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::TypeTrace;
  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::CompareImplItemObligation {
 306                     impl_item_def_id,
 307                     trait_item_def_id,
 308                     kind: _,
 309                 } = *obligation.cause.code()
 310                 {
 311                     self.report_extra_impl_obligation(
 312                         span,
 313                         impl_item_def_id,
 314                         trait_item_def_id,
 315                         &format!("`{}`", obligation.predicate),
 316                     )
 317                     .emit();
 318                     return;
 319                 }
 320
 321                 let bound_predicate = obligation.predicate.kind();
 322                 match bound_predicate.skip_binder() {
 323                     ty::PredicateKind::Trait(trait_predicate) => {
 324                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 325                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 326
 327                         trait_predicate.remap_constness_diag(obligation.param_env);
 328                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 329                             == trait_predicate.skip_binder().constness;
 330
 331                         if self.tcx.sess.has_errors().is_some()
 332                             && trait_predicate.references_error()
 333                         {
 334                             return;
 335                         }
 336                         let trait_ref = trait_predicate.to_poly_trait_ref();
 337                         let (post_message, pre_message, type_def) = self
 338                             .get_parent_trait_ref(obligation.cause.code())
 339                             .map(|(t, s)| {
 340                                 (
 341                                     format!(" in `{}`", t),
 342                                     format!("within `{}`, ", t),
 343                                     s.map(|s| (format!("within this `{}`", t), s)),
 344                                 )
 345                             })
 346                             .unwrap_or_default();
 347
 348                         let OnUnimplementedNote {
 349                             message,
 350                             label,
 351                             note,
 352                             parent_label,
 353                             append_const_msg,
 354                         } = self.on_unimplemented_note(trait_ref, &obligation);
 355                         let have_alt_message = message.is_some() || label.is_some();
 356                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 357                         let is_unsize =
 358                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 359                         let (message, note, append_const_msg) = if is_try_conversion {
 360                             (
 361                                 Some(format!(
 362                                     "`?` couldn't convert the error to `{}`",
 363                                     trait_ref.skip_binder().self_ty(),
 364                                 )),
 365                                 Some(
 366                                     "the question mark operation (`?`) implicitly performs a \
 367                                      conversion on the error value using the `From` trait"
 368                                         .to_owned(),
 369                                 ),
 370                                 Some(None),
 371                             )
 372                         } else {
 373                             (message, note, append_const_msg)
 374                         };
 375
 376                         let mut err = struct_span_err!(
 377                             self.tcx.sess,
 378                             span,
 379                             E0277,
 380                             "{}",
 381                             message
 382                                 .and_then(|cannot_do_this| {
 383                                     match (predicate_is_const, append_const_msg) {
 384                                         // do nothing if predicate is not const
 385                                         (false, _) => Some(cannot_do_this),
 386                                         // suggested using default post message
 387                                         (true, Some(None)) => {
 388                                             Some(format!("{cannot_do_this} in const contexts"))
 389                                         }
 390                                         // overridden post message
 391                                         (true, Some(Some(post_message))) => {
 392                                             Some(format!("{cannot_do_this}{post_message}"))
 393                                         }
 394                                         // fallback to generic message
 395                                         (true, None) => None,
 396                                     }
 397                                 })
 398                                 .unwrap_or_else(|| format!(
 399                                     "the trait bound `{}` is not satisfied{}",
 400                                     trait_predicate, post_message,
 401                                 ))
 402                         );
 403
 404                         if is_try_conversion {
 405                             let none_error = self
 406                                 .tcx
 407                                 .get_diagnostic_item(sym::none_error)
 408                                 .map(|def_id| tcx.type_of(def_id));
 409                             let should_convert_option_to_result =
 410                                 Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
 411                             let should_convert_result_to_option =
 412                                 Some(trait_ref.self_ty().skip_binder()) == none_error;
 413                             if should_convert_option_to_result {
 414                                 err.span_suggestion_verbose(
 415                                     span.shrink_to_lo(),
 416                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 417                                      using `Option::ok_or` or `Option::ok_or_else`",
 418                                     ".ok_or_else(|| /* error value */)",
 419                                     Applicability::HasPlaceholders,
 420                                 );
 421                             } else if should_convert_result_to_option {
 422                                 err.span_suggestion_verbose(
 423                                     span.shrink_to_lo(),
 424                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 425                                      using `Result::ok`",
 426                                     ".ok()",
 427                                     Applicability::MachineApplicable,
 428                                 );
 429                             }
 430                             if let Some(ret_span) = self.return_type_span(&obligation) {
 431                                 err.span_label(
 432                                     ret_span,
 433                                     &format!(
 434                                         "expected `{}` because of this",
 435                                         trait_ref.skip_binder().self_ty()
 436                                     ),
 437                                 );
 438                             }
 439                         }
 440
 441                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 442                             && predicate_is_const
 443                         {
 444                             err.note("`~const Drop` was renamed to `~const Destruct`");
 445                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 446                         }
 447
 448                         let explanation = if let ObligationCauseCode::MainFunctionType =
 449                             obligation.cause.code()
 450                         {
 451                             "consider using `()`, or a `Result`".to_owned()
 452                         } else {
 453                             let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
 454                                 ty::FnDef(_, _) => Some("fn item"),
 455                                 ty::Closure(_, _) => Some("closure"),
 456                                 _ => None,
 457                             };
 458
 459                             match ty_desc {
 460                                 Some(desc) => format!(
 461                                     "{}the trait `{}` is not implemented for {} `{}`",
 462                                     pre_message,
 463                                     trait_predicate.print_modifiers_and_trait_path(),
 464                                     desc,
 465                                     trait_ref.skip_binder().self_ty(),
 466                                 ),
 467                                 None => format!(
 468                                     "{}the trait `{}` is not implemented for `{}`",
 469                                     pre_message,
 470                                     trait_predicate.print_modifiers_and_trait_path(),
 471                                     trait_ref.skip_binder().self_ty(),
 472                                 ),
 473                             }
 474                         };
 475
 476                         if self.suggest_add_reference_to_arg(
 477                             &obligation,
 478                             &mut err,
 479                             trait_predicate,
 480                             have_alt_message,
 481                         ) {
 482                             self.note_obligation_cause(&mut err, &obligation);
 483                             err.emit();
 484                             return;
 485                         }
 486                         if let Some(ref s) = label {
 487                             // If it has a custom `#[rustc_on_unimplemented]`
 488                             // error message, let's display it as the label!
 489                             err.span_label(span, s);
 490                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 491                                 // When the self type is a type param We don't need to "the trait
 492                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 493                                 // at the type param with a label to suggest constraining it.
 494                                 err.help(&explanation);
 495                             }
 496                         } else {
 497                             err.span_label(span, explanation);
 498                         }
 499
 500                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 501                            Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 502                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 503                         }
 504
 505                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 506                             let non_const_predicate = trait_ref.without_const();
 507                             let non_const_obligation = Obligation {
 508                                 cause: obligation.cause.clone(),
 509                                 param_env: obligation.param_env.without_const(),
 510                                 predicate: non_const_predicate.to_predicate(tcx),
 511                                 recursion_depth: obligation.recursion_depth,
 512                             };
 513                             if self.predicate_may_hold(&non_const_obligation) {
 514                                 err.span_note(
 515                                     span,
 516                                     &format!(
 517                                         "the trait `{}` is implemented for `{}`, \
 518                                         but that implementation is not `const`",
 519                                         non_const_predicate.print_modifiers_and_trait_path(),
 520                                         trait_ref.skip_binder().self_ty(),
 521                                     ),
 522                                 );
 523                             }
 524                         }
 525
 526                         if let Some((msg, span)) = type_def {
 527                             err.span_label(span, &msg);
 528                         }
 529                         if let Some(ref s) = note {
 530                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 531                             err.note(s.as_str());
 532                         }
 533                         if let Some(ref s) = parent_label {
 534                             let body = tcx
 535                                 .hir()
 536                                 .opt_local_def_id(obligation.cause.body_id)
 537                                 .unwrap_or_else(|| {
 538                                     tcx.hir().body_owner_def_id(hir::BodyId {
 539                                         hir_id: obligation.cause.body_id,
 540                                     })
 541                                 });
 542                             err.span_label(tcx.def_span(body), s);
 543                         }
 544
 545                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 546                         self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
 547                         let mut suggested =
 548                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 549                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 550                         suggested |=
 551                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 552                         suggested |= self.suggest_semicolon_removal(
 553                             &obligation,
 554                             &mut err,
 555                             span,
 556                             trait_predicate,
 557                         );
 558                         self.note_version_mismatch(&mut err, &trait_ref);
 559                         self.suggest_remove_await(&obligation, &mut err);
 560                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 561
 562                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 563                             self.suggest_await_before_try(
 564                                 &mut err,
 565                                 &obligation,
 566                                 trait_predicate,
 567                                 span,
 568                             );
 569                         }
 570
 571                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 572                             err.emit();
 573                             return;
 574                         }
 575
 576                         if is_unsize {
 577                             // If the obligation failed due to a missing implementation of the
 578                             // `Unsize` trait, give a pointer to why that might be the case
 579                             err.note(
 580                                 "all implementations of `Unsize` are provided \
 581                                 automatically by the compiler, see \
 582                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 583                                 for more information",
 584                             );
 585                         }
 586
 587                         let is_fn_trait = [
 588                             self.tcx.lang_items().fn_trait(),
 589                             self.tcx.lang_items().fn_mut_trait(),
 590                             self.tcx.lang_items().fn_once_trait(),
 591                         ]
 592                         .contains(&Some(trait_ref.def_id()));
 593                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 594                             *trait_ref.skip_binder().self_ty().kind()
 595                         {
 596                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 597                         } else {
 598                             false
 599                         };
 600                         if is_fn_trait && is_target_feature_fn {
 601                             err.note(
 602                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 603                             );
 604                         }
 605
 606                         // Try to report a help message
 607                         if is_fn_trait
 608                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 609                                 obligation.param_env,
 610                                 trait_ref.self_ty(),
 611                                 trait_predicate.skip_binder().constness,
 612                                 trait_predicate.skip_binder().polarity,
 613                             )
 614                         {
 615                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 616                             // suggestion to add trait bounds for the type, since we only typically implement
 617                             // these traits once.
