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                     let ct_def_id = tcx.hir().local_def_id(ct.value.hir_id);
 890                     ty::Const::from_anon_const(tcx, ct_def_id).into()
 891                 }
 892                 _ => unreachable!(),
 893             },
 894             // Provide substitutions for parameters for which arguments are inferred.
 895             |substs, param, infer_args| {
 896                 match param.kind {
 897                     GenericParamDefKind::Lifetime => tcx.lifetimes.re_static.into(),
 898                     GenericParamDefKind::Type { has_default, .. } => {
 899                         if !infer_args && has_default {
 900                             // No type parameter provided, but a default exists.
 901
 902                             // If we are converting an object type, then the
 903                             // `Self` parameter is unknown. However, some of the
 904                             // other type parameters may reference `Self` in their
 905                             // defaults. This will lead to an ICE if we are not
 906                             // careful!
 907                             if default_needs_object_self(param) {
 908                                 missing_type_params.push(param.name.to_string());
 909                                 tcx.ty_error().into()
 910                             } else {
 911                                 // This is a default type parameter.
 912                                 self.normalize_ty(
 913                                     span,
 914                                     tcx.at(span).type_of(param.def_id).subst_spanned(
 915                                         tcx,
 916                                         substs.unwrap(),
 917                                         Some(span),
 918                                     ),
 919                                 )
 920                                 .into()
 921                             }
 922                         } else if infer_args {
 923                             // No type parameters were provided, we can infer all.
 924                             let param =
 925                                 if !default_needs_object_self(param) { Some(param) } else { None };
 926                             self.ty_infer(param, span).into()
 927                         } else {
 928                             // We've already errored above about the mismatch.
 929                             tcx.ty_error().into()
 930                         }
 931                     }
 932                     GenericParamDefKind::Const => {
 933                         let ty = tcx.at(span).type_of(param.def_id);
 934                         // FIXME(const_generics:defaults)
 935                         if infer_args {
 936                             // No const parameters were provided, we can infer all.
 937                             self.ct_infer(ty, Some(param), span).into()
 938                         } else {
 939                             // We've already errored above about the mismatch.
 940                             tcx.const_error(ty).into()
 941                         }
 942                     }
 943                 }
 944             },
 945         );
 946
 947         self.complain_about_missing_type_params(
 948             missing_type_params,
 949             def_id,
 950             span,
 951             generic_args.args.is_empty(),
 952         );
 953
 954         // Convert associated-type bindings or constraints into a separate vector.
 955         // Example: Given this:
 956         //
 957         //     T: Iterator<Item = u32>
 958         //
 959         // The `T` is passed in as a self-type; the `Item = u32` is
 960         // not a "type parameter" of the `Iterator` trait, but rather
 961         // a restriction on `<T as Iterator>::Item`, so it is passed
 962         // back separately.
 963         let assoc_bindings = generic_args
 964             .bindings
 965             .iter()
 966             .map(|binding| {
 967                 let kind = match binding.kind {
 968                     hir::TypeBindingKind::Equality { ref ty } => {
 969                         ConvertedBindingKind::Equality(self.ast_ty_to_ty(ty))
 970                     }
 971                     hir::TypeBindingKind::Constraint { ref bounds } => {
 972                         ConvertedBindingKind::Constraint(bounds)
 973                     }
 974                 };
 975                 ConvertedBinding { item_name: binding.ident, kind, span: binding.span }
 976             })
 977             .collect();
 978
 979         debug!(
 980             "create_substs_for_ast_path(generic_params={:?}, self_ty={:?}) -> {:?}",
 981             generic_params, self_ty, substs
 982         );
 983
 984         (substs, assoc_bindings, arg_count)
 985     }
 986
 987     crate fn create_substs_for_associated_item(
 988         &self,
 989         tcx: TyCtxt<'tcx>,
 990         span: Span,
 991         item_def_id: DefId,
 992         item_segment: &hir::PathSegment<'_>,
 993         parent_substs: SubstsRef<'tcx>,
 994     ) -> SubstsRef<'tcx> {
 995         if tcx.generics_of(item_def_id).params.is_empty() {
 996             self.prohibit_generics(slice::from_ref(item_segment));
 997
 998             parent_substs
 999         } else {
1000             self.create_substs_for_ast_path(
1001                 span,
1002                 item_def_id,
1003                 parent_substs,
1004                 item_segment.generic_args(),
1005                 item_segment.infer_args,
1006                 None,
1007             )
1008             .0
1009         }
1010     }
1011
1012     /// On missing type parameters, emit an E0393 error and provide a structured suggestion using
1013     /// the type parameter's name as a placeholder.
1014     fn complain_about_missing_type_params(
1015         &self,
1016         missing_type_params: Vec<String>,
1017         def_id: DefId,
1018         span: Span,
1019         empty_generic_args: bool,
1020     ) {
1021         if missing_type_params.is_empty() {
1022             return;
1023         }
1024         let display =
1025             missing_type_params.iter().map(|n| format!("`{}`", n)).collect::<Vec<_>>().join(", ");
1026         let mut err = struct_span_err!(
1027             self.tcx().sess,
1028             span,
1029             E0393,
1030             "the type parameter{} {} must be explicitly specified",
1031             pluralize!(missing_type_params.len()),
1032             display,
1033         );
1034         err.span_label(
1035             self.tcx().def_span(def_id),
1036             &format!(
1037                 "type parameter{} {} must be specified for this",
1038                 pluralize!(missing_type_params.len()),
1039                 display,
1040             ),
1041         );
1042         let mut suggested = false;
1043         if let (Ok(snippet), true) = (
1044             self.tcx().sess.source_map().span_to_snippet(span),
1045             // Don't suggest setting the type params if there are some already: the order is
1046             // tricky to get right and the user will already know what the syntax is.
1047             empty_generic_args,
1048         ) {
1049             if snippet.ends_with('>') {
1050                 // The user wrote `Trait<'a, T>` or similar. To provide an accurate suggestion
1051                 // we would have to preserve the right order. For now, as clearly the user is
1052                 // aware of the syntax, we do nothing.
1053             } else {
1054                 // The user wrote `Iterator`, so we don't have a type we can suggest, but at
1055                 // least we can clue them to the correct syntax `Iterator<Type>`.
1056                 err.span_suggestion(
1057                     span,
1058                     &format!(
1059                         "set the type parameter{plural} to the desired type{plural}",
1060                         plural = pluralize!(missing_type_params.len()),
1061                     ),
1062                     format!("{}<{}>", snippet, missing_type_params.join(", ")),
1063                     Applicability::HasPlaceholders,
1064                 );
1065                 suggested = true;
1066             }
1067         }
1068         if !suggested {
1069             err.span_label(
1070                 span,
1071                 format!(
1072                     "missing reference{} to {}",
1073                     pluralize!(missing_type_params.len()),
1074                     display,
1075                 ),
1076             );
1077         }
1078         err.note(
1079             "because of the default `Self` reference, type parameters must be \
1080                   specified on object types",
1081         );
1082         err.emit();
1083     }
1084
1085     /// Instantiates the path for the given trait reference, assuming that it's
1086     /// bound to a valid trait type. Returns the `DefId` of the defining trait.
1087     /// The type _cannot_ be a type other than a trait type.
1088     ///
1089     /// If the `projections` argument is `None`, then assoc type bindings like `Foo<T = X>`
1090     /// are disallowed. Otherwise, they are pushed onto the vector given.
1091     pub fn instantiate_mono_trait_ref(
1092         &self,
1093         trait_ref: &hir::TraitRef<'_>,
1094         self_ty: Ty<'tcx>,
1095     ) -> ty::TraitRef<'tcx> {
1096         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1);
1097
1098         self.ast_path_to_mono_trait_ref(
1099             trait_ref.path.span,
1100             trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
1101             self_ty,
1102             trait_ref.path.segments.last().unwrap(),
1103         )
1104     }
1105
1106     /// The given trait-ref must actually be a trait.
1107     pub(super) fn instantiate_poly_trait_ref_inner(
1108         &self,
1109         trait_ref: &hir::TraitRef<'_>,
1110         span: Span,
1111         constness: Constness,
1112         self_ty: Ty<'tcx>,
1113         bounds: &mut Bounds<'tcx>,
1114         speculative: bool,
1115     ) -> GenericArgCountResult {
1116         let trait_def_id = trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise());
1117
1118         debug!("instantiate_poly_trait_ref({:?}, def_id={:?})", trait_ref, trait_def_id);
1119
1120         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1);
1121
1122         let (substs, assoc_bindings, arg_count) = self.create_substs_for_ast_trait_ref(
1123             trait_ref.path.span,
1124             trait_def_id,
1125             self_ty,
1126             trait_ref.path.segments.last().unwrap(),
1127         );
1128         let poly_trait_ref = ty::Binder::bind(ty::TraitRef::new(trait_def_id, substs));
1129
1130         bounds.trait_bounds.push((poly_trait_ref, span, constness));
1131
1132         let mut dup_bindings = FxHashMap::default();
1133         for binding in &assoc_bindings {
1134             // Specify type to assert that error was already reported in `Err` case.
1135             let _: Result<_, ErrorReported> = self.add_predicates_for_ast_type_binding(
1136                 trait_ref.hir_ref_id,
1137                 poly_trait_ref,
1138                 binding,
1139                 bounds,
1140                 speculative,
1141                 &mut dup_bindings,
1142                 binding.span,
1143             );
1144             // Okay to ignore `Err` because of `ErrorReported` (see above).
1145         }
1146
1147         debug!(
1148             "instantiate_poly_trait_ref({:?}, bounds={:?}) -> {:?}",
1149             trait_ref, bounds, poly_trait_ref
1150         );
1151
1152         arg_count
1153     }
1154
1155     /// Given a trait bound like `Debug`, applies that trait bound the given self-type to construct
1156     /// a full trait reference. The resulting trait reference is returned. This may also generate
1157     /// auxiliary bounds, which are added to `bounds`.
1158     ///
1159     /// Example:
1160     ///
1161     /// ```
1162     /// poly_trait_ref = Iterator<Item = u32>
1163     /// self_ty = Foo
1164     /// ```
1165     ///
1166     /// this would return `Foo: Iterator` and add `<Foo as Iterator>::Item = u32` into `bounds`.
1167     ///
1168     /// **A note on binders:** against our usual convention, there is an implied bounder around
1169     /// the `self_ty` and `poly_trait_ref` parameters here. So they may reference bound regions.
1170     /// If for example you had `for<'a> Foo<'a>: Bar<'a>`, then the `self_ty` would be `Foo<'a>`
1171     /// where `'a` is a bound region at depth 0. Similarly, the `poly_trait_ref` would be
1172     /// `Bar<'a>`. The returned poly-trait-ref will have this binder instantiated explicitly,
1173     /// however.
1174     pub fn instantiate_poly_trait_ref(
1175         &self,
1176         poly_trait_ref: &hir::PolyTraitRef<'_>,
1177         constness: Constness,
1178         self_ty: Ty<'tcx>,
1179         bounds: &mut Bounds<'tcx>,
1180     ) -> GenericArgCountResult {
1181         self.instantiate_poly_trait_ref_inner(
1182             &poly_trait_ref.trait_ref,
1183             poly_trait_ref.span,
1184             constness,
1185             self_ty,
1186             bounds,
1187             false,
1188         )
1189     }
1190
1191     fn ast_path_to_mono_trait_ref(
1192         &self,
1193         span: Span,
1194         trait_def_id: DefId,
1195         self_ty: Ty<'tcx>,
1196         trait_segment: &hir::PathSegment<'_>,
1197     ) -> ty::TraitRef<'tcx> {
1198         let (substs, assoc_bindings, _) =
1199             self.create_substs_for_ast_trait_ref(span, trait_def_id, self_ty, trait_segment);
1200         if let Some(b) = assoc_bindings.first() {
1201             AstConv::prohibit_assoc_ty_binding(self.tcx(), b.span);
1202         }
1203         ty::TraitRef::new(trait_def_id, substs)
1204     }
1205
1206     /// When the code is using the `Fn` traits directly, instead of the `Fn(A) -> B` syntax, emit
1207     /// an error and attempt to build a reasonable structured suggestion.
