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::traits::TraitEngine;
  26 use rustc_middle::traits::select::OverflowError;
  27 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  28 use rustc_middle::ty::error::ExpectedFound;
  29 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  30 use rustc_middle::ty::{
  31     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  32     TypeVisitable,
  33 };
  34 use rustc_span::symbol::{kw, sym};
  35 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  36 use std::fmt;
  37 use std::iter;
  38 use std::ops::ControlFlow;
  39
  40 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  41 use crate::traits::query::normalize::AtExt as _;
  42 use crate::traits::specialize::to_pretty_impl_header;
  43 use on_unimplemented::InferCtxtExt as _;
  44 use suggestions::InferCtxtExt as _;
  45
  46 pub use rustc_infer::traits::error_reporting::*;
  47
  48 // When outputting impl candidates, prefer showing those that are more similar.
  49 //
  50 // We also compare candidates after skipping lifetimes, which has a lower
  51 // priority than exact matches.
  52 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  53 pub enum CandidateSimilarity {
  54     Exact { ignoring_lifetimes: bool },
  55     Fuzzy { ignoring_lifetimes: bool },
  56 }
  57
  58 #[derive(Debug, Clone, Copy)]
  59 pub struct ImplCandidate<'tcx> {
  60     pub trait_ref: ty::TraitRef<'tcx>,
  61     pub similarity: CandidateSimilarity,
  62 }
  63
  64 pub trait InferCtxtExt<'tcx> {
  65     fn report_fulfillment_errors(
  66         &self,
  67         errors: &[FulfillmentError<'tcx>],
  68         body_id: Option<hir::BodyId>,
  69         fallback_has_occurred: bool,
  70     ) -> ErrorGuaranteed;
  71
  72     fn report_overflow_error<T>(
  73         &self,
  74         obligation: &Obligation<'tcx, T>,
  75         suggest_increasing_limit: bool,
  76     ) -> !
  77     where
  78         T: fmt::Display + TypeFoldable<'tcx>;
  79
  80     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
  81
  82     /// The `root_obligation` parameter should be the `root_obligation` field
  83     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
  84     /// then it should be the same as `obligation`.
  85     fn report_selection_error(
  86         &self,
  87         obligation: PredicateObligation<'tcx>,
  88         root_obligation: &PredicateObligation<'tcx>,
  89         error: &SelectionError<'tcx>,
  90         fallback_has_occurred: bool,
  91     );
  92
  93     /// Given some node representing a fn-like thing in the HIR map,
  94     /// returns a span and `ArgKind` information that describes the
  95     /// arguments it expects. This can be supplied to
  96     /// `report_arg_count_mismatch`.
  97     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  98
  99     /// Reports an error when the number of arguments needed by a
 100     /// trait match doesn't match the number that the expression
 101     /// provides.
 102     fn report_arg_count_mismatch(
 103         &self,
 104         span: Span,
 105         found_span: Option<Span>,
 106         expected_args: Vec<ArgKind>,
 107         found_args: Vec<ArgKind>,
 108         is_closure: bool,
 109     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 110
 111     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
 112     /// in that order, and returns the generic type corresponding to the
 113     /// argument of that trait (corresponding to the closure arguments).
 114     fn type_implements_fn_trait(
 115         &self,
 116         param_env: ty::ParamEnv<'tcx>,
 117         ty: ty::Binder<'tcx, Ty<'tcx>>,
 118         constness: ty::BoundConstness,
 119         polarity: ty::ImplPolarity,
 120     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
 121 }
 122
 123 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 124     fn report_fulfillment_errors(
 125         &self,
 126         errors: &[FulfillmentError<'tcx>],
 127         body_id: Option<hir::BodyId>,
 128         fallback_has_occurred: bool,
 129     ) -> ErrorGuaranteed {
 130         #[derive(Debug)]
 131         struct ErrorDescriptor<'tcx> {
 132             predicate: ty::Predicate<'tcx>,
 133             index: Option<usize>, // None if this is an old error
 134         }
 135
 136         let mut error_map: FxHashMap<_, Vec<_>> = self
 137             .reported_trait_errors
 138             .borrow()
 139             .iter()
 140             .map(|(&span, predicates)| {
 141                 (
 142                     span,
 143                     predicates
 144                         .iter()
 145                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 146                         .collect(),
 147                 )
 148             })
 149             .collect();
 150
 151         for (index, error) in errors.iter().enumerate() {
 152             // We want to ignore desugarings here: spans are equivalent even
 153             // if one is the result of a desugaring and the other is not.
 154             let mut span = error.obligation.cause.span;
 155             let expn_data = span.ctxt().outer_expn_data();
 156             if let ExpnKind::Desugaring(_) = expn_data.kind {
 157                 span = expn_data.call_site;
 158             }
 159
 160             error_map.entry(span).or_default().push(ErrorDescriptor {
 161                 predicate: error.obligation.predicate,
 162                 index: Some(index),
 163             });
 164
 165             self.reported_trait_errors
 166                 .borrow_mut()
 167                 .entry(span)
 168                 .or_default()
 169                 .push(error.obligation.predicate);
 170         }
 171
 172         // We do this in 2 passes because we want to display errors in order, though
 173         // maybe it *is* better to sort errors by span or something.
 174         let mut is_suppressed = vec![false; errors.len()];
 175         for (_, error_set) in error_map.iter() {
 176             // We want to suppress "duplicate" errors with the same span.
 177             for error in error_set {
 178                 if let Some(index) = error.index {
 179                     // Suppress errors that are either:
 180                     // 1) strictly implied by another error.
 181                     // 2) implied by an error with a smaller index.
 182                     for error2 in error_set {
 183                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 184                             // Avoid errors being suppressed by already-suppressed
 185                             // errors, to prevent all errors from being suppressed
 186                             // at once.
 187                             continue;
 188                         }
 189
 190                         if self.error_implies(error2.predicate, error.predicate)
 191                             && !(error2.index >= error.index
 192                                 && self.error_implies(error.predicate, error2.predicate))
 193                         {
 194                             info!("skipping {:?} (implied by {:?})", error, error2);
 195                             is_suppressed[index] = true;
 196                             break;
 197                         }
 198                     }
 199                 }
 200             }
 201         }
 202
 203         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 204             if !suppressed {
 205                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 206             }
 207         }
 208
 209         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 210     }
 211
 212     /// Reports that an overflow has occurred and halts compilation. We
 213     /// halt compilation unconditionally because it is important that
 214     /// overflows never be masked -- they basically represent computations
 215     /// whose result could not be truly determined and thus we can't say
 216     /// if the program type checks or not -- and they are unusual
 217     /// occurrences in any case.
 218     fn report_overflow_error<T>(
 219         &self,
 220         obligation: &Obligation<'tcx, T>,
 221         suggest_increasing_limit: bool,
 222     ) -> !
 223     where
 224         T: fmt::Display + TypeFoldable<'tcx>,
 225     {
 226         let predicate = self.resolve_vars_if_possible(obligation.predicate.clone());
 227         let mut err = struct_span_err!(
 228             self.tcx.sess,
 229             obligation.cause.span,
 230             E0275,
 231             "overflow evaluating the requirement `{}`",
 232             predicate
 233         );
 234
 235         if suggest_increasing_limit {
 236             self.suggest_new_overflow_limit(&mut err);
 237         }
 238
 239         self.note_obligation_cause_code(
 240             &mut err,
 241             &obligation.predicate,
 242             obligation.param_env,
 243             obligation.cause.code(),
 244             &mut vec![],
 245             &mut Default::default(),
 246         );
 247
 248         err.emit();
 249         self.tcx.sess.abort_if_errors();
 250         bug!();
 251     }
 252
 253     /// Reports that a cycle was detected which led to overflow and halts
 254     /// compilation. This is equivalent to `report_overflow_error` except
 255     /// that we can give a more helpful error message (and, in particular,
 256     /// we do not suggest increasing the overflow limit, which is not
 257     /// going to help).
 258     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 259         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 260         assert!(!cycle.is_empty());
 261
 262         debug!(?cycle, "report_overflow_error_cycle");
 263
 264         // The 'deepest' obligation is most likely to have a useful
 265         // cause 'backtrace'
 266         self.report_overflow_error(cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(), false);
 267     }
 268
 269     fn report_selection_error(
 270         &self,
 271         mut obligation: PredicateObligation<'tcx>,
 272         root_obligation: &PredicateObligation<'tcx>,
 273         error: &SelectionError<'tcx>,
 274         fallback_has_occurred: bool,
 275     ) {
 276         self.set_tainted_by_errors();
 277         let tcx = self.tcx;
 278         let mut span = obligation.cause.span;
 279
 280         let mut err = match *error {
 281             SelectionError::Ambiguous(ref impls) => {
 282                 let mut err = self.tcx.sess.struct_span_err(
 283                     obligation.cause.span,
 284                     &format!("multiple applicable `impl`s for `{}`", obligation.predicate),
 285                 );
 286                 self.annotate_source_of_ambiguity(&mut err, impls, obligation.predicate);
 287                 err.emit();
 288                 return;
 289             }
 290             SelectionError::Unimplemented => {
 291                 // If this obligation was generated as a result of well-formedness checking, see if we
 292                 // can get a better error message by performing HIR-based well-formedness checking.