 618
 619                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 620                             // to implement.
 621                             let selected_kind =
 622                                 ty::ClosureKind::from_def_id(self.tcx, trait_ref.def_id())
 623                                     .expect("expected to map DefId to ClosureKind");
 624                             if !implemented_kind.extends(selected_kind) {
 625                                 err.note(
 626                                     &format!(
 627                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 628                                         trait_ref.skip_binder().self_ty(),
 629                                         implemented_kind,
 630                                         selected_kind
 631                                     )
 632                                 );
 633                             }
 634
 635                             // Note any argument mismatches
 636                             let given_ty = params.skip_binder();
 637                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 638                             if let ty::Tuple(given) = given_ty.kind()
 639                                 && let ty::Tuple(expected) = expected_ty.kind()
 640                             {
 641                                 if expected.len() != given.len() {
 642                                     // Note number of types that were expected and given
 643                                     err.note(
 644                                         &format!(
 645                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 646                                             given.len(),
 647                                             pluralize!(given.len()),
 648                                             expected.len(),
 649                                             pluralize!(expected.len()),
 650                                         )
 651                                     );
 652                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
 653                                     // Print type mismatch
 654                                     let (expected_args, given_args) =
 655                                         self.cmp(given_ty, expected_ty);
 656                                     err.note_expected_found(
 657                                         &"a closure with arguments",
 658                                         expected_args,
 659                                         &"a closure with arguments",
 660                                         given_args,
 661                                     );
 662                                 }
 663                             }
 664                         } else if !trait_ref.has_infer_types_or_consts()
 665                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 666                         {
 667                             // If a where-clause may be useful, remind the
 668                             // user that they can add it.
 669                             //
 670                             // don't display an on-unimplemented note, as
 671                             // these notes will often be of the form
 672                             //     "the type `T` can't be frobnicated"
 673                             // which is somewhat confusing.
 674                             self.suggest_restricting_param_bound(
 675                                 &mut err,
 676                                 trait_predicate,
 677                                 None,
 678                                 obligation.cause.body_id,
 679                             );
 680                         } else if !suggested {
 681                             // Can't show anything else useful, try to find similar impls.
 682                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
 683                             if !self.report_similar_impl_candidates(
 684                                 impl_candidates,
 685                                 trait_ref,
 686                                 obligation.cause.body_id,
 687                                 &mut err,
 688                             ) {
 689                                 // This is *almost* equivalent to
 690                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 691                                 // trait predicate for that corresponding root obligation. This
 692                                 // lets us get a derived obligation from a type parameter, like
 693                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 694                                 // implementer of `Pattern<'_>`.
 695                                 let mut code = obligation.cause.code();
 696                                 let mut trait_pred = trait_predicate;
 697                                 let mut peeled = false;
 698                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
 699                                     code = parent_code;
 700                                     if let Some(parent_trait_pred) = parent_trait_pred {
 701                                         trait_pred = parent_trait_pred;
 702                                         peeled = true;
 703                                     }
 704                                 }
 705                                 let def_id = trait_pred.def_id();
 706                                 // Mention *all* the `impl`s for the *top most* obligation, the
 707                                 // user might have meant to use one of them, if any found. We skip
 708                                 // auto-traits or fundamental traits that might not be exactly what
 709                                 // the user might expect to be presented with. Instead this is
 710                                 // useful for less general traits.
 711                                 if peeled
 712                                     && !self.tcx.trait_is_auto(def_id)
 713                                     && !self.tcx.lang_items().items().contains(&Some(def_id))
 714                                 {
 715                                     let trait_ref = trait_pred.to_poly_trait_ref();
 716                                     let impl_candidates =
 717                                         self.find_similar_impl_candidates(trait_pred);
 718                                     self.report_similar_impl_candidates(
 719                                         impl_candidates,
 720                                         trait_ref,
 721                                         obligation.cause.body_id,
 722                                         &mut err,
 723                                     );
 724                                 }
 725                             }
 726                         }
 727
 728                         // Changing mutability doesn't make a difference to whether we have
 729                         // an `Unsize` impl (Fixes ICE in #71036)
 730                         if !is_unsize {
 731                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
 732                         }
 733
 734                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 735                         // implemented, and fallback has occurred, then it could be due to a
 736                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 737                         // note informing about the change in behaviour.
 738                         if trait_predicate.skip_binder().self_ty().is_never()
 739                             && fallback_has_occurred
 740                         {
 741                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 742                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 743                                     self.tcx.mk_unit(),
 744                                     &trait_pred.trait_ref.substs[1..],
 745                                 );
 746                                 trait_pred
 747                             });
 748                             let unit_obligation = obligation.with(predicate.to_predicate(tcx));
 749                             if self.predicate_may_hold(&unit_obligation) {
 750                                 err.note(
 751                                     "this error might have been caused by changes to \
 752                                     Rust's type-inference algorithm (see issue #48950 \
 753                                     <https://github.com/rust-lang/rust/issues/48950> \
 754                                     for more information)",
 755                                 );
 756                                 err.help("did you intend to use the type `()` here instead?");
 757                             }
 758                         }
 759
 760                         // Return early if the trait is Debug or Display and the invocation
 761                         // originates within a standard library macro, because the output
 762                         // is otherwise overwhelming and unhelpful (see #85844 for an
 763                         // example).
 764
 765                         let in_std_macro =
 766                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
 767                                 Some(macro_def_id) => {
 768                                     let crate_name = tcx.crate_name(macro_def_id.krate);
 769                                     crate_name == sym::std || crate_name == sym::core
 770                                 }
 771                                 None => false,
 772                             };
 773
 774                         if in_std_macro
 775                             && matches!(
 776                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
 777                                 Some(sym::Debug | sym::Display)
 778                             )
 779                         {
 780                             err.emit();
 781                             return;
 782                         }
 783
 784                         err
 785                     }
 786
 787                     ty::PredicateKind::Subtype(predicate) => {
 788                         // Errors for Subtype predicates show up as
 789                         // `FulfillmentErrorCode::CodeSubtypeError`,
 790                         // not selection error.
 791                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 792                     }
 793
 794                     ty::PredicateKind::Coerce(predicate) => {
 795                         // Errors for Coerce predicates show up as
 796                         // `FulfillmentErrorCode::CodeSubtypeError`,
 797                         // not selection error.
 798                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
 799                     }
 800
 801                     ty::PredicateKind::RegionOutlives(..)
 802                     | ty::PredicateKind::Projection(..)
 803                     | ty::PredicateKind::TypeOutlives(..) => {
 804                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
 805                         struct_span_err!(
 806                             self.tcx.sess,
 807                             span,
 808                             E0280,
 809                             "the requirement `{}` is not satisfied",
 810                             predicate
 811                         )
 812                     }
 813
 814                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
 815                         let violations = self.tcx.object_safety_violations(trait_def_id);
 816                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 817                     }
 818
 819                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 820                         let found_kind = self.closure_kind(closure_substs).unwrap();
 821                         let closure_span = self.tcx.def_span(closure_def_id);
 822                         let mut err = struct_span_err!(
 823                             self.tcx.sess,
 824                             closure_span,
 825                             E0525,
 826                             "expected a closure that implements the `{}` trait, \
 827                              but this closure only implements `{}`",
 828                             kind,
 829                             found_kind
 830                         );
 831
 832                         err.span_label(
 833                             closure_span,
 834                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 835                         );
 836                         err.span_label(
 837                             obligation.cause.span,
 838                             format!("the requirement to implement `{}` derives from here", kind),
 839                         );
 840
 841                         // Additional context information explaining why the closure only implements
 842                         // a particular trait.
 843                         if let Some(typeck_results) = self.in_progress_typeck_results {
 844                             let hir_id = self
 845                                 .tcx
 846                                 .hir()
 847                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
 848                             let typeck_results = typeck_results.borrow();
 849                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
 850                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
 851                                     err.span_label(
 852                                         *span,
 853                                         format!(
 854                                             "closure is `FnOnce` because it moves the \
 855                                          variable `{}` out of its environment",
 856                                             ty::place_to_string_for_capture(tcx, place)
 857                                         ),
 858                                     );
 859                                 }
 860                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
 861                                     err.span_label(
 862                                         *span,
 863                                         format!(
 864                                             "closure is `FnMut` because it mutates the \
 865                                          variable `{}` here",
 866                                             ty::place_to_string_for_capture(tcx, place)
 867                                         ),
 868                                     );
 869                                 }
 870                                 _ => {}
 871                             }
 872                         }
 873
 874                         err
 875                     }
 876
 877                     ty::PredicateKind::WellFormed(ty) => {
 878                         if !self.tcx.sess.opts.unstable_opts.chalk {
 879                             // WF predicates cannot themselves make
 880                             // errors. They can only block due to
 881                             // ambiguity; otherwise, they always
 882                             // degenerate into other obligations
 883                             // (which may fail).
 884                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 885                         } else {
 886                             // FIXME: we'll need a better message which takes into account
 887                             // which bounds actually failed to hold.
 888                             self.tcx.sess.struct_span_err(
 889                                 span,
 890                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 891                             )
 892                         }
 893                     }
 894
 895                     ty::PredicateKind::ConstEvaluatable(..) => {
 896                         // Errors for `ConstEvaluatable` predicates show up as
 897                         // `SelectionError::ConstEvalFailure`,
 898                         // not `Unimplemented`.
 899                         span_bug!(
 900                             span,
 901                             "const-evaluatable requirement gave wrong error: `{:?}`",
 902                             obligation
 903                         )
 904                     }
 905
 906                     ty::PredicateKind::ConstEquate(..) => {
 907                         // Errors for `ConstEquate` predicates show up as
 908                         // `SelectionError::ConstEvalFailure`,
 909                         // not `Unimplemented`.
