compiler/rustc_typeck/src/astconv/generics.rs

   1 use crate::astconv::{
   2     AstConv, CreateSubstsForGenericArgsCtxt, ExplicitLateBound, GenericArgCountMismatch,
   3     GenericArgCountResult, GenericArgPosition,
   4 };
   5 use crate::errors::AssocTypeBindingNotAllowed;
   6 use rustc_ast::ast::ParamKindOrd;
   7 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticId, ErrorReported};
   8 use rustc_hir as hir;
   9 use rustc_hir::def_id::DefId;
  10 use rustc_hir::GenericArg;
  11 use rustc_middle::ty::{
  12     self, subst, subst::SubstsRef, GenericParamDef, GenericParamDefKind, Ty, TyCtxt,
  13 };
  14 use rustc_session::{lint::builtin::LATE_BOUND_LIFETIME_ARGUMENTS, Session};
  15 use rustc_span::{symbol::kw, MultiSpan, Span};
  16
  17 use smallvec::SmallVec;
  18
  19 impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
  20     /// Report an error that a generic argument did not match the generic parameter that was
  21     /// expected.
  22     fn generic_arg_mismatch_err(
  23         sess: &Session,
  24         arg: &GenericArg<'_>,
  25         kind: &'static str,
  26         possible_ordering_error: bool,
  27         help: Option<&str>,
  28     ) {
  29         let mut err = struct_span_err!(
  30             sess,
  31             arg.span(),
  32             E0747,
  33             "{} provided when a {} was expected",
  34             arg.descr(),
  35             kind,
  36         );
  37
  38         let unordered = sess.features_untracked().const_generics;
  39         let kind_ord = match kind {
  40             "lifetime" => ParamKindOrd::Lifetime,
  41             "type" => ParamKindOrd::Type,
  42             "constant" => ParamKindOrd::Const { unordered },
  43             // It's more concise to match on the string representation, though it means
  44             // the match is non-exhaustive.
  45             _ => bug!("invalid generic parameter kind {}", kind),
  46         };
  47         let arg_ord = match arg {
  48             GenericArg::Lifetime(_) => ParamKindOrd::Lifetime,
  49             GenericArg::Type(_) => ParamKindOrd::Type,
  50             GenericArg::Const(_) => ParamKindOrd::Const { unordered },
  51         };
  52
  53         if matches!(arg, GenericArg::Type(hir::Ty { kind: hir::TyKind::Path { .. }, .. }))
  54             && matches!(kind_ord, ParamKindOrd::Const { .. })
  55         {
  56             let suggestions = vec![
  57                 (arg.span().shrink_to_lo(), String::from("{ ")),
  58                 (arg.span().shrink_to_hi(), String::from(" }")),
  59             ];
  60             err.multipart_suggestion(
  61                 "if this generic argument was intended as a const parameter, \
  62                 try surrounding it with braces:",
  63                 suggestions,
  64                 Applicability::MaybeIncorrect,
  65             );
  66         }
  67
  68         // This note is only true when generic parameters are strictly ordered by their kind.
  69         if possible_ordering_error && kind_ord.cmp(&arg_ord) != core::cmp::Ordering::Equal {
  70             let (first, last) =
  71                 if kind_ord < arg_ord { (kind, arg.descr()) } else { (arg.descr(), kind) };
  72             err.note(&format!("{} arguments must be provided before {} arguments", first, last));
  73             if let Some(help) = help {
  74                 err.help(help);
  75             }
  76         }
  77
  78         err.emit();
  79     }
  80
  81     /// Creates the relevant generic argument substitutions
  82     /// corresponding to a set of generic parameters. This is a
  83     /// rather complex function. Let us try to explain the role
  84     /// of each of its parameters:
  85     ///
  86     /// To start, we are given the `def_id` of the thing we are
  87     /// creating the substitutions for, and a partial set of
  88     /// substitutions `parent_substs`. In general, the substitutions
  89     /// for an item begin with substitutions for all the "parents" of
  90     /// that item -- e.g., for a method it might include the
  91     /// parameters from the impl.
