compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

   1 pub mod on_unimplemented;
   2 pub mod suggestions;
   3
   4 use super::{
   5     DerivedObligationCause, EvaluationResult, FulfillmentContext, FulfillmentError,
   6     FulfillmentErrorCode, ImplDerivedObligationCause, MismatchedProjectionTypes, Obligation,
   7     ObligationCause, ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote,
   8     OutputTypeParameterMismatch, Overflow, PredicateObligation, SelectionContext, SelectionError,
   9     TraitNotObjectSafe,
  10 };
  11
  12 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  13 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  14 use crate::infer::{self, InferCtxt, TyCtxtInferExt};
  15 use rustc_data_structures::fx::FxHashMap;
  16 use rustc_errors::{
  17     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
  18     MultiSpan, Style,
  19 };
  20 use rustc_hir as hir;
  21 use rustc_hir::def_id::DefId;
  22 use rustc_hir::intravisit::Visitor;
  23 use rustc_hir::GenericParam;
  24 use rustc_hir::Item;
  25 use rustc_hir::Node;
  26 use rustc_infer::infer::error_reporting::same_type_modulo_infer;
  27 use rustc_infer::traits::TraitEngine;
  28 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
  29 use rustc_middle::traits::select::OverflowError;
  30 use rustc_middle::ty::error::ExpectedFound;
  31 use rustc_middle::ty::fold::TypeFolder;
  32 use rustc_middle::ty::{
  33     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  34 };
  35 use rustc_span::symbol::{kw, sym};
  36 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  37 use std::fmt;
  38 use std::iter;
  39 use std::ops::ControlFlow;
  40
  41 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  42 use crate::traits::query::normalize::AtExt as _;
  43 use crate::traits::specialize::to_pretty_impl_header;
  44 use on_unimplemented::InferCtxtExt as _;
  45 use suggestions::InferCtxtExt as _;
  46
  47 pub use rustc_infer::traits::error_reporting::*;
  48
  49 // When outputting impl candidates, prefer showing those that are more similar.
  50 //
  51 // We also compare candidates after skipping lifetimes, which has a lower
  52 // priority than exact matches.
  53 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  54 pub enum CandidateSimilarity {
  55     Exact { ignoring_lifetimes: bool },
  56     Fuzzy { ignoring_lifetimes: bool },
  57 }
  58
  59 #[derive(Debug, Clone, Copy)]
  60 pub struct ImplCandidate<'tcx> {
  61     pub trait_ref: ty::TraitRef<'tcx>,
  62     pub similarity: CandidateSimilarity,
  63 }
  64
  65 pub trait InferCtxtExt<'tcx> {
  66     fn report_fulfillment_errors(
  67         &self,
  68         errors: &[FulfillmentError<'tcx>],
  69         body_id: Option<hir::BodyId>,
  70         fallback_has_occurred: bool,
  71     ) -> ErrorGuaranteed;
  72
  73     fn report_overflow_error<T>(
  74         &self,
  75         obligation: &Obligation<'tcx, T>,
  76         suggest_increasing_limit: bool,
  77     ) -> !
  78     where
  79         T: fmt::Display + TypeFoldable<'tcx>;
  80
  81     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
  82
  83     /// The `root_obligation` parameter should be the `root_obligation` field
  84     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
  85     /// then it should be the same as `obligation`.
  86     fn report_selection_error(
  87         &self,
  88         obligation: PredicateObligation<'tcx>,
  89         root_obligation: &PredicateObligation<'tcx>,
  90         error: &SelectionError<'tcx>,
  91         fallback_has_occurred: bool,
  92     );
  93
  94     /// Given some node representing a fn-like thing in the HIR map,
  95     /// returns a span and `ArgKind` information that describes the
  96     /// arguments it expects. This can be supplied to
  97     /// `report_arg_count_mismatch`.
  98     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  99
 100     /// Reports an error when the number of arguments needed by a
 101     /// trait match doesn't match the number that the expression
 102     /// provides.
 103     fn report_arg_count_mismatch(
 104         &self,
 105         span: Span,
 106         found_span: Option<Span>,
 107         expected_args: Vec<ArgKind>,
 108         found_args: Vec<ArgKind>,
 109         is_closure: bool,
 110     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 111
 112     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
 113     /// in that order, and returns the generic type corresponding to the
 114     /// argument of that trait (corresponding to the closure arguments).
 115     fn type_implements_fn_trait(
 116         &self,
 117         param_env: ty::ParamEnv<'tcx>,
 118         ty: ty::Binder<'tcx, Ty<'tcx>>,
 119         constness: ty::BoundConstness,
 120         polarity: ty::ImplPolarity,
 121     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
 122 }
 123
 124 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 125     fn report_fulfillment_errors(
 126         &self,
 127         errors: &[FulfillmentError<'tcx>],
 128         body_id: Option<hir::BodyId>,
 129         fallback_has_occurred: bool,
 130     ) -> ErrorGuaranteed {
 131         #[derive(Debug)]
 132         struct ErrorDescriptor<'tcx> {
 133             predicate: ty::Predicate<'tcx>,
 134             index: Option<usize>, // None if this is an old error
 135         }
 136
 137         let mut error_map: FxHashMap<_, Vec<_>> = self
 138             .reported_trait_errors
 139             .borrow()
 140             .iter()
 141             .map(|(&span, predicates)| {
 142                 (
 143                     span,
 144                     predicates
 145                         .iter()
 146                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 147                         .collect(),
 148                 )
 149             })
 150             .collect();
 151
 152         for (index, error) in errors.iter().enumerate() {
 153             // We want to ignore desugarings here: spans are equivalent even
 154             // if one is the result of a desugaring and the other is not.
 155             let mut span = error.obligation.cause.span;
 156             let expn_data = span.ctxt().outer_expn_data();
 157             if let ExpnKind::Desugaring(_) = expn_data.kind {
 158                 span = expn_data.call_site;
 159             }
 160
 161             error_map.entry(span).or_default().push(ErrorDescriptor {
 162                 predicate: error.obligation.predicate,
 163                 index: Some(index),
 164             });
 165
 166             self.reported_trait_errors
 167                 .borrow_mut()
 168                 .entry(span)
 169                 .or_default()
 170                 .push(error.obligation.predicate);
 171         }
 172
 173         // We do this in 2 passes because we want to display errors in order, though
 174         // maybe it *is* better to sort errors by span or something.
 175         let mut is_suppressed = vec![false; errors.len()];
 176         for (_, error_set) in error_map.iter() {
 177             // We want to suppress "duplicate" errors with the same span.
 178             for error in error_set {
 179                 if let Some(index) = error.index {
 180                     // Suppress errors that are either:
 181                     // 1) strictly implied by another error.
 182                     // 2) implied by an error with a smaller index.
 183                     for error2 in error_set {
 184                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 185                             // Avoid errors being suppressed by already-suppressed
 186                             // errors, to prevent all errors from being suppressed
 187                             // at once.
 188                             continue;
 189                         }
 190
 191                         if self.error_implies(error2.predicate, error.predicate)
 192                             && !(error2.index >= error.index
 193                                 && self.error_implies(error.predicate, error2.predicate))
 194                         {
 195                             info!("skipping {:?} (implied by {:?})", error, error2);
 196                             is_suppressed[index] = true;
 197                             break;
 198                         }
 199                     }
 200                 }
 201             }
 202         }
 203
 204         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 205             if !suppressed {
 206                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 207             }
 208         }
 209
 210         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 211     }
 212
 213     /// Reports that an overflow has occurred and halts compilation. We
 214     /// halt compilation unconditionally because it is important that
 215     /// overflows never be masked -- they basically represent computations
 216     /// whose result could not be truly determined and thus we can't say
 217     /// if the program type checks or not -- and they are unusual
 218     /// occurrences in any case.
 219     fn report_overflow_error<T>(
 220         &self,
 221         obligation: &Obligation<'tcx, T>,
 222         suggest_increasing_limit: bool,
 223     ) -> !
 224     where
 225         T: fmt::Display + TypeFoldable<'tcx>,
 226     {
 227         let predicate = self.resolve_vars_if_possible(obligation.predicate.clone());
 228         let mut err = struct_span_err!(
 229             self.tcx.sess,
 230             obligation.cause.span,
 231             E0275,
 232             "overflow evaluating the requirement `{}`",
 233             predicate
 234         );
 235
 236         if suggest_increasing_limit {
 237             self.suggest_new_overflow_limit(&mut err);
 238         }
 239
 240         self.note_obligation_cause_code(
 241             &mut err,
 242             &obligation.predicate,
 243             obligation.param_env,
 244             obligation.cause.code(),
 245             &mut vec![],
 246             &mut Default::default(),
 247         );
 248
 249         err.emit();
 250         self.tcx.sess.abort_if_errors();
 251         bug!();
 252     }
 253
 254     /// Reports that a cycle was detected which led to overflow and halts
 255     /// compilation. This is equivalent to `report_overflow_error` except
 256     /// that we can give a more helpful error message (and, in particular,
 257     /// we do not suggest increasing the overflow limit, which is not
 258     /// going to help).
 259     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 260         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 261         assert!(!cycle.is_empty());
 262
 263         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 264
 265         // The 'deepest' obligation is most likely to have a useful
 266         // cause 'backtrace'
 267         self.report_overflow_error(cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(), false);
 268     }
 269
 270     fn report_selection_error(
 271         &self,
 272         mut obligation: PredicateObligation<'tcx>,
 273         root_obligation: &PredicateObligation<'tcx>,
 274         error: &SelectionError<'tcx>,
 275         fallback_has_occurred: bool,
 276     ) {
 277         let tcx = self.tcx;
 278         let mut span = obligation.cause.span;
 279
 280         let mut err = match *error {
 281             SelectionError::Ambiguous(ref impls) => {
 282                 let mut err = self.tcx.sess.struct_span_err(
 283                     obligation.cause.span,
 284                     &format!("multiple applicable `impl`s for `{}`", obligation.predicate),
 285                 );
 286                 self.annotate_source_of_ambiguity(&mut err, impls, obligation.predicate);
 287                 err.emit();
 288                 return;
 289             }
 290             SelectionError::Unimplemented => {
 291                 // If this obligation was generated as a result of well-formedness checking, see if we
 292                 // can get a better error message by performing HIR-based well-formedness checking.