1208     fn complain_about_internal_fn_trait(
1209         &self,
1210         span: Span,
1211         trait_def_id: DefId,
1212         trait_segment: &'a hir::PathSegment<'a>,
1213     ) {
1214         let trait_def = self.tcx().trait_def(trait_def_id);
1215
1216         if !self.tcx().features().unboxed_closures
1217             && trait_segment.generic_args().parenthesized != trait_def.paren_sugar
1218         {
1219             let sess = &self.tcx().sess.parse_sess;
1220             // For now, require that parenthetical notation be used only with `Fn()` etc.
1221             let (msg, sugg) = if trait_def.paren_sugar {
1222                 (
1223                     "the precise format of `Fn`-family traits' type parameters is subject to \
1224                      change",
1225                     Some(format!(
1226                         "{}{} -> {}",
1227                         trait_segment.ident,
1228                         trait_segment
1229                             .args
1230                             .as_ref()
1231                             .and_then(|args| args.args.get(0))
1232                             .and_then(|arg| match arg {
1233                                 hir::GenericArg::Type(ty) => match ty.kind {
1234                                     hir::TyKind::Tup(t) => t
1235                                         .iter()
1236                                         .map(|e| sess.source_map().span_to_snippet(e.span))
1237                                         .collect::<Result<Vec<_>, _>>()
1238                                         .map(|a| a.join(", ")),
1239                                     _ => sess.source_map().span_to_snippet(ty.span),
1240                                 }
1241                                 .map(|s| format!("({})", s))
1242                                 .ok(),
1243                                 _ => None,
1244                             })
1245                             .unwrap_or_else(|| "()".to_string()),
1246                         trait_segment
1247                             .generic_args()
1248                             .bindings
1249                             .iter()
1250                             .find_map(|b| match (b.ident.as_str() == "Output", &b.kind) {
1251                                 (true, hir::TypeBindingKind::Equality { ty }) => {
1252                                     sess.source_map().span_to_snippet(ty.span).ok()
1253                                 }
1254                                 _ => None,
1255                             })
1256                             .unwrap_or_else(|| "()".to_string()),
1257                     )),
1258                 )
1259             } else {
1260                 ("parenthetical notation is only stable when used with `Fn`-family traits", None)
1261             };
1262             let mut err = feature_err(sess, sym::unboxed_closures, span, msg);
1263             if let Some(sugg) = sugg {
1264                 let msg = "use parenthetical notation instead";
1265                 err.span_suggestion(span, msg, sugg, Applicability::MaybeIncorrect);
1266             }
1267             err.emit();
1268         }
1269     }
1270
1271     fn create_substs_for_ast_trait_ref<'a>(
1272         &self,
1273         span: Span,
1274         trait_def_id: DefId,
1275         self_ty: Ty<'tcx>,
1276         trait_segment: &'a hir::PathSegment<'a>,
1277     ) -> (SubstsRef<'tcx>, Vec<ConvertedBinding<'a, 'tcx>>, GenericArgCountResult) {
1278         debug!("create_substs_for_ast_trait_ref(trait_segment={:?})", trait_segment);
1279
1280         self.complain_about_internal_fn_trait(span, trait_def_id, trait_segment);
1281
1282         self.create_substs_for_ast_path(
1283             span,
1284             trait_def_id,
1285             &[],
1286             trait_segment.generic_args(),
1287             trait_segment.infer_args,
1288             Some(self_ty),
1289         )
1290     }
1291
1292     fn trait_defines_associated_type_named(&self, trait_def_id: DefId, assoc_name: Ident) -> bool {
1293         self.tcx()
1294             .associated_items(trait_def_id)
1295             .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, trait_def_id)
1296             .is_some()
1297     }
1298
1299     // Returns `true` if a bounds list includes `?Sized`.
1300     pub fn is_unsized(&self, ast_bounds: &[hir::GenericBound<'_>], span: Span) -> bool {
1301         let tcx = self.tcx();
1302
1303         // Try to find an unbound in bounds.
1304         let mut unbound = None;
1305         for ab in ast_bounds {
1306             if let &hir::GenericBound::Trait(ref ptr, hir::TraitBoundModifier::Maybe) = ab {
1307                 if unbound.is_none() {
1308                     unbound = Some(&ptr.trait_ref);
1309                 } else {
1310                     struct_span_err!(
1311                         tcx.sess,
1312                         span,
1313                         E0203,
1314                         "type parameter has more than one relaxed default \
1315                         bound, only one is supported"
1316                     )
1317                     .emit();
1318                 }
1319             }
1320         }
1321
1322         let kind_id = tcx.lang_items().require(SizedTraitLangItem);
1323         match unbound {
1324             Some(tpb) => {
1325                 // FIXME(#8559) currently requires the unbound to be built-in.
1326                 if let Ok(kind_id) = kind_id {
1327                     if tpb.path.res != Res::Def(DefKind::Trait, kind_id) {
1328                         tcx.sess.span_warn(
1329                             span,
1330                             "default bound relaxed for a type parameter, but \
1331                              this does nothing because the given bound is not \
1332                              a default; only `?Sized` is supported",
1333                         );
1334                     }
1335                 }
1336             }
1337             _ if kind_id.is_ok() => {
1338                 return false;
1339             }
1340             // No lang item for `Sized`, so we can't add it as a bound.
1341             None => {}
1342         }
1343
1344         true
1345     }
1346
1347     /// This helper takes a *converted* parameter type (`param_ty`)
1348     /// and an *unconverted* list of bounds:
1349     ///
1350     /// ```text
1351     /// fn foo<T: Debug>
1352     ///        ^  ^^^^^ `ast_bounds` parameter, in HIR form
1353     ///        |
1354     ///        `param_ty`, in ty form
1355     /// ```
1356     ///
1357     /// It adds these `ast_bounds` into the `bounds` structure.
1358     ///
1359     /// **A note on binders:** there is an implied binder around
1360     /// `param_ty` and `ast_bounds`. See `instantiate_poly_trait_ref`
1361     /// for more details.
1362     fn add_bounds(
1363         &self,
1364         param_ty: Ty<'tcx>,
1365         ast_bounds: &[hir::GenericBound<'_>],
1366         bounds: &mut Bounds<'tcx>,
1367     ) {
1368         let mut trait_bounds = Vec::new();
1369         let mut region_bounds = Vec::new();
1370
1371         let constness = self.default_constness_for_trait_bounds();
1372         for ast_bound in ast_bounds {
1373             match *ast_bound {
1374                 hir::GenericBound::Trait(ref b, hir::TraitBoundModifier::None) => {
1375                     trait_bounds.push((b, constness))
1376                 }
1377                 hir::GenericBound::Trait(ref b, hir::TraitBoundModifier::MaybeConst) => {
1378                     trait_bounds.push((b, Constness::NotConst))
1379                 }
1380                 hir::GenericBound::Trait(_, hir::TraitBoundModifier::Maybe) => {}
1381                 hir::GenericBound::Outlives(ref l) => region_bounds.push(l),
1382             }
1383         }
1384
1385         for (bound, constness) in trait_bounds {
1386             let _ = self.instantiate_poly_trait_ref(bound, constness, param_ty, bounds);
1387         }
1388
1389         bounds.region_bounds.extend(
1390             region_bounds.into_iter().map(|r| (self.ast_region_to_region(r, None), r.span)),
1391         );
1392     }
1393
1394     /// Translates a list of bounds from the HIR into the `Bounds` data structure.
1395     /// The self-type for the bounds is given by `param_ty`.
1396     ///
1397     /// Example:
1398     ///
1399     /// ```
1400     /// fn foo<T: Bar + Baz>() { }
1401     ///        ^  ^^^^^^^^^ ast_bounds
1402     ///        param_ty
1403     /// ```
1404     ///
1405     /// The `sized_by_default` parameter indicates if, in this context, the `param_ty` should be
1406     /// considered `Sized` unless there is an explicit `?Sized` bound.  This would be true in the
1407     /// example above, but is not true in supertrait listings like `trait Foo: Bar + Baz`.
1408     ///
1409     /// `span` should be the declaration size of the parameter.
1410     pub fn compute_bounds(
1411         &self,
1412         param_ty: Ty<'tcx>,
1413         ast_bounds: &[hir::GenericBound<'_>],
1414         sized_by_default: SizedByDefault,
1415         span: Span,
1416     ) -> Bounds<'tcx> {
1417         let mut bounds = Bounds::default();
1418
1419         self.add_bounds(param_ty, ast_bounds, &mut bounds);
1420         bounds.trait_bounds.sort_by_key(|(t, _, _)| t.def_id());
1421
1422         bounds.implicitly_sized = if let SizedByDefault::Yes = sized_by_default {
1423             if !self.is_unsized(ast_bounds, span) { Some(span) } else { None }
1424         } else {
1425             None
1426         };
1427
1428         bounds
1429     }
1430
1431     /// Given an HIR binding like `Item = Foo` or `Item: Foo`, pushes the corresponding predicates
1432     /// onto `bounds`.
1433     ///
1434     /// **A note on binders:** given something like `T: for<'a> Iterator<Item = &'a u32>`, the
1435     /// `trait_ref` here will be `for<'a> T: Iterator`. The `binding` data however is from *inside*
1436     /// the binder (e.g., `&'a u32`) and hence may reference bound regions.
1437     fn add_predicates_for_ast_type_binding(
1438         &self,
1439         hir_ref_id: hir::HirId,
1440         trait_ref: ty::PolyTraitRef<'tcx>,
1441         binding: &ConvertedBinding<'_, 'tcx>,
1442         bounds: &mut Bounds<'tcx>,
1443         speculative: bool,
1444         dup_bindings: &mut FxHashMap<DefId, Span>,
1445         path_span: Span,
1446     ) -> Result<(), ErrorReported> {
1447         let tcx = self.tcx();
1448
1449         if !speculative {
1450             // Given something like `U: SomeTrait<T = X>`, we want to produce a
1451             // predicate like `<U as SomeTrait>::T = X`. This is somewhat
1452             // subtle in the event that `T` is defined in a supertrait of
1453             // `SomeTrait`, because in that case we need to upcast.
1454             //
1455             // That is, consider this case:
1456             //
1457             // ```
1458             // trait SubTrait: SuperTrait<i32> { }
1459             // trait SuperTrait<A> { type T; }
1460             //
1461             // ... B: SubTrait<T = foo> ...
1462             // ```
1463             //
1464             // We want to produce `<B as SuperTrait<i32>>::T == foo`.
1465
1466             // Find any late-bound regions declared in `ty` that are not
1467             // declared in the trait-ref. These are not well-formed.
1468             //
1469             // Example:
1470             //
1471             //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
1472             //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
1473             if let ConvertedBindingKind::Equality(ty) = binding.kind {
1474                 let late_bound_in_trait_ref =
1475                     tcx.collect_constrained_late_bound_regions(&trait_ref);
1476                 let late_bound_in_ty =
1477                     tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(ty));
1478                 debug!("late_bound_in_trait_ref = {:?}", late_bound_in_trait_ref);
1479                 debug!("late_bound_in_ty = {:?}", late_bound_in_ty);
1480                 for br in late_bound_in_ty.difference(&late_bound_in_trait_ref) {
1481                     let br_name = match *br {
1482                         ty::BrNamed(_, name) => name,
1483                         _ => {
1484                             span_bug!(
1485                                 binding.span,
1486                                 "anonymous bound region {:?} in binding but not trait ref",
1487                                 br
1488                             );
1489                         }
1490                     };
1491                     // FIXME: point at the type params that don't have appropriate lifetimes:
1492                     // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
1493                     //                         ----  ----     ^^^^^^^
1494                     struct_span_err!(
1495                         tcx.sess,
1496                         binding.span,
1497                         E0582,
1498                         "binding for associated type `{}` references lifetime `{}`, \
1499                          which does not appear in the trait input types",
1500                         binding.item_name,
1501                         br_name
1502                     )
1503                     .emit();
1504                 }
1505             }
1506         }
1507
1508         let candidate =
1509             if self.trait_defines_associated_type_named(trait_ref.def_id(), binding.item_name) {
1510                 // Simple case: X is defined in the current trait.
