_id = tcx.parent(def_id).unwrap()
2718                     }
2719
2720                     // `DefKind::Variant` -> `DefKind::Enum`
2721                     let enum_def_id = tcx.parent(def_id).unwrap();
2722                     (enum_def_id, last - 1)
2723                 } else {
2724                     // FIXME: lint here recommending `Enum::<...>::Variant` form
2725                     // instead of `Enum::Variant::<...>` form.
2726
2727                     // Everything but the final segment should have no
2728                     // parameters at all.
2729                     let generics = tcx.generics_of(def_id);
2730                     // Variant and struct constructors use the
2731                     // generics of their parent type definition.
2732                     (generics.parent.unwrap_or(def_id), last)
2733                 };
2734                 path_segs.push(PathSeg(generics_def_id, index));
2735             }
2736
2737             // Case 3. Reference to a top-level value.
2738             DefKind::Fn | DefKind::Const | DefKind::ConstParam | DefKind::Static => {
2739                 path_segs.push(PathSeg(def_id, last));
2740             }
2741
2742             // Case 4. Reference to a method or associated const.
2743             DefKind::AssocFn | DefKind::AssocConst => {
2744                 if segments.len() >= 2 {
2745                     let generics = tcx.generics_of(def_id);
2746                     path_segs.push(PathSeg(generics.parent.unwrap(), last - 1));
2747                 }
2748                 path_segs.push(PathSeg(def_id, last));
2749             }
2750
2751             kind => bug!("unexpected definition kind {:?} for {:?}", kind, def_id),
2752         }
2753
2754         debug!("path_segs = {:?}", path_segs);
2755
2756         path_segs
2757     }
2758
2759     // Check a type `Path` and convert it to a `Ty`.
2760     pub fn res_to_ty(
2761         &self,
2762         opt_self_ty: Option<Ty<'tcx>>,
2763         path: &hir::Path<'_>,
2764         permit_variants: bool,
2765     ) -> Ty<'tcx> {
2766         let tcx = self.tcx();
2767
2768         debug!(
2769             "res_to_ty(res={:?}, opt_self_ty={:?}, path_segments={:?})",
2770             path.res, opt_self_ty, path.segments
2771         );
2772
2773         let span = path.span;
2774         match path.res {
2775             Res::Def(DefKind::OpaqueTy, did) => {
2776                 // Check for desugared `impl Trait`.
2777                 assert!(ty::is_impl_trait_defn(tcx, did).is_none());
2778                 let item_segment = path.segments.split_last().unwrap();
2779                 self.prohibit_generics(item_segment.1);
2780                 let substs = self.ast_path_substs_for_ty(span, did, item_segment.0);
2781                 self.normalize_ty(span, tcx.mk_opaque(did, substs))
2782             }
2783             Res::Def(
2784                 DefKind::Enum
2785                 | DefKind::TyAlias
2786                 | DefKind::Struct
2787                 | DefKind::Union
2788                 | DefKind::ForeignTy,
2789                 did,
2790             ) => {
2791                 assert_eq!(opt_self_ty, None);
2792                 self.prohibit_generics(path.segments.split_last().unwrap().1);
2793                 self.ast_path_to_ty(span, did, path.segments.last().unwrap())
2794             }
2795             Res::Def(kind @ DefKind::Variant, def_id) if permit_variants => {
2796                 // Convert "variant type" as if it were a real type.
2797                 // The resulting `Ty` is type of the variant's enum for now.