 910                         span_bug!(
 911                             span,
 912                             "const-equate requirement gave wrong error: `{:?}`",
 913                             obligation
 914                         )
 915                     }
 916
 917                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
 918                         span,
 919                         "TypeWellFormedFromEnv predicate should only exist in the environment"
 920                     ),
 921                 }
 922             }
 923
 924             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
 925                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
 926                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
 927
 928                 if expected_trait_ref.self_ty().references_error() {
 929                     return;
 930                 }
 931
 932                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
 933                     return;
 934                 };
 935
 936                 let found_did = match *found_trait_ty.kind() {
 937                     ty::Closure(did, _)
 938                     | ty::Foreign(did)
 939                     | ty::FnDef(did, _)
 940                     | ty::Generator(did, ..) => Some(did),
 941                     ty::Adt(def, _) => Some(def.did()),
 942                     _ => None,
 943                 };
 944
 945                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
 946
 947                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 948                     // We check closures twice, with obligations flowing in different directions,
 949                     // but we want to complain about them only once.
 950                     return;
 951                 }
 952
 953                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 954
 955                 let mut not_tupled = false;
 956
 957                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
 958                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 959                     _ => {
 960                         not_tupled = true;
 961                         vec![ArgKind::empty()]
 962                     }
 963                 };
 964
 965                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 966                 let expected = match expected_ty.kind() {
 967                     ty::Tuple(ref tys) => {
 968                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
 969                     }
 970                     _ => {
 971                         not_tupled = true;
 972                         vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
 973                     }
 974                 };
 975
 976                 // If this is a `Fn` family trait and either the expected or found
 977                 // is not tupled, then fall back to just a regular mismatch error.
 978                 // This shouldn't be common unless manually implementing one of the
 979                 // traits manually, but don't make it more confusing when it does
 980                 // happen.
 981                 if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
 982                     self.report_and_explain_type_error(
 983                         TypeTrace::poly_trait_refs(
 984                             &obligation.cause,
 985                             true,
 986                             expected_trait_ref,
 987                             found_trait_ref,
 988                         ),
 989                         ty::error::TypeError::Mismatch,
 990                     )
 991                 } else if found.len() == expected.len() {
 992                     self.report_closure_arg_mismatch(
 993                         span,
 994                         found_span,
 995                         found_trait_ref,
 996                         expected_trait_ref,
 997                     )
 998                 } else {
 999                     let (closure_span, found) = found_did
1000                         .and_then(|did| {
1001                             let node = self.tcx.hir().get_if_local(did)?;
1002                             let (found_span, found) = self.get_fn_like_arguments(node)?;
1003                             Some((Some(found_span), found))
1004                         })
1005                         .unwrap_or((found_span, found));
1006
1007                     self.report_arg_count_mismatch(
1008                         span,
1009                         closure_span,
1010                         expected,
1011                         found,
1012                         found_trait_ty.is_closure(),
1013                     )
1014                 }
1015             }
1016
1017             TraitNotObjectSafe(did) => {
1018                 let violations = self.tcx.object_safety_violations(did);
1019                 report_object_safety_error(self.tcx, span, did, violations)
1020             }
1021
1022             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1023                 bug!(
1024                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1025                 )
1026             }
1027             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1028                 if !self.tcx.features().generic_const_exprs {
1029                     let mut err = self.tcx.sess.struct_span_err(
1030                         span,
1031                         "constant expression depends on a generic parameter",
1032                     );
1033                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1034                     // issue. However, this is currently not actually possible
1035                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1036                     //
1037                     // Note that with `feature(generic_const_exprs)` this case should not
1038                     // be reachable.
1039                     err.note("this may fail depending on what value the parameter takes");
1040                     err.emit();
1041                     return;
1042                 }
1043
1044                 match obligation.predicate.kind().skip_binder() {
1045                     ty::PredicateKind::ConstEvaluatable(uv) => {
1046                         let mut err =
1047                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1048                         let const_span = self.tcx.def_span(uv.def.did);
1049                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1050                             Ok(snippet) => err.help(&format!(
1051                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1052                                 snippet
1053                             )),
1054                             _ => err.help("consider adding a `where` bound using this expression"),
1055                         };
1056                         err
1057                     }
1058                     _ => {
1059                         span_bug!(
1060                             span,
1061                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1062                         )
1063                     }
1064                 }
1065             }
1066
1067             // Already reported in the query.
1068             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1069                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1070                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1071                 return;
1072             }
1073             // Already reported.
1074             Overflow(OverflowError::Error(_)) => {
1075                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1076                 return;
1077             }
1078             Overflow(_) => {
1079                 bug!("overflow should be handled before the `report_selection_error` path");
1080             }
1081             SelectionError::ErrorReporting => {
1082                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1083             }
1084         };
1085
1086         self.note_obligation_cause(&mut err, &obligation);
1087         self.point_at_returns_when_relevant(&mut err, &obligation);
1088
1089         err.emit();
1090     }
1091
1092     /// Given some node representing a fn-like thing in the HIR map,
1093     /// returns a span and `ArgKind` information that describes the
1094     /// arguments it expects. This can be supplied to
1095     /// `report_arg_count_mismatch`.
1096     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
1097         let sm = self.tcx.sess.source_map();
1098         let hir = self.tcx.hir();
1099         Some(match node {
1100             Node::Expr(&hir::Expr {
1101                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, .. }),
1102                 ..
1103             }) => (
1104                 fn_decl_span,
1105                 hir.body(body)
1106                     .params
1107                     .iter()
1108                     .map(|arg| {
1109                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
1110                             *arg.pat
1111                         {
1112                             Some(ArgKind::Tuple(
1113                                 Some(span),
1114                                 args.iter()
1115                                     .map(|pat| {
1116                                         sm.span_to_snippet(pat.span)
1117                                             .ok()
1118                                             .map(|snippet| (snippet, "_".to_owned()))
1119                                     })
1120                                     .collect::<Option<Vec<_>>>()?,
1121                             ))
1122                         } else {
1123                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
1124                             Some(ArgKind::Arg(name, "_".to_owned()))
1125                         }
1126                     })
1127                     .collect::<Option<Vec<ArgKind>>>()?,
1128             ),
1129             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
1130             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
1131             | Node::TraitItem(&hir::TraitItem {
1132                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
1133             }) => (
1134                 sig.span,
1135                 sig.decl
1136                     .inputs
1137                     .iter()
1138                     .map(|arg| match arg.kind {
1139                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1140                             Some(arg.span),
1141                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
1142                         ),
1143                         _ => ArgKind::empty(),
1144                     })
1145                     .collect::<Vec<ArgKind>>(),
1146             ),
1147             Node::Ctor(ref variant_data) => {
1148                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
1149                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1150             }
1151             _ => panic!("non-FnLike node found: {:?}", node),
1152         })
1153     }
1154
1155     /// Reports an error when the number of arguments needed by a
1156     /// trait match doesn't match the number that the expression
1157     /// provides.
1158     fn report_arg_count_mismatch(
1159         &self,
1160         span: Span,
1161         found_span: Option<Span>,
1162         expected_args: Vec<ArgKind>,
1163         found_args: Vec<ArgKind>,
1164         is_closure: bool,
1165     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
1166         let kind = if is_closure { "closure" } else { "function" };
1167
1168         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1169             let arg_length = arguments.len();
1170             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
1171             match (arg_length, arguments.get(0)) {
1172                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1173                     format!("a single {}-tuple as argument", fields.len())
1174                 }
1175                 _ => format!(
1176                     "{} {}argument{}",
1177                     arg_length,
1178                     if distinct && arg_length > 1 { "distinct " } else { "" },
1179                     pluralize!(arg_length)
1180                 ),
1181             }
1182         };
1183
1184         let expected_str = args_str(&expected_args, &found_args);
1185         let found_str = args_str(&found_args, &expected_args);
1186
1187         let mut err = struct_span_err!(
1188             self.tcx.sess,
1189             span,
1190             E0593,
1191             "{} is expected to take {}, but it takes {}",
1192             kind,
1193             expected_str,
1194             found_str,
1195         );
1196
1197         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1198
1199         if let Some(found_span) = found_span {
1200             err.span_label(found_span, format!("takes {}", found_str));
1201
1202             // move |_| { ... }
1203             // ^^^^^^^^-- def_span
1204             //
1205             // move |_| { ... }
1206             // ^^^^^-- prefix
1207             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1208             // move |_| { ... }
1209             //      ^^^-- pipe_span
1210             let pipe_span =
1211                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
1212
1213             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1214             // found arguments is empty (assume the user just wants to ignore args in this case).
1215             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1216             if found_args.is_empty() && is_closure {
1217                 let underscores = vec!["_"; expected_args.len()].join(", ");
1218                 err.span_suggestion_verbose(
1219                     pipe_span,
1220                     &format!(
1221                         "consider changing the closure to take and ignore the expected argument{}",
1222                         pluralize!(expected_args.len())
1223                     ),
1224                     format!("|{}|", underscores),
1225                     Applicability::MachineApplicable,
1226                 );
1227             }
1228
1229             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1230                 if fields.len() == expected_args.len() {
1231                     let sugg = fields
1232                         .iter()
1233                         .map(|(name, _)| name.to_owned())
1234                         .collect::<Vec<String>>()
1235                         .join(", ");
1236                     err.span_suggestion_verbose(
1237                         found_span,
1238                         "change the closure to take multiple arguments instead of a single tuple",
1239                         format!("|{}|", sugg),
1240                         Applicability::MachineApplicable,
1241                     );
1242                 }
1243             }
1244             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
1245                 && fields.len() == found_args.len()
1246                 && is_closure
1247             {
1248                 let sugg = format!(
1249                     "|({}){}|",
1250                     found_args
1251                         .iter()
1252                         .map(|arg| match arg {
1253                             ArgKind::Arg(name, _) => name.to_owned(),
1254                             _ => "_".to_owned(),
1255                         })
1256                         .collect::<Vec<String>>()
1257                         .join(", "),
1258                     // add type annotations if available
1259                     if found_args.iter().any(|arg| match arg {
1260                         ArgKind::Arg(_, ty) => ty != "_",
1261                         _ => false,
1262                     }) {
1263                         format!(
1264                             ": ({})",
1265                             fields
1266                                 .iter()
1267                                 .map(|(_, ty)| ty.to_owned())
1268                                 .collect::<Vec<String>>()
1269                                 .join(", ")
1270                         )
1271                     } else {
1272                         String::new()
1273                     },
1274                 );
1275                 err.span_suggestion_verbose(
1276                     found_span,
1277                     "change the closure to accept a tuple instead of individual arguments",
1278                     sugg,
1279                     Applicability::MachineApplicable,
1280                 );
1281             }
1282         }
1283
1284         err
1285     }
1286
1287     fn type_implements_fn_trait(
1288         &self,
1289         param_env: ty::ParamEnv<'tcx>,
1290         ty: ty::Binder<'tcx, Ty<'tcx>>,
1291         constness: ty::BoundConstness,
1292         polarity: ty::ImplPolarity,
1293     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
1294         self.commit_if_ok(|_| {
1295             for trait_def_id in [
1296                 self.tcx.lang_items().fn_trait(),
1297                 self.tcx.lang_items().fn_mut_trait(),
1298                 self.tcx.lang_items().fn_once_trait(),
1299             ] {
1300                 let Some(trait_def_id) = trait_def_id else { continue };
1301                 // Make a fresh inference variable so we can determine what the substitutions
1302                 // of the trait are.