1511                 trait_ref
1512             } else {
1513                 // Otherwise, we have to walk through the supertraits to find
1514                 // those that do.
1515                 self.one_bound_for_assoc_type(
1516                     || traits::supertraits(tcx, trait_ref),
1517                     || trait_ref.print_only_trait_path().to_string(),
1518                     binding.item_name,
1519                     path_span,
1520                     || match binding.kind {
1521                         ConvertedBindingKind::Equality(ty) => Some(ty.to_string()),
1522                         _ => None,
1523                     },
1524                 )?
1525             };
1526
1527         let (assoc_ident, def_scope) =
1528             tcx.adjust_ident_and_get_scope(binding.item_name, candidate.def_id(), hir_ref_id);
1529
1530         // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
1531         // of calling `filter_by_name_and_kind`.
1532         let assoc_ty = tcx
1533             .associated_items(candidate.def_id())
1534             .filter_by_name_unhygienic(assoc_ident.name)
1535             .find(|i| {
1536                 i.kind == ty::AssocKind::Type && i.ident.normalize_to_macros_2_0() == assoc_ident
1537             })
1538             .expect("missing associated type");
1539
1540         if !assoc_ty.vis.is_accessible_from(def_scope, tcx) {
1541             tcx.sess
1542                 .struct_span_err(
1543                     binding.span,
1544                     &format!("associated type `{}` is private", binding.item_name),
1545                 )
1546                 .span_label(binding.span, "private associated type")
1547                 .emit();
1548         }
1549         tcx.check_stability(assoc_ty.def_id, Some(hir_ref_id), binding.span);
1550
1551         if !speculative {
1552             dup_bindings
1553                 .entry(assoc_ty.def_id)
1554                 .and_modify(|prev_span| {
1555                     struct_span_err!(
1556                         self.tcx().sess,
1557                         binding.span,
1558                         E0719,
1559                         "the value of the associated type `{}` (from trait `{}`) \
1560                          is already specified",
1561                         binding.item_name,
1562                         tcx.def_path_str(assoc_ty.container.id())
1563                     )
1564                     .span_label(binding.span, "re-bound here")
1565                     .span_label(*prev_span, format!("`{}` bound here first", binding.item_name))
1566                     .emit();
1567                 })
1568                 .or_insert(binding.span);
1569         }
1570
1571         match binding.kind {
1572             ConvertedBindingKind::Equality(ref ty) => {
1573                 // "Desugar" a constraint like `T: Iterator<Item = u32>` this to
1574                 // the "projection predicate" for:
1575                 //
1576                 // `<T as Iterator>::Item = u32`
1577                 bounds.projection_bounds.push((
1578                     candidate.map_bound(|trait_ref| ty::ProjectionPredicate {
1579                         projection_ty: ty::ProjectionTy::from_ref_and_name(
1580                             tcx,
1581                             trait_ref,
1582                             binding.item_name,
1583                         ),
1584                         ty,
1585                     }),
1586                     binding.span,
1587                 ));
1588             }
1589             ConvertedBindingKind::Constraint(ast_bounds) => {
1590                 // "Desugar" a constraint like `T: Iterator<Item: Debug>` to
1591                 //
1592                 // `<T as Iterator>::Item: Debug`
1593                 //
1594                 // Calling `skip_binder` is okay, because `add_bounds` expects the `param_ty`
1595                 // parameter to have a skipped binder.
1596                 let param_ty = tcx.mk_projection(assoc_ty.def_id, candidate.skip_binder().substs);
1597                 self.add_bounds(param_ty, ast_bounds, bounds);
1598             }
1599         }
1600         Ok(())
1601     }
1602
1603     fn ast_path_to_ty(
1604         &self,
1605         span: Span,
1606         did: DefId,
1607         item_segment: &hir::PathSegment<'_>,
1608     ) -> Ty<'tcx> {
1609         let substs = self.ast_path_substs_for_ty(span, did, item_segment);
1610         self.normalize_ty(span, self.tcx().at(span).type_of(did).subst(self.tcx(), substs))
1611     }
1612
1613     fn conv_object_ty_poly_trait_ref(
1614         &self,
1615         span: Span,
1616         trait_bounds: &[hir::PolyTraitRef<'_>],
1617         lifetime: &hir::Lifetime,
1618     ) -> Ty<'tcx> {
1619         let tcx = self.tcx();
1620
1621         let mut bounds = Bounds::default();
1622         let mut potential_assoc_types = Vec::new();
1623         let dummy_self = self.tcx().types.trait_object_dummy_self;
1624         for trait_bound in trait_bounds.iter().rev() {
1625             if let GenericArgCountResult {
1626                 correct:
1627                     Err(GenericArgCountMismatch { invalid_args: cur_potential_assoc_types, .. }),
1628                 ..
1629             } = self.instantiate_poly_trait_ref(
1630                 trait_bound,
1631                 Constness::NotConst,
1632                 dummy_self,
1633                 &mut bounds,
1634             ) {
1635                 potential_assoc_types.extend(cur_potential_assoc_types.into_iter());
1636             }
1637         }
1638
1639         // Expand trait aliases recursively and check that only one regular (non-auto) trait
1640         // is used and no 'maybe' bounds are used.
1641         let expanded_traits =
1642             traits::expand_trait_aliases(tcx, bounds.trait_bounds.iter().map(|&(a, b, _)| (a, b)));
1643         let (mut auto_traits, regular_traits): (Vec<_>, Vec<_>) =
1644             expanded_traits.partition(|i| tcx.trait_is_auto(i.trait_ref().def_id()));
1645         if regular_traits.len() > 1 {
1646             let first_trait = &regular_traits[0];
1647             let additional_trait = &regular_traits[1];
1648             let mut err = struct_span_err!(
1649                 tcx.sess,
1650                 additional_trait.bottom().1,
1651                 E0225,
1652                 "only auto traits can be used as additional traits in a trait object"
1653             );
1654             additional_trait.label_with_exp_info(
1655                 &mut err,
1656                 "additional non-auto trait",
1657                 "additional use",
1658             );
1659             first_trait.label_with_exp_info(&mut err, "first non-auto trait", "first use");
1660             err.emit();
1661         }
1662
1663         if regular_traits.is_empty() && auto_traits.is_empty() {
1664             struct_span_err!(
1665                 tcx.sess,
1666                 span,
1667                 E0224,
1668                 "at least one trait is required for an object type"
1669             )
1670             .emit();
1671             return tcx.ty_error();
1672         }
1673
1674         // Check that there are no gross object safety violations;
1675         // most importantly, that the supertraits don't contain `Self`,
1676         // to avoid ICEs.
1677         for item in &regular_traits {
1678             let object_safety_violations =
1679                 astconv_object_safety_violations(tcx, item.trait_ref().def_id());
1680             if !object_safety_violations.is_empty() {
1681                 report_object_safety_error(
1682                     tcx,
1683                     span,
1684                     item.trait_ref().def_id(),
1685                     &object_safety_violations[..],
1686                 )
1687                 .emit();
1688                 return tcx.ty_error();
1689             }
1690         }
1691
1692         // Use a `BTreeSet` to keep output in a more consistent order.
1693         let mut associated_types: FxHashMap<Span, BTreeSet<DefId>> = FxHashMap::default();
1694
1695         let regular_traits_refs_spans = bounds
1696             .trait_bounds
1697             .into_iter()
1698             .filter(|(trait_ref, _, _)| !tcx.trait_is_auto(trait_ref.def_id()));
1699
1700         for (base_trait_ref, span, constness) in regular_traits_refs_spans {
1701             assert_eq!(constness, Constness::NotConst);
1702
1703             for obligation in traits::elaborate_trait_ref(tcx, base_trait_ref) {
1704                 debug!(
1705                     "conv_object_ty_poly_trait_ref: observing object predicate `{:?}`",
1706                     obligation.predicate
1707                 );
1708                 match obligation.predicate.kind() {
1709                     ty::PredicateKind::Trait(pred, _) => {
1710                         associated_types.entry(span).or_default().extend(
1711                             tcx.associated_items(pred.def_id())
1712                                 .in_definition_order()
1713                                 .filter(|item| item.kind == ty::AssocKind::Type)
1714                                 .map(|item| item.def_id),
1715                         );
1716                     }
1717                     &ty::PredicateKind::Projection(pred) => {
1718                         // A `Self` within the original bound will be substituted with a
1719                         // `trait_object_dummy_self`, so check for that.
1720                         let references_self =
1721                             pred.skip_binder().ty.walk().any(|arg| arg == dummy_self.into());
1722
1723                         // If the projection output contains `Self`, force the user to
1724                         // elaborate it explicitly to avoid a lot of complexity.
1725                         //
1726                         // The "classicaly useful" case is the following:
1727                         // ```
1728                         //     trait MyTrait: FnMut() -> <Self as MyTrait>::MyOutput {
1729                         //         type MyOutput;
1730                         //     }
1731                         // ```
1732                         //
1733                         // Here, the user could theoretically write `dyn MyTrait<Output = X>`,
1734                         // but actually supporting that would "expand" to an infinitely-long type
1735                         // `fix $ τ → dyn MyTrait<MyOutput = X, Output = <τ as MyTrait>::MyOutput`.
1736                         //
1737                         // Instead, we force the user to write
1738                         // `dyn MyTrait<MyOutput = X, Output = X>`, which is uglier but works. See
1739                         // the discussion in #56288 for alternatives.
1740                         if !references_self {
1741                             // Include projections defined on supertraits.
1742                             bounds.projection_bounds.push((pred, span));
1743                         }
1744                     }
1745                     _ => (),
1746                 }
1747             }
1748         }
1749
1750         for (projection_bound, _) in &bounds.projection_bounds {
1751             for def_ids in associated_types.values_mut() {
1752                 def_ids.remove(&projection_bound.projection_def_id());
1753             }
1754         }
1755
1756         self.complain_about_missing_associated_types(
1757             associated_types,
1758             potential_assoc_types,
1759             trait_bounds,
1760         );
1761
1762         // De-duplicate auto traits so that, e.g., `dyn Trait + Send + Send` is the same as
1763         // `dyn Trait + Send`.
1764         auto_traits.sort_by_key(|i| i.trait_ref().def_id());
1765         auto_traits.dedup_by_key(|i| i.trait_ref().def_id());
1766         debug!("regular_traits: {:?}", regular_traits);
1767         debug!("auto_traits: {:?}", auto_traits);
1768
1769         // Transform a `PolyTraitRef` into a `PolyExistentialTraitRef` by
1770         // removing the dummy `Self` type (`trait_object_dummy_self`).
1771         let trait_ref_to_existential = |trait_ref: ty::TraitRef<'tcx>| {
1772             if trait_ref.self_ty() != dummy_self {
1773                 // FIXME: There appears to be a missing filter on top of `expand_trait_aliases`,
1774                 // which picks up non-supertraits where clauses - but also, the object safety
1775                 // completely ignores trait aliases, which could be object safety hazards. We
1776                 // `delay_span_bug` here to avoid an ICE in stable even when the feature is
1777                 // disabled. (#66420)
1778                 tcx.sess.delay_span_bug(
1779                     DUMMY_SP,
1780                     &format!(
1781                         "trait_ref_to_existential called on {:?} with non-dummy Self",
1782                         trait_ref,
1783                     ),
1784                 );
1785             }
1786             ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
1787         };
1788
1789         // Erase the `dummy_self` (`trait_object_dummy_self`) used above.
1790         let existential_trait_refs =
1791             regular_traits.iter().map(|i| i.trait_ref().map_bound(trait_ref_to_existential));
1792         let existential_projections = bounds.projection_bounds.iter().map(|(bound, _)| {
1793             bound.map_bound(|b| {
1794                 let trait_ref = trait_ref_to_existential(b.projection_ty.trait_ref(tcx));
1795                 ty::ExistentialProjection {
1796                     ty: b.ty,
1797                     item_def_id: b.projection_ty.item_def_id,
1798                     substs: trait_ref.substs,
1799                 }
1800             })
1801         });
1802
1803         // Calling `skip_binder` is okay because the predicates are re-bound.