2798                 assert_eq!(opt_self_ty, None);
2799
2800                 let path_segs =
2801                     self.def_ids_for_value_path_segments(&path.segments, None, kind, def_id);
2802                 let generic_segs: FxHashSet<_> =
2803                     path_segs.iter().map(|PathSeg(_, index)| index).collect();
2804                 self.prohibit_generics(path.segments.iter().enumerate().filter_map(
2805                     |(index, seg)| {
2806                         if !generic_segs.contains(&index) { Some(seg) } else { None }
2807                     },
2808                 ));
2809
2810                 let PathSeg(def_id, index) = path_segs.last().unwrap();
2811                 self.ast_path_to_ty(span, *def_id, &path.segments[*index])
2812             }
2813             Res::Def(DefKind::TyParam, def_id) => {
2814                 assert_eq!(opt_self_ty, None);
2815                 self.prohibit_generics(path.segments);
2816
2817                 let hir_id = tcx.hir().as_local_hir_id(def_id.expect_local());
2818                 let item_id = tcx.hir().get_parent_node(hir_id);
2819                 let item_def_id = tcx.hir().local_def_id(item_id);
2820                 let generics = tcx.generics_of(item_def_id);
2821                 let index = generics.param_def_id_to_index[&def_id];
2822                 tcx.mk_ty_param(index, tcx.hir().name(hir_id))
2823             }
2824             Res::SelfTy(Some(_), None) => {
2825                 // `Self` in trait or type alias.
2826                 assert_eq!(opt_self_ty, None);
2827                 self.prohibit_generics(path.segments);
2828                 tcx.types.self_param
2829             }
2830             Res::SelfTy(_, Some(def_id)) => {
2831                 // `Self` in impl (we know the concrete type).
2832                 assert_eq!(opt_self_ty, None);
2833                 self.prohibit_generics(path.segments);
2834                 // Try to evaluate any array length constants.
2835                 self.normalize_ty(span, tcx.at(span).type_of(def_id))
2836             }
2837             Res::Def(DefKind::AssocTy, def_id) => {
2838                 debug_assert!(path.segments.len() >= 2);
2839                 self.prohibit_generics(&path.segments[..path.segments.len() - 2]);
2840                 self.qpath_to_ty(
2841                     span,
2842                     opt_self_ty,
2843                     def_id,
2844                     &path.segments[path.segments.len() - 2],
2845                     path.segments.last().unwrap(),
2846                 )
2847             }
2848             Res::PrimTy(prim_ty) => {
2849                 assert_eq!(opt_self_ty, None);
2850                 self.prohibit_generics(path.segments);
2851                 match prim_ty {
2852                     hir::PrimTy::Bool => tcx.types.bool,
2853                     hir::PrimTy::Char => tcx.types.char,
2854                     hir::PrimTy::Int(it) => tcx.mk_mach_int(it),
2855                     hir::PrimTy::Uint(uit) => tcx.mk_mach_uint(uit),
2856                     hir::PrimTy::Float(ft) => tcx.mk_mach_float(ft),
2857                     hir::PrimTy::Str => tcx.types.str_,
2858                 }
2859             }
2860             Res::Err => {
2861                 self.set_tainted_by_errors();
2862                 self.tcx().ty_error()
2863             }
2864             _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
2865         }
2866     }
2867
2868     /// Parses the programmer's textual representation of a type into our
2869     /// internal notion of a type.
2870     pub fn ast_ty_to_ty(&self, ast_ty: &hir::Ty<'_>) -> Ty<'tcx> {
2871         debug!("ast_ty_to_ty(id={:?}, ast_ty={:?} ty_ty={:?})", ast_ty.hir_id, ast_ty, ast_ty.kind);
2872
2873         let tcx = self.tcx();
2874
2875         let result_ty = match ast_ty.kind {
2876             hir::TyKind::Slice(ref ty) => tcx.mk_slice(self.ast_ty_to_ty(&ty)),
2877             hir::TyKind::Ptr(ref mt) => {
2878                 tcx.mk_ptr(ty::TypeAndMut { ty: self.ast_ty_to_ty(&mt.ty), mutbl: mt.mutbl })
2879             }
2880             hir::TyKind::Rptr(ref region, ref mt) => {
2881                 let r = self.ast_region_to_region(region, None);
2882                 debug!("ast_ty_to_ty: r={:?}", r);
2883                 let t = self.ast_ty_to_ty(&mt.ty);
2884                 tcx.mk_ref(r, ty::TypeAndMut { ty: t, mutbl: mt.mutbl })
2885             }
2886             hir::TyKind::Never => tcx.types.never,
2887             hir::TyKind::Tup(ref fields) => {