 293                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 294                     root_obligation.cause.code().peel_derives()
 295                 {
 296                     if let Some(cause) = self
 297                         .tcx
 298                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 299                     {
 300                         obligation.cause = cause.clone();
 301                         span = obligation.cause.span;
 302                     }
 303                 }
 304                 if let ObligationCauseCode::CompareImplMethodObligation {
 305                     impl_item_def_id,
 306                     trait_item_def_id,
 307                 }
 308                 | ObligationCauseCode::CompareImplTypeObligation {
 309                     impl_item_def_id,
 310                     trait_item_def_id,
 311                 } = *obligation.cause.code()
 312                 {
 313                     self.report_extra_impl_obligation(
 314                         span,
 315                         impl_item_def_id,
 316                         trait_item_def_id,
 317                         &format!("`{}`", obligation.predicate),
 318                     )
 319                     .emit();
 320                     return;
 321                 }
 322
 323                 let bound_predicate = obligation.predicate.kind();
 324                 match bound_predicate.skip_binder() {
 325                     ty::PredicateKind::Trait(trait_predicate) => {
 326                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 327                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 328
 329                         trait_predicate.remap_constness_diag(obligation.param_env);
 330                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 331                             == trait_predicate.skip_binder().constness;
 332
 333                         if self.tcx.sess.has_errors().is_some()
 334                             && trait_predicate.references_error()
 335                         {
 336                             return;
 337                         }
 338                         let trait_ref = trait_predicate.to_poly_trait_ref();
 339                         let (post_message, pre_message, type_def) = self
 340                             .get_parent_trait_ref(obligation.cause.code())
 341                             .map(|(t, s)| {
 342                                 (
 343                                     format!(" in `{}`", t),
 344                                     format!("within `{}`, ", t),
 345                                     s.map(|s| (format!("within this `{}`", t), s)),
 346                                 )
 347                             })
 348                             .unwrap_or_default();
 349
 350                         let OnUnimplementedNote {
 351                             message,
 352                             label,
 353                             note,
 354                             enclosing_scope,
 355                             append_const_msg,
 356                         } = self.on_unimplemented_note(trait_ref, &obligation);
 357                         let have_alt_message = message.is_some() || label.is_some();
 358                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 359                         let is_unsize =
 360                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 361                         let (message, note, append_const_msg) = if is_try_conversion {
 362                             (
 363                                 Some(format!(
 364                                     "`?` couldn't convert the error to `{}`",
 365                                     trait_ref.skip_binder().self_ty(),
 366                                 )),
 367                                 Some(
 368                                     "the question mark operation (`?`) implicitly performs a \
 369                                      conversion on the error value using the `From` trait"
 370                                         .to_owned(),
 371                                 ),
 372                                 Some(None),
 373                             )
 374                         } else {
 375                             (message, note, append_const_msg)
 376                         };
 377
 378                         let mut err = struct_span_err!(
 379                             self.tcx.sess,
 380                             span,
 381                             E0277,
 382                             "{}",
 383                             message
 384                                 .and_then(|cannot_do_this| {
 385                                     match (predicate_is_const, append_const_msg) {
 386                                         // do nothing if predicate is not const
 387                                         (false, _) => Some(cannot_do_this),
 388                                         // suggested using default post message
 389                                         (true, Some(None)) => {
 390                                             Some(format!("{cannot_do_this} in const contexts"))
 391                                         }
 392                                         // overridden post message
 393                                         (true, Some(Some(post_message))) => {
 394                                             Some(format!("{cannot_do_this}{post_message}"))
 395                                         }
 396                                         // fallback to generic message
 397                                         (true, None) => None,
 398                                     }
 399                                 })
 400                                 .unwrap_or_else(|| format!(
 401                                     "the trait bound `{}` is not satisfied{}",
 402                                     trait_predicate, post_message,
 403                                 ))
 404                         );
 405
 406                         if is_try_conversion {
 407                             let none_error = self
 408                                 .tcx
 409                                 .get_diagnostic_item(sym::none_error)
 410                                 .map(|def_id| tcx.type_of(def_id));
 411                             let should_convert_option_to_result =
 412                                 Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
 413                             let should_convert_result_to_option =
 414                                 Some(trait_ref.self_ty().skip_binder()) == none_error;
 415                             if should_convert_option_to_result {
 416                                 err.span_suggestion_verbose(
 417                                     span.shrink_to_lo(),
 418                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 419                                      using `Option::ok_or` or `Option::ok_or_else`",
 420                                     ".ok_or_else(|| /* error value */)".to_string(),
 421                                     Applicability::HasPlaceholders,
 422                                 );
 423                             } else if should_convert_result_to_option {
 424                                 err.span_suggestion_verbose(
 425                                     span.shrink_to_lo(),
 426                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 427                                      using `Result::ok`",
 428                                     ".ok()".to_string(),
 429                                     Applicability::MachineApplicable,
 430                                 );
 431                             }
 432                             if let Some(ret_span) = self.return_type_span(&obligation) {
 433                                 err.span_label(
 434                                     ret_span,
 435                                     &format!(
 436                                         "expected `{}` because of this",
 437                                         trait_ref.skip_binder().self_ty()
 438                                     ),
 439                                 );
 440                             }
 441                         }
 442
 443                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 444                             && predicate_is_const
 445                         {
 446                             err.note("`~const Drop` was renamed to `~const Destruct`");
 447                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 448                         }
 449
 450                         let explanation = if let ObligationCauseCode::MainFunctionType =
 451                             obligation.cause.code()
 452                         {
 453                             "consider using `()`, or a `Result`".to_owned()
 454                         } else {
 455                             format!(
 456                                 "{}the trait `{}` is not implemented for `{}`",
 457                                 pre_message,
 458                                 trait_predicate.print_modifiers_and_trait_path(),
 459                                 trait_ref.skip_binder().self_ty(),
 460                             )
 461                         };
 462
 463                         if self.suggest_add_reference_to_arg(
 464                             &obligation,
 465                             &mut err,
 466                             trait_predicate,
 467                             have_alt_message,
 468                         ) {
 469                             self.note_obligation_cause(&mut err, &obligation);
 470                             err.emit();
 471                             return;
 472                         }
 473                         if let Some(ref s) = label {
 474                             // If it has a custom `#[rustc_on_unimplemented]`
 475                             // error message, let's display it as the label!
 476                             err.span_label(span, s.as_str());
 477                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 478                                 // When the self type is a type param We don't need to "the trait
 479                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 480                                 // at the type param with a label to suggest constraining it.
 481                                 err.help(&explanation);
 482                             }
 483                         } else {
 484                             err.span_label(span, explanation);
 485                         }
 486
 487                         if let ObligationCauseCode::ObjectCastObligation(obj_ty) = obligation.cause.code().peel_derives() &&
 488                            let Some(self_ty) = trait_predicate.self_ty().no_bound_vars() &&
 489                            Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 490                             self.suggest_borrowing_for_object_cast(&mut err, &obligation, self_ty, *obj_ty);
 491                         }
 492
 493                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 494                             let non_const_predicate = trait_ref.without_const();
 495                             let non_const_obligation = Obligation {
 496                                 cause: obligation.cause.clone(),
 497                                 param_env: obligation.param_env.without_const(),
 498                                 predicate: non_const_predicate.to_predicate(tcx),
 499                                 recursion_depth: obligation.recursion_depth,
 500                             };
 501                             if self.predicate_may_hold(&non_const_obligation) {
 502                                 err.span_note(
 503                                     span,
 504                                     &format!(
 505                                         "the trait `{}` is implemented for `{}`, \
 506                                         but that implementation is not `const`",
 507                                         non_const_predicate.print_modifiers_and_trait_path(),
 508                                         trait_ref.skip_binder().self_ty(),
 509                                     ),
 510                                 );
 511                             }
 512                         }
 513
 514                         if let Some((msg, span)) = type_def {
 515                             err.span_label(span, &msg);
 516                         }
 517                         if let Some(ref s) = note {
 518                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 519                             err.note(s.as_str());
 520                         }
 521                         if let Some(ref s) = enclosing_scope {
 522                             let body = tcx
 523                                 .hir()
 524                                 .opt_local_def_id(obligation.cause.body_id)
 525                                 .unwrap_or_else(|| {
 526                                     tcx.hir().body_owner_def_id(hir::BodyId {
 527                                         hir_id: obligation.cause.body_id,
 528                                     })
 529                                 });
 530
 531                             let enclosing_scope_span =
 532                                 tcx.hir().span_with_body(tcx.hir().local_def_id_to_hir_id(body));
 533
 534                             err.span_label(enclosing_scope_span, s.as_str());
 535                         }
 536
 537                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 538                         let mut suggested =
 539                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 540                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 541                         suggested |=
 542                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 543                         suggested |= self.suggest_semicolon_removal(
 544                             &obligation,
 545                             &mut err,
 546                             span,
 547                             trait_predicate,
 548                         );
 549                         self.note_version_mismatch(&mut err, &trait_ref);
 550                         self.suggest_remove_await(&obligation, &mut err);
 551                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 552
 553                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 554                             self.suggest_await_before_try(
 555                                 &mut err,
 556                                 &obligation,
 557                                 trait_predicate,
 558                                 span,
 559                             );
 560                         }
 561
 562                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 563                             err.emit();
 564                             return;
 565                         }
 566
 567                         if is_unsize {
 568                             // If the obligation failed due to a missing implementation of the
 569                             // `Unsize` trait, give a pointer to why that might be the case
 570                             err.note(
 571                                 "all implementations of `Unsize` are provided \
 572                                 automatically by the compiler, see \
 573                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 574                                 for more information",
 575                             );
 576                         }
 577
 578                         let is_fn_trait = [
 579                             self.tcx.lang_items().fn_trait(),
 580                             self.tcx.lang_items().fn_mut_trait(),
 581                             self.tcx.lang_items().fn_once_trait(),
 582                         ]
 583                         .contains(&Some(trait_ref.def_id()));
 584                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 585                             *trait_ref.skip_binder().self_ty().kind()
 586                         {
 587                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 588                         } else {
 589                             false
 590                         };
 591                         if is_fn_trait && is_target_feature_fn {
 592                             err.note(
 593                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 594                             );
 595                         }
 596
 597                         // Try to report a help message
 598                         if is_fn_trait
 599                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 600                                 obligation.param_env,
 601                                 trait_ref.self_ty(),
 602                                 trait_predicate.skip_binder().constness,
 603                                 trait_predicate.skip_binder().polarity,
 604                             )
 605                         {
 606                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 607                             // suggestion to add trait bounds for the type, since we only typically implement
 608                             // these traits once.
 609
 610                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 611                             // to implement.
 612                             let selected_kind =
 613                                 ty::ClosureKind::from_def_id(self.tcx, trait_ref.def_id())
 614                                     .expect("expected to map DefId to ClosureKind");
 615                             if !implemented_kind.extends(selected_kind) {
 616                                 err.note(
 617                                     &format!(
 618                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 619                                         trait_ref.skip_binder().self_ty(),
 620                                         implemented_kind,
 621                                         selected_kind
 622                                     )
 623                                 );
 624                             }
 625
 626                             // Note any argument mismatches
 627                             let given_ty = params.skip_binder();
 628                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 629                             if let ty::Tuple(given) = given_ty.kind()
 630                                 && let ty::Tuple(expected) = expected_ty.kind()
 631                             {
 632                                 if expected.len() != given.len() {
 633                                     // Note number of types that were expected and given
 634                                     err.note(
 635                                         &format!(
 636                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 637                                             given.len(),
 638                                             if given.len() == 1 { "" } else { "s" },
 639                                             expected.len(),
 640                                             if expected.len() == 1 { "" } else { "s" },
 641                                         )
 642                                     );
 643                                 } else if !same_type_modulo_infer(given_ty, expected_ty) {
 644                                     // Print type mismatch
 645                                     let (expected_args, given_args) =
 646                                         self.cmp(given_ty, expected_ty);
 647                                     err.note_expected_found(
 648                                         &"a closure with arguments",
 649                                         expected_args,
 650                                         &"a closure with arguments",
 651                                         given_args,
 652                                     );
 653                                 }
 654                             }
 655                         } else if !trait_ref.has_infer_types_or_consts()
 656                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 657                         {
 658                             // If a where-clause may be useful, remind the
 659                             // user that they can add it.