1804         let regular_trait_predicates = existential_trait_refs
1805             .map(|trait_ref| ty::ExistentialPredicate::Trait(trait_ref.skip_binder()));
1806         let auto_trait_predicates = auto_traits
1807             .into_iter()
1808             .map(|trait_ref| ty::ExistentialPredicate::AutoTrait(trait_ref.trait_ref().def_id()));
1809         let mut v = regular_trait_predicates
1810             .chain(auto_trait_predicates)
1811             .chain(
1812                 existential_projections
1813                     .map(|x| ty::ExistentialPredicate::Projection(x.skip_binder())),
1814             )
1815             .collect::<SmallVec<[_; 8]>>();
1816         v.sort_by(|a, b| a.stable_cmp(tcx, b));
1817         v.dedup();
1818         let existential_predicates = ty::Binder::bind(tcx.mk_existential_predicates(v.into_iter()));
1819
1820         // Use explicitly-specified region bound.
1821         let region_bound = if !lifetime.is_elided() {
1822             self.ast_region_to_region(lifetime, None)
1823         } else {
1824             self.compute_object_lifetime_bound(span, existential_predicates).unwrap_or_else(|| {
1825                 if tcx.named_region(lifetime.hir_id).is_some() {
1826                     self.ast_region_to_region(lifetime, None)
1827                 } else {
1828                     self.re_infer(None, span).unwrap_or_else(|| {
1829                         // FIXME: these can be redundant with E0106, but not always.
1830                         struct_span_err!(
1831                             tcx.sess,
1832                             span,
1833                             E0228,
1834                             "the lifetime bound for this object type cannot be deduced \
1835                              from context; please supply an explicit bound"
1836                         )
1837                         .emit();
1838                         tcx.lifetimes.re_static
1839                     })
1840                 }
1841             })
1842         };
1843         debug!("region_bound: {:?}", region_bound);
1844
1845         let ty = tcx.mk_dynamic(existential_predicates, region_bound);
1846         debug!("trait_object_type: {:?}", ty);
1847         ty
1848     }
1849
1850     /// When there are any missing associated types, emit an E0191 error and attempt to supply a
1851     /// reasonable suggestion on how to write it. For the case of multiple associated types in the
1852     /// same trait bound have the same name (as they come from different super-traits), we instead
1853     /// emit a generic note suggesting using a `where` clause to constraint instead.
1854     fn complain_about_missing_associated_types(
1855         &self,
1856         associated_types: FxHashMap<Span, BTreeSet<DefId>>,
1857         potential_assoc_types: Vec<Span>,
1858         trait_bounds: &[hir::PolyTraitRef<'_>],
1859     ) {
1860         if associated_types.values().all(|v| v.is_empty()) {
1861             return;
1862         }
1863         let tcx = self.tcx();
1864         // FIXME: Marked `mut` so that we can replace the spans further below with a more
1865         // appropriate one, but this should be handled earlier in the span assignment.
1866         let mut associated_types: FxHashMap<Span, Vec<_>> = associated_types
1867             .into_iter()
1868             .map(|(span, def_ids)| {
1869                 (span, def_ids.into_iter().map(|did| tcx.associated_item(did)).collect())
1870             })
1871             .collect();
1872         let mut names = vec![];
1873
1874         // Account for things like `dyn Foo + 'a`, like in tests `issue-22434.rs` and
1875         // `issue-22560.rs`.
1876         let mut trait_bound_spans: Vec<Span> = vec![];
1877         for (span, items) in &associated_types {
1878             if !items.is_empty() {
1879                 trait_bound_spans.push(*span);
1880             }
1881             for assoc_item in items {
1882                 let trait_def_id = assoc_item.container.id();
1883                 names.push(format!(
1884                     "`{}` (from trait `{}`)",
1885                     assoc_item.ident,
1886                     tcx.def_path_str(trait_def_id),
1887                 ));
1888             }
1889         }
1890         if let ([], [bound]) = (&potential_assoc_types[..], &trait_bounds) {
1891             match &bound.trait_ref.path.segments[..] {
1892                 // FIXME: `trait_ref.path.span` can point to a full path with multiple
1893                 // segments, even though `trait_ref.path.segments` is of length `1`. Work
1894                 // around that bug here, even though it should be fixed elsewhere.
1895                 // This would otherwise cause an invalid suggestion. For an example, look at
1896                 // `src/test/ui/issues/issue-28344.rs` where instead of the following:
1897                 //
1898                 //   error[E0191]: the value of the associated type `Output`
1899                 //                 (from trait `std::ops::BitXor`) must be specified
1900                 //   --> $DIR/issue-28344.rs:4:17
1901                 //    |
1902                 // LL |     let x: u8 = BitXor::bitor(0 as u8, 0 as u8);
1903                 //    |                 ^^^^^^ help: specify the associated type:
1904                 //    |                              `BitXor<Output = Type>`
1905                 //
1906                 // we would output:
1907                 //
1908                 //   error[E0191]: the value of the associated type `Output`
1909                 //                 (from trait `std::ops::BitXor`) must be specified
1910                 //   --> $DIR/issue-28344.rs:4:17
1911                 //    |
1912                 // LL |     let x: u8 = BitXor::bitor(0 as u8, 0 as u8);
1913                 //    |                 ^^^^^^^^^^^^^ help: specify the associated type:
1914                 //    |                                     `BitXor::bitor<Output = Type>`
1915                 [segment] if segment.args.is_none() => {
1916                     trait_bound_spans = vec![segment.ident.span];
1917                     associated_types = associated_types
1918                         .into_iter()
1919                         .map(|(_, items)| (segment.ident.span, items))
1920                         .collect();
1921                 }
1922                 _ => {}
1923             }
1924         }
1925         names.sort();
1926         trait_bound_spans.sort();
1927         let mut err = struct_span_err!(
1928             tcx.sess,
1929             trait_bound_spans,
1930             E0191,
1931             "the value of the associated type{} {} must be specified",
1932             pluralize!(names.len()),
1933             names.join(", "),
1934         );
1935         let mut suggestions = vec![];
1936         let mut types_count = 0;
1937         let mut where_constraints = vec![];
1938         for (span, assoc_items) in &associated_types {
1939             let mut names: FxHashMap<_, usize> = FxHashMap::default();
1940             for item in assoc_items {
1941                 types_count += 1;
1942                 *names.entry(item.ident.name).or_insert(0) += 1;
1943             }
1944             let mut dupes = false;
1945             for item in assoc_items {
1946                 let prefix = if names[&item.ident.name] > 1 {
1947                     let trait_def_id = item.container.id();
1948                     dupes = true;
1949                     format!("{}::", tcx.def_path_str(trait_def_id))
1950                 } else {
1951                     String::new()
1952                 };
1953                 if let Some(sp) = tcx.hir().span_if_local(item.def_id) {
1954                     err.span_label(sp, format!("`{}{}` defined here", prefix, item.ident));
1955                 }
1956             }
1957             if potential_assoc_types.len() == assoc_items.len() {
1958                 // Only suggest when the amount of missing associated types equals the number of
1959                 // extra type arguments present, as that gives us a relatively high confidence
1960                 // that the user forgot to give the associtated type's name. The canonical
1961                 // example would be trying to use `Iterator<isize>` instead of
1962                 // `Iterator<Item = isize>`.
1963                 for (potential, item) in potential_assoc_types.iter().zip(assoc_items.iter()) {
1964                     if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(*potential) {
1965                         suggestions.push((*potential, format!("{} = {}", item.ident, snippet)));
1966                     }
1967                 }
1968             } else if let (Ok(snippet), false) =
1969                 (tcx.sess.source_map().span_to_snippet(*span), dupes)
1970             {
1971                 let types: Vec<_> =
1972                     assoc_items.iter().map(|item| format!("{} = Type", item.ident)).collect();
1973                 let code = if snippet.ends_with('>') {
1974                     // The user wrote `Trait<'a>` or similar and we don't have a type we can
1975                     // suggest, but at least we can clue them to the correct syntax
1976                     // `Trait<'a, Item = Type>` while accounting for the `<'a>` in the
1977                     // suggestion.
1978                     format!("{}, {}>", &snippet[..snippet.len() - 1], types.join(", "))
1979                 } else {
1980                     // The user wrote `Iterator`, so we don't have a type we can suggest, but at
1981                     // least we can clue them to the correct syntax `Iterator<Item = Type>`.
1982                     format!("{}<{}>", snippet, types.join(", "))
1983                 };
1984                 suggestions.push((*span, code));
1985             } else if dupes {
1986                 where_constraints.push(*span);
1987             }
1988         }
1989         let where_msg = "consider introducing a new type parameter, adding `where` constraints \
1990                          using the fully-qualified path to the associated types";
1991         if !where_constraints.is_empty() && suggestions.is_empty() {
1992             // If there are duplicates associated type names and a single trait bound do not
1993             // use structured suggestion, it means that there are multiple super-traits with
1994             // the same associated type name.
1995             err.help(where_msg);
1996         }
1997         if suggestions.len() != 1 {
1998             // We don't need this label if there's an inline suggestion, show otherwise.
1999             for (span, assoc_items) in &associated_types {
2000                 let mut names: FxHashMap<_, usize> = FxHashMap::default();
2001                 for item in assoc_items {
2002                     types_count += 1;
2003                     *names.entry(item.ident.name).or_insert(0) += 1;
2004                 }
2005                 let mut label = vec![];
2006                 for item in assoc_items {
2007                     let postfix = if names[&item.ident.name] > 1 {
2008                         let trait_def_id = item.container.id();
2009                         format!(" (from trait `{}`)", tcx.def_path_str(trait_def_id))
2010                     } else {
2011                         String::new()
2012                     };
2013                     label.push(format!("`{}`{}", item.ident, postfix));
2014                 }
2015                 if !label.is_empty() {
2016                     err.span_label(
2017                         *span,
2018                         format!(
2019                             "associated type{} {} must be specified",
2020                             pluralize!(label.len()),
2021                             label.join(", "),
2022                         ),
2023                     );
2024                 }
2025             }
2026         }
2027         if !suggestions.is_empty() {
2028             err.multipart_suggestion(
2029                 &format!("specify the associated type{}", pluralize!(types_count)),
2030                 suggestions,
2031                 Applicability::HasPlaceholders,
2032             );
2033             if !where_constraints.is_empty() {
2034                 err.span_help(where_constraints, where_msg);
2035             }
2036         }
2037         err.emit();
2038     }
2039
2040     fn report_ambiguous_associated_type(
2041         &self,
2042         span: Span,
2043         type_str: &str,
2044         trait_str: &str,
2045         name: Symbol,
2046     ) {
2047         let mut err = struct_span_err!(self.tcx().sess, span, E0223, "ambiguous associated type");
2048         if let (Some(_), Ok(snippet)) = (
2049             self.tcx().sess.confused_type_with_std_module.borrow().get(&span),
2050             self.tcx().sess.source_map().span_to_snippet(span),
2051         ) {
2052             err.span_suggestion(
2053                 span,
2054                 "you are looking for the module in `std`, not the primitive type",
2055                 format!("std::{}", snippet),
2056                 Applicability::MachineApplicable,
2057             );
2058         } else {
2059             err.span_suggestion(
2060                 span,
2061                 "use fully-qualified syntax",
2062                 format!("<{} as {}>::{}", type_str, trait_str, name),
2063                 Applicability::HasPlaceholders,
2064             );
2065         }
2066         err.emit();
2067     }
2068
2069     // Search for a bound on a type parameter which includes the associated item
2070     // given by `assoc_name`. `ty_param_def_id` is the `DefId` of the type parameter
2071     // This function will fail if there are no suitable bounds or there is
2072     // any ambiguity.