2888                 tcx.mk_tup(fields.iter().map(|t| self.ast_ty_to_ty(&t)))
2889             }
2890             hir::TyKind::BareFn(ref bf) => {
2891                 require_c_abi_if_c_variadic(tcx, &bf.decl, bf.abi, ast_ty.span);
2892                 tcx.mk_fn_ptr(self.ty_of_fn(
2893                     bf.unsafety,
2894                     bf.abi,
2895                     &bf.decl,
2896                     &hir::Generics::empty(),
2897                     None,
2898                 ))
2899             }
2900             hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
2901                 self.conv_object_ty_poly_trait_ref(ast_ty.span, bounds, lifetime)
2902             }
2903             hir::TyKind::Path(hir::QPath::Resolved(ref maybe_qself, ref path)) => {
2904                 debug!("ast_ty_to_ty: maybe_qself={:?} path={:?}", maybe_qself, path);
2905                 let opt_self_ty = maybe_qself.as_ref().map(|qself| self.ast_ty_to_ty(qself));
2906                 self.res_to_ty(opt_self_ty, path, false)
2907             }
2908             hir::TyKind::OpaqueDef(item_id, ref lifetimes) => {
2909                 let opaque_ty = tcx.hir().expect_item(item_id.id);
2910                 let def_id = tcx.hir().local_def_id(item_id.id).to_def_id();
2911
2912                 match opaque_ty.kind {
2913                     hir::ItemKind::OpaqueTy(hir::OpaqueTy { impl_trait_fn, .. }) => {
2914                         self.impl_trait_ty_to_ty(def_id, lifetimes, impl_trait_fn.is_some())
2915                     }
2916                     ref i => bug!("`impl Trait` pointed to non-opaque type?? {:#?}", i),
2917                 }
2918             }
2919             hir::TyKind::Path(hir::QPath::TypeRelative(ref qself, ref segment)) => {
2920                 debug!("ast_ty_to_ty: qself={:?} segment={:?}", qself, segment);
2921                 let ty = self.ast_ty_to_ty(qself);
2922
2923                 let res = if let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind {
2924                     path.res
2925                 } else {
2926                     Res::Err
2927                 };
2928                 self.associated_path_to_ty(ast_ty.hir_id, ast_ty.span, ty, res, segment, false)
2929                     .map(|(ty, _, _)| ty)
2930                     .unwrap_or_else(|_| tcx.ty_error())
2931             }
2932             hir::TyKind::Array(ref ty, ref length) => {
2933                 let length_def_id = tcx.hir().local_def_id(length.hir_id);
2934                 let length = ty::Const::from_anon_const(tcx, length_def_id);
2935                 let array_ty = tcx.mk_ty(ty::Array(self.ast_ty_to_ty(&ty), length));
2936                 self.normalize_ty(ast_ty.span, array_ty)
2937             }
2938             hir::TyKind::Typeof(ref _e) => {
2939                 struct_span_err!(
2940                     tcx.sess,
2941                     ast_ty.span,
2942                     E0516,
2943                     "`typeof` is a reserved keyword but unimplemented"
2944                 )
2945                 .span_label(ast_ty.span, "reserved keyword")
2946                 .emit();
2947
2948                 tcx.ty_error()
2949             }
2950             hir::TyKind::Infer => {
2951                 // Infer also appears as the type of arguments or return
2952                 // values in a ExprKind::Closure, or as
2953                 // the type of local variables. Both of these cases are
2954                 // handled specially and will not descend into this routine.
2955                 self.ty_infer(None, ast_ty.span)
2956             }
2957             hir::TyKind::Err => tcx.ty_error(),
2958         };
2959
2960         debug!("ast_ty_to_ty: result_ty={:?}", result_ty);
2961
2962         self.record_ty(ast_ty.hir_id, result_ty, ast_ty.span);
2963         result_ty
2964     }
2965
2966     pub fn impl_trait_ty_to_ty(
2967         &self,
2968         def_id: DefId,
2969         lifetimes: &[hir::GenericArg<'_>],
2970         replace_parent_lifetimes: bool,
2971     ) -> Ty<'tcx> {
2972         debug!("impl_trait_ty_to_ty(def_id={:?}, lifetimes={:?})", def_id, lifetimes);
2973         let tcx = self.tcx();
2974
2975         let generics = tcx.generics_of(def_id);
2976
2977         debug!("impl_trait_ty_to_ty: generics={:?}", generics);
2978         let substs = InternalSubsts::for_item(tcx, def_id, |param, _| {
2979             if let Some(i) = (param.index as usize).checked_sub(generics.parent_count) {
2980                 // Our own parameters are the resolved lifetimes.