 293                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 294                     root_obligation.cause.code().peel_derives()
 295                 {
 296                     if let Some(cause) = self
 297                         .tcx
 298                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 299                     {
 300                         obligation.cause = cause.clone();
 301                         span = obligation.cause.span;
 302                     }
 303                 }
 304                 if let ObligationCauseCode::CompareImplMethodObligation {
 305                     impl_item_def_id,
 306                     trait_item_def_id,
 307                 }
 308                 | ObligationCauseCode::CompareImplTypeObligation {
 309                     impl_item_def_id,
 310                     trait_item_def_id,
 311                 } = *obligation.cause.code()
 312                 {
 313                     self.report_extra_impl_obligation(
 314                         span,
 315                         impl_item_def_id,
 316                         trait_item_def_id,
 317                         &format!("`{}`", obligation.predicate),
 318                     )
 319                     .emit();
 320                     return;
 321                 }
 322
 323                 let bound_predicate = obligation.predicate.kind();
 324                 match bound_predicate.skip_binder() {
 325                     ty::PredicateKind::Trait(trait_predicate) => {
 326                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 327                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 328
 329                         trait_predicate.remap_constness_diag(obligation.param_env);
 330                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 331                             == trait_predicate.skip_binder().constness;
 332
 333                         if self.tcx.sess.has_errors().is_some()
 334                             && trait_predicate.references_error()
 335                         {
 336                             return;
 337                         }
 338                         let trait_ref = trait_predicate.to_poly_trait_ref();
 339                         let (post_message, pre_message, type_def) = self
 340                             .get_parent_trait_ref(obligation.cause.code())
 341                             .map(|(t, s)| {
 342                                 (
 343                                     format!(" in `{}`", t),
 344                                     format!("within `{}`, ", t),
 345                                     s.map(|s| (format!("within this `{}`", t), s)),
 346                                 )
 347                             })
 348                             .unwrap_or_default();
 349
 350                         let OnUnimplementedNote {
 351                             message,
 352                             label,
 353                             note,
 354                             enclosing_scope,
 355                             append_const_msg,
 356                         } = self.on_unimplemented_note(trait_ref, &obligation);
 357                         let have_alt_message = message.is_some() || label.is_some();
 358                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 359                         let is_unsize =
 360                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 361                         let (message, note, append_const_msg) = if is_try_conversion {
 362                             (
 363                                 Some(format!(
 364                                     "`?` couldn't convert the error to `{}`",
 365                                     trait_ref.skip_binder().self_ty(),
 366                                 )),
 367                                 Some(
 368                                     "the question mark operation (`?`) implicitly performs a \
 369                                      conversion on the error value using the `From` trait"
 370                                         .to_owned(),
 371                                 ),
 372                                 Some(None),
 373                             )
 374                         } else {
 375                             (message, note, append_const_msg)
 376                         };
 377
 378                         let mut err = struct_span_err!(
 379                             self.tcx.sess,
 380                             span,
 381                             E0277,
 382                             "{}",
 383                             message
 384                                 .and_then(|cannot_do_this| {
 385                                     match (predicate_is_const, append_const_msg) {
 386                                         // do nothing if predicate is not const
 387                                         (false, _) => Some(cannot_do_this),
 388                                         // suggested using default post message
 389                                         (true, Some(None)) => {
 390                                             Some(format!("{cannot_do_this} in const contexts"))
 391                                         }
 392                                         // overridden post message
 393                                         (true, Some(Some(post_message))) => {
 394                                             Some(format!("{cannot_do_this}{post_message}"))
 395                                         }
 396                                         // fallback to generic message
 397                                         (true, None) => None,
 398                                     }
 399                                 })
 400                                 .unwrap_or_else(|| format!(
 401                                     "the trait bound `{}` is not satisfied{}",
 402                                     trait_predicate, post_message,
 403                                 ))
 404                         );
 405
 406                         if is_try_conversion {
 407                             let none_error = self
 408                                 .tcx
 409                                 .get_diagnostic_item(sym::none_error)
 410                                 .map(|def_id| tcx.type_of(def_id));
 411                             let should_convert_option_to_result =
 412                                 Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
 413                             let should_convert_result_to_option =
 414                                 Some(trait_ref.self_ty().skip_binder()) == none_error;
 415                             if should_convert_option_to_result {
 416                                 err.span_suggestion_verbose(
 417                                     span.shrink_to_lo(),
 418                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 419                                      using `Option::ok_or` or `Option::ok_or_else`",
 420                                     ".ok_or_else(|| /* error value */)",
 421                                     Applicability::HasPlaceholders,
 422                                 );
 423                             } else if should_convert_result_to_option {
 424                                 err.span_suggestion_verbose(
 425                                     span.shrink_to_lo(),
 426                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 427                                      using `Result::ok`",
 428                                     ".ok()",
 429                                     Applicability::MachineApplicable,
 430                                 );
 431                             }
 432                             if let Some(ret_span) = self.return_type_span(&obligation) {
 433                                 err.span_label(
 434                                     ret_span,
 435                                     &format!(
 436                                         "expected `{}` because of this",
 437                                         trait_ref.skip_binder().self_ty()
 438                                     ),
 439                                 );
 440                             }
 441                         }
 442
 443                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 444                             && predicate_is_const
 445                         {
 446                             err.note("`~const Drop` was renamed to `~const Destruct`");
 447                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 448                         }
 449
 450                         let explanation = if let ObligationCauseCode::MainFunctionType =
 451                             obligation.cause.code()
 452                         {
 453                             "consider using `()`, or a `Result`".to_owned()
 454                         } else {
 455                             format!(
 456                                 "{}the trait `{}` is not implemented for `{}`",
 457                                 pre_message,
 458                                 trait_predicate.print_modifiers_and_trait_path(),
 459                                 trait_ref.skip_binder().self_ty(),
 460                             )
 461                         };
 462
 463                         if self.suggest_add_reference_to_arg(
 464                             &obligation,
 465                             &mut err,
 466                             trait_predicate,
 467                             have_alt_message,
 468                         ) {
 469                             self.note_obligation_cause(&mut err, &obligation);
 470                             err.emit();
 471                             return;
 472                         }
 473                         if let Some(ref s) = label {
 474                             // If it has a custom `#[rustc_on_unimplemented]`
 475                             // error message, let's display it as the label!
 476                             err.span_label(span, s);
 477                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 478                                 // When the self type is a type param We don't need to "the trait
 479                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 480                                 // at the type param with a label to suggest constraining it.
 481                                 err.help(&explanation);
 482                             }
 483                         } else {
 484                             err.span_label(span, explanation);
 485                         }
 486
 487                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 488                            Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 489                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 490                         }
 491
 492                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 493                             let non_const_predicate = trait_ref.without_const();
 494                             let non_const_obligation = Obligation {
 495                                 cause: obligation.cause.clone(),
 496                                 param_env: obligation.param_env.without_const(),
 497                                 predicate: non_const_predicate.to_predicate(tcx),
 498                                 recursion_depth: obligation.recursion_depth,
 499                             };
 500                             if self.predicate_may_hold(&non_const_obligation) {
 501                                 err.span_note(
 502                                     span,
 503                                     &format!(
 504                                         "the trait `{}` is implemented for `{}`, \
 505                                         but that implementation is not `const`",
 506                                         non_const_predicate.print_modifiers_and_trait_path(),
 507                                         trait_ref.skip_binder().self_ty(),
 508                                     ),
 509                                 );
 510                             }
 511                         }
 512
 513                         if let Some((msg, span)) = type_def {
 514                             err.span_label(span, &msg);
 515                         }
 516                         if let Some(ref s) = note {
 517                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 518                             err.note(s.as_str());
 519                         }
 520                         if let Some(ref s) = enclosing_scope {
 521                             let body = tcx
 522                                 .hir()
 523                                 .opt_local_def_id(obligation.cause.body_id)
 524                                 .unwrap_or_else(|| {
 525                                     tcx.hir().body_owner_def_id(hir::BodyId {
 526                                         hir_id: obligation.cause.body_id,
 527                                     })
 528                                 });
 529
 530                             let enclosing_scope_span =
 531                                 tcx.hir().span_with_body(tcx.hir().local_def_id_to_hir_id(body));
 532
 533                             err.span_label(enclosing_scope_span, s);
 534                         }
 535
 536                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 537                         let mut suggested =
 538                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 539                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 540                         suggested |=
 541                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 542                         suggested |= self.suggest_semicolon_removal(
 543                             &obligation,
 544                             &mut err,
 545                             span,
 546                             trait_predicate,
 547                         );
 548                         self.note_version_mismatch(&mut err, &trait_ref);
 549                         self.suggest_remove_await(&obligation, &mut err);
 550                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 551
 552                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 553                             self.suggest_await_before_try(
 554                                 &mut err,
 555                                 &obligation,
 556                                 trait_predicate,
 557                                 span,
 558                             );
 559                         }
 560
 561                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 562                             err.emit();
 563                             return;
 564                         }
 565
 566                         if is_unsize {
 567                             // If the obligation failed due to a missing implementation of the
 568                             // `Unsize` trait, give a pointer to why that might be the case
 569                             err.note(
 570                                 "all implementations of `Unsize` are provided \
 571                                 automatically by the compiler, see \
 572                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 573                                 for more information",
 574                             );
 575                         }
 576
 577                         let is_fn_trait = [
 578                             self.tcx.lang_items().fn_trait(),
 579                             self.tcx.lang_items().fn_mut_trait(),
 580                             self.tcx.lang_items().fn_once_trait(),
 581                         ]
 582                         .contains(&Some(trait_ref.def_id()));
 583                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 584                             *trait_ref.skip_binder().self_ty().kind()
 585                         {
 586                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 587                         } else {
 588                             false
 589                         };
 590                         if is_fn_trait && is_target_feature_fn {
 591                             err.note(
 592                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 593                             );
 594                         }
 595
 596                         // Try to report a help message
 597                         if is_fn_trait
 598                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 599                                 obligation.param_env,
 600                                 trait_ref.self_ty(),
 601                                 trait_predicate.skip_binder().constness,
 602                                 trait_predicate.skip_binder().polarity,
 603                             )
 604                         {
 605                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 606                             // suggestion to add trait bounds for the type, since we only typically implement
 607                             // these traits once.