2073     fn find_bound_for_assoc_item(
2074         &self,
2075         ty_param_def_id: LocalDefId,
2076         assoc_name: Ident,
2077         span: Span,
2078     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorReported> {
2079         let tcx = self.tcx();
2080
2081         debug!(
2082             "find_bound_for_assoc_item(ty_param_def_id={:?}, assoc_name={:?}, span={:?})",
2083             ty_param_def_id, assoc_name, span,
2084         );
2085
2086         let predicates =
2087             &self.get_type_parameter_bounds(span, ty_param_def_id.to_def_id()).predicates;
2088
2089         debug!("find_bound_for_assoc_item: predicates={:#?}", predicates);
2090
2091         let param_hir_id = tcx.hir().as_local_hir_id(ty_param_def_id);
2092         let param_name = tcx.hir().ty_param_name(param_hir_id);
2093         self.one_bound_for_assoc_type(
2094             || {
2095                 traits::transitive_bounds(
2096                     tcx,
2097                     predicates.iter().filter_map(|(p, _)| p.to_opt_poly_trait_ref()),
2098                 )
2099             },
2100             || param_name.to_string(),
2101             assoc_name,
2102             span,
2103             || None,
2104         )
2105     }
2106
2107     // Checks that `bounds` contains exactly one element and reports appropriate
2108     // errors otherwise.
2109     fn one_bound_for_assoc_type<I>(
2110         &self,
2111         all_candidates: impl Fn() -> I,
2112         ty_param_name: impl Fn() -> String,
2113         assoc_name: Ident,
2114         span: Span,
2115         is_equality: impl Fn() -> Option<String>,
2116     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorReported>
2117     where
2118         I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
2119     {
2120         let mut matching_candidates = all_candidates()
2121             .filter(|r| self.trait_defines_associated_type_named(r.def_id(), assoc_name));
2122
2123         let bound = match matching_candidates.next() {
2124             Some(bound) => bound,
2125             None => {
2126                 self.complain_about_assoc_type_not_found(
2127                     all_candidates,
2128                     &ty_param_name(),
2129                     assoc_name,
2130                     span,
2131                 );
2132                 return Err(ErrorReported);
2133             }
2134         };
2135
2136         debug!("one_bound_for_assoc_type: bound = {:?}", bound);
2137
2138         if let Some(bound2) = matching_candidates.next() {
2139             debug!("one_bound_for_assoc_type: bound2 = {:?}", bound2);
2140
2141             let is_equality = is_equality();
2142             let bounds = iter::once(bound).chain(iter::once(bound2)).chain(matching_candidates);
2143             let mut err = if is_equality.is_some() {
2144                 // More specific Error Index entry.
2145                 struct_span_err!(
2146                     self.tcx().sess,
2147                     span,
2148                     E0222,
2149                     "ambiguous associated type `{}` in bounds of `{}`",
2150                     assoc_name,
2151                     ty_param_name()
2152                 )
2153             } else {
2154                 struct_span_err!(
2155                     self.tcx().sess,
2156                     span,
2157                     E0221,
2158                     "ambiguous associated type `{}` in bounds of `{}`",
2159                     assoc_name,
2160                     ty_param_name()
2161                 )
2162             };
2163             err.span_label(span, format!("ambiguous associated type `{}`", assoc_name));
2164
2165             let mut where_bounds = vec![];
2166             for bound in bounds {
2167                 let bound_id = bound.def_id();
2168                 let bound_span = self
2169                     .tcx()
2170                     .associated_items(bound_id)
2171                     .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, bound_id)
2172                     .and_then(|item| self.tcx().hir().span_if_local(item.def_id));
2173
2174                 if let Some(bound_span) = bound_span {
2175                     err.span_label(
2176                         bound_span,
2177                         format!(
2178                             "ambiguous `{}` from `{}`",
2179                             assoc_name,
2180                             bound.print_only_trait_path(),
2181                         ),
2182                     );
2183                     if let Some(constraint) = &is_equality {
2184                         where_bounds.push(format!(
2185                             "        T: {trait}::{assoc} = {constraint}",
2186                             trait=bound.print_only_trait_path(),
2187                             assoc=assoc_name,
2188                             constraint=constraint,
2189                         ));
2190                     } else {
2191                         err.span_suggestion(
2192                             span,
2193                             "use fully qualified syntax to disambiguate",
2194                             format!(
2195                                 "<{} as {}>::{}",
2196                                 ty_param_name(),
2197                                 bound.print_only_trait_path(),
2198                                 assoc_name,
2199                             ),
2200                             Applicability::MaybeIncorrect,
2201                         );
2202                     }
2203                 } else {
2204                     err.note(&format!(
2205                         "associated type `{}` could derive from `{}`",
2206                         ty_param_name(),
2207                         bound.print_only_trait_path(),
2208                     ));
2209                 }
2210             }
2211             if !where_bounds.is_empty() {
2212                 err.help(&format!(
2213                     "consider introducing a new type parameter `T` and adding `where` constraints:\
2214                      \n    where\n        T: {},\n{}",
2215                     ty_param_name(),
2216                     where_bounds.join(",\n"),
2217                 ));
2218             }
2219             err.emit();
2220             if !where_bounds.is_empty() {
2221                 return Err(ErrorReported);
2222             }
2223         }
2224         Ok(bound)
2225     }
2226
2227     fn complain_about_assoc_type_not_found<I>(
2228         &self,
2229         all_candidates: impl Fn() -> I,
2230         ty_param_name: &str,
2231         assoc_name: Ident,
2232         span: Span,
2233     ) where
2234         I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
2235     {
2236         // The fallback span is needed because `assoc_name` might be an `Fn()`'s `Output` without a
2237         // valid span, so we point at the whole path segment instead.
2238         let span = if assoc_name.span != DUMMY_SP { assoc_name.span } else { span };
2239         let mut err = struct_span_err!(
2240             self.tcx().sess,
2241             span,
2242             E0220,
2243             "associated type `{}` not found for `{}`",
2244             assoc_name,
2245             ty_param_name
2246         );
2247
2248         let all_candidate_names: Vec<_> = all_candidates()
2249             .map(|r| self.tcx().associated_items(r.def_id()).in_definition_order())
2250             .flatten()
2251             .filter_map(
2252                 |item| if item.kind == ty::AssocKind::Type { Some(item.ident.name) } else { None },
2253             )
2254             .collect();
2255
2256         if let (Some(suggested_name), true) = (
2257             find_best_match_for_name(all_candidate_names.iter(), &assoc_name.as_str(), None),
2258             assoc_name.span != DUMMY_SP,
2259         ) {
2260             err.span_suggestion(
2261                 assoc_name.span,
2262                 "there is an associated type with a similar name",
2263                 suggested_name.to_string(),
2264                 Applicability::MaybeIncorrect,
2265             );
2266         } else {
2267             err.span_label(span, format!("associated type `{}` not found", assoc_name));
2268         }
2269
2270         err.emit();
2271     }
2272
2273     // Create a type from a path to an associated type.
2274     // For a path `A::B::C::D`, `qself_ty` and `qself_def` are the type and def for `A::B::C`
2275     // and item_segment is the path segment for `D`. We return a type and a def for
2276     // the whole path.
2277     // Will fail except for `T::A` and `Self::A`; i.e., if `qself_ty`/`qself_def` are not a type
2278     // parameter or `Self`.
2279     pub fn associated_path_to_ty(
2280         &self,
2281         hir_ref_id: hir::HirId,
2282         span: Span,
2283         qself_ty: Ty<'tcx>,
2284         qself_res: Res,
2285         assoc_segment: &hir::PathSegment<'_>,
2286         permit_variants: bool,
2287     ) -> Result<(Ty<'tcx>, DefKind, DefId), ErrorReported> {
2288         let tcx = self.tcx();
2289         let assoc_ident = assoc_segment.ident;
2290
2291         debug!("associated_path_to_ty: {:?}::{}", qself_ty, assoc_ident);
2292
2293         // Check if we have an enum variant.
2294         let mut variant_resolution = None;
2295         if let ty::Adt(adt_def, _) = qself_ty.kind {
2296             if adt_def.is_enum() {
2297                 let variant_def = adt_def
2298                     .variants
2299                     .iter()
2300                     .find(|vd| tcx.hygienic_eq(assoc_ident, vd.ident, adt_def.did));
2301                 if let Some(variant_def) = variant_def {
2302                     if permit_variants {
2303                         tcx.check_stability(variant_def.def_id, Some(hir_ref_id), span);
2304                         self.prohibit_generics(slice::from_ref(assoc_segment));
2305                         return Ok((qself_ty, DefKind::Variant, variant_def.def_id));
2306                     } else {
2307                         variant_resolution = Some(variant_def.def_id);
2308                     }
2309                 }
2310             }
2311         }
2312
2313         // Find the type of the associated item, and the trait where the associated
2314         // item is declared.
2315         let bound = match (&qself_ty.kind, qself_res) {
2316             (_, Res::SelfTy(Some(_), Some(impl_def_id))) => {
2317                 // `Self` in an impl of a trait -- we have a concrete self type and a
2318                 // trait reference.
2319                 let trait_ref = match tcx.impl_trait_ref(impl_def_id) {
2320                     Some(trait_ref) => trait_ref,
2321                     None => {
2322                         // A cycle error occurred, most likely.
2323                         return Err(ErrorReported);
2324                     }
2325                 };
2326
2327                 self.one_bound_for_assoc_type(
2328                     || traits::supertraits(tcx, ty::Binder::bind(trait_ref)),
2329                     || "Self".to_string(),
2330                     assoc_ident,
2331                     span,
2332                     || None,
2333                 )?
2334             }
2335             (
2336                 &ty::Param(_),
2337                 Res::SelfTy(Some(param_did), None) | Res::Def(DefKind::TyParam, param_did),
2338             ) => self.find_bound_for_assoc_item(param_did.expect_local(), assoc_ident, span)?,
2339             _ => {
2340                 if variant_resolution.is_some() {
2341                     // Variant in type position
2342                     let msg = format!("expected type, found variant `{}`", assoc_ident);
2343                     tcx.sess.span_err(span, &msg);
2344                 } else if qself_ty.is_enum() {
2345                     let mut err = struct_span_err!(
2346                         tcx.sess,
2347                         assoc_ident.span,
2348                         E0599,
2349                         "no variant named `{}` found for enum `{}`",
2350                         assoc_ident,
2351                         qself_ty,
2352                     );
2353
2354                     let adt_def = qself_ty.ty_adt_def().expect("enum is not an ADT");
2355                     if let Some(suggested_name) = find_best_match_for_name(
2356                         adt_def.variants.iter().map(|variant| &variant.ident.name),
2357                         &assoc_ident.as_str(),
2358                         None,
2359                     ) {
2360                         err.span_suggestion(
2361                             assoc_ident.span,
2362                             "there is a variant with a similar name",
2363                             suggested_name.to_string(),
2364                             Applicability::MaybeIncorrect,
2365                         );
2366                     } else {
2367                         err.span_label(
2368                             assoc_ident.span,
2369                             format!("variant not found in `{}`", qself_ty),
2370                         );
2371                     }
2372
2373                     if let Some(sp) = tcx.hir().span_if_local(adt_def.did) {
2374                         let sp = tcx.sess.source_map().guess_head_span(sp);
2375                         err.span_label(sp, format!("variant `{}` not found here", assoc_ident));
2376                     }
2377
2378                     err.emit();
2379                 } else if !qself_ty.references_error() {
2380                     // Don't print `TyErr` to the user.
2381                     self.report_ambiguous_associated_type(
2382                         span,
2383                         &qself_ty.to_string(),
2384                         "Trait",
2385                         assoc_ident.name,
2386                     );
2387                 }
2388                 return Err(ErrorReported);
2389             }
2390         };
2391
2392         let trait_did = bound.def_id();
2393         let (assoc_ident, def_scope) =
2394             tcx.adjust_ident_and_get_scope(assoc_ident, trait_did, hir_ref_id);
2395
2396         // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
2397         // of calling `filter_by_name_and_kind`.