 660                             //
 661                             // don't display an on-unimplemented note, as
 662                             // these notes will often be of the form
 663                             //     "the type `T` can't be frobnicated"
 664                             // which is somewhat confusing.
 665                             self.suggest_restricting_param_bound(
 666                                 &mut err,
 667                                 trait_predicate,
 668                                 obligation.cause.body_id,
 669                             );
 670                         } else if !suggested {
 671                             // Can't show anything else useful, try to find similar impls.
 672                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 673                             if !self.report_similar_impl_candidates(
 674                                 impl_candidates,
 675                                 trait_ref,
 676                                 &mut err,
 677                             ) {
 678                                 // This is *almost* equivalent to
 679                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 680                                 // trait predicate for that corresponding root obligation. This
 681                                 // lets us get a derived obligation from a type parameter, like
 682                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 683                                 // implementer of `Pattern<'_>`.
 684                                 let mut code = obligation.cause.code();
 685                                 let mut trait_pred = trait_predicate;
 686                                 let mut peeled = false;
 687                                 loop {
 688                                     match &*code {
 689                                         ObligationCauseCode::FunctionArgumentObligation {
 690                                             parent_code,
 691                                             ..
 692                                         } => {
 693                                             code = &parent_code;
 694                                         }
 695                                         ObligationCauseCode::ImplDerivedObligation(
 696                                             box ImplDerivedObligationCause {
 697                                                 derived:
 698                                                     DerivedObligationCause {
 699                                                         parent_code,
 700                                                         parent_trait_pred,
 701                                                     },
 702                                                 ..
 703                                             },
 704                                         )
 705                                         | ObligationCauseCode::BuiltinDerivedObligation(
 706                                             DerivedObligationCause {
 707                                                 parent_code,
 708                                                 parent_trait_pred,
 709                                             },
 710                                         )
 711                                         | ObligationCauseCode::DerivedObligation(
 712                                             DerivedObligationCause {
 713                                                 parent_code,
 714                                                 parent_trait_pred,
 715                                             },
 716                                         ) => {
 717                                             peeled = true;
 718                                             code = &parent_code;
 719                                             trait_pred = *parent_trait_pred;
 720                                         }
 721                                         _ => break,
 722                                     };
 723                                 }
 724                                 let def_id = trait_pred.def_id();
 725                                 // Mention *all* the `impl`s for the *top most* obligation, the
 726                                 // user might have meant to use one of them, if any found. We skip
 727                                 // auto-traits or fundamental traits that might not be exactly what
 728                                 // the user might expect to be presented with. Instead this is
 729                                 // useful for less general traits.
 730                                 if peeled
 731                                     && !self.tcx.trait_is_auto(def_id)
 732                                     && !self.tcx.lang_items().items().contains(&Some(def_id))
 733                                 {
 734                                     let trait_ref = trait_pred.to_poly_trait_ref();
 735                                     let impl_candidates =
 736                                         self.find_similar_impl_candidates(trait_ref);
 737                                     self.report_similar_impl_candidates(
 738                                         impl_candidates,
 739                                         trait_ref,
 740                                         &mut err,
 741                                     );
 742                                 }
 743                             }
 744                         }
 745
 746                         // Changing mutability doesn't make a difference to whether we have
 747                         // an `Unsize` impl (Fixes ICE in #71036)
 748                         if !is_unsize {
 749                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
 750                         }
 751
 752                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 753                         // implemented, and fallback has occurred, then it could be due to a
 754                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 755                         // note informing about the change in behaviour.
 756                         if trait_predicate.skip_binder().self_ty().is_never()
 757                             && fallback_has_occurred
 758                         {
 759                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 760                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 761                                     self.tcx.mk_unit(),
 762                                     &trait_pred.trait_ref.substs[1..],
 763                                 );
 764                                 trait_pred
 765                             });
 766                             let unit_obligation = obligation.with(predicate.to_predicate(tcx));
 767                             if self.predicate_may_hold(&unit_obligation) {
 768                                 err.note(
 769                                     "this error might have been caused by changes to \
 770                                     Rust's type-inference algorithm (see issue #48950 \
 771                                     <https://github.com/rust-lang/rust/issues/48950> \
 772                                     for more information)",
 773                                 );
 774                                 err.help("did you intend to use the type `()` here instead?");
 775                             }
 776                         }
 777
 778                         // Return early if the trait is Debug or Display and the invocation
 779                         // originates within a standard library macro, because the output
 780                         // is otherwise overwhelming and unhelpful (see #85844 for an
 781                         // example).
 782
 783                         let in_std_macro =
 784                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
 785                                 Some(macro_def_id) => {
 786                                     let crate_name = tcx.crate_name(macro_def_id.krate);
 787                                     crate_name == sym::std || crate_name == sym::core
 788                                 }
 789                                 None => false,
 790                             };
 791
 792                         if in_std_macro
 793                             && matches!(
 794                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
 795                                 Some(sym::Debug | sym::Display)
 796                             )
 797                         {
 798                             err.emit();
 799                             return;
 800                         }
 801
 802                         err
 803                     }
 804
 805                     ty::PredicateKind::Subtype(predicate) => {
 806                         // Errors for Subtype predicates show up as
 807                         // `FulfillmentErrorCode::CodeSubtypeError`,
 808                         // not selection error.
 809                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 810                     }
 811
 812                     ty::PredicateKind::Coerce(predicate) => {
 813                         // Errors for Coerce predicates show up as
 814                         // `FulfillmentErrorCode::CodeSubtypeError`,
 815                         // not selection error.
 816                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
 817                     }
 818
 819                     ty::PredicateKind::RegionOutlives(predicate) => {
 820                         let predicate = bound_predicate.rebind(predicate);
 821                         let predicate = self.resolve_vars_if_possible(predicate);
 822                         let err = self
 823                             .region_outlives_predicate(&obligation.cause, predicate)
 824                             .err()
 825                             .unwrap();
 826                         struct_span_err!(
 827                             self.tcx.sess,
 828                             span,
 829                             E0279,
 830                             "the requirement `{}` is not satisfied (`{}`)",
 831                             predicate,
 832                             err,
 833                         )
 834                     }
 835
 836                     ty::PredicateKind::Projection(..) | ty::PredicateKind::TypeOutlives(..) => {
 837                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
 838                         struct_span_err!(
 839                             self.tcx.sess,
 840                             span,
 841                             E0280,
 842                             "the requirement `{}` is not satisfied",
 843                             predicate
 844                         )
 845                     }
 846
 847                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
 848                         let violations = self.tcx.object_safety_violations(trait_def_id);
 849                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 850                     }
 851
 852                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 853                         let found_kind = self.closure_kind(closure_substs).unwrap();
 854                         let closure_span =
 855                             self.tcx.sess.source_map().guess_head_span(
 856                                 self.tcx.hir().span_if_local(closure_def_id).unwrap(),
 857                             );
 858                         let mut err = struct_span_err!(
 859                             self.tcx.sess,
 860                             closure_span,
 861                             E0525,
 862                             "expected a closure that implements the `{}` trait, \
 863                              but this closure only implements `{}`",
 864                             kind,
 865                             found_kind
 866                         );
 867
 868                         err.span_label(
 869                             closure_span,
 870                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 871                         );
 872                         err.span_label(
 873                             obligation.cause.span,
 874                             format!("the requirement to implement `{}` derives from here", kind),
 875                         );
 876
 877                         // Additional context information explaining why the closure only implements
 878                         // a particular trait.
 879                         if let Some(typeck_results) = self.in_progress_typeck_results {
 880                             let hir_id = self
 881                                 .tcx
 882                                 .hir()
 883                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
 884                             let typeck_results = typeck_results.borrow();
 885                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
 886                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
 887                                     err.span_label(
 888                                         *span,
 889                                         format!(
 890                                             "closure is `FnOnce` because it moves the \
 891                                          variable `{}` out of its environment",
 892                                             ty::place_to_string_for_capture(tcx, place)
 893                                         ),
 894                                     );
 895                                 }
 896                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
 897                                     err.span_label(
 898                                         *span,
 899                                         format!(
 900                                             "closure is `FnMut` because it mutates the \
 901                                          variable `{}` here",
 902                                             ty::place_to_string_for_capture(tcx, place)
 903                                         ),
 904                                     );
 905                                 }
 906                                 _ => {}
 907                             }
 908                         }
 909
 910                         err.emit();
 911                         return;
 912                     }
 913
 914                     ty::PredicateKind::WellFormed(ty) => {
 915                         if !self.tcx.sess.opts.debugging_opts.chalk {
 916                             // WF predicates cannot themselves make
 917                             // errors. They can only block due to
 918                             // ambiguity; otherwise, they always
 919                             // degenerate into other obligations
 920                             // (which may fail).
 921                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 922                         } else {
 923                             // FIXME: we'll need a better message which takes into account
 924                             // which bounds actually failed to hold.
 925                             self.tcx.sess.struct_span_err(
 926                                 span,
 927                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 928                             )
 929                         }
 930                     }
 931
 932                     ty::PredicateKind::ConstEvaluatable(..) => {
 933                         // Errors for `ConstEvaluatable` predicates show up as
 934                         // `SelectionError::ConstEvalFailure`,
 935                         // not `Unimplemented`.
 936                         span_bug!(
 937                             span,
 938                             "const-evaluatable requirement gave wrong error: `{:?}`",
 939                             obligation
 940                         )
 941                     }
 942
 943                     ty::PredicateKind::ConstEquate(..) => {
 944                         // Errors for `ConstEquate` predicates show up as
 945                         // `SelectionError::ConstEvalFailure`,
 946                         // not `Unimplemented`.
 947                         span_bug!(
 948                             span,
 949                             "const-equate requirement gave wrong error: `{:?}`",
 950                             obligation
 951                         )
 952                     }
 953
 954                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
 955                         span,
 956                         "TypeWellFormedFromEnv predicate should only exist in the environment"
 957                     ),
 958                 }
 959             }
 960
 961             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
 962                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
 963                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
 964
 965                 if expected_trait_ref.self_ty().references_error() {
 966                     return;
 967                 }
 968
 969                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
 970                     return;
 971                 };
 972
 973                 let found_did = match *found_trait_ty.kind() {
 974                     ty::Closure(did, _)
 975                     | ty::Foreign(did)
 976                     | ty::FnDef(did, _)
 977                     | ty::Generator(did, ..) => Some(did),
 978                     ty::Adt(def, _) => Some(def.did()),
 979                     _ => None,
 980                 };
 981
 982                 let found_span = found_did
 983                     .and_then(|did| self.tcx.hir().span_if_local(did))
 984                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 985
 986                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 987                     // We check closures twice, with obligations flowing in different directions,
 988                     // but we want to complain about them only once.