 608
 609                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 610                             // to implement.
 611                             let selected_kind =
 612                                 ty::ClosureKind::from_def_id(self.tcx, trait_ref.def_id())
 613                                     .expect("expected to map DefId to ClosureKind");
 614                             if !implemented_kind.extends(selected_kind) {
 615                                 err.note(
 616                                     &format!(
 617                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 618                                         trait_ref.skip_binder().self_ty(),
 619                                         implemented_kind,
 620                                         selected_kind
 621                                     )
 622                                 );
 623                             }
 624
 625                             // Note any argument mismatches
 626                             let given_ty = params.skip_binder();
 627                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 628                             if let ty::Tuple(given) = given_ty.kind()
 629                                 && let ty::Tuple(expected) = expected_ty.kind()
 630                             {
 631                                 if expected.len() != given.len() {
 632                                     // Note number of types that were expected and given
 633                                     err.note(
 634                                         &format!(
 635                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 636                                             given.len(),
 637                                             if given.len() == 1 { "" } else { "s" },
 638                                             expected.len(),
 639                                             if expected.len() == 1 { "" } else { "s" },
 640                                         )
 641                                     );
 642                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
 643                                     // Print type mismatch
 644                                     let (expected_args, given_args) =
 645                                         self.cmp(given_ty, expected_ty);
 646                                     err.note_expected_found(
 647                                         &"a closure with arguments",
 648                                         expected_args,
 649                                         &"a closure with arguments",
 650                                         given_args,
 651                                     );
 652                                 }
 653                             }
 654                         } else if !trait_ref.has_infer_types_or_consts()
 655                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 656                         {
 657                             // If a where-clause may be useful, remind the
 658                             // user that they can add it.
 659                             //
 660                             // don't display an on-unimplemented note, as
 661                             // these notes will often be of the form
 662                             //     "the type `T` can't be frobnicated"
 663                             // which is somewhat confusing.
 664                             self.suggest_restricting_param_bound(
 665                                 &mut err,
 666                                 trait_predicate,
 667                                 None,
 668                                 obligation.cause.body_id,
 669                             );
 670                         } else if !suggested {
 671                             // Can't show anything else useful, try to find similar impls.
 672                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 673                             if !self.report_similar_impl_candidates(
 674                                 impl_candidates,
 675                                 trait_ref,
 676                                 obligation.cause.body_id,
 677                                 &mut err,
 678                             ) {
 679                                 // This is *almost* equivalent to
 680                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 681                                 // trait predicate for that corresponding root obligation. This
 682                                 // lets us get a derived obligation from a type parameter, like
 683                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 684                                 // implementer of `Pattern<'_>`.
 685                                 let mut code = obligation.cause.code();
 686                                 let mut trait_pred = trait_predicate;
 687                                 let mut peeled = false;
 688                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
 689                                     code = parent_code;
 690                                     if let Some(parent_trait_pred) = parent_trait_pred {
 691                                         trait_pred = parent_trait_pred;
 692                                         peeled = true;
 693                                     }
 694                                 }
 695                                 let def_id = trait_pred.def_id();
 696                                 // Mention *all* the `impl`s for the *top most* obligation, the
 697                                 // user might have meant to use one of them, if any found. We skip
 698                                 // auto-traits or fundamental traits that might not be exactly what
 699                                 // the user might expect to be presented with. Instead this is
 700                                 // useful for less general traits.
 701                                 if peeled
 702                                     && !self.tcx.trait_is_auto(def_id)
 703                                     && !self.tcx.lang_items().items().contains(&Some(def_id))
 704                                 {
 705                                     let trait_ref = trait_pred.to_poly_trait_ref();
 706                                     let impl_candidates =
 707                                         self.find_similar_impl_candidates(trait_ref);
 708                                     self.report_similar_impl_candidates(
 709                                         impl_candidates,
 710                                         trait_ref,
 711                                         obligation.cause.body_id,
 712                                         &mut err,
 713                                     );
 714                                 }
 715                             }
 716                         }
 717
 718                         // Changing mutability doesn't make a difference to whether we have
 719                         // an `Unsize` impl (Fixes ICE in #71036)
 720                         if !is_unsize {
 721                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
 722                         }
 723
 724                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 725                         // implemented, and fallback has occurred, then it could be due to a
 726                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 727                         // note informing about the change in behaviour.
 728                         if trait_predicate.skip_binder().self_ty().is_never()
 729                             && fallback_has_occurred
 730                         {
 731                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 732                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 733                                     self.tcx.mk_unit(),
 734                                     &trait_pred.trait_ref.substs[1..],
 735                                 );
 736                                 trait_pred
 737                             });
 738                             let unit_obligation = obligation.with(predicate.to_predicate(tcx));
 739                             if self.predicate_may_hold(&unit_obligation) {
 740                                 err.note(
 741                                     "this error might have been caused by changes to \
 742                                     Rust's type-inference algorithm (see issue #48950 \
 743                                     <https://github.com/rust-lang/rust/issues/48950> \
 744                                     for more information)",
 745                                 );
 746                                 err.help("did you intend to use the type `()` here instead?");
 747                             }
 748                         }
 749
 750                         // Return early if the trait is Debug or Display and the invocation
 751                         // originates within a standard library macro, because the output
 752                         // is otherwise overwhelming and unhelpful (see #85844 for an
 753                         // example).
 754
 755                         let in_std_macro =
 756                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
 757                                 Some(macro_def_id) => {
 758                                     let crate_name = tcx.crate_name(macro_def_id.krate);
 759                                     crate_name == sym::std || crate_name == sym::core
 760                                 }
 761                                 None => false,
 762                             };
 763
 764                         if in_std_macro
 765                             && matches!(
 766                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
 767                                 Some(sym::Debug | sym::Display)
 768                             )
 769                         {
 770                             err.emit();
 771                             return;
 772                         }
 773
 774                         err
 775                     }
 776
 777                     ty::PredicateKind::Subtype(predicate) => {
 778                         // Errors for Subtype predicates show up as
 779                         // `FulfillmentErrorCode::CodeSubtypeError`,
 780                         // not selection error.
 781                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 782                     }
 783
 784                     ty::PredicateKind::Coerce(predicate) => {
 785                         // Errors for Coerce predicates show up as
 786                         // `FulfillmentErrorCode::CodeSubtypeError`,
 787                         // not selection error.
 788                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
 789                     }
 790
 791                     ty::PredicateKind::RegionOutlives(..)
 792                     | ty::PredicateKind::Projection(..)
 793                     | ty::PredicateKind::TypeOutlives(..) => {
 794                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
 795                         struct_span_err!(
 796                             self.tcx.sess,
 797                             span,
 798                             E0280,
 799                             "the requirement `{}` is not satisfied",
 800                             predicate
 801                         )
 802                     }
 803
 804                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
 805                         let violations = self.tcx.object_safety_violations(trait_def_id);
 806                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 807                     }
 808
 809                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 810                         let found_kind = self.closure_kind(closure_substs).unwrap();
 811                         let closure_span = self.tcx.def_span(closure_def_id);
 812                         let mut err = struct_span_err!(
 813                             self.tcx.sess,
 814                             closure_span,
 815                             E0525,
 816                             "expected a closure that implements the `{}` trait, \
 817                              but this closure only implements `{}`",
 818                             kind,
 819                             found_kind
 820                         );
 821
 822                         err.span_label(
 823                             closure_span,
 824                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 825                         );
 826                         err.span_label(
 827                             obligation.cause.span,
 828                             format!("the requirement to implement `{}` derives from here", kind),
 829                         );
 830
 831                         // Additional context information explaining why the closure only implements
 832                         // a particular trait.
 833                         if let Some(typeck_results) = self.in_progress_typeck_results {
 834                             let hir_id = self
 835                                 .tcx
 836                                 .hir()
 837                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
 838                             let typeck_results = typeck_results.borrow();
 839                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
 840                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
 841                                     err.span_label(
 842                                         *span,
 843                                         format!(
 844                                             "closure is `FnOnce` because it moves the \
 845                                          variable `{}` out of its environment",
 846                                             ty::place_to_string_for_capture(tcx, place)
 847                                         ),
 848                                     );
 849                                 }
 850                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
 851                                     err.span_label(
 852                                         *span,
 853                                         format!(
 854                                             "closure is `FnMut` because it mutates the \
 855                                          variable `{}` here",
 856                                             ty::place_to_string_for_capture(tcx, place)
 857                                         ),
 858                                     );
 859                                 }
 860                                 _ => {}
 861                             }
 862                         }
 863
 864                         err.emit();
 865                         return;
 866                     }
 867
 868                     ty::PredicateKind::WellFormed(ty) => {
 869                         if !self.tcx.sess.opts.unstable_opts.chalk {
 870                             // WF predicates cannot themselves make
 871                             // errors. They can only block due to
 872                             // ambiguity; otherwise, they always
 873                             // degenerate into other obligations
 874                             // (which may fail).