2981                 match param.kind {
2982                     GenericParamDefKind::Lifetime => {
2983                         if let hir::GenericArg::Lifetime(lifetime) = &lifetimes[i] {
2984                             self.ast_region_to_region(lifetime, None).into()
2985                         } else {
2986                             bug!()
2987                         }
2988                     }
2989                     _ => bug!(),
2990                 }
2991             } else {
2992                 match param.kind {
2993                     // For RPIT (return position impl trait), only lifetimes
2994                     // mentioned in the impl Trait predicate are captured by
2995                     // the opaque type, so the lifetime parameters from the
2996                     // parent item need to be replaced with `'static`.
2997                     //
2998                     // For `impl Trait` in the types of statics, constants,
2999                     // locals and type aliases. These capture all parent
3000                     // lifetimes, so they can use their identity subst.
3001                     GenericParamDefKind::Lifetime if replace_parent_lifetimes => {
3002                         tcx.lifetimes.re_static.into()
3003                     }
3004                     _ => tcx.mk_param_from_def(param),
3005                 }
3006             }
3007         });
3008         debug!("impl_trait_ty_to_ty: substs={:?}", substs);
3009
3010         let ty = tcx.mk_opaque(def_id, substs);
3011         debug!("impl_trait_ty_to_ty: {}", ty);
3012         ty
3013     }
3014
3015     pub fn ty_of_arg(&self, ty: &hir::Ty<'_>, expected_ty: Option<Ty<'tcx>>) -> Ty<'tcx> {
3016         match ty.kind {
3017             hir::TyKind::Infer if expected_ty.is_some() => {
3018                 self.record_ty(ty.hir_id, expected_ty.unwrap(), ty.span);
3019                 expected_ty.unwrap()
3020             }
3021             _ => self.ast_ty_to_ty(ty),
3022         }
3023     }
3024
3025     pub fn ty_of_fn(
3026         &self,
3027         unsafety: hir::Unsafety,
3028         abi: abi::Abi,
3029         decl: &hir::FnDecl<'_>,
3030         generics: &hir::Generics<'_>,
3031         ident_span: Option<Span>,
3032     ) -> ty::PolyFnSig<'tcx> {
3033         debug!("ty_of_fn");
3034
3035         let tcx = self.tcx();
3036
3037         // We proactively collect all the inferred type params to emit a single error per fn def.
3038         let mut visitor = PlaceholderHirTyCollector::default();
3039         for ty in decl.inputs {
3040             visitor.visit_ty(ty);
3041         }
3042         walk_generics(&mut visitor, generics);
3043
3044         let input_tys = decl.inputs.iter().map(|a| self.ty_of_arg(a, None));
3045         let output_ty = match decl.output {
3046             hir::FnRetTy::Return(ref output) => {
3047                 visitor.visit_ty(output);
3048                 self.ast_ty_to_ty(output)
3049             }
3050             hir::FnRetTy::DefaultReturn(..) => tcx.mk_unit(),
3051         };
3052
3053         debug!("ty_of_fn: output_ty={:?}", output_ty);
3054
3055         let bare_fn_ty =
3056             ty::Binder::bind(tcx.mk_fn_sig(input_tys, output_ty, decl.c_variadic, unsafety, abi));
3057
3058         if !self.allow_ty_infer() {
3059             // We always collect the spans for placeholder types when evaluating `fn`s, but we
3060             // only want to emit an error complaining about them if infer types (`_`) are not
3061             // allowed. `allow_ty_infer` gates this behavior. We check for the presence of
3062             // `ident_span` to not emit an error twice when we have `fn foo(_: fn() -> _)`.