 989                     return;
 990                 }
 991
 992                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 993
 994                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
 995                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 996                     _ => vec![ArgKind::empty()],
 997                 };
 998
 999                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1000                 let expected = match expected_ty.kind() {
1001                     ty::Tuple(ref tys) => {
1002                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
1003                     }
1004                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
1005                 };
1006
1007                 if found.len() == expected.len() {
1008                     self.report_closure_arg_mismatch(
1009                         span,
1010                         found_span,
1011                         found_trait_ref,
1012                         expected_trait_ref,
1013                     )
1014                 } else {
1015                     let (closure_span, found) = found_did
1016                         .and_then(|did| {
1017                             let node = self.tcx.hir().get_if_local(did)?;
1018                             let (found_span, found) = self.get_fn_like_arguments(node)?;
1019                             Some((Some(found_span), found))
1020                         })
1021                         .unwrap_or((found_span, found));
1022
1023                     self.report_arg_count_mismatch(
1024                         span,
1025                         closure_span,
1026                         expected,
1027                         found,
1028                         found_trait_ty.is_closure(),
1029                     )
1030                 }
1031             }
1032
1033             TraitNotObjectSafe(did) => {
1034                 let violations = self.tcx.object_safety_violations(did);
1035                 report_object_safety_error(self.tcx, span, did, violations)
1036             }
1037
1038             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1039                 bug!(
1040                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1041                 )
1042             }
1043             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1044                 if !self.tcx.features().generic_const_exprs {
1045                     let mut err = self.tcx.sess.struct_span_err(
1046                         span,
1047                         "constant expression depends on a generic parameter",
1048                     );
1049                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1050                     // issue. However, this is currently not actually possible
1051                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1052                     //
1053                     // Note that with `feature(generic_const_exprs)` this case should not
1054                     // be reachable.
1055                     err.note("this may fail depending on what value the parameter takes");
1056                     err.emit();
1057                     return;
1058                 }
1059
1060                 match obligation.predicate.kind().skip_binder() {
1061                     ty::PredicateKind::ConstEvaluatable(uv) => {
1062                         let mut err =
1063                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1064                         let const_span = self.tcx.def_span(uv.def.did);
1065                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1066                             Ok(snippet) => err.help(&format!(
1067                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1068                                 snippet
1069                             )),
1070                             _ => err.help("consider adding a `where` bound using this expression"),
1071                         };
1072                         err
1073                     }
1074                     _ => {
1075                         span_bug!(
1076                             span,
1077                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1078                         )
1079                     }
1080                 }
1081             }
1082
1083             // Already reported in the query.
1084             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1085                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1086                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1087                 return;
1088             }
1089             // Already reported.
1090             Overflow(OverflowError::Error(_)) => {
1091                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1092                 return;
1093             }
1094             Overflow(_) => {
1095                 bug!("overflow should be handled before the `report_selection_error` path");
1096             }
1097             SelectionError::ErrorReporting => {
1098                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1099             }
1100         };
1101
1102         self.note_obligation_cause(&mut err, &obligation);
1103         self.point_at_returns_when_relevant(&mut err, &obligation);
1104
1105         err.emit();
1106     }
1107
1108     /// Given some node representing a fn-like thing in the HIR map,
1109     /// returns a span and `ArgKind` information that describes the
1110     /// arguments it expects. This can be supplied to
1111     /// `report_arg_count_mismatch`.
1112     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
1113         let sm = self.tcx.sess.source_map();
1114         let hir = self.tcx.hir();
1115         Some(match node {
1116             Node::Expr(&hir::Expr {
1117                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
1118                 ..
1119             }) => (
1120                 sm.guess_head_span(span),
1121                 hir.body(id)
1122                     .params
1123                     .iter()
1124                     .map(|arg| {
1125                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
1126                             *arg.pat
1127                         {
1128                             Some(ArgKind::Tuple(
1129                                 Some(span),
1130                                 args.iter()
1131                                     .map(|pat| {
1132                                         sm.span_to_snippet(pat.span)
1133                                             .ok()
1134                                             .map(|snippet| (snippet, "_".to_owned()))
1135                                     })
1136                                     .collect::<Option<Vec<_>>>()?,
1137                             ))
1138                         } else {
1139                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
1140                             Some(ArgKind::Arg(name, "_".to_owned()))
1141                         }
1142                     })
1143                     .collect::<Option<Vec<ArgKind>>>()?,
1144             ),
1145             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
1146             | Node::ImplItem(&hir::ImplItem {
1147                 span,
1148                 kind: hir::ImplItemKind::Fn(ref sig, _),
1149                 ..
1150             })
1151             | Node::TraitItem(&hir::TraitItem {
1152                 span,
1153                 kind: hir::TraitItemKind::Fn(ref sig, _),
1154                 ..
1155             }) => (
1156                 sm.guess_head_span(span),
1157                 sig.decl
1158                     .inputs
1159                     .iter()
1160                     .map(|arg| match arg.kind {
1161                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1162                             Some(arg.span),
1163                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
1164                         ),
1165                         _ => ArgKind::empty(),
1166                     })
1167                     .collect::<Vec<ArgKind>>(),
1168             ),
1169             Node::Ctor(ref variant_data) => {
1170                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
1171                 let span = sm.guess_head_span(span);
1172                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1173             }
1174             _ => panic!("non-FnLike node found: {:?}", node),
1175         })
1176     }
1177
1178     /// Reports an error when the number of arguments needed by a
1179     /// trait match doesn't match the number that the expression
1180     /// provides.
1181     fn report_arg_count_mismatch(
1182         &self,
1183         span: Span,
1184         found_span: Option<Span>,
1185         expected_args: Vec<ArgKind>,
1186         found_args: Vec<ArgKind>,
1187         is_closure: bool,
1188     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
1189         let kind = if is_closure { "closure" } else { "function" };
1190
1191         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1192             let arg_length = arguments.len();
1193             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
1194             match (arg_length, arguments.get(0)) {
1195                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1196                     format!("a single {}-tuple as argument", fields.len())
1197                 }
1198                 _ => format!(
1199                     "{} {}argument{}",
1200                     arg_length,
1201                     if distinct && arg_length > 1 { "distinct " } else { "" },
1202                     pluralize!(arg_length)
1203                 ),
1204             }
1205         };
1206
1207         let expected_str = args_str(&expected_args, &found_args);
1208         let found_str = args_str(&found_args, &expected_args);
1209
1210         let mut err = struct_span_err!(
1211             self.tcx.sess,
1212             span,
1213             E0593,
1214             "{} is expected to take {}, but it takes {}",
1215             kind,
1216             expected_str,
1217             found_str,
1218         );
1219
1220         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1221
1222         if let Some(found_span) = found_span {
1223             err.span_label(found_span, format!("takes {}", found_str));
1224
1225             // move |_| { ... }
1226             // ^^^^^^^^-- def_span
1227             //
1228             // move |_| { ... }
1229             // ^^^^^-- prefix
1230             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1231             // move |_| { ... }
1232             //      ^^^-- pipe_span
1233             let pipe_span =
1234                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
1235
1236             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1237             // found arguments is empty (assume the user just wants to ignore args in this case).
1238             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1239             if found_args.is_empty() && is_closure {
1240                 let underscores = vec!["_"; expected_args.len()].join(", ");
1241                 err.span_suggestion_verbose(
1242                     pipe_span,
1243                     &format!(
1244                         "consider changing the closure to take and ignore the expected argument{}",
1245                         pluralize!(expected_args.len())
1246                     ),
1247                     format!("|{}|", underscores),
1248                     Applicability::MachineApplicable,
1249                 );
1250             }
1251
1252             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1253                 if fields.len() == expected_args.len() {
1254                     let sugg = fields
1255                         .iter()
1256                         .map(|(name, _)| name.to_owned())
1257                         .collect::<Vec<String>>()
1258                         .join(", ");
1259                     err.span_suggestion_verbose(
1260                         found_span,
1261                         "change the closure to take multiple arguments instead of a single tuple",
1262                         format!("|{}|", sugg),
1263                         Applicability::MachineApplicable,
1264                     );
1265                 }
1266             }
1267             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
1268                 && fields.len() == found_args.len()
1269                 && is_closure
1270             {
1271                 let sugg = format!(
1272                     "|({}){}|",
1273                     found_args
1274                         .iter()
1275                         .map(|arg| match arg {
1276                             ArgKind::Arg(name, _) => name.to_owned(),
1277                             _ => "_".to_owned(),
1278                         })
1279                         .collect::<Vec<String>>()
1280                         .join(", "),
1281                     // add type annotations if available
1282                     if found_args.iter().any(|arg| match arg {
1283                         ArgKind::Arg(_, ty) => ty != "_",
1284                         _ => false,
1285                     }) {
1286                         format!(
1287                             ": ({})",
1288                             fields
1289                                 .iter()
1290                                 .map(|(_, ty)| ty.to_owned())
1291                                 .collect::<Vec<String>>()
1292                                 .join(", ")
1293                         )
1294                     } else {
1295                         String::new()
1296                     },
1297                 );
1298                 err.span_suggestion_verbose(
1299                     found_span,
1300                     "change the closure to accept a tuple instead of individual arguments",
1301                     sugg,
1302                     Applicability::MachineApplicable,
1303                 );
1304             }
1305         }
1306
1307         err
1308     }
1309
1310     fn type_implements_fn_trait(
1311         &self,
1312         param_env: ty::ParamEnv<'tcx>,
1313         ty: ty::Binder<'tcx, Ty<'tcx>>,
1314         constness: ty::BoundConstness,
1315         polarity: ty::ImplPolarity,
1316     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
1317         self.commit_if_ok(|_| {
1318             for trait_def_id in [
1319                 self.tcx.lang_items().fn_trait(),
1320                 self.tcx.lang_items().fn_mut_trait(),
1321                 self.tcx.lang_items().fn_once_trait(),
1322             ] {
1323                 let Some(trait_def_id) = trait_def_id else { continue };
1324                 // Make a fresh inference variable so we can determine what the substitutions
1325                 // of the trait are.