 875                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 876                         } else {
 877                             // FIXME: we'll need a better message which takes into account
 878                             // which bounds actually failed to hold.
 879                             self.tcx.sess.struct_span_err(
 880                                 span,
 881                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 882                             )
 883                         }
 884                     }
 885
 886                     ty::PredicateKind::ConstEvaluatable(..) => {
 887                         // Errors for `ConstEvaluatable` predicates show up as
 888                         // `SelectionError::ConstEvalFailure`,
 889                         // not `Unimplemented`.
 890                         span_bug!(
 891                             span,
 892                             "const-evaluatable requirement gave wrong error: `{:?}`",
 893                             obligation
 894                         )
 895                     }
 896
 897                     ty::PredicateKind::ConstEquate(..) => {
 898                         // Errors for `ConstEquate` predicates show up as
 899                         // `SelectionError::ConstEvalFailure`,
 900                         // not `Unimplemented`.
 901                         span_bug!(
 902                             span,
 903                             "const-equate requirement gave wrong error: `{:?}`",
 904                             obligation
 905                         )
 906                     }
 907
 908                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
 909                         span,
 910                         "TypeWellFormedFromEnv predicate should only exist in the environment"
 911                     ),
 912                 }
 913             }
 914
 915             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
 916                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
 917                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
 918
 919                 if expected_trait_ref.self_ty().references_error() {
 920                     return;
 921                 }
 922
 923                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
 924                     return;
 925                 };
 926
 927                 let found_did = match *found_trait_ty.kind() {
 928                     ty::Closure(did, _)
 929                     | ty::Foreign(did)
 930                     | ty::FnDef(did, _)
 931                     | ty::Generator(did, ..) => Some(did),
 932                     ty::Adt(def, _) => Some(def.did()),
 933                     _ => None,
 934                 };
 935
 936                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
 937
 938                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 939                     // We check closures twice, with obligations flowing in different directions,
 940                     // but we want to complain about them only once.
 941                     return;
 942                 }
 943
 944                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 945
 946                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
 947                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 948                     _ => vec![ArgKind::empty()],
 949                 };
 950
 951                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 952                 let expected = match expected_ty.kind() {
 953                     ty::Tuple(ref tys) => {
 954                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
 955                     }
 956                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 957                 };
 958
 959                 if found.len() == expected.len() {
 960                     self.report_closure_arg_mismatch(
 961                         span,
 962                         found_span,
 963                         found_trait_ref,
 964                         expected_trait_ref,
 965                     )
 966                 } else {
 967                     let (closure_span, found) = found_did
 968                         .and_then(|did| {
 969                             let node = self.tcx.hir().get_if_local(did)?;
 970                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 971                             Some((Some(found_span), found))
 972                         })
 973                         .unwrap_or((found_span, found));
 974
 975                     self.report_arg_count_mismatch(
 976                         span,
 977                         closure_span,
 978                         expected,
 979                         found,
 980                         found_trait_ty.is_closure(),
 981                     )
 982                 }
 983             }
 984
 985             TraitNotObjectSafe(did) => {
 986                 let violations = self.tcx.object_safety_violations(did);
 987                 report_object_safety_error(self.tcx, span, did, violations)
 988             }
 989
 990             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
 991                 bug!(
 992                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
 993                 )
 994             }
 995             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
 996                 if !self.tcx.features().generic_const_exprs {
 997                     let mut err = self.tcx.sess.struct_span_err(
 998                         span,
 999                         "constant expression depends on a generic parameter",
1000                     );
1001                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1002                     // issue. However, this is currently not actually possible
1003                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1004                     //
1005                     // Note that with `feature(generic_const_exprs)` this case should not
1006                     // be reachable.
1007                     err.note("this may fail depending on what value the parameter takes");
1008                     err.emit();
1009                     return;
1010                 }
1011
1012                 match obligation.predicate.kind().skip_binder() {
1013                     ty::PredicateKind::ConstEvaluatable(uv) => {
1014                         let mut err =
1015                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1016                         let const_span = self.tcx.def_span(uv.def.did);
1017                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1018                             Ok(snippet) => err.help(&format!(
1019                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1020                                 snippet
1021                             )),
1022                             _ => err.help("consider adding a `where` bound using this expression"),
1023                         };
1024                         err
1025                     }
1026                     _ => {
1027                         span_bug!(
1028                             span,
1029                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1030                         )
1031                     }
1032                 }
1033             }
1034
1035             // Already reported in the query.
1036             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1037                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1038                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1039                 return;
1040             }
1041             // Already reported.
1042             Overflow(OverflowError::Error(_)) => {
1043                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1044                 return;
1045             }
1046             Overflow(_) => {
1047                 bug!("overflow should be handled before the `report_selection_error` path");
1048             }
1049             SelectionError::ErrorReporting => {
1050                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1051             }
1052         };
1053
1054         self.note_obligation_cause(&mut err, &obligation);
1055         self.point_at_returns_when_relevant(&mut err, &obligation);
1056
1057         err.emit();
1058     }
1059
1060     /// Given some node representing a fn-like thing in the HIR map,
1061     /// returns a span and `ArgKind` information that describes the
1062     /// arguments it expects. This can be supplied to
1063     /// `report_arg_count_mismatch`.
1064     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
1065         let sm = self.tcx.sess.source_map();
1066         let hir = self.tcx.hir();
1067         Some(match node {
1068             Node::Expr(&hir::Expr {
1069                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, .. }),
1070                 ..
1071             }) => (
1072                 fn_decl_span,
1073                 hir.body(body)
1074                     .params
1075                     .iter()
1076                     .map(|arg| {
1077                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
1078                             *arg.pat
1079                         {
1080                             Some(ArgKind::Tuple(
1081                                 Some(span),
1082                                 args.iter()
1083                                     .map(|pat| {
1084                                         sm.span_to_snippet(pat.span)
1085                                             .ok()
1086                                             .map(|snippet| (snippet, "_".to_owned()))
1087                                     })
1088                                     .collect::<Option<Vec<_>>>()?,
1089                             ))
1090                         } else {
1091                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
1092                             Some(ArgKind::Arg(name, "_".to_owned()))
1093                         }
1094                     })
1095                     .collect::<Option<Vec<ArgKind>>>()?,
1096             ),
1097             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
1098             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
1099             | Node::TraitItem(&hir::TraitItem {
1100                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
1101             }) => (
1102                 sig.span,
1103                 sig.decl
1104                     .inputs
1105                     .iter()
1106                     .map(|arg| match arg.kind {
1107                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1108                             Some(arg.span),
1109                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
1110                         ),
1111                         _ => ArgKind::empty(),
1112                     })
1113                     .collect::<Vec<ArgKind>>(),
1114             ),
1115             Node::Ctor(ref variant_data) => {
1116                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
1117                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1118             }
1119             _ => panic!("non-FnLike node found: {:?}", node),
1120         })
1121     }
1122
1123     /// Reports an error when the number of arguments needed by a
1124     /// trait match doesn't match the number that the expression
1125     /// provides.
1126     fn report_arg_count_mismatch(
1127         &self,
1128         span: Span,
1129         found_span: Option<Span>,
1130         expected_args: Vec<ArgKind>,
1131         found_args: Vec<ArgKind>,
1132         is_closure: bool,
1133     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
1134         let kind = if is_closure { "closure" } else { "function" };
1135
1136         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1137             let arg_length = arguments.len();
1138             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
1139             match (arg_length, arguments.get(0)) {
1140                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1141                     format!("a single {}-tuple as argument", fields.len())
1142                 }
1143                 _ => format!(
1144                     "{} {}argument{}",
1145                     arg_length,
1146                     if distinct && arg_length > 1 { "distinct " } else { "" },
1147                     pluralize!(arg_length)
1148                 ),
1149             }
1150         };
1151
1152         let expected_str = args_str(&expected_args, &found_args);
1153         let found_str = args_str(&found_args, &expected_args);
1154
1155         let mut err = struct_span_err!(
1156             self.tcx.sess,
1157             span,
1158             E0593,
1159             "{} is expected to take {}, but it takes {}",
1160             kind,
1161             expected_str,
1162             found_str,
1163         );
1164
1165         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1166
1167         if let Some(found_span) = found_span {
1168             err.span_label(found_span, format!("takes {}", found_str));
1169
1170             // move |_| { ... }
1171             // ^^^^^^^^-- def_span
1172             //
1173             // move |_| { ... }
1174             // ^^^^^-- prefix
1175             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1176             // move |_| { ... }
1177             //      ^^^-- pipe_span
1178             let pipe_span =
1179                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
1180
1181             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1182             // found arguments is empty (assume the user just wants to ignore args in this case).