3063             crate::collect::placeholder_type_error(
3064                 tcx,
3065                 ident_span.map(|sp| sp.shrink_to_hi()),
3066                 &generics.params[..],
3067                 visitor.0,
3068                 true,
3069             );
3070         }
3071
3072         // Find any late-bound regions declared in return type that do
3073         // not appear in the arguments. These are not well-formed.
3074         //
3075         // Example:
3076         //     for<'a> fn() -> &'a str <-- 'a is bad
3077         //     for<'a> fn(&'a String) -> &'a str <-- 'a is ok
3078         let inputs = bare_fn_ty.inputs();
3079         let late_bound_in_args =
3080             tcx.collect_constrained_late_bound_regions(&inputs.map_bound(|i| i.to_owned()));
3081         let output = bare_fn_ty.output();
3082         let late_bound_in_ret = tcx.collect_referenced_late_bound_regions(&output);
3083         for br in late_bound_in_ret.difference(&late_bound_in_args) {
3084             let lifetime_name = match *br {
3085                 ty::BrNamed(_, name) => format!("lifetime `{}`,", name),
3086                 ty::BrAnon(_) | ty::BrEnv => "an anonymous lifetime".to_string(),
3087             };
3088             let mut err = struct_span_err!(
3089                 tcx.sess,
3090                 decl.output.span(),
3091                 E0581,
3092                 "return type references {} which is not constrained by the fn input types",
3093                 lifetime_name
3094             );
3095             if let ty::BrAnon(_) = *br {
3096                 // The only way for an anonymous lifetime to wind up
3097                 // in the return type but **also** be unconstrained is
3098                 // if it only appears in "associated types" in the
3099                 // input. See #47511 for an example. In this case,
3100                 // though we can easily give a hint that ought to be
3101                 // relevant.
3102                 err.note(
3103                     "lifetimes appearing in an associated type are not considered constrained",
3104                 );
3105             }
3106             err.emit();
3107         }
3108
3109         bare_fn_ty
3110     }
3111
3112     /// Given the bounds on an object, determines what single region bound (if any) we can
3113     /// use to summarize this type. The basic idea is that we will use the bound the user
3114     /// provided, if they provided one, and otherwise search the supertypes of trait bounds
3115     /// for region bounds. It may be that we can derive no bound at all, in which case
3116     /// we return `None`.
3117     fn compute_object_lifetime_bound(
3118         &self,
3119         span: Span,
3120         existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
3121     ) -> Option<ty::Region<'tcx>> // if None, use the default
3122     {
3123         let tcx = self.tcx();
3124
3125         debug!("compute_opt_region_bound(existential_predicates={:?})", existential_predicates);
3126
3127         // No explicit region bound specified. Therefore, examine trait
3128         // bounds and see if we can derive region bounds from those.
3129         let derived_region_bounds = object_region_bounds(tcx, existential_predicates);
3130
3131         // If there are no derived region bounds, then report back that we
3132         // can find no region bound. The caller will use the default.
3133         if derived_region_bounds.is_empty() {
3134             return None;
3135         }
3136
3137         // If any of the derived region bounds are 'static, that is always
3138         // the best choice.
3139         if derived_region_bounds.iter().any(|&r| ty::ReStatic == *r) {
3140             return Some(tcx.lifetimes.re_static);
3141         }
3142
3143         // Determine whether there is exactly one unique region in the set
3144         // of derived region bounds. If so, use that. Otherwise, report an
3145         // error.