  92     ///
  93     /// Therefore, the method begins by walking down these parents,
  94     /// starting with the outermost parent and proceed inwards until
  95     /// it reaches `def_id`. For each parent `P`, it will check `parent_substs`
  96     /// first to see if the parent's substitutions are listed in there. If so,
  97     /// we can append those and move on. Otherwise, it invokes the
  98     /// three callback functions:
  99     ///
 100     /// - `args_for_def_id`: given the `DefId` `P`, supplies back the
 101     ///   generic arguments that were given to that parent from within
 102     ///   the path; so e.g., if you have `<T as Foo>::Bar`, the `DefId`
 103     ///   might refer to the trait `Foo`, and the arguments might be
 104     ///   `[T]`. The boolean value indicates whether to infer values
 105     ///   for arguments whose values were not explicitly provided.
 106     /// - `provided_kind`: given the generic parameter and the value from `args_for_def_id`,
 107     ///   instantiate a `GenericArg`.
 108     /// - `inferred_kind`: if no parameter was provided, and inference is enabled, then
 109     ///   creates a suitable inference variable.
 110     pub fn create_substs_for_generic_args<'a>(
 111         tcx: TyCtxt<'tcx>,
 112         def_id: DefId,
 113         parent_substs: &[subst::GenericArg<'tcx>],
 114         has_self: bool,
 115         self_ty: Option<Ty<'tcx>>,
 116         arg_count: GenericArgCountResult,
 117         ctx: &mut impl CreateSubstsForGenericArgsCtxt<'a, 'tcx>,
 118     ) -> SubstsRef<'tcx> {
 119         // Collect the segments of the path; we need to substitute arguments
 120         // for parameters throughout the entire path (wherever there are
 121         // generic parameters).
 122         let mut parent_defs = tcx.generics_of(def_id);
 123         let count = parent_defs.count();
 124         let mut stack = vec![(def_id, parent_defs)];
 125         while let Some(def_id) = parent_defs.parent {
 126             parent_defs = tcx.generics_of(def_id);
 127             stack.push((def_id, parent_defs));
 128         }
 129
 130         // We manually build up the substitution, rather than using convenience
 131         // methods in `subst.rs`, so that we can iterate over the arguments and
 132         // parameters in lock-step linearly, instead of trying to match each pair.
 133         let mut substs: SmallVec<[subst::GenericArg<'tcx>; 8]> = SmallVec::with_capacity(count);
 134         // Iterate over each segment of the path.
 135         while let Some((def_id, defs)) = stack.pop() {
 136             let mut params = defs.params.iter().peekable();
 137
 138             // If we have already computed substitutions for parents, we can use those directly.
 139             while let Some(&param) = params.peek() {
 140                 if let Some(&kind) = parent_substs.get(param.index as usize) {
 141                     substs.push(kind);
 142                     params.next();
 143                 } else {
 144                     break;
 145                 }
 146             }
 147
 148             // `Self` is handled first, unless it's been handled in `parent_substs`.
 149             if has_self {
 150                 if let Some(&param) = params.peek() {
 151                     if param.index == 0 {
 152                         if let GenericParamDefKind::Type { .. } = param.kind {
 153                             substs.push(
 154                                 self_ty
 155                                     .map(|ty| ty.into())
 156                                     .unwrap_or_else(|| ctx.inferred_kind(None, param, true)),
 157                             );
 158                             params.next();
 159                         }
 160                     }
 161                 }
 162             }
 163
 164             // Check whether this segment takes generic arguments and the user has provided any.
 165             let (generic_args, infer_args) = ctx.args_for_def_id(def_id);
 166
 167             let args_iter = generic_args.iter().flat_map(|generic_args| generic_args.args.iter());
 168             let mut args = args_iter.clone().peekable();
 169
 170             // If we encounter a type or const when we expect a lifetime, we infer the lifetimes.
 171             // If we later encounter a lifetime, we know that the arguments were provided in the
 172             // wrong order. `force_infer_lt` records the type or const that forced lifetimes to be
 173             // inferred, so we can use it for diagnostics later.
 174             let mut force_infer_lt = None;
 175
 176             loop {
 177                 // We're going to iterate through the generic arguments that the user
 178                 // provided, matching them with the generic parameters we expect.
 179                 // Mismatches can occur as a result of elided lifetimes, or for malformed
 180                 // input. We try to handle both sensibly.