2398         let item = tcx
2399             .associated_items(trait_did)
2400             .in_definition_order()
2401             .find(|i| {
2402                 i.kind.namespace() == Namespace::TypeNS
2403                     && i.ident.normalize_to_macros_2_0() == assoc_ident
2404             })
2405             .expect("missing associated type");
2406
2407         let ty = self.projected_ty_from_poly_trait_ref(span, item.def_id, assoc_segment, bound);
2408         let ty = self.normalize_ty(span, ty);
2409
2410         let kind = DefKind::AssocTy;
2411         if !item.vis.is_accessible_from(def_scope, tcx) {
2412             let kind = kind.descr(item.def_id);
2413             let msg = format!("{} `{}` is private", kind, assoc_ident);
2414             tcx.sess
2415                 .struct_span_err(span, &msg)
2416                 .span_label(span, &format!("private {}", kind))
2417                 .emit();
2418         }
2419         tcx.check_stability(item.def_id, Some(hir_ref_id), span);
2420
2421         if let Some(variant_def_id) = variant_resolution {
2422             tcx.struct_span_lint_hir(AMBIGUOUS_ASSOCIATED_ITEMS, hir_ref_id, span, |lint| {
2423                 let mut err = lint.build("ambiguous associated item");
2424                 let mut could_refer_to = |kind: DefKind, def_id, also| {
2425                     let note_msg = format!(
2426                         "`{}` could{} refer to the {} defined here",
2427                         assoc_ident,
2428                         also,
2429                         kind.descr(def_id)
2430                     );
2431                     err.span_note(tcx.def_span(def_id), &note_msg);
2432                 };
2433
2434                 could_refer_to(DefKind::Variant, variant_def_id, "");
2435                 could_refer_to(kind, item.def_id, " also");
2436
2437                 err.span_suggestion(
2438                     span,
2439                     "use fully-qualified syntax",
2440                     format!("<{} as {}>::{}", qself_ty, tcx.item_name(trait_did), assoc_ident),
2441                     Applicability::MachineApplicable,
2442                 );
2443
2444                 err.emit();
2445             });
2446         }
2447         Ok((ty, kind, item.def_id))
2448     }
2449
2450     fn qpath_to_ty(
2451         &self,
2452         span: Span,
2453         opt_self_ty: Option<Ty<'tcx>>,
2454         item_def_id: DefId,
2455         trait_segment: &hir::PathSegment<'_>,
2456         item_segment: &hir::PathSegment<'_>,
2457     ) -> Ty<'tcx> {
2458         let tcx = self.tcx();
2459
2460         let trait_def_id = tcx.parent(item_def_id).unwrap();
2461
2462         debug!("qpath_to_ty: trait_def_id={:?}", trait_def_id);
2463
2464         let self_ty = if let Some(ty) = opt_self_ty {
2465             ty
2466         } else {
2467             let path_str = tcx.def_path_str(trait_def_id);
2468
2469             let def_id = self.item_def_id();
2470
2471             debug!("qpath_to_ty: self.item_def_id()={:?}", def_id);
2472
2473             let parent_def_id = def_id
2474                 .and_then(|def_id| {
2475                     def_id.as_local().map(|def_id| tcx.hir().as_local_hir_id(def_id))
2476                 })
2477                 .map(|hir_id| tcx.hir().get_parent_did(hir_id).to_def_id());
2478
2479             debug!("qpath_to_ty: parent_def_id={:?}", parent_def_id);
2480
2481             // If the trait in segment is the same as the trait defining the item,
2482             // use the `<Self as ..>` syntax in the error.
2483             let is_part_of_self_trait_constraints = def_id == Some(trait_def_id);
2484             let is_part_of_fn_in_self_trait = parent_def_id == Some(trait_def_id);
2485
2486             let type_name = if is_part_of_self_trait_constraints || is_part_of_fn_in_self_trait {
2487                 "Self"
2488             } else {
2489                 "Type"
2490             };
2491
2492             self.report_ambiguous_associated_type(
2493                 span,
2494                 type_name,
2495                 &path_str,
2496                 item_segment.ident.name,
2497             );
2498             return tcx.ty_error();
2499         };
2500
2501         debug!("qpath_to_ty: self_type={:?}", self_ty);
2502
2503         let trait_ref = self.ast_path_to_mono_trait_ref(span, trait_def_id, self_ty, trait_segment);
2504
2505         let item_substs = self.create_substs_for_associated_item(
2506             tcx,
2507             span,
2508             item_def_id,
2509             item_segment,
2510             trait_ref.substs,
2511         );
2512
2513         debug!("qpath_to_ty: trait_ref={:?}", trait_ref);
2514
2515         self.normalize_ty(span, tcx.mk_projection(item_def_id, item_substs))
2516     }
2517
2518     pub fn prohibit_generics<'a, T: IntoIterator<Item = &'a hir::PathSegment<'a>>>(
2519         &self,
2520         segments: T,
2521     ) -> bool {
2522         let mut has_err = false;
2523         for segment in segments {
2524             let (mut err_for_lt, mut err_for_ty, mut err_for_ct) = (false, false, false);
2525             for arg in segment.generic_args().args {
2526                 let (span, kind) = match arg {
2527                     hir::GenericArg::Lifetime(lt) => {
2528                         if err_for_lt {
2529                             continue;
2530                         }
2531                         err_for_lt = true;
2532                         has_err = true;
2533                         (lt.span, "lifetime")
2534                     }
2535                     hir::GenericArg::Type(ty) => {
2536                         if err_for_ty {
2537                             continue;
2538                         }
2539                         err_for_ty = true;
2540                         has_err = true;
2541                         (ty.span, "type")
2542                     }
2543                     hir::GenericArg::Const(ct) => {
2544                         if err_for_ct {
2545                             continue;
2546                         }
2547                         err_for_ct = true;
2548                         has_err = true;
2549                         (ct.span, "const")
2550                     }
2551                 };
2552                 let mut err = struct_span_err!(
2553                     self.tcx().sess,
2554                     span,
2555                     E0109,
2556                     "{} arguments are not allowed for this type",
2557                     kind,
2558                 );
2559                 err.span_label(span, format!("{} argument not allowed", kind));
2560                 err.emit();
2561                 if err_for_lt && err_for_ty && err_for_ct {
2562                     break;
2563                 }
2564             }
2565
2566             // Only emit the first error to avoid overloading the user with error messages.
2567             if let [binding, ..] = segment.generic_args().bindings {
2568                 has_err = true;
2569                 Self::prohibit_assoc_ty_binding(self.tcx(), binding.span);
2570             }
2571         }
2572         has_err
2573     }
2574
2575     pub fn prohibit_assoc_ty_binding(tcx: TyCtxt<'_>, span: Span) {
2576         let mut err = struct_span_err!(
2577             tcx.sess,
2578             span,
2579             E0229,
2580             "associated type bindings are not allowed here"
2581         );
2582         err.span_label(span, "associated type not allowed here").emit();
2583     }
2584
2585     /// Prohibits explicit lifetime arguments if late-bound lifetime parameters
2586     /// are present. This is used both for datatypes and function calls.
2587     fn prohibit_explicit_late_bound_lifetimes(
2588         tcx: TyCtxt<'_>,
2589         def: &ty::Generics,
2590         args: &hir::GenericArgs<'_>,
2591         position: GenericArgPosition,
2592     ) -> ExplicitLateBound {
2593         let param_counts = def.own_counts();
2594         let arg_counts = args.own_counts();
2595         let infer_lifetimes = position != GenericArgPosition::Type && arg_counts.lifetimes == 0;
2596
2597         if infer_lifetimes {
2598             ExplicitLateBound::No
2599         } else if let Some(span_late) = def.has_late_bound_regions {
2600             let msg = "cannot specify lifetime arguments explicitly \
2601                        if late bound lifetime parameters are present";
2602             let note = "the late bound lifetime parameter is introduced here";
2603             let span = args.args[0].span();
2604             if position == GenericArgPosition::Value
2605                 && arg_counts.lifetimes != param_counts.lifetimes
2606             {
2607                 let mut err = tcx.sess.struct_span_err(span, msg);
2608                 err.span_note(span_late, note);
2609                 err.emit();
2610             } else {
2611                 let mut multispan = MultiSpan::from_span(span);
2612                 multispan.push_span_label(span_late, note.to_string());
2613                 tcx.struct_span_lint_hir(
2614                     LATE_BOUND_LIFETIME_ARGUMENTS,
2615                     args.args[0].id(),
2616                     multispan,
2617                     |lint| lint.build(msg).emit(),
2618                 );
2619             }
2620             ExplicitLateBound::Yes
2621         } else {
2622             ExplicitLateBound::No
2623         }
2624     }
2625
2626     // FIXME(eddyb, varkor) handle type paths here too, not just value ones.
2627     pub fn def_ids_for_value_path_segments(
2628         &self,
2629         segments: &[hir::PathSegment<'_>],
2630         self_ty: Option<Ty<'tcx>>,
2631         kind: DefKind,
2632         def_id: DefId,
2633     ) -> Vec<PathSeg> {
2634         // We need to extract the type parameters supplied by the user in
2635         // the path `path`. Due to the current setup, this is a bit of a
2636         // tricky-process; the problem is that resolve only tells us the
2637         // end-point of the path resolution, and not the intermediate steps.
2638         // Luckily, we can (at least for now) deduce the intermediate steps
2639         // just from the end-point.
2640         //
2641         // There are basically five cases to consider:
2642         //
2643         // 1. Reference to a constructor of a struct:
2644         //
2645         //        struct Foo<T>(...)
2646         //
2647         //    In this case, the parameters are declared in the type space.
2648         //
2649         // 2. Reference to a constructor of an enum variant:
2650         //
2651         //        enum E<T> { Foo(...) }
2652         //
2653         //    In this case, the parameters are defined in the type space,
2654         //    but may be specified either on the type or the variant.
2655         //
2656         // 3. Reference to a fn item or a free constant:
2657         //
2658         //        fn foo<T>() { }
2659         //
2660         //    In this case, the path will again always have the form
2661         //    `a::b::foo::<T>` where only the final segment should have
2662         //    type parameters. However, in this case, those parameters are
2663         //    declared on a value, and hence are in the `FnSpace`.
2664         //
2665         // 4. Reference to a method or an associated constant:
2666         //
2667         //        impl<A> SomeStruct<A> {
2668         //            fn foo<B>(...)
2669         //        }
2670         //
2671         //    Here we can have a path like
2672         //    `a::b::SomeStruct::<A>::foo::<B>`, in which case parameters
2673         //    may appear in two places. The penultimate segment,
2674         //    `SomeStruct::<A>`, contains parameters in TypeSpace, and the
2675         //    final segment, `foo::<B>` contains parameters in fn space.
2676         //
2677         // The first step then is to categorize the segments appropriately.
2678
2679         let tcx = self.tcx();
2680
2681         assert!(!segments.is_empty());
2682         let last = segments.len() - 1;
2683
2684         let mut path_segs = vec![];
2685
2686         match kind {
2687             // Case 1. Reference to a struct constructor.
2688             DefKind::Ctor(CtorOf::Struct, ..) => {
2689                 // Everything but the final segment should have no
2690                 // parameters at all.
2691                 let generics = tcx.generics_of(def_id);
2692                 // Variant and struct constructors use the
2693                 // generics of their parent type definition.
2694                 let generics_def_id = generics.parent.unwrap_or(def_id);
2695                 path_segs.push(PathSeg(generics_def_id, last));
2696             }
2697
2698             // Case 2. Reference to a variant constructor.
2699             DefKind::Ctor(CtorOf::Variant, ..) | DefKind::Variant => {
2700                 let adt_def = self_ty.map(|t| t.ty_adt_def().unwrap());
2701                 let (generics_def_id, index) = if let Some(adt_def) = adt_def {
2702                     debug_assert!(adt_def.is_enum());
2703                     (adt_def.did, last)
2704                 } else if last >= 1 && segments[last - 1].args.is_some() {
2705                     // Everything but the penultimate segment should have no
2706                     // parameters at all.