1183             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1184             if found_args.is_empty() && is_closure {
1185                 let underscores = vec!["_"; expected_args.len()].join(", ");
1186                 err.span_suggestion_verbose(
1187                     pipe_span,
1188                     &format!(
1189                         "consider changing the closure to take and ignore the expected argument{}",
1190                         pluralize!(expected_args.len())
1191                     ),
1192                     format!("|{}|", underscores),
1193                     Applicability::MachineApplicable,
1194                 );
1195             }
1196
1197             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1198                 if fields.len() == expected_args.len() {
1199                     let sugg = fields
1200                         .iter()
1201                         .map(|(name, _)| name.to_owned())
1202                         .collect::<Vec<String>>()
1203                         .join(", ");
1204                     err.span_suggestion_verbose(
1205                         found_span,
1206                         "change the closure to take multiple arguments instead of a single tuple",
1207                         format!("|{}|", sugg),
1208                         Applicability::MachineApplicable,
1209                     );
1210                 }
1211             }
1212             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
1213                 && fields.len() == found_args.len()
1214                 && is_closure
1215             {
1216                 let sugg = format!(
1217                     "|({}){}|",
1218                     found_args
1219                         .iter()
1220                         .map(|arg| match arg {
1221                             ArgKind::Arg(name, _) => name.to_owned(),
1222                             _ => "_".to_owned(),
1223                         })
1224                         .collect::<Vec<String>>()
1225                         .join(", "),
1226                     // add type annotations if available
1227                     if found_args.iter().any(|arg| match arg {
1228                         ArgKind::Arg(_, ty) => ty != "_",
1229                         _ => false,
1230                     }) {
1231                         format!(
1232                             ": ({})",
1233                             fields
1234                                 .iter()
1235                                 .map(|(_, ty)| ty.to_owned())
1236                                 .collect::<Vec<String>>()
1237                                 .join(", ")
1238                         )
1239                     } else {
1240                         String::new()
1241                     },
1242                 );
1243                 err.span_suggestion_verbose(
1244                     found_span,
1245                     "change the closure to accept a tuple instead of individual arguments",
1246                     sugg,
1247                     Applicability::MachineApplicable,
1248                 );
1249             }
1250         }
1251
1252         err
1253     }
1254
1255     fn type_implements_fn_trait(
1256         &self,
1257         param_env: ty::ParamEnv<'tcx>,
1258         ty: ty::Binder<'tcx, Ty<'tcx>>,
1259         constness: ty::BoundConstness,
1260         polarity: ty::ImplPolarity,
1261     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
1262         self.commit_if_ok(|_| {
1263             for trait_def_id in [
1264                 self.tcx.lang_items().fn_trait(),
1265                 self.tcx.lang_items().fn_mut_trait(),
1266                 self.tcx.lang_items().fn_once_trait(),
1267             ] {
1268                 let Some(trait_def_id) = trait_def_id else { continue };
1269                 // Make a fresh inference variable so we can determine what the substitutions
1270                 // of the trait are.
1271                 let var = self.next_ty_var(TypeVariableOrigin {
1272                     span: DUMMY_SP,
1273                     kind: TypeVariableOriginKind::MiscVariable,
1274                 });
1275                 let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
1276                 let obligation = Obligation::new(
1277                     ObligationCause::dummy(),
1278                     param_env,
1279                     ty.rebind(ty::TraitPredicate {
1280                         trait_ref: ty::TraitRef::new(trait_def_id, substs),
1281                         constness,
1282                         polarity,
1283                     })
1284                     .to_predicate(self.tcx),
1285                 );
1286                 let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
1287                 fulfill_cx.register_predicate_obligation(self, obligation);
1288                 if fulfill_cx.select_all_or_error(self).is_empty() {
1289                     return Ok((
1290                         ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
1291                             .expect("expected to map DefId to ClosureKind"),
1292                         ty.rebind(self.resolve_vars_if_possible(var)),
1293                     ));
1294                 }
1295             }
1296
1297             Err(())
1298         })
1299     }
1300 }
1301
1302 trait InferCtxtPrivExt<'hir, 'tcx> {
1303     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1304     // `error` occurring implies that `cond` occurs.
1305     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1306
1307     fn report_fulfillment_error(
1308         &self,
1309         error: &FulfillmentError<'tcx>,
1310         body_id: Option<hir::BodyId>,
1311         fallback_has_occurred: bool,
1312     );
1313
1314     fn report_projection_error(
1315         &self,
1316         obligation: &PredicateObligation<'tcx>,
1317         error: &MismatchedProjectionTypes<'tcx>,
1318     );
1319
1320     fn fuzzy_match_tys(
1321         &self,
1322         a: Ty<'tcx>,
1323         b: Ty<'tcx>,
1324         ignoring_lifetimes: bool,
1325     ) -> Option<CandidateSimilarity>;
1326
1327     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1328
1329     fn find_similar_impl_candidates(
1330         &self,
1331         trait_ref: ty::PolyTraitRef<'tcx>,
1332     ) -> Vec<ImplCandidate<'tcx>>;
1333
1334     fn report_similar_impl_candidates(
1335         &self,
1336         impl_candidates: Vec<ImplCandidate<'tcx>>,
1337         trait_ref: ty::PolyTraitRef<'tcx>,
1338         body_id: hir::HirId,
1339         err: &mut Diagnostic,
1340     ) -> bool;
1341
1342     /// Gets the parent trait chain start
1343     fn get_parent_trait_ref(
1344         &self,
1345         code: &ObligationCauseCode<'tcx>,
1346     ) -> Option<(String, Option<Span>)>;
1347
1348     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1349     /// with the same path as `trait_ref`, a help message about
1350     /// a probable version mismatch is added to `err`
1351     fn note_version_mismatch(
1352         &self,
1353         err: &mut Diagnostic,
1354         trait_ref: &ty::PolyTraitRef<'tcx>,
1355     ) -> bool;
1356
1357     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1358     /// `trait_ref`.
1359     ///
1360     /// For this to work, `new_self_ty` must have no escaping bound variables.
1361     fn mk_trait_obligation_with_new_self_ty(
1362         &self,
1363         param_env: ty::ParamEnv<'tcx>,
1364         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1365     ) -> PredicateObligation<'tcx>;
1366
1367     fn maybe_report_ambiguity(
1368         &self,
1369         obligation: &PredicateObligation<'tcx>,
1370         body_id: Option<hir::BodyId>,
1371     );
1372
1373     fn predicate_can_apply(
1374         &self,
1375         param_env: ty::ParamEnv<'tcx>,
1376         pred: ty::PolyTraitRef<'tcx>,
1377     ) -> bool;
1378
1379     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1380
1381     fn suggest_unsized_bound_if_applicable(
1382         &self,
1383         err: &mut Diagnostic,
1384         obligation: &PredicateObligation<'tcx>,
1385     );
1386
1387     fn annotate_source_of_ambiguity(
1388         &self,
1389         err: &mut Diagnostic,
1390         impls: &[DefId],
1391         predicate: ty::Predicate<'tcx>,
1392     );
1393
1394     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'hir>);
1395
1396     fn maybe_indirection_for_unsized(
1397         &self,
1398         err: &mut Diagnostic,
1399         item: &'hir Item<'hir>,
1400         param: &'hir GenericParam<'hir>,
1401     ) -> bool;
1402
1403     fn is_recursive_obligation(
1404         &self,
1405         obligated_types: &mut Vec<Ty<'tcx>>,
1406         cause_code: &ObligationCauseCode<'tcx>,
1407     ) -> bool;
1408 }
1409
1410 impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
1411     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1412     // `error` occurring implies that `cond` occurs.
1413     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1414         if cond == error {
1415             return true;
1416         }
1417
1418         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1419         let bound_error = error.kind();
1420         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1421             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
1422                 (cond, bound_error.rebind(error))
1423             }
1424             _ => {
1425                 // FIXME: make this work in other cases too.
1426                 return false;
1427             }
1428         };
1429
1430         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1431             let bound_predicate = obligation.predicate.kind();
1432             if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
1433                 let error = error.to_poly_trait_ref();
1434                 let implication = bound_predicate.rebind(implication.trait_ref);
1435                 // FIXME: I'm just not taking associated types at all here.
1436                 // Eventually I'll need to implement param-env-aware
1437                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1438                 let param_env = ty::ParamEnv::empty();
1439                 if self.can_sub(param_env, error, implication).is_ok() {
1440                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1441                     return true;
1442                 }
1443             }
1444         }
1445
1446         false
1447     }
1448
1449     #[instrument(skip(self), level = "debug")]
1450     fn report_fulfillment_error(
1451         &self,
1452         error: &FulfillmentError<'tcx>,
1453         body_id: Option<hir::BodyId>,
1454         fallback_has_occurred: bool,
1455     ) {
1456         match error.code {
1457             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1458                 self.report_selection_error(
1459                     error.obligation.clone(),
1460                     &error.root_obligation,
1461                     selection_error,
1462                     fallback_has_occurred,
1463                 );
1464             }
1465             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1466                 self.report_projection_error(&error.obligation, e);
1467             }
1468             FulfillmentErrorCode::CodeAmbiguity => {
1469                 self.maybe_report_ambiguity(&error.obligation, body_id);
1470             }
1471             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1472                 self.report_mismatched_types(
1473                     &error.obligation.cause,
1474                     expected_found.expected,
1475                     expected_found.found,
1476                     err.clone(),
1477                 )
1478                 .emit();
1479             }
1480             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1481                 self.report_mismatched_consts(
1482                     &error.obligation.cause,
1483                     expected_found.expected,
1484                     expected_found.found,
1485                     err.clone(),
1486                 )
1487                 .emit();
1488             }
1489         }
1490     }
1491
1492     #[instrument(level = "debug", skip_all)]
1493     fn report_projection_error(
1494         &self,
1495         obligation: &PredicateObligation<'tcx>,
1496         error: &MismatchedProjectionTypes<'tcx>,
1497     ) {
1498         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1499
1500         if predicate.references_error() {
1501             return;
1502         }
1503
1504         self.probe(|_| {
1505             let err_buf;
1506             let mut err = &error.err;
1507             let mut values = None;
1508
1509             // try to find the mismatched types to report the error with.