 181                 match (args.peek(), params.peek()) {
 182                     (Some(&arg), Some(&param)) => {
 183                         match (arg, &param.kind, arg_count.explicit_late_bound) {
 184                             (GenericArg::Lifetime(_), GenericParamDefKind::Lifetime, _)
 185                             | (GenericArg::Type(_), GenericParamDefKind::Type { .. }, _)
 186                             | (GenericArg::Const(_), GenericParamDefKind::Const, _) => {
 187                                 substs.push(ctx.provided_kind(param, arg));
 188                                 args.next();
 189                                 params.next();
 190                             }
 191                             (
 192                                 GenericArg::Type(_) | GenericArg::Const(_),
 193                                 GenericParamDefKind::Lifetime,
 194                                 _,
 195                             ) => {
 196                                 // We expected a lifetime argument, but got a type or const
 197                                 // argument. That means we're inferring the lifetimes.
 198                                 substs.push(ctx.inferred_kind(None, param, infer_args));
 199                                 force_infer_lt = Some(arg);
 200                                 params.next();
 201                             }
 202                             (GenericArg::Lifetime(_), _, ExplicitLateBound::Yes) => {
 203                                 // We've come across a lifetime when we expected something else in
 204                                 // the presence of explicit late bounds. This is most likely
 205                                 // due to the presence of the explicit bound so we're just going to
 206                                 // ignore it.
 207                                 args.next();
 208                             }
 209                             (_, kind, _) => {
 210                                 // We expected one kind of parameter, but the user provided
 211                                 // another. This is an error. However, if we already know that
 212                                 // the arguments don't match up with the parameters, we won't issue
 213                                 // an additional error, as the user already knows what's wrong.
 214                                 if arg_count.correct.is_ok()
 215                                     && arg_count.explicit_late_bound == ExplicitLateBound::No
 216                                 {
 217                                     // We're going to iterate over the parameters to sort them out, and
 218                                     // show that order to the user as a possible order for the parameters
 219                                     let mut param_types_present = defs
 220                                         .params
 221                                         .clone()
 222                                         .into_iter()
 223                                         .map(|param| {
 224                                             (
 225                                                 match param.kind {
 226                                                     GenericParamDefKind::Lifetime => {
 227                                                         ParamKindOrd::Lifetime
 228                                                     }
 229                                                     GenericParamDefKind::Type { .. } => {
 230                                                         ParamKindOrd::Type
 231                                                     }
 232                                                     GenericParamDefKind::Const => {
 233                                                         ParamKindOrd::Const {
 234                                                             unordered: tcx
 235                                                                 .features()
 236                                                                 .const_generics,
 237                                                         }
 238                                                     }
 239                                                 },
 240                                                 param,
 241                                             )
 242                                         })
 243                                         .collect::<Vec<(ParamKindOrd, GenericParamDef)>>();
 244                                     param_types_present.sort_by_key(|(ord, _)| *ord);
 245                                     let (mut param_types_present, ordered_params): (
 246                                         Vec<ParamKindOrd>,
 247                                         Vec<GenericParamDef>,
 248                                     ) = param_types_present.into_iter().unzip();
 249                                     param_types_present.dedup();
 250
 251                                     Self::generic_arg_mismatch_err(
 252                                         tcx.sess,
 253                                         arg,
 254                                         kind.descr(),
 255                                         !args_iter.clone().is_sorted_by_key(|arg| match arg {
 256                                             GenericArg::Lifetime(_) => ParamKindOrd::Lifetime,
 257                                             GenericArg::Type(_) => ParamKindOrd::Type,
 258                                             GenericArg::Const(_) => ParamKindOrd::Const {
 259                                                 unordered: tcx.features().const_generics,
 260                                             },
 261                                         }),
 262                                         Some(&format!(
 263                                             "reorder the arguments: {}: `<{}>`",
 264                                             param_types_present
 265                                                 .into_iter()
 266                                                 .map(|ord| format!("{}s", ord.to_string()))
 267                                                 .collect::<Vec<String>>()
 268                                                 .join(", then "),
 269                                             ordered_params
 270                                                 .into_iter()
 271                                                 .filter_map(|param| {
 272                                                     if param.name == kw::SelfUpper {
 273                                                         None
 274                                                     } else {
 275                                                         Some(param.name.to_string())
 276                                                     }
 277                                                 })
 278                                                 .collect::<Vec<String>>()
 279                                                 .join(", ")
 280                                         )),
 281                                     );
 282                                 }
 283
 284                                 // We've reported the error, but we want to make sure that this
 285                                 // problem doesn't bubble down and create additional, irrelevant
 286                                 // errors. In this case, we're simply going to ignore the argument
 287                                 // and any following arguments. The rest of the parameters will be
 288                                 // inferred.