3146         let r = derived_region_bounds[0];
3147         if derived_region_bounds[1..].iter().any(|r1| r != *r1) {
3148             struct_span_err!(
3149                 tcx.sess,
3150                 span,
3151                 E0227,
3152                 "ambiguous lifetime bound, explicit lifetime bound required"
3153             )
3154             .emit();
3155         }
3156         Some(r)
3157     }
3158 }
3159
3160 /// Collects together a list of bounds that are applied to some type,
3161 /// after they've been converted into `ty` form (from the HIR
3162 /// representations). These lists of bounds occur in many places in
3163 /// Rust's syntax:
3164 ///
3165 /// ```text
3166 /// trait Foo: Bar + Baz { }
3167 ///            ^^^^^^^^^ supertrait list bounding the `Self` type parameter
3168 ///
3169 /// fn foo<T: Bar + Baz>() { }
3170 ///           ^^^^^^^^^ bounding the type parameter `T`
3171 ///
3172 /// impl dyn Bar + Baz
3173 ///          ^^^^^^^^^ bounding the forgotten dynamic type
3174 /// ```
3175 ///
3176 /// Our representation is a bit mixed here -- in some cases, we
3177 /// include the self type (e.g., `trait_bounds`) but in others we do
3178 #[derive(Default, PartialEq, Eq, Clone, Debug)]
3179 pub struct Bounds<'tcx> {
3180     /// A list of region bounds on the (implicit) self type. So if you
3181     /// had `T: 'a + 'b` this might would be a list `['a, 'b]` (but
3182     /// the `T` is not explicitly included).
3183     pub region_bounds: Vec<(ty::Region<'tcx>, Span)>,
3184
3185     /// A list of trait bounds. So if you had `T: Debug` this would be
3186     /// `T: Debug`. Note that the self-type is explicit here.
3187     pub trait_bounds: Vec<(ty::PolyTraitRef<'tcx>, Span, Constness)>,
3188
3189     /// A list of projection equality bounds. So if you had `T:
3190     /// Iterator<Item = u32>` this would include `<T as
3191     /// Iterator>::Item => u32`. Note that the self-type is explicit
3192     /// here.
3193     pub projection_bounds: Vec<(ty::PolyProjectionPredicate<'tcx>, Span)>,
3194
3195     /// `Some` if there is *no* `?Sized` predicate. The `span`
3196     /// is the location in the source of the `T` declaration which can
3197     /// be cited as the source of the `T: Sized` requirement.
3198     pub implicitly_sized: Option<Span>,
3199 }
3200
3201 impl<'tcx> Bounds<'tcx> {
3202     /// Converts a bounds list into a flat set of predicates (like
3203     /// where-clauses). Because some of our bounds listings (e.g.,
3204     /// regions) don't include the self-type, you must supply the
3205     /// self-type here (the `param_ty` parameter).
3206     pub fn predicates(
3207         &self,
3208         tcx: TyCtxt<'tcx>,
3209         param_ty: Ty<'tcx>,
3210     ) -> Vec<(ty::Predicate<'tcx>, Span)> {
3211         // If it could be sized, and is, add the `Sized` predicate.
3212         let sized_predicate = self.implicitly_sized.and_then(|span| {
3213             tcx.lang_items().sized_trait().map(|sized| {
3214                 let trait_ref = ty::Binder::bind(ty::TraitRef {
3215                     def_id: sized,
3216                     substs: tcx.mk_substs_trait(param_ty, &[]),
3217                 });
3218                 (trait_ref.without_const().to_predicate(tcx), span)
3219             })
3220         });
3221
3222         sized_predicate
3223             .into_iter()
3224             .chain(
3225                 self.region_bounds
3226                     .iter()
3227                     .map(|&(region_bound, span)| {
3228                         // Account for the binder being introduced below; no need to shift `param_ty`
3229                         // because, at present at least, it either only refers to early-bound regions,
3230                         // or it's a generic associated type that deliberately has escaping bound vars.
3231                         let region_bound = ty::fold::shift_region(tcx, region_bound, 1);
3232                         let outlives = ty::OutlivesPredicate(param_ty, region_bound);
3233                         (ty::Binder::bind(outlives).to_predicate(tcx), span)
3234                     })
3235                     .chain(self.trait_bounds.iter().map(|&(bound_trait_ref, span, constness)| {
3236                         let predicate = bound_trait_ref.with_constness(constness).to_predicate(tcx);
3237                         (predicate, span)
3238                     }))
3239                     .chain(
3240                         self.projection_bounds
3241                             .iter()
3242                             .map(|&(projection, span)| (projection.to_predicate(tcx), span)),
3243                     ),
3244             )
3245             .collect()
3246     }
3247 }