1510             //
1511             // this can fail if the problem was higher-ranked, in which
1512             // cause I have no idea for a good error message.
1513             let bound_predicate = predicate.kind();
1514             if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
1515                 let mut selcx = SelectionContext::new(self);
1516                 let data = self.replace_bound_vars_with_fresh_vars(
1517                     obligation.cause.span,
1518                     infer::LateBoundRegionConversionTime::HigherRankedType,
1519                     bound_predicate.rebind(data),
1520                 );
1521                 let mut obligations = vec![];
1522                 let normalized_ty = super::normalize_projection_type(
1523                     &mut selcx,
1524                     obligation.param_env,
1525                     data.projection_ty,
1526                     obligation.cause.clone(),
1527                     0,
1528                     &mut obligations,
1529                 );
1530
1531                 debug!(?obligation.cause, ?obligation.param_env);
1532
1533                 debug!(?normalized_ty, data.ty = ?data.term);
1534
1535                 let is_normalized_ty_expected = !matches!(
1536                     obligation.cause.code().peel_derives(),
1537                     ObligationCauseCode::ItemObligation(_)
1538                         | ObligationCauseCode::BindingObligation(_, _)
1539                         | ObligationCauseCode::ObjectCastObligation(..)
1540                         | ObligationCauseCode::OpaqueType
1541                 );
1542                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1543                     is_normalized_ty_expected,
1544                     normalized_ty,
1545                     data.term,
1546                 ) {
1547                     values = Some(infer::ValuePairs::Terms(ExpectedFound::new(
1548                         is_normalized_ty_expected,
1549                         normalized_ty,
1550                         data.term,
1551                     )));
1552                     err_buf = error;
1553                     err = &err_buf;
1554                 }
1555             }
1556
1557             let mut diag = struct_span_err!(
1558                 self.tcx.sess,
1559                 obligation.cause.span,
1560                 E0271,
1561                 "type mismatch resolving `{}`",
1562                 predicate
1563             );
1564             let secondary_span = match predicate.kind().skip_binder() {
1565                 ty::PredicateKind::Projection(proj) => self
1566                     .tcx
1567                     .opt_associated_item(proj.projection_ty.item_def_id)
1568                     .and_then(|trait_assoc_item| {
1569                         self.tcx
1570                             .trait_of_item(proj.projection_ty.item_def_id)
1571                             .map(|id| (trait_assoc_item, id))
1572                     })
1573                     .and_then(|(trait_assoc_item, id)| {
1574                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1575                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1576                             self.tcx
1577                                 .associated_items(did)
1578                                 .in_definition_order()
1579                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1580                         })
1581                     })
1582                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1583                         Some(
1584                             hir::Node::TraitItem(hir::TraitItem {
1585                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1586                                 ..
1587                             })
1588                             | hir::Node::ImplItem(hir::ImplItem {
1589                                 kind: hir::ImplItemKind::TyAlias(ty),
1590                                 ..
1591                             }),
1592                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1593                         _ => None,
1594                     }),
1595                 _ => None,
1596             };
1597             self.note_type_err(
1598                 &mut diag,
1599                 &obligation.cause,
1600                 secondary_span,
1601                 values,
1602                 err,
1603                 true,
1604                 false,
1605             );
1606             self.note_obligation_cause(&mut diag, obligation);
1607             diag.emit();
1608         });
1609     }
1610
1611     fn fuzzy_match_tys(
1612         &self,
1613         mut a: Ty<'tcx>,
1614         mut b: Ty<'tcx>,
1615         ignoring_lifetimes: bool,
1616     ) -> Option<CandidateSimilarity> {
1617         /// returns the fuzzy category of a given type, or None
1618         /// if the type can be equated to any type.
1619         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1620             match t.kind() {
1621                 ty::Bool => Some(0),
1622                 ty::Char => Some(1),
1623                 ty::Str => Some(2),
1624                 ty::Adt(def, _) if tcx.is_diagnostic_item(sym::String, def.did()) => Some(2),
1625                 ty::Int(..)
1626                 | ty::Uint(..)
1627                 | ty::Float(..)
1628                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1629                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1630                 ty::Array(..) | ty::Slice(..) => Some(6),
1631                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1632                 ty::Dynamic(..) => Some(8),
1633                 ty::Closure(..) => Some(9),
1634                 ty::Tuple(..) => Some(10),
1635                 ty::Param(..) => Some(11),
1636                 ty::Projection(..) => Some(12),
1637                 ty::Opaque(..) => Some(13),
1638                 ty::Never => Some(14),
1639                 ty::Adt(..) => Some(15),
1640                 ty::Generator(..) => Some(16),
1641                 ty::Foreign(..) => Some(17),
1642                 ty::GeneratorWitness(..) => Some(18),
1643                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1644             }
1645         }
1646
1647         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1648             loop {
1649                 match t.kind() {
1650                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1651                         t = *inner
1652                     }
1653                     _ => break t,
1654                 }
1655             }
1656         };
1657
1658         if !ignoring_lifetimes {
1659             a = strip_references(a);
1660             b = strip_references(b);
1661         }
1662
1663         let cat_a = type_category(self.tcx, a)?;
1664         let cat_b = type_category(self.tcx, b)?;
1665         if a == b {
1666             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1667         } else if cat_a == cat_b {
1668             match (a.kind(), b.kind()) {
1669                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1670                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1671                 // Matching on references results in a lot of unhelpful
1672                 // suggestions, so let's just not do that for now.
1673                 //
1674                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1675                 // to prioritize that impl.
1676                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1677                     self.fuzzy_match_tys(a, b, true).is_some()
1678                 }
1679                 _ => true,
1680             }
1681             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1682         } else if ignoring_lifetimes {
1683             None
1684         } else {
1685             self.fuzzy_match_tys(a, b, true)
1686         }
1687     }
1688
1689     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1690         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1691             hir::GeneratorKind::Gen => "a generator",
1692             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1693             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1694             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1695         })
1696     }
1697
1698     fn find_similar_impl_candidates(
1699         &self,
1700         trait_ref: ty::PolyTraitRef<'tcx>,
1701     ) -> Vec<ImplCandidate<'tcx>> {
1702         self.tcx
1703             .all_impls(trait_ref.def_id())
1704             .filter_map(|def_id| {
1705                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative {
1706                     return None;
1707                 }
1708
1709                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1710
1711                 self.fuzzy_match_tys(trait_ref.skip_binder().self_ty(), imp.self_ty(), false)
1712                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1713             })
1714             .collect()
1715     }
1716
1717     fn report_similar_impl_candidates(
1718         &self,
1719         impl_candidates: Vec<ImplCandidate<'tcx>>,
1720         trait_ref: ty::PolyTraitRef<'tcx>,
1721         body_id: hir::HirId,
1722         err: &mut Diagnostic,
1723     ) -> bool {
1724         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1725             candidates.sort();
1726             candidates.dedup();
1727             let len = candidates.len();
1728             if candidates.len() == 0 {
1729                 return false;
1730             }
1731             if candidates.len() == 1 {
1732                 err.highlighted_help(vec![
1733                     (
1734                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1735                         Style::NoStyle,
1736                     ),
1737                     ("is".to_string(), Style::Highlight),
1738                     (" implemented for `".to_string(), Style::NoStyle),
1739                     (candidates[0].self_ty().to_string(), Style::Highlight),
1740                     ("`".to_string(), Style::NoStyle),
1741                 ]);
1742                 return true;
1743             }
1744             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1745             // Check if the trait is the same in all cases. If so, we'll only show the type.
1746             let mut traits: Vec<_> =
1747                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1748             traits.sort();
1749             traits.dedup();
1750
1751             let mut candidates: Vec<String> = candidates
1752                 .into_iter()
1753                 .map(|c| {
1754                     if traits.len() == 1 {
1755                         format!("\n  {}", c.self_ty())
1756                     } else {
1757                         format!("\n  {}", c)
1758                     }
1759                 })
1760                 .collect();
1761
1762             candidates.sort();
1763             candidates.dedup();
1764             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1765             err.help(&format!(
1766                 "the following other types implement trait `{}`:{}{}",
1767                 trait_ref.print_only_trait_path(),
1768                 candidates[..end].join(""),
1769                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1770             ));
1771             true
1772         };
1773
1774         let def_id = trait_ref.def_id();
1775         if impl_candidates.is_empty() {
1776             if self.tcx.trait_is_auto(def_id)
1777                 || self.tcx.lang_items().items().contains(&Some(def_id))
1778                 || self.tcx.get_diagnostic_name(def_id).is_some()
1779             {
1780                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1781                 return false;
1782             }
1783             let normalized_impl_candidates: Vec<_> = self
1784                 .tcx
1785                 .all_impls(def_id)
1786                 // Ignore automatically derived impls and `!Trait` impls.
1787                 .filter(|&def_id| {
1788                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1789                         || self.tcx.is_builtin_derive(def_id)
1790                 })
1791                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1792                 .filter(|trait_ref| {
1793                     let self_ty = trait_ref.self_ty();
1794                     // Avoid mentioning type parameters.