 289                                 while args.next().is_some() {}
 290                             }
 291                         }
 292                     }
 293
 294                     (Some(&arg), None) => {
 295                         // We should never be able to reach this point with well-formed input.
 296                         // There are three situations in which we can encounter this issue.
 297                         //
 298                         //  1.  The number of arguments is incorrect. In this case, an error
 299                         //      will already have been emitted, and we can ignore it.
 300                         //  2.  There are late-bound lifetime parameters present, yet the
 301                         //      lifetime arguments have also been explicitly specified by the
 302                         //      user.
 303                         //  3.  We've inferred some lifetimes, which have been provided later (i.e.
 304                         //      after a type or const). We want to throw an error in this case.
 305
 306                         if arg_count.correct.is_ok()
 307                             && arg_count.explicit_late_bound == ExplicitLateBound::No
 308                         {
 309                             let kind = arg.descr();
 310                             assert_eq!(kind, "lifetime");
 311                             let provided =
 312                                 force_infer_lt.expect("lifetimes ought to have been inferred");
 313                             Self::generic_arg_mismatch_err(tcx.sess, provided, kind, false, None);
 314                         }
 315
 316                         break;
 317                     }
 318
 319                     (None, Some(&param)) => {
 320                         // If there are fewer arguments than parameters, it means
 321                         // we're inferring the remaining arguments.
 322                         substs.push(ctx.inferred_kind(Some(&substs), param, infer_args));
 323                         params.next();
 324                     }
 325
 326                     (None, None) => break,
 327                 }
 328             }
 329         }
 330
 331         tcx.intern_substs(&substs)
 332     }
 333
 334     /// Checks that the correct number of generic arguments have been provided.
 335     /// Used specifically for function calls.
 336     pub fn check_generic_arg_count_for_call(
 337         tcx: TyCtxt<'_>,
 338         span: Span,
 339         def: &ty::Generics,
 340         seg: &hir::PathSegment<'_>,
 341         is_method_call: bool,
 342     ) -> GenericArgCountResult {
 343         let empty_args = hir::GenericArgs::none();
 344         let suppress_mismatch = Self::check_impl_trait(tcx, seg, &def);
 345         Self::check_generic_arg_count(
 346             tcx,
 347             span,
 348             def,
 349             if let Some(ref args) = seg.args { args } else { &empty_args },
 350             if is_method_call { GenericArgPosition::MethodCall } else { GenericArgPosition::Value },
 351             def.parent.is_none() && def.has_self, // `has_self`
 352             seg.infer_args || suppress_mismatch,  // `infer_args`
 353         )
 354     }
 355
 356     /// Checks that the correct number of generic arguments have been provided.
 357     /// This is used both for datatypes and function calls.
 358     pub(crate) fn check_generic_arg_count(
 359         tcx: TyCtxt<'_>,
 360         span: Span,
 361         def: &ty::Generics,
 362         args: &hir::GenericArgs<'_>,
 363         position: GenericArgPosition,
 364         has_self: bool,
 365         infer_args: bool,
 366     ) -> GenericArgCountResult {
 367         // At this stage we are guaranteed that the generic arguments are in the correct order, e.g.
 368         // that lifetimes will proceed types. So it suffices to check the number of each generic
 369         // arguments in order to validate them with respect to the generic parameters.