1326                 let var = self.next_ty_var(TypeVariableOrigin {
1327                     span: DUMMY_SP,
1328                     kind: TypeVariableOriginKind::MiscVariable,
1329                 });
1330                 let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
1331                 let obligation = Obligation::new(
1332                     ObligationCause::dummy(),
1333                     param_env,
1334                     ty.rebind(ty::TraitPredicate {
1335                         trait_ref: ty::TraitRef::new(trait_def_id, substs),
1336                         constness,
1337                         polarity,
1338                     })
1339                     .to_predicate(self.tcx),
1340                 );
1341                 let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
1342                 fulfill_cx.register_predicate_obligation(self, obligation);
1343                 if fulfill_cx.select_all_or_error(self).is_empty() {
1344                     return Ok((
1345                         ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
1346                             .expect("expected to map DefId to ClosureKind"),
1347                         ty.rebind(self.resolve_vars_if_possible(var)),
1348                     ));
1349                 }
1350             }
1351
1352             Err(())
1353         })
1354     }
1355 }
1356
1357 trait InferCtxtPrivExt<'hir, 'tcx> {
1358     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1359     // `error` occurring implies that `cond` occurs.
1360     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1361
1362     fn report_fulfillment_error(
1363         &self,
1364         error: &FulfillmentError<'tcx>,
1365         body_id: Option<hir::BodyId>,
1366         fallback_has_occurred: bool,
1367     );
1368
1369     fn report_projection_error(
1370         &self,
1371         obligation: &PredicateObligation<'tcx>,
1372         error: &MismatchedProjectionTypes<'tcx>,
1373     );
1374
1375     fn fuzzy_match_tys(
1376         &self,
1377         a: Ty<'tcx>,
1378         b: Ty<'tcx>,
1379         ignoring_lifetimes: bool,
1380     ) -> Option<CandidateSimilarity>;
1381
1382     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1383
1384     fn find_similar_impl_candidates(
1385         &self,
1386         trait_ref: ty::PolyTraitRef<'tcx>,
1387     ) -> Vec<ImplCandidate<'tcx>>;
1388
1389     fn report_similar_impl_candidates(
1390         &self,
1391         impl_candidates: Vec<ImplCandidate<'tcx>>,
1392         trait_ref: ty::PolyTraitRef<'tcx>,
1393         err: &mut Diagnostic,
1394     ) -> bool;
1395
1396     /// Gets the parent trait chain start
1397     fn get_parent_trait_ref(
1398         &self,
1399         code: &ObligationCauseCode<'tcx>,
1400     ) -> Option<(String, Option<Span>)>;
1401
1402     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1403     /// with the same path as `trait_ref`, a help message about
1404     /// a probable version mismatch is added to `err`
1405     fn note_version_mismatch(
1406         &self,
1407         err: &mut Diagnostic,
1408         trait_ref: &ty::PolyTraitRef<'tcx>,
1409     ) -> bool;
1410
1411     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1412     /// `trait_ref`.
1413     ///
1414     /// For this to work, `new_self_ty` must have no escaping bound variables.
1415     fn mk_trait_obligation_with_new_self_ty(
1416         &self,
1417         param_env: ty::ParamEnv<'tcx>,
1418         trait_ref: ty::PolyTraitPredicate<'tcx>,
1419         new_self_ty: Ty<'tcx>,
1420     ) -> PredicateObligation<'tcx>;
1421
1422     fn maybe_report_ambiguity(
1423         &self,
1424         obligation: &PredicateObligation<'tcx>,
1425         body_id: Option<hir::BodyId>,
1426     );
1427
1428     fn predicate_can_apply(
1429         &self,
1430         param_env: ty::ParamEnv<'tcx>,
1431         pred: ty::PolyTraitRef<'tcx>,
1432     ) -> bool;
1433
1434     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1435
1436     fn suggest_unsized_bound_if_applicable(
1437         &self,
1438         err: &mut Diagnostic,
1439         obligation: &PredicateObligation<'tcx>,
1440     );
1441
1442     fn annotate_source_of_ambiguity(
1443         &self,
1444         err: &mut Diagnostic,
1445         impls: &[DefId],
1446         predicate: ty::Predicate<'tcx>,
1447     );
1448
1449     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'hir>);
1450
1451     fn maybe_indirection_for_unsized(
1452         &self,
1453         err: &mut Diagnostic,
1454         item: &'hir Item<'hir>,
1455         param: &'hir GenericParam<'hir>,
1456     ) -> bool;
1457
1458     fn is_recursive_obligation(
1459         &self,
1460         obligated_types: &mut Vec<Ty<'tcx>>,
1461         cause_code: &ObligationCauseCode<'tcx>,
1462     ) -> bool;
1463 }
1464
1465 impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
1466     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1467     // `error` occurring implies that `cond` occurs.
1468     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1469         if cond == error {
1470             return true;
1471         }
1472
1473         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1474         let bound_error = error.kind();
1475         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1476             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
1477                 (cond, bound_error.rebind(error))
1478             }
1479             _ => {
1480                 // FIXME: make this work in other cases too.
1481                 return false;
1482             }
1483         };
1484
1485         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1486             let bound_predicate = obligation.predicate.kind();
1487             if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
1488                 let error = error.to_poly_trait_ref();
1489                 let implication = bound_predicate.rebind(implication.trait_ref);
1490                 // FIXME: I'm just not taking associated types at all here.
1491                 // Eventually I'll need to implement param-env-aware
1492                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1493                 let param_env = ty::ParamEnv::empty();
1494                 if self.can_sub(param_env, error, implication).is_ok() {
1495                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1496                     return true;
1497                 }
1498             }
1499         }
1500
1501         false
1502     }
1503
1504     #[instrument(skip(self), level = "debug")]
1505     fn report_fulfillment_error(
1506         &self,
1507         error: &FulfillmentError<'tcx>,
1508         body_id: Option<hir::BodyId>,
1509         fallback_has_occurred: bool,
1510     ) {
1511         match error.code {
1512             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1513                 self.report_selection_error(
1514                     error.obligation.clone(),
1515                     &error.root_obligation,
1516                     selection_error,
1517                     fallback_has_occurred,
1518                 );
1519             }
1520             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1521                 self.report_projection_error(&error.obligation, e);
1522             }
1523             FulfillmentErrorCode::CodeAmbiguity => {
1524                 self.maybe_report_ambiguity(&error.obligation, body_id);
1525             }
1526             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1527                 self.report_mismatched_types(
1528                     &error.obligation.cause,
1529                     expected_found.expected,
1530                     expected_found.found,
1531                     err.clone(),
1532                 )
1533                 .emit();
1534             }
1535             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1536                 self.report_mismatched_consts(
1537                     &error.obligation.cause,
1538                     expected_found.expected,
1539                     expected_found.found,
1540                     err.clone(),
1541                 )
1542                 .emit();
1543             }
1544         }
1545     }
1546
1547     fn report_projection_error(
1548         &self,
1549         obligation: &PredicateObligation<'tcx>,
1550         error: &MismatchedProjectionTypes<'tcx>,
1551     ) {
1552         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1553
1554         if predicate.references_error() {
1555             return;
1556         }
1557
1558         self.probe(|_| {
1559             let err_buf;
1560             let mut err = &error.err;
1561             let mut values = None;
1562
1563             // try to find the mismatched types to report the error with.
1564             //
1565             // this can fail if the problem was higher-ranked, in which
1566             // cause I have no idea for a good error message.
1567             let bound_predicate = predicate.kind();
1568             if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
1569                 let mut selcx = SelectionContext::new(self);
1570                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
1571                     obligation.cause.span,
1572                     infer::LateBoundRegionConversionTime::HigherRankedType,
1573                     bound_predicate.rebind(data),
1574                 );
1575                 let mut obligations = vec![];
1576                 let normalized_ty = super::normalize_projection_type(
1577                     &mut selcx,
1578                     obligation.param_env,
1579                     data.projection_ty,
1580                     obligation.cause.clone(),
1581                     0,
1582                     &mut obligations,
1583                 );
1584
1585                 debug!(
1586                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
1587                     obligation.cause, obligation.param_env
1588                 );
1589
1590                 debug!(
1591                     "report_projection_error normalized_ty={:?} data.ty={:?}",
1592                     normalized_ty, data.term,
1593                 );
1594
1595                 let is_normalized_ty_expected = !matches!(
1596                     obligation.cause.code().peel_derives(),
1597                     ObligationCauseCode::ItemObligation(_)
1598                         | ObligationCauseCode::BindingObligation(_, _)
1599                         | ObligationCauseCode::ObjectCastObligation(_)
1600                         | ObligationCauseCode::OpaqueType
1601                 );
1602                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1603                     is_normalized_ty_expected,
1604                     normalized_ty,
1605                     data.term,
1606                 ) {
1607                     values = Some(infer::ValuePairs::Terms(ExpectedFound::new(
1608                         is_normalized_ty_expected,
1609                         normalized_ty,
1610                         data.term,
1611                     )));
1612                     err_buf = error;
1613                     err = &err_buf;
1614                 }
1615             }
1616
1617             let mut diag = struct_span_err!(
1618                 self.tcx.sess,
1619                 obligation.cause.span,
1620                 E0271,
1621                 "type mismatch resolving `{}`",
1622                 predicate
1623             );
1624             let secondary_span = match predicate.kind().skip_binder() {
1625                 ty::PredicateKind::Projection(proj) => self
1626                     .tcx
1627                     .opt_associated_item(proj.projection_ty.item_def_id)
1628                     .and_then(|trait_assoc_item| {
1629                         self.tcx
1630                             .trait_of_item(proj.projection_ty.item_def_id)
1631                             .map(|id| (trait_assoc_item, id))
1632                     })
1633                     .and_then(|(trait_assoc_item, id)| {
1634                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1635                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1636                             self.tcx
1637                                 .associated_items(did)
1638                                 .in_definition_order()
1639                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1640                         })
1641                     })
1642                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1643                         Some(
1644                             hir::Node::TraitItem(hir::TraitItem {
1645                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1646                                 ..
1647                             })
1648                             | hir::Node::ImplItem(hir::ImplItem {
1649                                 kind: hir::ImplItemKind::TyAlias(ty),
1650                                 ..
1651                             }),
1652                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1653                         _ => None,
1654                     }),
1655                 _ => None,
1656             };
1657             self.note_type_err(&mut diag, &obligation.cause, secondary_span, values, err, true);
1658             self.note_obligation_cause(&mut diag, obligation);
1659             diag.emit();
1660         });
1661     }
1662
1663     fn fuzzy_match_tys(
1664         &self,
1665         mut a: Ty<'tcx>,
1666         mut b: Ty<'tcx>,
1667         ignoring_lifetimes: bool,
1668     ) -> Option<CandidateSimilarity> {
1669         /// returns the fuzzy category of a given type, or None
1670         /// if the type can be equated to any type.
1671         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1672             match t.kind() {
1673                 ty::Bool => Some(0),
1674                 ty::Char => Some(1),
1675                 ty::Str => Some(2),
1676                 ty::Adt(def, _) if tcx.is_diagnostic_item(sym::String, def.did()) => Some(2),
1677                 ty::Int(..)
1678                 | ty::Uint(..)
1679                 | ty::Float(..)