1795                     if let ty::Param(_) = self_ty.kind() {
1796                         false
1797                     }
1798                     // Avoid mentioning types that are private to another crate
1799                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
1800                         // FIXME(compiler-errors): This could be generalized, both to
1801                         // be more granular, and probably look past other `#[fundamental]`
1802                         // types, too.
1803                         self.tcx
1804                             .visibility(def.did())
1805                             .is_accessible_from(body_id.owner.to_def_id(), self.tcx)
1806                     } else {
1807                         true
1808                     }
1809                 })
1810                 .collect();
1811             return report(normalized_impl_candidates, err);
1812         }
1813
1814         let normalize = |candidate| {
1815             self.tcx.infer_ctxt().enter(|ref infcx| {
1816                 let normalized = infcx
1817                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1818                     .normalize(candidate)
1819                     .ok();
1820                 match normalized {
1821                     Some(normalized) => normalized.value,
1822                     None => candidate,
1823                 }
1824             })
1825         };
1826
1827         // Sort impl candidates so that ordering is consistent for UI tests.
1828         // because the ordering of `impl_candidates` may not be deterministic:
1829         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1830         //
1831         // Prefer more similar candidates first, then sort lexicographically
1832         // by their normalized string representation.
1833         let mut normalized_impl_candidates_and_similarities = impl_candidates
1834             .into_iter()
1835             .map(|ImplCandidate { trait_ref, similarity }| {
1836                 let normalized = normalize(trait_ref);
1837                 (similarity, normalized)
1838             })
1839             .collect::<Vec<_>>();
1840         normalized_impl_candidates_and_similarities.sort();
1841         normalized_impl_candidates_and_similarities.dedup();
1842
1843         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
1844             .into_iter()
1845             .map(|(_, normalized)| normalized)
1846             .collect::<Vec<_>>();
1847
1848         report(normalized_impl_candidates, err)
1849     }
1850
1851     /// Gets the parent trait chain start
1852     fn get_parent_trait_ref(
1853         &self,
1854         code: &ObligationCauseCode<'tcx>,
1855     ) -> Option<(String, Option<Span>)> {
1856         match code {
1857             ObligationCauseCode::BuiltinDerivedObligation(data) => {
1858                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
1859                 match self.get_parent_trait_ref(&data.parent_code) {
1860                     Some(t) => Some(t),
1861                     None => {
1862                         let ty = parent_trait_ref.skip_binder().self_ty();
1863                         let span = TyCategory::from_ty(self.tcx, ty)
1864                             .map(|(_, def_id)| self.tcx.def_span(def_id));
1865                         Some((ty.to_string(), span))
1866                     }
1867                 }
1868             }
1869             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
1870                 self.get_parent_trait_ref(&parent_code)
1871             }
1872             _ => None,
1873         }
1874     }
1875
1876     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1877     /// with the same path as `trait_ref`, a help message about
1878     /// a probable version mismatch is added to `err`
1879     fn note_version_mismatch(
1880         &self,
1881         err: &mut Diagnostic,
1882         trait_ref: &ty::PolyTraitRef<'tcx>,
1883     ) -> bool {
1884         let get_trait_impl = |trait_def_id| {
1885             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
1886         };
1887         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1888         let traits_with_same_path: std::collections::BTreeSet<_> = self
1889             .tcx
1890             .all_traits()
1891             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
1892             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
1893             .collect();
1894         let mut suggested = false;
1895         for trait_with_same_path in traits_with_same_path {
1896             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
1897                 let impl_span = self.tcx.def_span(impl_def_id);
1898                 err.span_help(impl_span, "trait impl with same name found");
1899                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1900                 let crate_msg = format!(
1901                     "perhaps two different versions of crate `{}` are being used?",
1902                     trait_crate
1903                 );
1904                 err.note(&crate_msg);
1905                 suggested = true;
1906             }
1907         }
1908         suggested
1909     }
1910
1911     fn mk_trait_obligation_with_new_self_ty(
1912         &self,
1913         param_env: ty::ParamEnv<'tcx>,
1914         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1915     ) -> PredicateObligation<'tcx> {
1916         let trait_pred = trait_ref_and_ty.map_bound_ref(|(tr, new_self_ty)| ty::TraitPredicate {
1917             trait_ref: ty::TraitRef {
1918                 substs: self.tcx.mk_substs_trait(*new_self_ty, &tr.trait_ref.substs[1..]),
1919                 ..tr.trait_ref
1920             },
1921             ..*tr
1922         });
1923
1924         Obligation::new(ObligationCause::dummy(), param_env, trait_pred.to_predicate(self.tcx))
1925     }
1926
1927     #[instrument(skip(self), level = "debug")]
1928     fn maybe_report_ambiguity(
1929         &self,
1930         obligation: &PredicateObligation<'tcx>,
1931         body_id: Option<hir::BodyId>,
1932     ) {
1933         // Unable to successfully determine, probably means
1934         // insufficient type information, but could mean
1935         // ambiguous impls. The latter *ought* to be a
1936         // coherence violation, so we don't report it here.
1937
1938         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1939         let span = obligation.cause.span;
1940
1941         debug!(?predicate, obligation.cause.code = tracing::field::debug(&obligation.cause.code()));
1942
1943         // Ambiguity errors are often caused as fallout from earlier errors.
1944         // We ignore them if this `infcx` is tainted in some cases below.
1945
1946         let bound_predicate = predicate.kind();
1947         let mut err = match bound_predicate.skip_binder() {
1948             ty::PredicateKind::Trait(data) => {
1949                 let trait_ref = bound_predicate.rebind(data.trait_ref);
1950                 debug!(?trait_ref);
1951
1952                 if predicate.references_error() {
1953                     return;
1954                 }
1955
1956                 // This is kind of a hack: it frequently happens that some earlier
1957                 // error prevents types from being fully inferred, and then we get
1958                 // a bunch of uninteresting errors saying something like "<generic
1959                 // #0> doesn't implement Sized".  It may even be true that we
1960                 // could just skip over all checks where the self-ty is an
1961                 // inference variable, but I was afraid that there might be an
1962                 // inference variable created, registered as an obligation, and
1963                 // then never forced by writeback, and hence by skipping here we'd
1964                 // be ignoring the fact that we don't KNOW the type works
1965                 // out. Though even that would probably be harmless, given that
1966                 // we're only talking about builtin traits, which are known to be
1967                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1968                 // avoid inundating the user with unnecessary errors, but we now
1969                 // check upstream for type errors and don't add the obligations to
1970                 // begin with in those cases.
1971                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
1972                     if !self.is_tainted_by_errors() {
1973                         self.emit_inference_failure_err(
1974                             body_id,
1975                             span,
1976                             trait_ref.self_ty().skip_binder().into(),
1977                             ErrorCode::E0282,
1978                             false,
1979                         )
1980                         .emit();
1981                     }
1982                     return;
1983                 }
1984
1985                 // Typically, this ambiguity should only happen if
1986                 // there are unresolved type inference variables
1987                 // (otherwise it would suggest a coherence
1988                 // failure). But given #21974 that is not necessarily
1989                 // the case -- we can have multiple where clauses that
1990                 // are only distinguished by a region, which results
1991                 // in an ambiguity even when all types are fully
1992                 // known, since we don't dispatch based on region
1993                 // relationships.
1994
1995                 // Pick the first substitution that still contains inference variables as the one
1996                 // we're going to emit an error for. If there are none (see above), fall back to
1997                 // a more general error.
1998                 let subst = data.trait_ref.substs.iter().find(|s| s.has_infer_types_or_consts());
1999
2000                 let mut err = if let Some(subst) = subst {
2001                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2002                 } else {
2003                     struct_span_err!(
2004                         self.tcx.sess,
2005                         span,
2006                         E0283,
2007                         "type annotations needed: cannot satisfy `{}`",
2008                         predicate,
2009                     )
2010                 };
2011
2012                 let obligation = Obligation::new(
2013                     obligation.cause.clone(),
2014                     obligation.param_env,
2015                     trait_ref.to_poly_trait_predicate(),
2016                 );
2017                 let mut selcx = SelectionContext::with_query_mode(
2018                     &self,
2019                     crate::traits::TraitQueryMode::Standard,
2020                 );
2021                 match selcx.select_from_obligation(&obligation) {
2022                     Err(SelectionError::Ambiguous(impls)) if impls.len() > 1 => {
2023                         self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2024                     }
2025                     _ => {
2026                         if self.is_tainted_by_errors() {
2027                             err.cancel();
2028                             return;
2029                         }
2030                         err.note(&format!("cannot satisfy `{}`", predicate));
2031                     }
2032                 }
2033
2034                 if let ObligationCauseCode::ItemObligation(def_id) = *obligation.cause.code() {
2035                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2036                 } else if let (
2037                     Ok(ref snippet),
2038                     &ObligationCauseCode::BindingObligation(def_id, _),
2039                 ) =
2040                     (self.tcx.sess.source_map().span_to_snippet(span), obligation.cause.code())
2041                 {
2042                     let generics = self.tcx.generics_of(def_id);
2043                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2044                         && !snippet.ends_with('>')
2045                         && !generics.has_impl_trait()
2046                         && !self.tcx.fn_trait_kind_from_lang_item(def_id).is_some()
2047                     {
2048                         // FIXME: To avoid spurious suggestions in functions where type arguments
2049                         // where already supplied, we check the snippet to make sure it doesn't
2050                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2051                         // instead. Otherwise we would produce the following output:
2052                         //
2053                         // error[E0283]: type annotations needed
2054                         //   --> $DIR/issue-54954.rs:3:24
2055                         //    |
2056                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2057                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2058                         //    |                        |
2059                         //    |                        cannot infer type
2060                         //    |                        help: consider specifying the type argument
2061                         //    |                        in the function call:
2062                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2063                         // ...
