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

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