1680                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1681                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1682                 ty::Array(..) | ty::Slice(..) => Some(6),
1683                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1684                 ty::Dynamic(..) => Some(8),
1685                 ty::Closure(..) => Some(9),
1686                 ty::Tuple(..) => Some(10),
1687                 ty::Param(..) => Some(11),
1688                 ty::Projection(..) => Some(12),
1689                 ty::Opaque(..) => Some(13),
1690                 ty::Never => Some(14),
1691                 ty::Adt(..) => Some(15),
1692                 ty::Generator(..) => Some(16),
1693                 ty::Foreign(..) => Some(17),
1694                 ty::GeneratorWitness(..) => Some(18),
1695                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1696             }
1697         }
1698
1699         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1700             loop {
1701                 match t.kind() {
1702                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1703                         t = *inner
1704                     }
1705                     _ => break t,
1706                 }
1707             }
1708         };
1709
1710         if !ignoring_lifetimes {
1711             a = strip_references(a);
1712             b = strip_references(b);
1713         }
1714
1715         let cat_a = type_category(self.tcx, a)?;
1716         let cat_b = type_category(self.tcx, b)?;
1717         if a == b {
1718             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1719         } else if cat_a == cat_b {
1720             match (a.kind(), b.kind()) {
1721                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1722                 // Matching on references results in a lot of unhelpful
1723                 // suggestions, so let's just not do that for now.
1724                 //
1725                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1726                 // to prioritize that impl.
1727                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1728                     self.fuzzy_match_tys(a, b, true).is_some()
1729                 }
1730                 _ => true,
1731             }
1732             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1733         } else if ignoring_lifetimes {
1734             None
1735         } else {
1736             self.fuzzy_match_tys(a, b, true)
1737         }
1738     }
1739
1740     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1741         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1742             hir::GeneratorKind::Gen => "a generator",
1743             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1744             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1745             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1746         })
1747     }
1748
1749     fn find_similar_impl_candidates(
1750         &self,
1751         trait_ref: ty::PolyTraitRef<'tcx>,
1752     ) -> Vec<ImplCandidate<'tcx>> {
1753         self.tcx
1754             .all_impls(trait_ref.def_id())
1755             .filter_map(|def_id| {
1756                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative {
1757                     return None;
1758                 }
1759
1760                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1761
1762                 self.fuzzy_match_tys(trait_ref.skip_binder().self_ty(), imp.self_ty(), false)
1763                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1764             })
1765             .collect()
1766     }
1767
1768     fn report_similar_impl_candidates(
1769         &self,
1770         impl_candidates: Vec<ImplCandidate<'tcx>>,
1771         trait_ref: ty::PolyTraitRef<'tcx>,
1772         err: &mut Diagnostic,
1773     ) -> bool {
1774         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1775             candidates.sort();
1776             candidates.dedup();
1777             let len = candidates.len();
1778             if candidates.len() == 0 {
1779                 return false;
1780             }
1781             if candidates.len() == 1 {
1782                 err.highlighted_help(vec![
1783                     (
1784                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1785                         Style::NoStyle,
1786                     ),
1787                     ("is".to_string(), Style::Highlight),
1788                     (" implemented for `".to_string(), Style::NoStyle),
1789                     (candidates[0].self_ty().to_string(), Style::Highlight),
1790                     ("`".to_string(), Style::NoStyle),
1791                 ]);
1792                 return true;
1793             }
1794             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1795             // Check if the trait is the same in all cases. If so, we'll only show the type.
1796             let mut traits: Vec<_> =
1797                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1798             traits.sort();
1799             traits.dedup();
1800
1801             let mut candidates: Vec<String> = candidates
1802                 .into_iter()
1803                 .map(|c| {
1804                     if traits.len() == 1 {
1805                         format!("\n  {}", c.self_ty())
1806                     } else {
1807                         format!("\n  {}", c)
1808                     }
1809                 })
1810                 .collect();
1811
1812             candidates.sort();
1813             candidates.dedup();
1814             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1815             err.help(&format!(
1816                 "the following other types implement trait `{}`:{}{}",
1817                 trait_ref.print_only_trait_path(),
1818                 candidates[..end].join(""),
1819                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1820             ));
1821             true
1822         };
1823
1824         let def_id = trait_ref.def_id();
1825         if impl_candidates.is_empty() {
1826             if self.tcx.trait_is_auto(def_id)
1827                 || self.tcx.lang_items().items().contains(&Some(def_id))
1828                 || self.tcx.get_diagnostic_name(def_id).is_some()
1829             {
1830                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1831                 return false;
1832             }
1833             let normalized_impl_candidates: Vec<_> = self
1834                 .tcx
1835                 .all_impls(def_id)
1836                 // Ignore automatically derived impls and `!Trait` impls.
1837                 .filter(|&def_id| {
1838                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1839                         || self.tcx.is_builtin_derive(def_id)
1840                 })
1841                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1842                 // Avoid mentioning type parameters.
1843                 .filter(|trait_ref| !matches!(trait_ref.self_ty().kind(), ty::Param(_)))
1844                 .collect();
1845             return report(normalized_impl_candidates, err);
1846         }
1847
1848         let normalize = |candidate| {
1849             self.tcx.infer_ctxt().enter(|ref infcx| {
1850                 let normalized = infcx
1851                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1852                     .normalize(candidate)
1853                     .ok();
1854                 match normalized {
1855                     Some(normalized) => normalized.value,
1856                     None => candidate,
1857                 }
1858             })
1859         };
1860
1861         // Sort impl candidates so that ordering is consistent for UI tests.
1862         // because the ordering of `impl_candidates` may not be deterministic:
1863         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1864         //
1865         // Prefer more similar candidates first, then sort lexicographically
1866         // by their normalized string representation.
1867         let mut normalized_impl_candidates_and_similarities = impl_candidates
1868             .into_iter()
1869             .map(|ImplCandidate { trait_ref, similarity }| {
1870                 let normalized = normalize(trait_ref);
1871                 (similarity, normalized)
1872             })
1873             .collect::<Vec<_>>();
1874         normalized_impl_candidates_and_similarities.sort();
1875         normalized_impl_candidates_and_similarities.dedup();
1876
1877         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
1878             .into_iter()
1879             .map(|(_, normalized)| normalized)
1880             .collect::<Vec<_>>();
1881
1882         report(normalized_impl_candidates, err)
1883     }
1884
1885     /// Gets the parent trait chain start
1886     fn get_parent_trait_ref(
1887         &self,
1888         code: &ObligationCauseCode<'tcx>,
1889     ) -> Option<(String, Option<Span>)> {
1890         match code {
1891             ObligationCauseCode::BuiltinDerivedObligation(data) => {
1892                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
1893                 match self.get_parent_trait_ref(&data.parent_code) {
1894                     Some(t) => Some(t),
1895                     None => {
1896                         let ty = parent_trait_ref.skip_binder().self_ty();
1897                         let span = TyCategory::from_ty(self.tcx, ty)
1898                             .map(|(_, def_id)| self.tcx.def_span(def_id));
1899                         Some((ty.to_string(), span))
1900                     }
1901                 }
1902             }
1903             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
1904                 self.get_parent_trait_ref(&parent_code)
1905             }
1906             _ => None,
1907         }
1908     }
1909
1910     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1911     /// with the same path as `trait_ref`, a help message about
1912     /// a probable version mismatch is added to `err`
1913     fn note_version_mismatch(
1914         &self,
1915         err: &mut Diagnostic,
1916         trait_ref: &ty::PolyTraitRef<'tcx>,
1917     ) -> bool {
1918         let get_trait_impl = |trait_def_id| {
1919             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
1920         };
1921         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1922         let traits_with_same_path: std::collections::BTreeSet<_> = self
1923             .tcx
1924             .all_traits()
1925             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
1926             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
1927             .collect();
1928         let mut suggested = false;
1929         for trait_with_same_path in traits_with_same_path {
1930             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
1931                 let impl_span = self.tcx.def_span(impl_def_id);
1932                 err.span_help(impl_span, "trait impl with same name found");
1933                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1934                 let crate_msg = format!(
1935                     "perhaps two different versions of crate `{}` are being used?",
1936                     trait_crate
1937                 );
1938                 err.note(&crate_msg);
1939                 suggested = true;
1940             }
1941         }
1942         suggested
1943     }
1944
1945     fn mk_trait_obligation_with_new_self_ty(
1946         &self,
1947         param_env: ty::ParamEnv<'tcx>,
1948         trait_ref: ty::PolyTraitPredicate<'tcx>,
1949         new_self_ty: Ty<'tcx>,
1950     ) -> PredicateObligation<'tcx> {
1951         assert!(!new_self_ty.has_escaping_bound_vars());
1952
1953         let trait_pred = trait_ref.map_bound_ref(|tr| ty::TraitPredicate {
1954             trait_ref: ty::TraitRef {
1955                 substs: self.tcx.mk_substs_trait(new_self_ty, &tr.trait_ref.substs[1..]),
1956                 ..tr.trait_ref
1957             },
1958             ..*tr
1959         });
1960
1961         Obligation::new(ObligationCause::dummy(), param_env, trait_pred.to_predicate(self.tcx))
1962     }
1963
1964     #[instrument(skip(self), level = "debug")]
1965     fn maybe_report_ambiguity(
1966         &self,
1967         obligation: &PredicateObligation<'tcx>,
1968         body_id: Option<hir::BodyId>,
1969     ) {
1970         // Unable to successfully determine, probably means
1971         // insufficient type information, but could mean
1972         // ambiguous impls. The latter *ought* to be a
1973         // coherence violation, so we don't report it here.
1974
1975         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1976         let span = obligation.cause.span;
1977
1978         debug!(?predicate, obligation.cause.code = tracing::field::debug(&obligation.cause.code()));
1979
1980         // Ambiguity errors are often caused as fallout from earlier errors.
1981         // We ignore them if this `infcx` is tainted in some cases below.
1982
1983         let bound_predicate = predicate.kind();
1984         let mut err = match bound_predicate.skip_binder() {
1985             ty::PredicateKind::Trait(data) => {
1986                 let trait_ref = bound_predicate.rebind(data.trait_ref);
1987                 debug!(?trait_ref);
1988
1989                 if predicate.references_error() {
1990                     return;
1991                 }
1992                 // Typically, this ambiguity should only happen if
1993                 // there are unresolved type inference variables
1994                 // (otherwise it would suggest a coherence
1995                 // failure). But given #21974 that is not necessarily
1996                 // the case -- we can have multiple where clauses that
1997                 // are only distinguished by a region, which results
1998                 // in an ambiguity even when all types are fully
1999                 // known, since we don't dispatch based on region
2000                 // relationships.
2001
2002                 // Pick the first substitution that still contains inference variables as the one
2003                 // we're going to emit an error for. If there are none (see above), fall back to
2004                 // the substitution for `Self`.