2707                     let mut def_id = def_id;
2708
2709                     // `DefKind::Ctor` -> `DefKind::Variant`
2710                     if let DefKind::Ctor(..) = kind {
2711                         def_id = tcx.parent(def_id).unwrap()
2712                     }
2713
2714                     // `DefKind::Variant` -> `DefKind::Enum`
2715                     let enum_def_id = tcx.parent(def_id).unwrap();
2716                     (enum_def_id, last - 1)
2717                 } else {
2718                     // FIXME: lint here recommending `Enum::<...>::Variant` form
2719                     // instead of `Enum::Variant::<...>` form.
2720
2721                     // Everything but the final segment should have no
2722                     // parameters at all.
2723                     let generics = tcx.generics_of(def_id);
2724                     // Variant and struct constructors use the
2725                     // generics of their parent type definition.
2726                     (generics.parent.unwrap_or(def_id), last)
2727                 };
2728                 path_segs.push(PathSeg(generics_def_id, index));
2729             }
2730
2731             // Case 3. Reference to a top-level value.
2732             DefKind::Fn | DefKind::Const | DefKind::ConstParam | DefKind::Static => {
2733                 path_segs.push(PathSeg(def_id, last));
2734             }
2735
2736             // Case 4. Reference to a method or associated const.
2737             DefKind::AssocFn | DefKind::AssocConst => {
2738                 if segments.len() >= 2 {
2739                     let generics = tcx.generics_of(def_id);
2740                     path_segs.push(PathSeg(generics.parent.unwrap(), last - 1));
2741                 }
2742                 path_segs.push(PathSeg(def_id, last));
2743             }
2744
2745             kind => bug!("unexpected definition kind {:?} for {:?}", kind, def_id),
2746         }
2747
2748         debug!("path_segs = {:?}", path_segs);
2749
2750         path_segs
2751     }
2752
2753     // Check a type `Path` and convert it to a `Ty`.
2754     pub fn res_to_ty(
2755         &self,
2756         opt_self_ty: Option<Ty<'tcx>>,
2757         path: &hir::Path<'_>,
2758         permit_variants: bool,
2759     ) -> Ty<'tcx> {
2760         let tcx = self.tcx();
2761
2762         debug!(
2763             "res_to_ty(res={:?}, opt_self_ty={:?}, path_segments={:?})",
2764             path.res, opt_self_ty, path.segments
2765         );
2766
2767         let span = path.span;
2768         match path.res {
2769             Res::Def(DefKind::OpaqueTy, did) => {
2770                 // Check for desugared `impl Trait`.
2771                 assert!(ty::is_impl_trait_defn(tcx, did).is_none());
2772                 let item_segment = path.segments.split_last().unwrap();
2773                 self.prohibit_generics(item_segment.1);
2774                 let substs = self.ast_path_substs_for_ty(span, did, item_segment.0);
2775                 self.normalize_ty(span, tcx.mk_opaque(did, substs))
2776             }
2777             Res::Def(
2778                 DefKind::Enum
2779                 | DefKind::TyAlias
2780                 | DefKind::Struct
2781                 | DefKind::Union
2782                 | DefKind::ForeignTy,
2783                 did,
2784             ) => {
2785                 assert_eq!(opt_self_ty, None);
2786                 self.prohibit_generics(path.segments.split_last().unwrap().1);
2787                 self.ast_path_to_ty(span, did, path.segments.last().unwrap())
2788             }
2789             Res::Def(kind @ DefKind::Variant, def_id) if permit_variants => {
2790                 // Convert "variant type" as if it were a real type.
2791                 // The resulting `Ty` is type of the variant's enum for now.
2792                 assert_eq!(opt_self_ty, None);
2793
2794                 let path_segs =
2795                     self.def_ids_for_value_path_segments(&path.segments, None, kind, def_id);
2796                 let generic_segs: FxHashSet<_> =
2797                     path_segs.iter().map(|PathSeg(_, index)| index).collect();
2798                 self.prohibit_generics(path.segments.iter().enumerate().filter_map(
2799                     |(index, seg)| {
2800                         if !generic_segs.contains(&index) { Some(seg) } else { None }
2801                     },
2802                 ));
2803
2804                 let PathSeg(def_id, index) = path_segs.last().unwrap();
2805                 self.ast_path_to_ty(span, *def_id, &path.segments[*index])
2806             }
2807             Res::Def(DefKind::TyParam, def_id) => {
2808                 assert_eq!(opt_self_ty, None);
2809                 self.prohibit_generics(path.segments);
2810
2811                 let hir_id = tcx.hir().as_local_hir_id(def_id.expect_local());
2812                 let item_id = tcx.hir().get_parent_node(hir_id);
2813                 let item_def_id = tcx.hir().local_def_id(item_id);
2814                 let generics = tcx.generics_of(item_def_id);
2815                 let index = generics.param_def_id_to_index[&def_id];
2816                 tcx.mk_ty_param(index, tcx.hir().name(hir_id))
2817             }
2818             Res::SelfTy(Some(_), None) => {
2819                 // `Self` in trait or type alias.
2820                 assert_eq!(opt_self_ty, None);
2821                 self.prohibit_generics(path.segments);
2822                 tcx.types.self_param
2823             }
2824             Res::SelfTy(_, Some(def_id)) => {
2825                 // `Self` in impl (we know the concrete type).
2826                 assert_eq!(opt_self_ty, None);
2827                 self.prohibit_generics(path.segments);
2828                 // Try to evaluate any array length constants.
2829                 self.normalize_ty(span, tcx.at(span).type_of(def_id))
2830             }
2831             Res::Def(DefKind::AssocTy, def_id) => {
2832                 debug_assert!(path.segments.len() >= 2);
2833                 self.prohibit_generics(&path.segments[..path.segments.len() - 2]);
2834                 self.qpath_to_ty(
2835                     span,
2836                     opt_self_ty,
2837                     def_id,
2838                     &path.segments[path.segments.len() - 2],
2839                     path.segments.last().unwrap(),
2840                 )
2841             }
2842             Res::PrimTy(prim_ty) => {
2843                 assert_eq!(opt_self_ty, None);
2844                 self.prohibit_generics(path.segments);
2845                 match prim_ty {
2846                     hir::PrimTy::Bool => tcx.types.bool,
2847                     hir::PrimTy::Char => tcx.types.char,
2848                     hir::PrimTy::Int(it) => tcx.mk_mach_int(it),
2849                     hir::PrimTy::Uint(uit) => tcx.mk_mach_uint(uit),
2850                     hir::PrimTy::Float(ft) => tcx.mk_mach_float(ft),
2851                     hir::PrimTy::Str => tcx.types.str_,
2852                 }
2853             }
2854             Res::Err => {
2855                 self.set_tainted_by_errors();
2856                 self.tcx().ty_error()
2857             }
2858             _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
2859         }
2860     }
2861
2862     /// Parses the programmer's textual representation of a type into our
2863     /// internal notion of a type.
2864     pub fn ast_ty_to_ty(&self, ast_ty: &hir::Ty<'_>) -> Ty<'tcx> {
2865         debug!("ast_ty_to_ty(id={:?}, ast_ty={:?} ty_ty={:?})", ast_ty.hir_id, ast_ty, ast_ty.kind);
2866
2867         let tcx = self.tcx();
2868
2869         let result_ty = match ast_ty.kind {
2870             hir::TyKind::Slice(ref ty) => tcx.mk_slice(self.ast_ty_to_ty(&ty)),
2871             hir::TyKind::Ptr(ref mt) => {
2872                 tcx.mk_ptr(ty::TypeAndMut { ty: self.ast_ty_to_ty(&mt.ty), mutbl: mt.mutbl })
2873             }
2874             hir::TyKind::Rptr(ref region, ref mt) => {
2875                 let r = self.ast_region_to_region(region, None);
2876                 debug!("ast_ty_to_ty: r={:?}", r);
2877                 let t = self.ast_ty_to_ty(&mt.ty);
2878                 tcx.mk_ref(r, ty::TypeAndMut { ty: t, mutbl: mt.mutbl })
2879             }
2880             hir::TyKind::Never => tcx.types.never,
2881             hir::TyKind::Tup(ref fields) => {
2882                 tcx.mk_tup(fields.iter().map(|t| self.ast_ty_to_ty(&t)))
2883             }
2884             hir::TyKind::BareFn(ref bf) => {
2885                 require_c_abi_if_c_variadic(tcx, &bf.decl, bf.abi, ast_ty.span);
2886                 tcx.mk_fn_ptr(self.ty_of_fn(
2887                     bf.unsafety,
2888                     bf.abi,
2889                     &bf.decl,
2890                     &hir::Generics::empty(),
2891                     None,
2892                 ))
2893             }
2894             hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
2895                 self.conv_object_ty_poly_trait_ref(ast_ty.span, bounds, lifetime)
2896             }
2897             hir::TyKind::Path(hir::QPath::Resolved(ref maybe_qself, ref path)) => {
2898                 debug!("ast_ty_to_ty: maybe_qself={:?} path={:?}", maybe_qself, path);
2899                 let opt_self_ty = maybe_qself.as_ref().map(|qself| self.ast_ty_to_ty(qself));
2900                 self.res_to_ty(opt_self_ty, path, false)
2901             }
2902             hir::TyKind::OpaqueDef(item_id, ref lifetimes) => {
2903                 let opaque_ty = tcx.hir().expect_item(item_id.id);
2904                 let def_id = tcx.hir().local_def_id(item_id.id).to_def_id();
2905
2906                 match opaque_ty.kind {
2907                     hir::ItemKind::OpaqueTy(hir::OpaqueTy { impl_trait_fn, .. }) => {
2908                         self.impl_trait_ty_to_ty(def_id, lifetimes, impl_trait_fn.is_some())
2909                     }
2910                     ref i => bug!("`impl Trait` pointed to non-opaque type?? {:#?}", i),
2911                 }
2912             }
2913             hir::TyKind::Path(hir::QPath::TypeRelative(ref qself, ref segment)) => {
2914                 debug!("ast_ty_to_ty: qself={:?} segment={:?}", qself, segment);
2915                 let ty = self.ast_ty_to_ty(qself);
2916
2917                 let res = if let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind {
2918                     path.res
2919                 } else {
2920                     Res::Err
2921                 };
2922                 self.associated_path_to_ty(ast_ty.hir_id, ast_ty.span, ty, res, segment, false)
2923                     .map(|(ty, _, _)| ty)
2924                     .unwrap_or_else(|_| tcx.ty_error())
2925             }
2926             hir::TyKind::Array(ref ty, ref length) => {
2927                 let length_def_id = tcx.hir().local_def_id(length.hir_id);
2928                 let length = ty::Const::from_anon_const(tcx, length_def_id);
2929                 let array_ty = tcx.mk_ty(ty::Array(self.ast_ty_to_ty(&ty), length));
2930                 self.normalize_ty(ast_ty.span, array_ty)
2931             }
2932             hir::TyKind::Typeof(ref _e) => {
2933                 struct_span_err!(
2934                     tcx.sess,
2935                     ast_ty.span,
2936                     E0516,
2937                     "`typeof` is a reserved keyword but unimplemented"
2938                 )
2939                 .span_label(ast_ty.span, "reserved keyword")
2940                 .emit();
2941
2942                 tcx.ty_error()
2943             }
2944             hir::TyKind::Infer => {
2945                 // Infer also appears as the type of arguments or return
2946                 // values in a ExprKind::Closure, or as
2947                 // the type of local variables. Both of these cases are
2948                 // handled specially and will not descend into this routine.
2949                 self.ty_infer(None, ast_ty.span)
2950             }
2951             hir::TyKind::Err => tcx.ty_error(),
2952         };
2953
2954         debug!("ast_ty_to_ty: result_ty={:?}", result_ty);
2955
2956         self.record_ty(ast_ty.hir_id, result_ty, ast_ty.span);
2957         result_ty
2958     }
2959
2960     pub fn impl_trait_ty_to_ty(
2961         &self,
2962         def_id: DefId,
2963         lifetimes: &[hir::GenericArg<'_>],
2964         replace_parent_lifetimes: bool,
2965     ) -> Ty<'tcx> {
2966         debug!("impl_trait_ty_to_ty(def_id={:?}, lifetimes={:?})", def_id, lifetimes);
2967         let tcx = self.tcx();
2968
2969         let generics = tcx.generics_of(def_id);
2970
2971         debug!("impl_trait_ty_to_ty: generics={:?}", generics);
2972         let substs = InternalSubsts::for_item(tcx, def_id, |param, _| {
2973             if let Some(i) = (param.index as usize).checked_sub(generics.parent_count) {
2974                 // Our own parameters are the resolved lifetimes.