1303                 let var = self.next_ty_var(TypeVariableOrigin {
1304                     span: DUMMY_SP,
1305                     kind: TypeVariableOriginKind::MiscVariable,
1306                 });
1307                 let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
1308                 let obligation = Obligation::new(
1309                     ObligationCause::dummy(),
1310                     param_env,
1311                     ty.rebind(ty::TraitPredicate {
1312                         trait_ref: ty::TraitRef::new(trait_def_id, substs),
1313                         constness,
1314                         polarity,
1315                     })
1316                     .to_predicate(self.tcx),
1317                 );
1318                 let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
1319                 fulfill_cx.register_predicate_obligation(self, obligation);
1320                 if fulfill_cx.select_all_or_error(self).is_empty() {
1321                     return Ok((
1322                         ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
1323                             .expect("expected to map DefId to ClosureKind"),
1324                         ty.rebind(self.resolve_vars_if_possible(var)),
1325                     ));
1326                 }
1327             }
1328
1329             Err(())
1330         })
1331     }
1332 }
1333
1334 trait InferCtxtPrivExt<'hir, 'tcx> {
1335     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1336     // `error` occurring implies that `cond` occurs.
1337     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1338
1339     fn report_fulfillment_error(
1340         &self,
1341         error: &FulfillmentError<'tcx>,
1342         body_id: Option<hir::BodyId>,
1343         fallback_has_occurred: bool,
1344     );
1345
1346     fn report_projection_error(
1347         &self,
1348         obligation: &PredicateObligation<'tcx>,
1349         error: &MismatchedProjectionTypes<'tcx>,
1350     );
1351
1352     fn maybe_detailed_projection_msg(
1353         &self,
1354         pred: ty::ProjectionPredicate<'tcx>,
1355         normalized_ty: ty::Term<'tcx>,
1356         expected_ty: ty::Term<'tcx>,
1357     ) -> Option<String>;
1358
1359     fn fuzzy_match_tys(
1360         &self,
1361         a: Ty<'tcx>,
1362         b: Ty<'tcx>,
1363         ignoring_lifetimes: bool,
1364     ) -> Option<CandidateSimilarity>;
1365
1366     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1367
1368     fn find_similar_impl_candidates(
1369         &self,
1370         trait_pred: ty::PolyTraitPredicate<'tcx>,
1371     ) -> Vec<ImplCandidate<'tcx>>;
1372
1373     fn report_similar_impl_candidates(
1374         &self,
1375         impl_candidates: Vec<ImplCandidate<'tcx>>,
1376         trait_ref: ty::PolyTraitRef<'tcx>,
1377         body_id: hir::HirId,
1378         err: &mut Diagnostic,
1379     ) -> bool;
1380
1381     /// Gets the parent trait chain start
1382     fn get_parent_trait_ref(
1383         &self,
1384         code: &ObligationCauseCode<'tcx>,
1385     ) -> Option<(String, Option<Span>)>;
1386
1387     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1388     /// with the same path as `trait_ref`, a help message about
1389     /// a probable version mismatch is added to `err`
1390     fn note_version_mismatch(
1391         &self,
1392         err: &mut Diagnostic,
1393         trait_ref: &ty::PolyTraitRef<'tcx>,
1394     ) -> bool;
1395
1396     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1397     /// `trait_ref`.
1398     ///
1399     /// For this to work, `new_self_ty` must have no escaping bound variables.
1400     fn mk_trait_obligation_with_new_self_ty(
1401         &self,
1402         param_env: ty::ParamEnv<'tcx>,
1403         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1404     ) -> PredicateObligation<'tcx>;
1405
1406     fn maybe_report_ambiguity(
1407         &self,
1408         obligation: &PredicateObligation<'tcx>,
1409         body_id: Option<hir::BodyId>,
1410     );
1411
1412     fn predicate_can_apply(
1413         &self,
1414         param_env: ty::ParamEnv<'tcx>,
1415         pred: ty::PolyTraitRef<'tcx>,
1416     ) -> bool;
1417
1418     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1419
1420     fn suggest_unsized_bound_if_applicable(
1421         &self,
1422         err: &mut Diagnostic,
1423         obligation: &PredicateObligation<'tcx>,
1424     );
1425
1426     fn annotate_source_of_ambiguity(
1427         &self,
1428         err: &mut Diagnostic,
1429         impls: &[DefId],
1430         predicate: ty::Predicate<'tcx>,
1431     );
1432
1433     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'hir>);
1434
1435     fn maybe_indirection_for_unsized(
1436         &self,
1437         err: &mut Diagnostic,
1438         item: &'hir Item<'hir>,
1439         param: &'hir GenericParam<'hir>,
1440     ) -> bool;
1441
1442     fn is_recursive_obligation(
1443         &self,
1444         obligated_types: &mut Vec<Ty<'tcx>>,
1445         cause_code: &ObligationCauseCode<'tcx>,
1446     ) -> bool;
1447 }
1448
1449 impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
1450     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1451     // `error` occurring implies that `cond` occurs.
1452     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1453         if cond == error {
1454             return true;
1455         }
1456
1457         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1458         let bound_error = error.kind();
1459         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1460             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
1461                 (cond, bound_error.rebind(error))
1462             }
1463             _ => {
1464                 // FIXME: make this work in other cases too.
1465                 return false;
1466             }
1467         };
1468
1469         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1470             let bound_predicate = obligation.predicate.kind();
1471             if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
1472                 let error = error.to_poly_trait_ref();
1473                 let implication = bound_predicate.rebind(implication.trait_ref);
1474                 // FIXME: I'm just not taking associated types at all here.
1475                 // Eventually I'll need to implement param-env-aware
1476                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1477                 let param_env = ty::ParamEnv::empty();
1478                 if self.can_sub(param_env, error, implication).is_ok() {
1479                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1480                     return true;
1481                 }
1482             }
1483         }
1484
1485         false
1486     }
1487
1488     #[instrument(skip(self), level = "debug")]
1489     fn report_fulfillment_error(
1490         &self,
1491         error: &FulfillmentError<'tcx>,
1492         body_id: Option<hir::BodyId>,
1493         fallback_has_occurred: bool,
1494     ) {
1495         match error.code {
1496             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1497                 self.report_selection_error(
1498                     error.obligation.clone(),
1499                     &error.root_obligation,
1500                     selection_error,
1501                     fallback_has_occurred,
1502                 );
1503             }
1504             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1505                 self.report_projection_error(&error.obligation, e);
1506             }
1507             FulfillmentErrorCode::CodeAmbiguity => {
1508                 self.maybe_report_ambiguity(&error.obligation, body_id);
1509             }
1510             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1511                 self.report_mismatched_types(
1512                     &error.obligation.cause,
1513                     expected_found.expected,
1514                     expected_found.found,
1515                     err.clone(),
1516                 )
1517                 .emit();
1518             }
1519             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1520                 let mut diag = self.report_mismatched_consts(
1521                     &error.obligation.cause,
1522                     expected_found.expected,
1523                     expected_found.found,
1524                     err.clone(),
1525                 );
1526                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
1527                 if let ObligationCauseCode::BindingObligation(..)
1528                 | ObligationCauseCode::ItemObligation(..)
1529                 | ObligationCauseCode::ExprBindingObligation(..)
1530                 | ObligationCauseCode::ExprItemObligation(..) = code
1531                 {
1532                     self.note_obligation_cause_code(
1533                         &mut diag,
1534                         &error.obligation.predicate,
1535                         error.obligation.param_env,
1536                         code,
1537                         &mut vec![],
1538                         &mut Default::default(),
1539                     );
1540                 }
1541                 diag.emit();
1542             }
1543         }
1544     }
1545
1546     #[instrument(level = "debug", skip_all)]
1547     fn report_projection_error(
1548         &self,
1549         obligation: &PredicateObligation<'tcx>,
1550         error: &MismatchedProjectionTypes<'tcx>,
1551     ) {
1552         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1553
1554         if predicate.references_error() {
1555             return;
1556         }
1557
1558         self.probe(|_| {
1559             let mut err = error.err;
1560             let mut values = None;
1561
1562             // try to find the mismatched types to report the error with.
1563             //
1564             // this can fail if the problem was higher-ranked, in which
1565             // cause I have no idea for a good error message.
1566             let bound_predicate = predicate.kind();
1567             if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
1568                 let mut selcx = SelectionContext::new(self);
1569                 let data = self.replace_bound_vars_with_fresh_vars(
1570                     obligation.cause.span,
1571                     infer::LateBoundRegionConversionTime::HigherRankedType,
1572                     bound_predicate.rebind(data),
1573                 );
1574                 let mut obligations = vec![];
1575                 let normalized_ty = super::normalize_projection_type(
1576                     &mut selcx,
1577                     obligation.param_env,
1578                     data.projection_ty,
1579                     obligation.cause.clone(),
1580                     0,
1581                     &mut obligations,
1582                 );
1583
1584                 debug!(?obligation.cause, ?obligation.param_env);
1585
1586                 debug!(?normalized_ty, data.ty = ?data.term);
1587
1588                 let is_normalized_ty_expected = !matches!(
1589                     obligation.cause.code().peel_derives(),
1590                     ObligationCauseCode::ItemObligation(_)
1591                         | ObligationCauseCode::BindingObligation(_, _)
1592                         | ObligationCauseCode::ExprItemObligation(..)