2005                 let subst = {
2006                     let substs = data.trait_ref.substs;
2007                     substs
2008                         .iter()
2009                         .find(|s| s.has_infer_types_or_consts())
2010                         .unwrap_or_else(|| substs[0])
2011                 };
2012
2013                 // This is kind of a hack: it frequently happens that some earlier
2014                 // error prevents types from being fully inferred, and then we get
2015                 // a bunch of uninteresting errors saying something like "<generic
2016                 // #0> doesn't implement Sized".  It may even be true that we
2017                 // could just skip over all checks where the self-ty is an
2018                 // inference variable, but I was afraid that there might be an
2019                 // inference variable created, registered as an obligation, and
2020                 // then never forced by writeback, and hence by skipping here we'd
2021                 // be ignoring the fact that we don't KNOW the type works
2022                 // out. Though even that would probably be harmless, given that
2023                 // we're only talking about builtin traits, which are known to be
2024                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2025                 // avoid inundating the user with unnecessary errors, but we now
2026                 // check upstream for type errors and don't add the obligations to
2027                 // begin with in those cases.
2028                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2029                     if !self.is_tainted_by_errors() {
2030                         self.emit_inference_failure_err(
2031                             body_id,
2032                             span,
2033                             subst,
2034                             vec![],
2035                             ErrorCode::E0282,
2036                         )
2037                         .emit();
2038                     }
2039                     return;
2040                 }
2041
2042                 let impl_candidates = self
2043                     .find_similar_impl_candidates(trait_ref)
2044                     .into_iter()
2045                     .map(|candidate| candidate.trait_ref)
2046                     .collect();
2047                 let mut err = self.emit_inference_failure_err(
2048                     body_id,
2049                     span,
2050                     subst,
2051                     impl_candidates,
2052                     ErrorCode::E0283,
2053                 );
2054
2055                 let obligation = Obligation::new(
2056                     obligation.cause.clone(),
2057                     obligation.param_env,
2058                     trait_ref.to_poly_trait_predicate(),
2059                 );
2060                 let mut selcx = SelectionContext::with_query_mode(
2061                     &self,
2062                     crate::traits::TraitQueryMode::Standard,
2063                 );
2064                 match selcx.select_from_obligation(&obligation) {
2065                     Err(SelectionError::Ambiguous(impls)) if impls.len() > 1 => {
2066                         self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2067                     }
2068                     _ => {
2069                         if self.is_tainted_by_errors() {
2070                             err.cancel();
2071                             return;
2072                         }
2073                         err.note(&format!("cannot satisfy `{}`", predicate));
2074                     }
2075                 }
2076
2077                 if let ObligationCauseCode::ItemObligation(def_id) = *obligation.cause.code() {
2078                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2079                 } else if let (
2080                     Ok(ref snippet),
2081                     &ObligationCauseCode::BindingObligation(def_id, _),
2082                 ) =
2083                     (self.tcx.sess.source_map().span_to_snippet(span), obligation.cause.code())
2084                 {
2085                     let generics = self.tcx.generics_of(def_id);
2086                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2087                         && !snippet.ends_with('>')
2088                         && !generics.has_impl_trait()
2089                         && !self.tcx.fn_trait_kind_from_lang_item(def_id).is_some()
2090                     {
2091                         // FIXME: To avoid spurious suggestions in functions where type arguments
2092                         // where already supplied, we check the snippet to make sure it doesn't
2093                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2094                         // instead. Otherwise we would produce the following output:
2095                         //
2096                         // error[E0283]: type annotations needed
2097                         //   --> $DIR/issue-54954.rs:3:24
2098                         //    |
2099                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2100                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2101                         //    |                        |
2102                         //    |                        cannot infer type
2103                         //    |                        help: consider specifying the type argument
2104                         //    |                        in the function call:
2105                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2106                         // ...
2107                         // LL |     const fn const_val<T: Sized>() -> usize {
2108                         //    |                        - required by this bound in `Tt::const_val`
2109                         //    |
2110                         //    = note: cannot satisfy `_: Tt`
2111
2112                         err.span_suggestion_verbose(
2113                             span.shrink_to_hi(),
2114                             &format!(
2115                                 "consider specifying the type argument{} in the function call",
2116                                 pluralize!(generics.params.len()),
2117                             ),
2118                             format!(
2119                                 "::<{}>",
2120                                 generics
2121                                     .params
2122                                     .iter()
2123                                     .map(|p| p.name.to_string())
2124                                     .collect::<Vec<String>>()
2125                                     .join(", ")
2126                             ),
2127                             Applicability::HasPlaceholders,
2128                         );
2129                     }
2130                 }
2131                 err
2132             }
2133
2134             ty::PredicateKind::WellFormed(arg) => {
2135                 // Same hacky approach as above to avoid deluging user
2136                 // with error messages.
2137                 if arg.references_error()
2138                     || self.tcx.sess.has_errors().is_some()
2139                     || self.is_tainted_by_errors()
2140                 {
2141                     return;
2142                 }
2143
2144                 self.emit_inference_failure_err(body_id, span, arg, vec![], ErrorCode::E0282)
2145             }
2146
2147             ty::PredicateKind::Subtype(data) => {
2148                 if data.references_error()
2149                     || self.tcx.sess.has_errors().is_some()
2150                     || self.is_tainted_by_errors()
2151                 {
2152                     // no need to overload user in such cases
2153                     return;
2154                 }
2155                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2156                 // both must be type variables, or the other would've been instantiated
2157                 assert!(a.is_ty_var() && b.is_ty_var());
2158                 self.emit_inference_failure_err(body_id, span, a.into(), vec![], ErrorCode::E0282)
2159             }
2160             ty::PredicateKind::Projection(data) => {
2161                 let self_ty = data.projection_ty.self_ty();
2162                 let term = data.term;
2163                 if predicate.references_error() || self.is_tainted_by_errors() {
2164                     return;
2165                 }
2166                 if self_ty.needs_infer() && term.needs_infer() {
2167                     // We do this for the `foo.collect()?` case to produce a suggestion.
2168                     let mut err = self.emit_inference_failure_err(
2169                         body_id,
2170                         span,
2171                         self_ty.into(),
2172                         vec![],
2173                         ErrorCode::E0284,
2174                     );
2175                     err.note(&format!("cannot satisfy `{}`", predicate));
2176                     err
2177                 } else {
2178                     let mut err = struct_span_err!(
2179                         self.tcx.sess,
2180                         span,
2181                         E0284,
2182                         "type annotations needed: cannot satisfy `{}`",
2183                         predicate,
2184                     );
2185                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2186                     err
2187                 }
2188             }
2189
2190             _ => {
2191                 if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
2192                     return;
2193                 }
2194                 let mut err = struct_span_err!(
2195                     self.tcx.sess,
2196                     span,
2197                     E0284,
2198                     "type annotations needed: cannot satisfy `{}`",
2199                     predicate,
2200                 );
2201                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2202                 err
2203             }
2204         };
2205         self.note_obligation_cause(&mut err, obligation);
2206         err.emit();
2207     }
2208
2209     fn annotate_source_of_ambiguity(
2210         &self,
2211         err: &mut Diagnostic,
2212         impls: &[DefId],
2213         predicate: ty::Predicate<'tcx>,
2214     ) {
2215         let mut spans = vec![];
2216         let mut crates = vec![];
2217         let mut post = vec![];
2218         for def_id in impls {
2219             match self.tcx.span_of_impl(*def_id) {
2220                 Ok(span) => spans.push(self.tcx.sess.source_map().guess_head_span(span)),
2221                 Err(name) => {
2222                     crates.push(name);
2223                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2224                         post.push(header);
2225                     }
2226                 }
2227             }
2228         }
2229         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2230         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2231         crate_names.sort();
2232         crate_names.dedup();
2233         post.sort();
2234         post.dedup();
2235
2236         if self.is_tainted_by_errors()
2237             && (crate_names.len() == 1
2238                 && spans.len() == 0
2239                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2240                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2241         {
2242             // Avoid complaining about other inference issues for expressions like
2243             // `42 >> 1`, where the types are still `{integer}`, but we want to
2244             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2245             // NOTE(eddyb) this was `.cancel()`, but `err`
2246             // is borrowed, so we can't fully defuse it.
2247             err.downgrade_to_delayed_bug();
2248             return;
2249         }
2250         let post = if post.len() > 4 {
2251             format!(
2252                 ":\n{}\nand {} more",
2253                 post.iter().map(|p| format!("- {}", p)).take(4).collect::<Vec<_>>().join("\n"),
2254                 post.len() - 4,
2255             )
2256         } else if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2257             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2258         } else if post.len() == 1 {
2259             format!(": `{}`", post[0])
2260         } else {
2261             String::new()
2262         };
2263
2264         match (spans.len(), crates.len(), crate_names.len()) {
2265             (0, 0, 0) => {
2266                 err.note(&format!("cannot satisfy `{}`", predicate));
2267             }
2268             (0, _, 1) => {
2269                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2270             }
2271             (0, _, _) => {
2272                 err.note(&format!(
2273                     "{} in the following crates: {}{}",
2274                     msg,
2275                     crate_names.join(", "),
2276                     post,
2277                 ));
2278             }
2279             (_, 0, 0) => {
2280                 let span: MultiSpan = spans.into();
2281                 err.span_note(span, &msg);
2282             }
2283             (_, 1, 1) => {
2284                 let span: MultiSpan = spans.into();
2285                 err.span_note(span, &msg);
2286                 err.note(
2287                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2288                 );
2289             }
2290             _ => {
2291                 let span: MultiSpan = spans.into();
2292                 err.span_note(span, &msg);
2293                 err.note(&format!(
2294                     "and more `impl`s found in the following crates: {}{}",
2295                     crate_names.join(", "),
2296                     post,
2297                 ));
2298             }
2299         }
2300     }
2301
2302     /// Returns `true` if the trait predicate may apply for *some* assignment
2303     /// to the type parameters.
2304     fn predicate_can_apply(
2305         &self,
2306         param_env: ty::ParamEnv<'tcx>,
2307         pred: ty::PolyTraitRef<'tcx>,
2308     ) -> bool {
2309         struct ParamToVarFolder<'a, 'tcx> {
2310             infcx: &'a InferCtxt<'a, 'tcx>,
2311             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2312         }
2313
2314         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2315             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2316                 self.infcx.tcx
2317             }
2318
2319             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2320                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2321                     let infcx = self.infcx;
2322                     *self.var_map.entry(ty).or_insert_with(|| {
2323                         infcx.next_ty_var(TypeVariableOrigin {
2324                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2325                             span: DUMMY_SP,
2326                         })
2327                     })
2328                 } else {
2329                     ty.super_fold_with(self)
2330                 }
2331             }
2332         }
2333
2334         self.probe(|_| {
2335             let mut selcx = SelectionContext::new(self);
2336
2337             let cleaned_pred =
2338                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2339
2340             let cleaned_pred = super::project::normalize(
2341                 &mut selcx,
2342                 param_env,
2343                 ObligationCause::dummy(),
2344                 cleaned_pred,
2345             )
2346             .value;
2347
2348             let obligation = Obligation::new(
2349                 ObligationCause::dummy(),
2350                 param_env,
2351                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
2352             );
2353
2354             self.predicate_may_hold(&obligation)
2355         })
2356     }
2357
2358     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2359         // First, attempt to add note to this error with an async-await-specific
2360         // message, and fall back to regular note otherwise.