2975                 match param.kind {
2976                     GenericParamDefKind::Lifetime => {
2977                         if let hir::GenericArg::Lifetime(lifetime) = &lifetimes[i] {
2978                             self.ast_region_to_region(lifetime, None).into()
2979                         } else {
2980                             bug!()
2981                         }
2982                     }
2983                     _ => bug!(),
2984                 }
2985             } else {
2986                 match param.kind {
2987                     // For RPIT (return position impl trait), only lifetimes
2988                     // mentioned in the impl Trait predicate are captured by
2989                     // the opaque type, so the lifetime parameters from the
2990                     // parent item need to be replaced with `'static`.
2991                     //
2992                     // For `impl Trait` in the types of statics, constants,
2993                     // locals and type aliases. These capture all parent
2994                     // lifetimes, so they can use their identity subst.
2995                     GenericParamDefKind::Lifetime if replace_parent_lifetimes => {
2996                         tcx.lifetimes.re_static.into()
2997                     }
2998                     _ => tcx.mk_param_from_def(param),
2999                 }
3000             }
3001         });
3002         debug!("impl_trait_ty_to_ty: substs={:?}", substs);
3003
3004         let ty = tcx.mk_opaque(def_id, substs);
3005         debug!("impl_trait_ty_to_ty: {}", ty);
3006         ty
3007     }
3008
3009     pub fn ty_of_arg(&self, ty: &hir::Ty<'_>, expected_ty: Option<Ty<'tcx>>) -> Ty<'tcx> {
3010         match ty.kind {
3011             hir::TyKind::Infer if expected_ty.is_some() => {
3012                 self.record_ty(ty.hir_id, expected_ty.unwrap(), ty.span);
3013                 expected_ty.unwrap()
3014             }
3015             _ => self.ast_ty_to_ty(ty),
3016         }
3017     }
3018
3019     pub fn ty_of_fn(
3020         &self,
3021         unsafety: hir::Unsafety,
3022         abi: abi::Abi,
3023         decl: &hir::FnDecl<'_>,
3024         generics: &hir::Generics<'_>,
3025         ident_span: Option<Span>,
3026     ) -> ty::PolyFnSig<'tcx> {
3027         debug!("ty_of_fn");
3028
3029         let tcx = self.tcx();
3030
3031         // We proactively collect all the inferred type params to emit a single error per fn def.
3032         let mut visitor = PlaceholderHirTyCollector::default();
3033         for ty in decl.inputs {
3034             visitor.visit_ty(ty);
3035         }
3036         walk_generics(&mut visitor, generics);
3037
3038         let input_tys = decl.inputs.iter().map(|a| self.ty_of_arg(a, None));
3039         let output_ty = match decl.output {
3040             hir::FnRetTy::Return(ref output) => {
3041                 visitor.visit_ty(output);
3042                 self.ast_ty_to_ty(output)
3043             }
3044             hir::FnRetTy::DefaultReturn(..) => tcx.mk_unit(),
3045         };
3046
3047         debug!("ty_of_fn: output_ty={:?}", output_ty);
3048
3049         let bare_fn_ty =
3050             ty::Binder::bind(tcx.mk_fn_sig(input_tys, output_ty, decl.c_variadic, unsafety, abi));
3051
3052         if let (false, Some(ident_span)) = (self.allow_ty_infer(), ident_span) {
3053             // We always collect the spans for placeholder types when evaluating `fn`s, but we
3054             // only want to emit an error complaining about them if infer types (`_`) are not
3055             // allowed. `allow_ty_infer` gates this behavior. We check for the presence of
3056             // `ident_span` to not emit an error twice when we have `fn foo(_: fn() -> _)`.
3057             crate::collect::placeholder_type_error(
3058                 tcx,
3059                 ident_span.shrink_to_hi(),
3060                 &generics.params[..],
3061                 visitor.0,
3062                 true,
3063             );
3064         }
3065
3066         // Find any late-bound regions declared in return type that do
3067         // not appear in the arguments. These are not well-formed.
3068         //
3069         // Example:
3070         //     for<'a> fn() -> &'a str <-- 'a is bad
3071         //     for<'a> fn(&'a String) -> &'a str <-- 'a is ok
3072         let inputs = bare_fn_ty.inputs();
3073         let late_bound_in_args =
3074             tcx.collect_constrained_late_bound_regions(&inputs.map_bound(|i| i.to_owned()));
3075         let output = bare_fn_ty.output();
3076         let late_bound_in_ret = tcx.collect_referenced_late_bound_regions(&output);
3077         for br in late_bound_in_ret.difference(&late_bound_in_args) {
3078             let lifetime_name = match *br {
3079                 ty::BrNamed(_, name) => format!("lifetime `{}`,", name),
3080                 ty::BrAnon(_) | ty::BrEnv => "an anonymous lifetime".to_string(),
3081             };
3082             let mut err = struct_span_err!(
3083                 tcx.sess,
3084                 decl.output.span(),
3085                 E0581,
3086                 "return type references {} which is not constrained by the fn input types",
3087                 lifetime_name
3088             );
3089             if let ty::BrAnon(_) = *br {
3090                 // The only way for an anonymous lifetime to wind up
3091                 // in the return type but **also** be unconstrained is
3092                 // if it only appears in "associated types" in the
3093                 // input. See #47511 for an example. In this case,
3094                 // though we can easily give a hint that ought to be
3095                 // relevant.
3096                 err.note(
3097                     "lifetimes appearing in an associated type are not considered constrained",
3098                 );
3099             }
3100             err.emit();
3101         }
3102
3103         bare_fn_ty
3104     }
3105
3106     /// Given the bounds on an object, determines what single region bound (if any) we can
3107     /// use to summarize this type. The basic idea is that we will use the bound the user
3108     /// provided, if they provided one, and otherwise search the supertypes of trait bounds
3109     /// for region bounds. It may be that we can derive no bound at all, in which case
3110     /// we return `None`.
3111     fn compute_object_lifetime_bound(
3112         &self,
3113         span: Span,
3114         existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
3115     ) -> Option<ty::Region<'tcx>> // if None, use the default
3116     {
3117         let tcx = self.tcx();
3118
3119         debug!("compute_opt_region_bound(existential_predicates={:?})", existential_predicates);
3120
3121         // No explicit region bound specified. Therefore, examine trait
3122         // bounds and see if we can derive region bounds from those.
3123         let derived_region_bounds = object_region_bounds(tcx, existential_predicates);
3124
3125         // If there are no derived region bounds, then report back that we
3126         // can find no region bound. The caller will use the default.
3127         if derived_region_bounds.is_empty() {
3128             return None;
3129         }
3130
3131         // If any of the derived region bounds are 'static, that is always
3132         // the best choice.
3133         if derived_region_bounds.iter().any(|&r| ty::ReStatic == *r) {
3134             return Some(tcx.lifetimes.re_static);
3135         }
3136
3137         // Determine whether there is exactly one unique region in the set
3138         // of derived region bounds. If so, use that. Otherwise, report an
3139         // error.
3140         let r = derived_region_bounds[0];
3141         if derived_region_bounds[1..].iter().any(|r1| r != *r1) {
3142             struct_span_err!(
3143                 tcx.sess,
3144                 span,
3145                 E0227,
3146                 "ambiguous lifetime bound, explicit lifetime bound required"
3147             )
3148             .emit();
3149         }
3150         Some(r)
3151     }
3152 }
3153
3154 /// Collects together a list of bounds that are applied to some type,
3155 /// after they've been converted into `ty` form (from the HIR
3156 /// representations). These lists of bounds occur in many places in
3157 /// Rust's syntax:
3158 ///
3159 /// ```text
3160 /// trait Foo: Bar + Baz { }
3161 ///            ^^^^^^^^^ supertrait list bounding the `Self` type parameter
3162 ///
3163 /// fn foo<T: Bar + Baz>() { }
3164 ///           ^^^^^^^^^ bounding the type parameter `T`
3165 ///
3166 /// impl dyn Bar + Baz
3167 ///          ^^^^^^^^^ bounding the forgotten dynamic type
3168 /// ```
3169 ///
3170 /// Our representation is a bit mixed here -- in some cases, we
3171 /// include the self type (e.g., `trait_bounds`) but in others we do
3172 #[derive(Default, PartialEq, Eq, Clone, Debug)]
3173 pub struct Bounds<'tcx> {
3174     /// A list of region bounds on the (implicit) self type. So if you
3175     /// had `T: 'a + 'b` this might would be a list `['a, 'b]` (but
3176     /// the `T` is not explicitly included).
3177     pub region_bounds: Vec<(ty::Region<'tcx>, Span)>,
3178
3179     /// A list of trait bounds. So if you had `T: Debug` this would be
3180     /// `T: Debug`. Note that the self-type is explicit here.
3181     pub trait_bounds: Vec<(ty::PolyTraitRef<'tcx>, Span, Constness)>,
3182
3183     /// A list of projection equality bounds. So if you had `T:
3184     /// Iterator<Item = u32>` this would include `<T as
3185     /// Iterator>::Item => u32`. Note that the self-type is explicit
3186     /// here.
3187     pub projection_bounds: Vec<(ty::PolyProjectionPredicate<'tcx>, Span)>,
3188
3189     /// `Some` if there is *no* `?Sized` predicate. The `span`
3190     /// is the location in the source of the `T` declaration which can
3191     /// be cited as the source of the `T: Sized` requirement.
3192     pub implicitly_sized: Option<Span>,
3193 }
3194
3195 impl<'tcx> Bounds<'tcx> {
3196     /// Converts a bounds list into a flat set of predicates (like
3197     /// where-clauses). Because some of our bounds listings (e.g.,
3198     /// regions) don't include the self-type, you must supply the
3199     /// self-type here (the `param_ty` parameter).
3200     pub fn predicates(
3201         &self,
3202         tcx: TyCtxt<'tcx>,
3203         param_ty: Ty<'tcx>,
3204     ) -> Vec<(ty::Predicate<'tcx>, Span)> {
3205         // If it could be sized, and is, add the `Sized` predicate.
3206         let sized_predicate = self.implicitly_sized.and_then(|span| {
3207             tcx.lang_items().sized_trait().map(|sized| {
3208                 let trait_ref = ty::Binder::bind(ty::TraitRef {
3209                     def_id: sized,
3210                     substs: tcx.mk_substs_trait(param_ty, &[]),
3211                 });
3212                 (trait_ref.without_const().to_predicate(tcx), span)
3213             })
3214         });
3215
3216         sized_predicate
3217             .into_iter()
3218             .chain(
3219                 self.region_bounds
3220                     .iter()
3221                     .map(|&(region_bound, span)| {
3222                         // Account for the binder being introduced below; no need to shift `param_ty`
3223                         // because, at present at least, it either only refers to early-bound regions,
3224                         // or it's a generic associated type that deliberately has escaping bound vars.
3225                         let region_bound = ty::fold::shift_region(tcx, region_bound, 1);
3226                         let outlives = ty::OutlivesPredicate(param_ty, region_bound);
3227                         (ty::Binder::bind(outlives).to_predicate(tcx), span)
3228                     })
3229                     .chain(self.trait_bounds.iter().map(|&(bound_trait_ref, span, constness)| {
3230                         let predicate = bound_trait_ref.with_constness(constness).to_predicate(tcx);
3231                         (predicate, span)
3232                     }))
3233                     .chain(
3234                         self.projection_bounds
3235                             .iter()
3236                             .map(|&(projection, span)| (projection.to_predicate(tcx), span)),
3237                     ),
3238             )
3239             .collect()
3240     }
3241 }