1593                         | ObligationCauseCode::ExprBindingObligation(..)
1594                         | ObligationCauseCode::ObjectCastObligation(..)
1595                         | ObligationCauseCode::OpaqueType
1596                 );
1597                 if let Err(new_err) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1598                     is_normalized_ty_expected,
1599                     normalized_ty,
1600                     data.term,
1601                 ) {
1602                     values = Some((data, is_normalized_ty_expected, normalized_ty, data.term));
1603                     err = new_err;
1604                 }
1605             }
1606
1607             let msg = values
1608                 .and_then(|(predicate, _, normalized_ty, expected_ty)| {
1609                     self.maybe_detailed_projection_msg(predicate, normalized_ty, expected_ty)
1610                 })
1611                 .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
1612             let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
1613
1614             let secondary_span = match predicate.kind().skip_binder() {
1615                 ty::PredicateKind::Projection(proj) => self
1616                     .tcx
1617                     .opt_associated_item(proj.projection_ty.item_def_id)
1618                     .and_then(|trait_assoc_item| {
1619                         self.tcx
1620                             .trait_of_item(proj.projection_ty.item_def_id)
1621                             .map(|id| (trait_assoc_item, id))
1622                     })
1623                     .and_then(|(trait_assoc_item, id)| {
1624                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1625                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1626                             self.tcx
1627                                 .associated_items(did)
1628                                 .in_definition_order()
1629                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1630                         })
1631                     })
1632                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1633                         Some(
1634                             hir::Node::TraitItem(hir::TraitItem {
1635                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1636                                 ..
1637                             })
1638                             | hir::Node::ImplItem(hir::ImplItem {
1639                                 kind: hir::ImplItemKind::TyAlias(ty),
1640                                 ..
1641                             }),
1642                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1643                         _ => None,
1644                     }),
1645                 _ => None,
1646             };
1647             self.note_type_err(
1648                 &mut diag,
1649                 &obligation.cause,
1650                 secondary_span,
1651                 values.map(|(_, is_normalized_ty_expected, normalized_ty, term)| {
1652                     infer::ValuePairs::Terms(ExpectedFound::new(
1653                         is_normalized_ty_expected,
1654                         normalized_ty,
1655                         term,
1656                     ))
1657                 }),
1658                 err,
1659                 true,
1660                 false,
1661             );
1662             self.note_obligation_cause(&mut diag, obligation);
1663             diag.emit();
1664         });
1665     }
1666
1667     fn maybe_detailed_projection_msg(
1668         &self,
1669         pred: ty::ProjectionPredicate<'tcx>,
1670         normalized_ty: ty::Term<'tcx>,
1671         expected_ty: ty::Term<'tcx>,
1672     ) -> Option<String> {
1673         let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
1674         let self_ty = pred.projection_ty.self_ty();
1675
1676         if Some(pred.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output() {
1677             Some(format!(
1678                 "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it returns `{normalized_ty}`",
1679                 fn_kind = self_ty.prefix_string(self.tcx)
1680             ))
1681         } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
1682             Some(format!(
1683                 "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it resolves to `{normalized_ty}`"
1684             ))
1685         } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
1686             Some(format!(
1687                 "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it yields `{normalized_ty}`"
1688             ))
1689         } else {
1690             None
1691         }
1692     }
1693
1694     fn fuzzy_match_tys(
1695         &self,
1696         mut a: Ty<'tcx>,
1697         mut b: Ty<'tcx>,
1698         ignoring_lifetimes: bool,
1699     ) -> Option<CandidateSimilarity> {
1700         /// returns the fuzzy category of a given type, or None
1701         /// if the type can be equated to any type.
1702         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1703             match t.kind() {
1704                 ty::Bool => Some(0),
1705                 ty::Char => Some(1),
1706                 ty::Str => Some(2),
1707                 ty::Adt(def, _) if tcx.is_diagnostic_item(sym::String, def.did()) => Some(2),
1708                 ty::Int(..)
1709                 | ty::Uint(..)
1710                 | ty::Float(..)
1711                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1712                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1713                 ty::Array(..) | ty::Slice(..) => Some(6),
1714                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1715                 ty::Dynamic(..) => Some(8),
1716                 ty::Closure(..) => Some(9),
1717                 ty::Tuple(..) => Some(10),
1718                 ty::Param(..) => Some(11),
1719                 ty::Projection(..) => Some(12),
1720                 ty::Opaque(..) => Some(13),
1721                 ty::Never => Some(14),
1722                 ty::Adt(..) => Some(15),
1723                 ty::Generator(..) => Some(16),
1724                 ty::Foreign(..) => Some(17),
1725                 ty::GeneratorWitness(..) => Some(18),
1726                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1727             }
1728         }
1729
1730         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1731             loop {
1732                 match t.kind() {
1733                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1734                         t = *inner
1735                     }
1736                     _ => break t,
1737                 }
1738             }
1739         };
1740
1741         if !ignoring_lifetimes {
1742             a = strip_references(a);
1743             b = strip_references(b);
1744         }
1745
1746         let cat_a = type_category(self.tcx, a)?;
1747         let cat_b = type_category(self.tcx, b)?;
1748         if a == b {
1749             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1750         } else if cat_a == cat_b {
1751             match (a.kind(), b.kind()) {
1752                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1753                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1754                 // Matching on references results in a lot of unhelpful
1755                 // suggestions, so let's just not do that for now.
1756                 //
1757                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1758                 // to prioritize that impl.
1759                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1760                     self.fuzzy_match_tys(a, b, true).is_some()
1761                 }
1762                 _ => true,
1763             }
1764             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1765         } else if ignoring_lifetimes {
1766             None
1767         } else {
1768             self.fuzzy_match_tys(a, b, true)
1769         }
1770     }
1771
1772     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1773         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1774             hir::GeneratorKind::Gen => "a generator",
1775             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1776             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1777             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1778         })
1779     }
1780
1781     fn find_similar_impl_candidates(
1782         &self,
1783         trait_pred: ty::PolyTraitPredicate<'tcx>,
1784     ) -> Vec<ImplCandidate<'tcx>> {
1785         self.tcx
1786             .all_impls(trait_pred.def_id())
1787             .filter_map(|def_id| {
1788                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
1789                     || !trait_pred
1790                         .skip_binder()
1791                         .is_constness_satisfied_by(self.tcx.constness(def_id))
1792                 {
1793                     return None;
1794                 }
1795
1796                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1797
1798                 self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
1799                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1800             })
1801             .collect()
1802     }
1803
1804     fn report_similar_impl_candidates(
1805         &self,
1806         impl_candidates: Vec<ImplCandidate<'tcx>>,
1807         trait_ref: ty::PolyTraitRef<'tcx>,
1808         body_id: hir::HirId,
1809         err: &mut Diagnostic,
1810     ) -> bool {
1811         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1812             candidates.sort();
1813             candidates.dedup();
1814             let len = candidates.len();
1815             if candidates.len() == 0 {
1816                 return false;
1817             }
1818             if candidates.len() == 1 {
1819                 let ty_desc = match candidates[0].self_ty().kind() {
1820                     ty::FnPtr(_) => Some("fn pointer"),
1821                     _ => None,
1822                 };
1823                 let the_desc = match ty_desc {
1824                     Some(desc) => format!(" implemented for {} `", desc),
1825                     None => " implemented for `".to_string(),
1826                 };
1827                 err.highlighted_help(vec![
1828                     (
1829                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1830                         Style::NoStyle,
1831                     ),
1832                     ("is".to_string(), Style::Highlight),
1833                     (the_desc, Style::NoStyle),
1834                     (candidates[0].self_ty().to_string(), Style::Highlight),
1835                     ("`".to_string(), Style::NoStyle),
1836                 ]);
1837                 return true;
1838             }
1839             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1840             // Check if the trait is the same in all cases. If so, we'll only show the type.
1841             let mut traits: Vec<_> =
1842                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1843             traits.sort();
1844             traits.dedup();
1845
1846             let mut candidates: Vec<String> = candidates
1847                 .into_iter()
1848                 .map(|c| {
1849                     if traits.len() == 1 {
1850                         format!("\n  {}", c.self_ty())
1851                     } else {
1852                         format!("\n  {}", c)
1853                     }
1854                 })
1855                 .collect();
1856
1857             candidates.sort();
1858             candidates.dedup();
1859             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1860             err.help(&format!(
1861                 "the following other types implement trait `{}`:{}{}",
1862                 trait_ref.print_only_trait_path(),
1863                 candidates[..end].join(""),
1864                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1865             ));
1866             true
1867         };
1868
1869         let def_id = trait_ref.def_id();
1870         if impl_candidates.is_empty() {
1871             if self.tcx.trait_is_auto(def_id)
1872                 || self.tcx.lang_items().items().contains(&Some(def_id))
1873                 || self.tcx.get_diagnostic_name(def_id).is_some()
1874             {
1875                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1876                 return false;
1877             }
1878             let normalized_impl_candidates: Vec<_> = self
1879                 .tcx
1880                 .all_impls(def_id)
1881                 // Ignore automatically derived impls and `!Trait` impls.
1882                 .filter(|&def_id| {
1883                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1884                         || self.tcx.is_builtin_derive(def_id)
1885                 })
1886                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1887                 .filter(|trait_ref| {
1888                     let self_ty = trait_ref.self_ty();
1889                     // Avoid mentioning type parameters.
1890                     if let ty::Param(_) = self_ty.kind() {
1891                         false
1892                     }
1893                     // Avoid mentioning types that are private to another crate
1894                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
1895                         // FIXME(compiler-errors): This could be generalized, both to
1896                         // be more granular, and probably look past other `#[fundamental]`
1897                         // types, too.