2361         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2362             self.note_obligation_cause_code(
2363                 err,
2364                 &obligation.predicate,
2365                 obligation.param_env,
2366                 obligation.cause.code(),
2367                 &mut vec![],
2368                 &mut Default::default(),
2369             );
2370             self.suggest_unsized_bound_if_applicable(err, obligation);
2371         }
2372     }
2373
2374     fn suggest_unsized_bound_if_applicable(
2375         &self,
2376         err: &mut Diagnostic,
2377         obligation: &PredicateObligation<'tcx>,
2378     ) {
2379         let (
2380             ty::PredicateKind::Trait(pred),
2381             &ObligationCauseCode::BindingObligation(item_def_id, span),
2382         ) = (
2383             obligation.predicate.kind().skip_binder(),
2384             obligation.cause.code().peel_derives(),
2385         )  else {
2386             return;
2387         };
2388         debug!(
2389             "suggest_unsized_bound_if_applicable: pred={:?} item_def_id={:?} span={:?}",
2390             pred, item_def_id, span
2391         );
2392
2393         let (Some(node), true) = (
2394             self.tcx.hir().get_if_local(item_def_id),
2395             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2396         ) else {
2397             return;
2398         };
2399         self.maybe_suggest_unsized_generics(err, span, node);
2400     }
2401
2402     fn maybe_suggest_unsized_generics<'hir>(
2403         &self,
2404         err: &mut Diagnostic,
2405         span: Span,
2406         node: Node<'hir>,
2407     ) {
2408         let Some(generics) = node.generics() else {
2409             return;
2410         };
2411         let sized_trait = self.tcx.lang_items().sized_trait();
2412         debug!("maybe_suggest_unsized_generics: generics.params={:?}", generics.params);
2413         debug!("maybe_suggest_unsized_generics: generics.where_clause={:?}", generics.where_clause);
2414         let param = generics.params.iter().filter(|param| param.span == span).find(|param| {
2415             // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2416             // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2417             param
2418                 .bounds
2419                 .iter()
2420                 .all(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) != sized_trait)
2421         });
2422         let Some(param) = param else {
2423             return;
2424         };
2425         let param_def_id = self.tcx.hir().local_def_id(param.hir_id).to_def_id();
2426         let preds = generics.where_clause.predicates.iter();
2427         let explicitly_sized = preds
2428             .filter_map(|pred| match pred {
2429                 hir::WherePredicate::BoundPredicate(bp) => Some(bp),
2430                 _ => None,
2431             })
2432             .filter(|bp| bp.is_param_bound(param_def_id))
2433             .flat_map(|bp| bp.bounds)
2434             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2435         if explicitly_sized {
2436             return;
2437         }
2438         debug!("maybe_suggest_unsized_generics: param={:?}", param);
2439         match node {
2440             hir::Node::Item(
2441                 item @ hir::Item {
2442                     // Only suggest indirection for uses of type parameters in ADTs.
2443                     kind:
2444                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2445                     ..
2446                 },
2447             ) => {
2448                 if self.maybe_indirection_for_unsized(err, item, param) {
2449                     return;
2450                 }
2451             }
2452             _ => {}
2453         };
2454         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2455         let (span, separator) = match param.bounds {
2456             [] => (span.shrink_to_hi(), ":"),
2457             [.., bound] => (bound.span().shrink_to_hi(), " +"),
2458         };
2459         err.span_suggestion_verbose(
2460             span,
2461             "consider relaxing the implicit `Sized` restriction",
2462             format!("{} ?Sized", separator),
2463             Applicability::MachineApplicable,
2464         );
2465     }
2466
2467     fn maybe_indirection_for_unsized<'hir>(
2468         &self,
2469         err: &mut Diagnostic,
2470         item: &'hir Item<'hir>,
2471         param: &'hir GenericParam<'hir>,
2472     ) -> bool {
2473         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2474         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2475         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2476         let mut visitor =
2477             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2478         visitor.visit_item(item);
2479         if visitor.invalid_spans.is_empty() {
2480             return false;
2481         }
2482         let mut multispan: MultiSpan = param.span.into();
2483         multispan.push_span_label(
2484             param.span,
2485             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2486         );
2487         for sp in visitor.invalid_spans {
2488             multispan.push_span_label(
2489                 sp,
2490                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2491             );
2492         }
2493         err.span_help(
2494             multispan,
2495             &format!(
2496                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2497                 used through indirection like `&{T}` or `Box<{T}>`",
2498                 T = param.name.ident(),
2499             ),
2500         );
2501         true
2502     }
2503
2504     fn is_recursive_obligation(
2505         &self,
2506         obligated_types: &mut Vec<Ty<'tcx>>,
2507         cause_code: &ObligationCauseCode<'tcx>,
2508     ) -> bool {
2509         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2510             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2511             let self_ty = parent_trait_ref.skip_binder().self_ty();
2512             if obligated_types.iter().any(|ot| ot == &self_ty) {
2513                 return true;
2514             }
2515             if let ty::Adt(def, substs) = self_ty.kind()
2516                 && let [arg] = &substs[..]
2517                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2518                 && let ty::Adt(inner_def, _) = ty.kind()
2519                 && inner_def == def
2520             {
2521                 return true;
2522             }
2523         }
2524         false
2525     }
2526 }
2527
2528 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2529 /// `param: ?Sized` would be a valid constraint.
2530 struct FindTypeParam {
2531     param: rustc_span::Symbol,
2532     invalid_spans: Vec<Span>,
2533     nested: bool,
2534 }
2535
2536 impl<'v> Visitor<'v> for FindTypeParam {
2537     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2538         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2539     }
2540
2541     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2542         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2543         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2544         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2545         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2546         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2547         // in that case should make what happened clear enough.
2548         match ty.kind {
2549             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2550             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2551                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2552             {
2553                 if !self.nested {
2554                     debug!("FindTypeParam::visit_ty: ty={:?}", ty);
2555                     self.invalid_spans.push(ty.span);
2556                 }
2557             }
2558             hir::TyKind::Path(_) => {
2559                 let prev = self.nested;
2560                 self.nested = true;
2561                 hir::intravisit::walk_ty(self, ty);
2562                 self.nested = prev;
2563             }
2564             _ => {
2565                 hir::intravisit::walk_ty(self, ty);
2566             }
2567         }
2568     }
2569 }
2570
2571 pub fn recursive_type_with_infinite_size_error<'tcx>(
2572     tcx: TyCtxt<'tcx>,
2573     type_def_id: DefId,
2574     spans: Vec<(Span, Option<hir::HirId>)>,
2575 ) {
2576     assert!(type_def_id.is_local());
2577     let span = tcx.hir().span_if_local(type_def_id).unwrap();
2578     let span = tcx.sess.source_map().guess_head_span(span);
2579     let path = tcx.def_path_str(type_def_id);
2580     let mut err =
2581         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
2582     err.span_label(span, "recursive type has infinite size");
2583     for &(span, _) in &spans {
2584         err.span_label(span, "recursive without indirection");
2585     }
2586     let msg = format!(
2587         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
2588         path,
2589     );
2590     if spans.len() <= 4 {
2591         // FIXME(compiler-errors): This suggestion might be erroneous if Box is shadowed
2592         err.multipart_suggestion(
2593             &msg,
2594             spans
2595                 .into_iter()
2596                 .flat_map(|(span, field_id)| {
2597                     if let Some(generic_span) = get_option_generic_from_field_id(tcx, field_id) {
2598                         // If we match an `Option` and can grab the span of the Option's generic, then
2599                         // suggest boxing the generic arg for a non-null niche optimization.
2600                         vec![
2601                             (generic_span.shrink_to_lo(), "Box<".to_string()),
2602                             (generic_span.shrink_to_hi(), ">".to_string()),
2603                         ]
2604                     } else {
2605                         vec![
2606                             (span.shrink_to_lo(), "Box<".to_string()),
2607                             (span.shrink_to_hi(), ">".to_string()),
2608                         ]
2609                     }
2610                 })
2611                 .collect(),
2612             Applicability::HasPlaceholders,
2613         );
2614     } else {
2615         err.help(&msg);
2616     }
2617     err.emit();
2618 }
2619
2620 /// Extract the span for the generic type `T` of `Option<T>` in a field definition
2621 fn get_option_generic_from_field_id(tcx: TyCtxt<'_>, field_id: Option<hir::HirId>) -> Option<Span> {
2622     let node = tcx.hir().find(field_id?);
2623
2624     // Expect a field from our field_id
2625     let Some(hir::Node::Field(field_def)) = node
2626         else { bug!("Expected HirId corresponding to FieldDef, found: {:?}", node) };
2627
2628     // Match a type that is a simple QPath with no Self
2629     let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = &field_def.ty.kind
2630         else { return None };
2631
2632     // Check if the path we're checking resolves to Option
2633     let hir::def::Res::Def(_, did) = path.res
2634         else { return None };
2635
2636     // Bail if this path doesn't describe `::core::option::Option`
2637     if !tcx.is_diagnostic_item(sym::Option, did) {
2638         return None;
2639     }
2640
2641     // Match a single generic arg in the 0th path segment
2642     let generic_arg = path.segments.last()?.args?.args.get(0)?;
2643
2644     // Take the span out of the type, if it's a type
2645     if let hir::GenericArg::Type(generic_ty) = generic_arg { Some(generic_ty.span) } else { None }
2646 }
2647
2648 /// Summarizes information
2649 #[derive(Clone)]
2650 pub enum ArgKind {
2651     /// An argument of non-tuple type. Parameters are (name, ty)
2652     Arg(String, String),
2653
2654     /// An argument of tuple type. For a "found" argument, the span is
2655     /// the location in the source of the pattern. For an "expected"
2656     /// argument, it will be None. The vector is a list of (name, ty)
2657     /// strings for the components of the tuple.
2658     Tuple(Option<Span>, Vec<(String, String)>),
2659 }
2660
2661 impl ArgKind {
2662     fn empty() -> ArgKind {
2663         ArgKind::Arg("_".to_owned(), "_".to_owned())
2664     }
2665
2666     /// Creates an `ArgKind` from the expected type of an
2667     /// argument. It has no name (`_`) and an optional source span.
2668     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2669         match t.kind() {
2670             ty::Tuple(tys) => ArgKind::Tuple(
2671                 span,
2672                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2673             ),
2674             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2675         }
2676     }
2677 }
2678
2679 struct HasNumericInferVisitor;
2680
2681 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2682     type BreakTy = ();
2683
2684     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2685         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2686             ControlFlow::Break(())
2687         } else {
2688             ControlFlow::CONTINUE
2689         }
2690     }
2691 }