 370         let param_counts = def.own_counts();
 371         let named_type_param_count = param_counts.types - has_self as usize;
 372         let arg_counts = args.own_counts();
 373         let infer_lifetimes = position != GenericArgPosition::Type && arg_counts.lifetimes == 0;
 374
 375         let mut defaults: ty::GenericParamCount = Default::default();
 376         for param in &def.params {
 377             match param.kind {
 378                 GenericParamDefKind::Lifetime => {}
 379                 GenericParamDefKind::Type { has_default, .. } => {
 380                     defaults.types += has_default as usize
 381                 }
 382                 GenericParamDefKind::Const => {
 383                     // FIXME(const_generics:defaults)
 384                 }
 385             };
 386         }
 387
 388         if position != GenericArgPosition::Type && !args.bindings.is_empty() {
 389             AstConv::prohibit_assoc_ty_binding(tcx, args.bindings[0].span);
 390         }
 391
 392         let explicit_late_bound =
 393             Self::prohibit_explicit_late_bound_lifetimes(tcx, def, args, position);
 394
 395         let check_kind_count = |kind,
 396                                 required,
 397                                 permitted,
 398                                 provided,
 399                                 offset,
 400                                 unexpected_spans: &mut Vec<Span>,
 401                                 silent| {
 402             debug!(
 403                 "check_kind_count: kind: {} required: {} permitted: {} provided: {} offset: {}",
 404                 kind, required, permitted, provided, offset
 405             );
 406             // We enforce the following: `required` <= `provided` <= `permitted`.
 407             // For kinds without defaults (e.g.., lifetimes), `required == permitted`.
 408             // For other kinds (i.e., types), `permitted` may be greater than `required`.
 409             if required <= provided && provided <= permitted {
 410                 return true;
 411             }
 412
 413             if silent {
 414                 return false;
 415             }
 416
 417             // Unfortunately lifetime and type parameter mismatches are typically styled
 418             // differently in diagnostics, which means we have a few cases to consider here.
 419             let (bound, quantifier) = if required != permitted {
 420                 if provided < required {
 421                     (required, "at least ")
 422                 } else {
 423                     // provided > permitted
 424                     (permitted, "at most ")
 425                 }
 426             } else {
 427                 (required, "")
 428             };
 429
 430             let (spans, labels) = if provided > permitted {
 431                 // In the case when the user has provided too many arguments,
 432                 // we want to point to the unexpected arguments.
 433                 let (spans, labels): (Vec<Span>, Vec<String>) = args.args
 434                     [offset + permitted..offset + provided]
 435                     .iter()
 436                     .map(|arg| (arg.span(), format!("unexpected {} argument", arg.short_descr())))
 437                     .unzip();
 438                 unexpected_spans.extend(spans.clone());
 439                 (spans, labels)
 440             } else {
 441                 (
 442                     vec![span],
 443                     vec![format!(
 444                         "expected {}{} {} argument{}",
 445                         quantifier,
 446                         bound,
 447                         kind,
 448                         pluralize!(bound),
 449                     )],
 450                 )
 451             };
 452
 453             let mut err = tcx.sess.struct_span_err_with_code(
 454                 spans.clone(),
 455                 &format!(
 456                     "wrong number of {} arguments: expected {}{}, found {}",
 457                     kind, quantifier, bound, provided,
 458                 ),
 459                 DiagnosticId::Error("E0107".into()),
 460             );
 461             for (span, label) in spans.into_iter().zip(labels) {
 462                 err.span_label(span, label.as_str());
 463             }
 464             err.emit();
 465             false
 466         };
 467
 468         let mut unexpected_spans = vec![];
 469
 470         let lifetime_count_correct = check_kind_count(
 471             "lifetime",
 472             if infer_lifetimes { 0 } else { param_counts.lifetimes },
 473             param_counts.lifetimes,
 474             arg_counts.lifetimes,
 475             0,
 476             &mut unexpected_spans,
 477             explicit_late_bound == ExplicitLateBound::Yes,
 478         );
 479
 480         let kind_str = if param_counts.consts + arg_counts.consts == 0 {
 481             "type"
 482         } else if named_type_param_count + arg_counts.types == 0 {
 483             "const"
 484         } else {
 485             "generic"
 486         };
 487
 488         let arg_count_correct = check_kind_count(
 489             kind_str,
 490             if infer_args {
 491                 0
 492             } else {
 493                 param_counts.consts + named_type_param_count - defaults.types
 494             },
 495             param_counts.consts + named_type_param_count,
 496             arg_counts.consts + arg_counts.types,
 497             arg_counts.lifetimes,
 498             &mut unexpected_spans,
 499             false,
 500         );
 501
 502         GenericArgCountResult {
 503             explicit_late_bound,
 504             correct: if lifetime_count_correct && arg_count_correct {
 505                 Ok(())
 506             } else {
 507                 Err(GenericArgCountMismatch {
 508                     reported: Some(ErrorReported),
 509                     invalid_args: unexpected_spans,
 510                 })
 511             },
 512         }
 513     }
 514
 515     /// Report error if there is an explicit type parameter when using `impl Trait`.