1898                         self.tcx
1899                             .visibility(def.did())
1900                             .is_accessible_from(body_id.owner.to_def_id(), self.tcx)
1901                     } else {
1902                         true
1903                     }
1904                 })
1905                 .collect();
1906             return report(normalized_impl_candidates, err);
1907         }
1908
1909         let normalize = |candidate| {
1910             self.tcx.infer_ctxt().enter(|ref infcx| {
1911                 let normalized = infcx
1912                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1913                     .normalize(candidate)
1914                     .ok();
1915                 match normalized {
1916                     Some(normalized) => normalized.value,
1917                     None => candidate,
1918                 }
1919             })
1920         };
1921
1922         // Sort impl candidates so that ordering is consistent for UI tests.
1923         // because the ordering of `impl_candidates` may not be deterministic:
1924         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1925         //
1926         // Prefer more similar candidates first, then sort lexicographically
1927         // by their normalized string representation.
1928         let mut normalized_impl_candidates_and_similarities = impl_candidates
1929             .into_iter()
1930             .map(|ImplCandidate { trait_ref, similarity }| {
1931                 let normalized = normalize(trait_ref);
1932                 (similarity, normalized)
1933             })
1934             .collect::<Vec<_>>();
1935         normalized_impl_candidates_and_similarities.sort();
1936         normalized_impl_candidates_and_similarities.dedup();
1937
1938         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
1939             .into_iter()
1940             .map(|(_, normalized)| normalized)
1941             .collect::<Vec<_>>();
1942
1943         report(normalized_impl_candidates, err)
1944     }
1945
1946     /// Gets the parent trait chain start
1947     fn get_parent_trait_ref(
1948         &self,
1949         code: &ObligationCauseCode<'tcx>,
1950     ) -> Option<(String, Option<Span>)> {
1951         match code {
1952             ObligationCauseCode::BuiltinDerivedObligation(data) => {
1953                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
1954                 match self.get_parent_trait_ref(&data.parent_code) {
1955                     Some(t) => Some(t),
1956                     None => {
1957                         let ty = parent_trait_ref.skip_binder().self_ty();
1958                         let span = TyCategory::from_ty(self.tcx, ty)
1959                             .map(|(_, def_id)| self.tcx.def_span(def_id));
1960                         Some((ty.to_string(), span))
1961                     }
1962                 }
1963             }
1964             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
1965                 self.get_parent_trait_ref(&parent_code)
1966             }
1967             _ => None,
1968         }
1969     }
1970
1971     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1972     /// with the same path as `trait_ref`, a help message about
1973     /// a probable version mismatch is added to `err`
1974     fn note_version_mismatch(
1975         &self,
1976         err: &mut Diagnostic,
1977         trait_ref: &ty::PolyTraitRef<'tcx>,
1978     ) -> bool {
1979         let get_trait_impl = |trait_def_id| {
1980             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
1981         };
1982         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1983         let traits_with_same_path: std::collections::BTreeSet<_> = self
1984             .tcx
1985             .all_traits()
1986             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
1987             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
1988             .collect();
1989         let mut suggested = false;
1990         for trait_with_same_path in traits_with_same_path {
1991             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
1992                 let impl_span = self.tcx.def_span(impl_def_id);
1993                 err.span_help(impl_span, "trait impl with same name found");
1994                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1995                 let crate_msg = format!(
1996                     "perhaps two different versions of crate `{}` are being used?",
1997                     trait_crate
1998                 );
1999                 err.note(&crate_msg);
2000                 suggested = true;
2001             }
2002         }
2003         suggested
2004     }
2005
2006     fn mk_trait_obligation_with_new_self_ty(
2007         &self,
2008         param_env: ty::ParamEnv<'tcx>,
2009         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
2010     ) -> PredicateObligation<'tcx> {
2011         let trait_pred = trait_ref_and_ty.map_bound_ref(|(tr, new_self_ty)| ty::TraitPredicate {
2012             trait_ref: ty::TraitRef {
2013                 substs: self.tcx.mk_substs_trait(*new_self_ty, &tr.trait_ref.substs[1..]),
2014                 ..tr.trait_ref
2015             },
2016             ..*tr
2017         });
2018
2019         Obligation::new(ObligationCause::dummy(), param_env, trait_pred.to_predicate(self.tcx))
2020     }
2021
2022     #[instrument(skip(self), level = "debug")]
2023     fn maybe_report_ambiguity(
2024         &self,
2025         obligation: &PredicateObligation<'tcx>,
2026         body_id: Option<hir::BodyId>,
2027     ) {
2028         // Unable to successfully determine, probably means
2029         // insufficient type information, but could mean
2030         // ambiguous impls. The latter *ought* to be a
2031         // coherence violation, so we don't report it here.
2032
2033         let predicate = self.resolve_vars_if_possible(obligation.predicate);
2034         let span = obligation.cause.span;
2035
2036         debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
2037
2038         // Ambiguity errors are often caused as fallout from earlier errors.
2039         // We ignore them if this `infcx` is tainted in some cases below.
2040
2041         let bound_predicate = predicate.kind();
2042         let mut err = match bound_predicate.skip_binder() {
2043             ty::PredicateKind::Trait(data) => {
2044                 let trait_ref = bound_predicate.rebind(data.trait_ref);
2045                 debug!(?trait_ref);
2046
2047                 if predicate.references_error() {
2048                     return;
2049                 }
2050
2051                 // This is kind of a hack: it frequently happens that some earlier
2052                 // error prevents types from being fully inferred, and then we get
2053                 // a bunch of uninteresting errors saying something like "<generic
2054                 // #0> doesn't implement Sized".  It may even be true that we
2055                 // could just skip over all checks where the self-ty is an
2056                 // inference variable, but I was afraid that there might be an
2057                 // inference variable created, registered as an obligation, and
2058                 // then never forced by writeback, and hence by skipping here we'd
2059                 // be ignoring the fact that we don't KNOW the type works
2060                 // out. Though even that would probably be harmless, given that
2061                 // we're only talking about builtin traits, which are known to be
2062                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2063                 // avoid inundating the user with unnecessary errors, but we now
2064                 // check upstream for type errors and don't add the obligations to
2065                 // begin with in those cases.
2066                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2067                     if !self.is_tainted_by_errors() {
2068                         self.emit_inference_failure_err(
2069                             body_id,
2070                             span,
2071                             trait_ref.self_ty().skip_binder().into(),
2072                             ErrorCode::E0282,
2073                             false,
2074                         )
2075                         .emit();
2076                     }
2077                     return;
2078                 }
2079
2080                 // Typically, this ambiguity should only happen if
2081                 // there are unresolved type inference variables
2082                 // (otherwise it would suggest a coherence
2083                 // failure). But given #21974 that is not necessarily
2084                 // the case -- we can have multiple where clauses that
2085                 // are only distinguished by a region, which results
2086                 // in an ambiguity even when all types are fully
2087                 // known, since we don't dispatch based on region
2088                 // relationships.
2089
2090                 // Pick the first substitution that still contains inference variables as the one
2091                 // we're going to emit an error for. If there are none (see above), fall back to
2092                 // a more general error.
2093                 let subst = data.trait_ref.substs.iter().find(|s| s.has_infer_types_or_consts());
2094
2095                 let mut err = if let Some(subst) = subst {
2096                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2097                 } else {
2098                     struct_span_err!(
2099                         self.tcx.sess,
2100                         span,
2101                         E0283,
2102                         "type annotations needed: cannot satisfy `{}`",
2103                         predicate,
2104                     )
2105                 };
2106
2107                 let obligation = Obligation::new(
2108                     obligation.cause.clone(),
2109                     obligation.param_env,
2110                     trait_ref.to_poly_trait_predicate(),
2111                 );
2112                 let mut selcx = SelectionContext::with_query_mode(
2113                     &self,
2114                     crate::traits::TraitQueryMode::Standard,
2115                 );
2116                 match selcx.select_from_obligation(&obligation) {
2117                     Err(SelectionError::Ambiguous(impls)) if impls.len() > 1 => {
2118                         self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2119                     }
2120                     _ => {
2121                         if self.is_tainted_by_errors() {
2122                             err.cancel();
2123                             return;
2124                         }
2125                         err.note(&format!("cannot satisfy `{}`", predicate));
2126                     }
2127                 }
2128
2129                 if let ObligationCauseCode::ItemObligation(def_id) | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code() {
2130                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2131                 } else if let Ok(snippet) = &self.tcx.sess.source_map().span_to_snippet(span)
2132                     && let ObligationCauseCode::BindingObligation(def_id, _) | ObligationCauseCode::ExprBindingObligation(def_id, ..)
2133                         = *obligation.cause.code()
2134                 {
2135                     let generics = self.tcx.generics_of(def_id);
2136                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2137                         && !snippet.ends_with('>')
2138                         && !generics.has_impl_trait()
2139                         && !self.tcx.fn_trait_kind_from_lang_item(def_id).is_some()
2140                     {
2141                         // FIXME: To avoid spurious suggestions in functions where type arguments
2142                         // where already supplied, we check the snippet to make sure it doesn't
2143                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2144                         // instead. Otherwise we would produce the following output:
2145                         //
2146                         // error[E0283]: type annotations needed
2147                         //   --> $DIR/issue-54954.rs:3:24
2148                         //    |
2149                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2150                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2151                         //    |                        |
2152                         //    |                        cannot infer type
2153                         //    |                        help: consider specifying the type argument
2154                         //    |                        in the function call:
2155                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2156                         // ...
2157                         // LL |     const fn const_val<T: Sized>() -> usize {
2158                         //    |                        - required by this bound in `Tt::const_val`
2159                         //    |
2160                         //    = note: cannot satisfy `_: Tt`
2161
2162                         // Clear any more general suggestions in favor of our specific one
2163                         err.clear_suggestions();
2164
2165                         err.span_suggestion_verbose(
2166                             span.shrink_to_hi(),
2167                             &format!(
2168                                 "consider specifying the type argument{} in the function call",
2169                                 pluralize!(generics.params.len()),
2170                             ),
2171                             format!(
2172                                 "::<{}>",
2173                                 generics
2174                                     .params
2175                                     .iter()
2176                                     .map(|p| p.name.to_string())
2177                                     .collect::<Vec<String>>()
2178                                     .join(", ")
2179                             ),
2180                             Applicability::HasPlaceholders,
2181                         );
2182                     }
2183                 }
2184
2185                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2186                     (body_id, subst.map(|subst| subst.unpack()))
2187                 {
2188                     struct FindExprBySpan<'hir> {
2189                         span: Span,
2190                         result: Option<&'hir hir::Expr<'hir>>,
2191                     }
2192
2193                     impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
2194                         fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2195                             if self.span == ex.span {
2196                                 self.result = Some(ex);
2197                             } else {
2198                                 hir::intravisit::walk_expr(self, ex);
2199                             }
2200                         }
2201                     }
2202
2203                     let mut expr_finder = FindExprBySpan { span, result: None };
2204
2205                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2206
2207                     if let Some(hir::Expr {
2208                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2209                     ) = expr_finder.result
2210                         && let [
2211                             ..,
2212                             trait_path_segment @ hir::PathSegment {
2213                                 res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
2214                                 ..
2215                             },
2216                             hir::PathSegment {
2217                                 ident: assoc_item_name,
2218                                 res: rustc_hir::def::Res::Def(_, item_id),
2219                                 ..
2220                             }
2221                         ] = path.segments
2222                         && data.trait_ref.def_id == *trait_id
2223                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
2224                         && !self.is_tainted_by_errors()
2225                     {
2226                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2227                             ty::AssocKind::Const => ("refer to the", "constant"),
2228                             ty::AssocKind::Fn => ("call", "function"),
2229                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2230                         };
2231
2232                         // Replace the more general E0283 with a more specific error
2233                         err.cancel();
2234                         err = self.tcx.sess.struct_span_err_with_code(
2235                             span,
2236                             &format!(
2237                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2238                              ),
2239                             rustc_errors::error_code!(E0790),
2240                         );
2241
2242                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2243                             && let Some(hir::Node::Item(hir::Item { ident: trait_name, kind: hir::ItemKind::Trait(_, _, _, _, trait_item_refs), .. })) = self.tcx.hir().find_by_def_id(local_def_id)
2244                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2245                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2246                         }
2247
2248                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2249
2250                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2251
2252                         if trait_impls.blanket_impls().is_empty()
2253                             && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
2254                             && let Some(impl_def_id) = impl_ty.def() {
2255                             let message = if trait_impls.non_blanket_impls().len() == 1 {
2256                                 "use the fully-qualified path to the only available implementation".to_string()
2257                             } else {
2258                                 format!(
2259                                     "use a fully-qualified path to a specific available implementation ({} found)",
2260                                     trait_impls.non_blanket_impls().len()
2261                                 )
2262                             };
2263
2264                             err.multipart_suggestion(
2265                                 message,
2266                                 vec![
2267                                     (trait_path_segment.ident.span.shrink_to_lo(), format!("<{} as ", self.tcx.def_path(impl_def_id).to_string_no_crate_verbose())),
2268                                     (trait_path_segment.ident.span.shrink_to_hi(), format!(">"))
2269                                 ],
2270                                 Applicability::MaybeIncorrect
2271                             );
2272                         }
2273                     }
2274                 };
2275
2276                 err
2277             }
2278
2279             ty::PredicateKind::WellFormed(arg) => {
2280                 // Same hacky approach as above to avoid deluging user
2281                 // with error messages.
2282                 if arg.references_error()
2283                     || self.tcx.sess.has_errors().is_some()
2284                     || self.is_tainted_by_errors()
2285                 {
2286                     return;
2287                 }
2288
2289                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2290             }
2291
2292             ty::PredicateKind::Subtype(data) => {
2293                 if data.references_error()
2294                     || self.tcx.sess.has_errors().is_some()
2295                     || self.is_tainted_by_errors()
2296                 {
2297                     // no need to overload user in such cases
2298                     return;
2299                 }
2300                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2301                 // both must be type variables, or the other would've been instantiated
2302                 assert!(a.is_ty_var() && b.is_ty_var());
2303                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2304             }
2305             ty::PredicateKind::Projection(data) => {
2306                 if predicate.references_error() || self.is_tainted_by_errors() {
2307                     return;
2308                 }
2309                 let subst = data
2310                     .projection_ty
2311                     .substs
2312                     .iter()
2313                     .chain(Some(data.term.into_arg()))
2314                     .find(|g| g.has_infer_types_or_consts());
2315                 if let Some(subst) = subst {
2316                     let mut err = self.emit_inference_failure_err(
2317                         body_id,
2318                         span,
2319                         subst,
2320                         ErrorCode::E0284,
2321                         true,
2322                     );
2323                     err.note(&format!("cannot satisfy `{}`", predicate));
2324                     err
2325                 } else {
2326                     // If we can't find a substitution, just print a generic error
2327                     let mut err = struct_span_err!(
2328                         self.tcx.sess,
2329                         span,
2330                         E0284,
2331                         "type annotations needed: cannot satisfy `{}`",
2332                         predicate,
2333                     );
2334                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2335                     err
2336                 }
2337             }
2338
2339             ty::PredicateKind::ConstEvaluatable(data) => {
2340                 if predicate.references_error() || self.is_tainted_by_errors() {
2341                     return;
2342                 }
2343                 let subst = data.substs.iter().find(|g| g.has_infer_types_or_consts());
2344                 if let Some(subst) = subst {
2345                     let err = self.emit_inference_failure_err(
2346                         body_id,
2347                         span,
2348                         subst,
2349                         ErrorCode::E0284,
2350                         true,
2351                     );
2352                     err
2353                 } else {
2354                     // If we can't find a substitution, just print a generic error
2355                     let mut err = struct_span_err!(
2356                         self.tcx.sess,
2357                         span,
2358                         E0284,
2359                         "type annotations needed: cannot satisfy `{}`",
2360                         predicate,
2361                     );
2362                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2363                     err
2364                 }
2365             }
2366             _ => {
2367                 if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
2368                     return;
2369                 }
2370                 let mut err = struct_span_err!(
2371                     self.tcx.sess,
2372                     span,
2373                     E0284,
2374                     "type annotations needed: cannot satisfy `{}`",
2375                     predicate,
2376                 );
2377                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2378                 err
2379             }
2380         };
2381         self.note_obligation_cause(&mut err, obligation);
2382         err.emit();
2383     }
2384
2385     fn annotate_source_of_ambiguity(
2386         &self,
2387         err: &mut Diagnostic,
2388         impls: &[DefId],
2389         predicate: ty::Predicate<'tcx>,
2390     ) {
2391         let mut spans = vec![];
2392         let mut crates = vec![];
2393         let mut post = vec![];
2394         for def_id in impls {
2395             match self.tcx.span_of_impl(*def_id) {
2396                 Ok(span) => spans.push(span),
2397                 Err(name) => {
2398                     crates.push(name);
2399                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2400                         post.push(header);
2401                     }
2402                 }
2403             }
2404         }
2405         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2406         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2407         crate_names.sort();
2408         crate_names.dedup();
2409         post.sort();
2410         post.dedup();
2411
2412         if self.is_tainted_by_errors()
2413             && (crate_names.len() == 1
2414                 && spans.len() == 0
2415                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2416                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2417         {
2418             // Avoid complaining about other inference issues for expressions like
2419             // `42 >> 1`, where the types are still `{integer}`, but we want to
2420             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2421             // NOTE(eddyb) this was `.cancel()`, but `err`
2422             // is borrowed, so we can't fully defuse it.
2423             err.downgrade_to_delayed_bug();
2424             return;
2425         }
2426         let post = if post.len() > 4 {
2427             format!(
2428                 ":\n{}\nand {} more",
2429                 post.iter().map(|p| format!("- {}", p)).take(4).collect::<Vec<_>>().join("\n"),
2430                 post.len() - 4,
2431             )
2432         } else if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2433             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2434         } else if post.len() == 1 {
2435             format!(": `{}`", post[0])
2436         } else {
2437             String::new()
2438         };
2439
2440         match (spans.len(), crates.len(), crate_names.len()) {
2441             (0, 0, 0) => {
2442                 err.note(&format!("cannot satisfy `{}`", predicate));
2443             }
2444             (0, _, 1) => {
2445                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2446             }
2447             (0, _, _) => {
2448                 err.note(&format!(
2449                     "{} in the following crates: {}{}",
2450                     msg,
2451                     crate_names.join(", "),
2452                     post,
2453                 ));
2454             }
2455             (_, 0, 0) => {
2456                 let span: MultiSpan = spans.into();
2457                 err.span_note(span, &msg);
2458             }
2459             (_, 1, 1) => {
2460                 let span: MultiSpan = spans.into();
2461                 err.span_note(span, &msg);
2462                 err.note(
2463                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2464                 );
2465             }
2466             _ => {
2467                 let span: MultiSpan = spans.into();
2468                 err.span_note(span, &msg);
2469                 err.note(&format!(
2470                     "and more `impl`s found in the following crates: {}{}",
2471                     crate_names.join(", "),
2472                     post,
2473                 ));
2474             }
2475         }
2476     }
2477
2478     /// Returns `true` if the trait predicate may apply for *some* assignment
2479     /// to the type parameters.
2480     fn predicate_can_apply(
2481         &self,
2482         param_env: ty::ParamEnv<'tcx>,
2483         pred: ty::PolyTraitRef<'tcx>,
2484     ) -> bool {
2485         struct ParamToVarFolder<'a, 'tcx> {
2486             infcx: &'a InferCtxt<'a, 'tcx>,
2487             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2488         }
2489
2490         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2491             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2492                 self.infcx.tcx
2493             }
2494
2495             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2496                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2497                     let infcx = self.infcx;
2498                     *self.var_map.entry(ty).or_insert_with(|| {
2499                         infcx.next_ty_var(TypeVariableOrigin {
2500                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2501                             span: DUMMY_SP,
2502                         })
2503                     })
2504                 } else {
2505                     ty.super_fold_with(self)
2506                 }
2507             }
2508         }
2509
2510         self.probe(|_| {
2511             let mut selcx = SelectionContext::new(self);
2512
2513             let cleaned_pred =
2514                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2515
2516             let cleaned_pred = super::project::normalize(
2517                 &mut selcx,
2518                 param_env,
2519                 ObligationCause::dummy(),
2520                 cleaned_pred,
2521             )
2522             .value;
2523
2524             let obligation = Obligation::new(
2525                 ObligationCause::dummy(),
2526                 param_env,
2527                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
2528             );
2529
2530             self.predicate_may_hold(&obligation)
2531         })
2532     }
2533
2534     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2535         // First, attempt to add note to this error with an async-await-specific
2536         // message, and fall back to regular note otherwise.