 516     pub(crate) fn check_impl_trait(
 517         tcx: TyCtxt<'_>,
 518         seg: &hir::PathSegment<'_>,
 519         generics: &ty::Generics,
 520     ) -> bool {
 521         let explicit = !seg.infer_args;
 522         let impl_trait =
 523             generics.params.iter().any(|param| match param.kind {
 524                 ty::GenericParamDefKind::Type {
 525                     synthetic:
 526                         Some(
 527                             hir::SyntheticTyParamKind::ImplTrait
 528                             | hir::SyntheticTyParamKind::FromAttr,
 529                         ),
 530                     ..
 531                 } => true,
 532                 _ => false,
 533             });
 534
 535         if explicit && impl_trait {
 536             let spans = seg
 537                 .generic_args()
 538                 .args
 539                 .iter()
 540                 .filter_map(|arg| match arg {
 541                     GenericArg::Type(_) | GenericArg::Const(_) => Some(arg.span()),
 542                     _ => None,
 543                 })
 544                 .collect::<Vec<_>>();
 545
 546             let mut err = struct_span_err! {
 547                 tcx.sess,
 548                 spans.clone(),
 549                 E0632,
 550                 "cannot provide explicit generic arguments when `impl Trait` is \
 551                 used in argument position"
 552             };
 553
 554             for span in spans {
 555                 err.span_label(span, "explicit generic argument not allowed");
 556             }
 557
 558             err.emit();
 559         }
 560
 561         impl_trait
 562     }
 563
 564     /// Emits an error regarding forbidden type binding associations
 565     pub fn prohibit_assoc_ty_binding(tcx: TyCtxt<'_>, span: Span) {
 566         tcx.sess.emit_err(AssocTypeBindingNotAllowed { span });
 567     }
 568
 569     /// Prohibits explicit lifetime arguments if late-bound lifetime parameters
 570     /// are present. This is used both for datatypes and function calls.
 571     pub(crate) fn prohibit_explicit_late_bound_lifetimes(
 572         tcx: TyCtxt<'_>,
 573         def: &ty::Generics,
 574         args: &hir::GenericArgs<'_>,
 575         position: GenericArgPosition,
 576     ) -> ExplicitLateBound {
 577         let param_counts = def.own_counts();
 578         let arg_counts = args.own_counts();
 579         let infer_lifetimes = position != GenericArgPosition::Type && arg_counts.lifetimes == 0;
 580
 581         if infer_lifetimes {
 582             ExplicitLateBound::No
 583         } else if let Some(span_late) = def.has_late_bound_regions {
 584             let msg = "cannot specify lifetime arguments explicitly \
 585                        if late bound lifetime parameters are present";
 586             let note = "the late bound lifetime parameter is introduced here";
 587             let span = args.args[0].span();
 588             if position == GenericArgPosition::Value
 589                 && arg_counts.lifetimes != param_counts.lifetimes
 590             {
 591                 let mut err = tcx.sess.struct_span_err(span, msg);
 592                 err.span_note(span_late, note);
 593                 err.emit();
 594             } else {
 595                 let mut multispan = MultiSpan::from_span(span);
 596                 multispan.push_span_label(span_late, note.to_string());
 597                 tcx.struct_span_lint_hir(
 598                     LATE_BOUND_LIFETIME_ARGUMENTS,
 599                     args.args[0].id(),
 600                     multispan,
 601                     |lint| lint.build(msg).emit(),
 602                 );
 603             }
 604             ExplicitLateBound::Yes
 605         } else {
 606             ExplicitLateBound::No
 607         }
 608     }
 609 }