2537         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2538             self.note_obligation_cause_code(
2539                 err,
2540                 &obligation.predicate,
2541                 obligation.param_env,
2542                 obligation.cause.code(),
2543                 &mut vec![],
2544                 &mut Default::default(),
2545             );
2546             self.suggest_unsized_bound_if_applicable(err, obligation);
2547         }
2548     }
2549
2550     #[instrument(level = "debug", skip_all)]
2551     fn suggest_unsized_bound_if_applicable(
2552         &self,
2553         err: &mut Diagnostic,
2554         obligation: &PredicateObligation<'tcx>,
2555     ) {
2556         let ty::PredicateKind::Trait(pred) = obligation.predicate.kind().skip_binder() else { return; };
2557         let (ObligationCauseCode::BindingObligation(item_def_id, span)
2558         | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
2559             = *obligation.cause.code().peel_derives() else { return; };
2560         debug!(?pred, ?item_def_id, ?span);
2561
2562         let (Some(node), true) = (
2563             self.tcx.hir().get_if_local(item_def_id),
2564             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2565         ) else {
2566             return;
2567         };
2568         self.maybe_suggest_unsized_generics(err, span, node);
2569     }
2570
2571     #[instrument(level = "debug", skip_all)]
2572     fn maybe_suggest_unsized_generics<'hir>(
2573         &self,
2574         err: &mut Diagnostic,
2575         span: Span,
2576         node: Node<'hir>,
2577     ) {
2578         let Some(generics) = node.generics() else {
2579             return;
2580         };
2581         let sized_trait = self.tcx.lang_items().sized_trait();
2582         debug!(?generics.params);
2583         debug!(?generics.predicates);
2584         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2585             return;
2586         };
2587         let param_def_id = self.tcx.hir().local_def_id(param.hir_id);
2588         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2589         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2590         let explicitly_sized = generics
2591             .bounds_for_param(param_def_id)
2592             .flat_map(|bp| bp.bounds)
2593             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2594         if explicitly_sized {
2595             return;
2596         }
2597         debug!(?param);
2598         match node {
2599             hir::Node::Item(
2600                 item @ hir::Item {
2601                     // Only suggest indirection for uses of type parameters in ADTs.
2602                     kind:
2603                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2604                     ..
2605                 },
2606             ) => {
2607                 if self.maybe_indirection_for_unsized(err, item, param) {
2608                     return;
2609                 }
2610             }
2611             _ => {}
2612         };
2613         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2614         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param_def_id)
2615         {
2616             (s, " +")
2617         } else {
2618             (span.shrink_to_hi(), ":")
2619         };
2620         err.span_suggestion_verbose(
2621             span,
2622             "consider relaxing the implicit `Sized` restriction",
2623             format!("{} ?Sized", separator),
2624             Applicability::MachineApplicable,
2625         );
2626     }
2627
2628     fn maybe_indirection_for_unsized<'hir>(
2629         &self,
2630         err: &mut Diagnostic,
2631         item: &'hir Item<'hir>,
2632         param: &'hir GenericParam<'hir>,
2633     ) -> bool {
2634         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2635         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2636         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2637         let mut visitor =
2638             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2639         visitor.visit_item(item);
2640         if visitor.invalid_spans.is_empty() {
2641             return false;
2642         }
2643         let mut multispan: MultiSpan = param.span.into();
2644         multispan.push_span_label(
2645             param.span,
2646             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2647         );
2648         for sp in visitor.invalid_spans {
2649             multispan.push_span_label(
2650                 sp,
2651                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2652             );
2653         }
2654         err.span_help(
2655             multispan,
2656             &format!(
2657                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2658                 used through indirection like `&{T}` or `Box<{T}>`",
2659                 T = param.name.ident(),
2660             ),
2661         );
2662         true
2663     }
2664
2665     fn is_recursive_obligation(
2666         &self,
2667         obligated_types: &mut Vec<Ty<'tcx>>,
2668         cause_code: &ObligationCauseCode<'tcx>,
2669     ) -> bool {
2670         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2671             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2672             let self_ty = parent_trait_ref.skip_binder().self_ty();
2673             if obligated_types.iter().any(|ot| ot == &self_ty) {
2674                 return true;
2675             }
2676             if let ty::Adt(def, substs) = self_ty.kind()
2677                 && let [arg] = &substs[..]
2678                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2679                 && let ty::Adt(inner_def, _) = ty.kind()
2680                 && inner_def == def
2681             {
2682                 return true;
2683             }
2684         }
2685         false
2686     }
2687 }
2688
2689 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2690 /// `param: ?Sized` would be a valid constraint.
2691 struct FindTypeParam {
2692     param: rustc_span::Symbol,
2693     invalid_spans: Vec<Span>,
2694     nested: bool,
2695 }
2696
2697 impl<'v> Visitor<'v> for FindTypeParam {
2698     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2699         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2700     }
2701
2702     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2703         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2704         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2705         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2706         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2707         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2708         // in that case should make what happened clear enough.
2709         match ty.kind {
2710             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2711             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2712                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2713             {
2714                 if !self.nested {
2715                     debug!(?ty, "FindTypeParam::visit_ty");
2716                     self.invalid_spans.push(ty.span);
2717                 }
2718             }
2719             hir::TyKind::Path(_) => {
2720                 let prev = self.nested;
2721                 self.nested = true;
2722                 hir::intravisit::walk_ty(self, ty);
2723                 self.nested = prev;
2724             }
2725             _ => {
2726                 hir::intravisit::walk_ty(self, ty);
2727             }
2728         }
2729     }
2730 }
2731
2732 pub fn recursive_type_with_infinite_size_error<'tcx>(
2733     tcx: TyCtxt<'tcx>,
2734     type_def_id: DefId,
2735     spans: Vec<(Span, Option<hir::HirId>)>,
2736 ) {
2737     assert!(type_def_id.is_local());
2738     let span = tcx.def_span(type_def_id);
2739     let path = tcx.def_path_str(type_def_id);
2740     let mut err =
2741         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
2742     err.span_label(span, "recursive type has infinite size");
2743     for &(span, _) in &spans {
2744         err.span_label(span, "recursive without indirection");
2745     }
2746     let msg = format!(
2747         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
2748         path,
2749     );
2750     if spans.len() <= 4 {
2751         // FIXME(compiler-errors): This suggestion might be erroneous if Box is shadowed
2752         err.multipart_suggestion(
2753             &msg,
2754             spans
2755                 .into_iter()
2756                 .flat_map(|(span, field_id)| {
2757                     if let Some(generic_span) = get_option_generic_from_field_id(tcx, field_id) {
2758                         // If we match an `Option` and can grab the span of the Option's generic, then
2759                         // suggest boxing the generic arg for a non-null niche optimization.
2760                         vec![
2761                             (generic_span.shrink_to_lo(), "Box<".to_string()),
2762                             (generic_span.shrink_to_hi(), ">".to_string()),
2763                         ]
2764                     } else {
2765                         vec![
2766                             (span.shrink_to_lo(), "Box<".to_string()),
2767                             (span.shrink_to_hi(), ">".to_string()),
2768                         ]
2769                     }
2770                 })
2771                 .collect(),
2772             Applicability::HasPlaceholders,
2773         );
2774     } else {
2775         err.help(&msg);
2776     }
2777     err.emit();
2778 }
2779
2780 /// Extract the span for the generic type `T` of `Option<T>` in a field definition
2781 fn get_option_generic_from_field_id(tcx: TyCtxt<'_>, field_id: Option<hir::HirId>) -> Option<Span> {
2782     let node = tcx.hir().find(field_id?);
2783
2784     // Expect a field from our field_id
2785     let Some(hir::Node::Field(field_def)) = node
2786         else { bug!("Expected HirId corresponding to FieldDef, found: {:?}", node) };
2787
2788     // Match a type that is a simple QPath with no Self
2789     let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = &field_def.ty.kind
2790         else { return None };
2791
2792     // Check if the path we're checking resolves to Option
2793     let hir::def::Res::Def(_, did) = path.res
2794         else { return None };
2795
2796     // Bail if this path doesn't describe `::core::option::Option`
2797     if !tcx.is_diagnostic_item(sym::Option, did) {
2798         return None;
2799     }
2800
2801     // Match a single generic arg in the 0th path segment
2802     let generic_arg = path.segments.last()?.args?.args.get(0)?;
2803
2804     // Take the span out of the type, if it's a type
2805     if let hir::GenericArg::Type(generic_ty) = generic_arg { Some(generic_ty.span) } else { None }
2806 }
2807
2808 /// Summarizes information
2809 #[derive(Clone)]
2810 pub enum ArgKind {
2811     /// An argument of non-tuple type. Parameters are (name, ty)
2812     Arg(String, String),
2813
2814     /// An argument of tuple type. For a "found" argument, the span is
2815     /// the location in the source of the pattern. For an "expected"
2816     /// argument, it will be None. The vector is a list of (name, ty)
2817     /// strings for the components of the tuple.
2818     Tuple(Option<Span>, Vec<(String, String)>),
2819 }
2820
2821 impl ArgKind {
2822     fn empty() -> ArgKind {
2823         ArgKind::Arg("_".to_owned(), "_".to_owned())
2824     }
2825
2826     /// Creates an `ArgKind` from the expected type of an
2827     /// argument. It has no name (`_`) and an optional source span.
2828     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2829         match t.kind() {
2830             ty::Tuple(tys) => ArgKind::Tuple(
2831                 span,
2832                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2833             ),
2834             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2835         }
2836     }
2837 }
2838
2839 struct HasNumericInferVisitor;
2840
2841 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2842     type BreakTy = ();
2843
2844     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2845         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2846             ControlFlow::Break(())
2847         } else {
2848             ControlFlow::CONTINUE
2849         }
2850     }
2851 }