2064                         // LL |     const fn const_val<T: Sized>() -> usize {
2065                         //    |                        - required by this bound in `Tt::const_val`
2066                         //    |
2067                         //    = note: cannot satisfy `_: Tt`
2068
2069                         // Clear any more general suggestions in favor of our specific one
2070                         err.clear_suggestions();
2071
2072                         err.span_suggestion_verbose(
2073                             span.shrink_to_hi(),
2074                             &format!(
2075                                 "consider specifying the type argument{} in the function call",
2076                                 pluralize!(generics.params.len()),
2077                             ),
2078                             format!(
2079                                 "::<{}>",
2080                                 generics
2081                                     .params
2082                                     .iter()
2083                                     .map(|p| p.name.to_string())
2084                                     .collect::<Vec<String>>()
2085                                     .join(", ")
2086                             ),
2087                             Applicability::HasPlaceholders,
2088                         );
2089                     }
2090                 }
2091
2092                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2093                     (body_id, subst.map(|subst| subst.unpack()))
2094                 {
2095                     struct FindExprBySpan<'hir> {
2096                         span: Span,
2097                         result: Option<&'hir hir::Expr<'hir>>,
2098                     }
2099
2100                     impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
2101                         fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2102                             if self.span == ex.span {
2103                                 self.result = Some(ex);
2104                             } else {
2105                                 hir::intravisit::walk_expr(self, ex);
2106                             }
2107                         }
2108                     }
2109
2110                     let mut expr_finder = FindExprBySpan { span, result: None };
2111
2112                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2113
2114                     if let Some(hir::Expr {
2115                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2116                     ) = expr_finder.result
2117                         && let [
2118                             ..,
2119                             trait_path_segment @ hir::PathSegment {
2120                                 res: Some(rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id)),
2121                                 ..
2122                             },
2123                             hir::PathSegment {
2124                                 ident: assoc_item_name,
2125                                 res: Some(rustc_hir::def::Res::Def(_, item_id)),
2126                                 ..
2127                             }
2128                         ] = path.segments
2129                         && data.trait_ref.def_id == *trait_id
2130                         && self.tcx.trait_of_item(item_id) == Some(*trait_id)
2131                         && !self.is_tainted_by_errors()
2132                     {
2133                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2134                             ty::AssocKind::Const => ("refer to the", "constant"),
2135                             ty::AssocKind::Fn => ("call", "function"),
2136                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2137                         };
2138
2139                         // Replace the more general E0283 with a more specific error
2140                         err.cancel();
2141                         err = self.tcx.sess.struct_span_err_with_code(
2142                             span,
2143                             &format!(
2144                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2145                              ),
2146                             rustc_errors::error_code!(E0790),
2147                         );
2148
2149                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2150                             && 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)
2151                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2152                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2153                         }
2154
2155                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2156
2157                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2158
2159                         if trait_impls.blanket_impls().is_empty()
2160                             && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
2161                             && let Some(impl_def_id) = impl_ty.def() {
2162                             let message = if trait_impls.non_blanket_impls().len() == 1 {
2163                                 "use the fully-qualified path to the only available implementation".to_string()
2164                             } else {
2165                                 format!(
2166                                     "use a fully-qualified path to a specific available implementation ({} found)",
2167                                     trait_impls.non_blanket_impls().len()
2168                                 )
2169                             };
2170
2171                             err.multipart_suggestion(
2172                                 message,
2173                                 vec![
2174                                     (trait_path_segment.ident.span.shrink_to_lo(), format!("<{} as ", self.tcx.def_path(impl_def_id).to_string_no_crate_verbose())),
2175                                     (trait_path_segment.ident.span.shrink_to_hi(), format!(">"))
2176                                 ],
2177                                 Applicability::MaybeIncorrect
2178                             );
2179                         }
2180                     }
2181                 };
2182
2183                 err
2184             }
2185
2186             ty::PredicateKind::WellFormed(arg) => {
2187                 // Same hacky approach as above to avoid deluging user
2188                 // with error messages.
2189                 if arg.references_error()
2190                     || self.tcx.sess.has_errors().is_some()
2191                     || self.is_tainted_by_errors()
2192                 {
2193                     return;
2194                 }
2195
2196                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2197             }
2198
2199             ty::PredicateKind::Subtype(data) => {
2200                 if data.references_error()
2201                     || self.tcx.sess.has_errors().is_some()
2202                     || self.is_tainted_by_errors()
2203                 {
2204                     // no need to overload user in such cases
2205                     return;
2206                 }
2207                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2208                 // both must be type variables, or the other would've been instantiated
2209                 assert!(a.is_ty_var() && b.is_ty_var());
2210                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2211             }
2212             ty::PredicateKind::Projection(data) => {
2213                 if predicate.references_error() || self.is_tainted_by_errors() {
2214                     return;
2215                 }
2216                 let subst = data
2217                     .projection_ty
2218                     .substs
2219                     .iter()
2220                     .chain(Some(data.term.into_arg()))
2221                     .find(|g| g.has_infer_types_or_consts());
2222                 if let Some(subst) = subst {
2223                     let mut err = self.emit_inference_failure_err(
2224                         body_id,
2225                         span,
2226                         subst,
2227                         ErrorCode::E0284,
2228                         true,
2229                     );
2230                     err.note(&format!("cannot satisfy `{}`", predicate));
2231                     err
2232                 } else {
2233                     // If we can't find a substitution, just print a generic error
2234                     let mut err = struct_span_err!(
2235                         self.tcx.sess,
2236                         span,
2237                         E0284,
2238                         "type annotations needed: cannot satisfy `{}`",
2239                         predicate,
2240                     );
2241                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2242                     err
2243                 }
2244             }
2245
2246             ty::PredicateKind::ConstEvaluatable(data) => {
2247                 if predicate.references_error() || self.is_tainted_by_errors() {
2248                     return;
2249                 }
2250                 let subst = data.substs.iter().find(|g| g.has_infer_types_or_consts());
2251                 if let Some(subst) = subst {
2252                     let err = self.emit_inference_failure_err(
2253                         body_id,
2254                         span,
2255                         subst,
2256                         ErrorCode::E0284,
2257                         true,
2258                     );
2259                     err
2260                 } else {
2261                     // If we can't find a substitution, just print a generic error
2262                     let mut err = struct_span_err!(
2263                         self.tcx.sess,
2264                         span,
2265                         E0284,
2266                         "type annotations needed: cannot satisfy `{}`",
2267                         predicate,
2268                     );
2269                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2270                     err
2271                 }
2272             }
2273             _ => {
2274                 if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
2275                     return;
2276                 }
2277                 let mut err = struct_span_err!(
2278                     self.tcx.sess,
2279                     span,
2280                     E0284,
2281                     "type annotations needed: cannot satisfy `{}`",
2282                     predicate,
2283                 );
2284                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2285                 err
2286             }
2287         };
2288         self.note_obligation_cause(&mut err, obligation);
2289         err.emit();
2290     }
2291
2292     fn annotate_source_of_ambiguity(
2293         &self,
2294         err: &mut Diagnostic,
2295         impls: &[DefId],
2296         predicate: ty::Predicate<'tcx>,
2297     ) {
2298         let mut spans = vec![];
2299         let mut crates = vec![];
2300         let mut post = vec![];
2301         for def_id in impls {
2302             match self.tcx.span_of_impl(*def_id) {
2303                 Ok(span) => spans.push(span),
2304                 Err(name) => {
2305                     crates.push(name);
2306                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2307                         post.push(header);
2308                     }
2309                 }
2310             }
2311         }
2312         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2313         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2314         crate_names.sort();
2315         crate_names.dedup();
2316         post.sort();
2317         post.dedup();
2318
2319         if self.is_tainted_by_errors()
2320             && (crate_names.len() == 1
2321                 && spans.len() == 0
2322                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2323                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2324         {
2325             // Avoid complaining about other inference issues for expressions like
2326             // `42 >> 1`, where the types are still `{integer}`, but we want to
2327             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2328             // NOTE(eddyb) this was `.cancel()`, but `err`
2329             // is borrowed, so we can't fully defuse it.
2330             err.downgrade_to_delayed_bug();
2331             return;
2332         }
2333         let post = if post.len() > 4 {
2334             format!(
2335                 ":\n{}\nand {} more",
2336                 post.iter().map(|p| format!("- {}", p)).take(4).collect::<Vec<_>>().join("\n"),
2337                 post.len() - 4,
2338             )
2339         } else if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2340             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2341         } else if post.len() == 1 {
2342             format!(": `{}`", post[0])
2343         } else {
2344             String::new()
2345         };
2346
2347         match (spans.len(), crates.len(), crate_names.len()) {
2348             (0, 0, 0) => {
2349                 err.note(&format!("cannot satisfy `{}`", predicate));
2350             }
2351             (0, _, 1) => {
2352                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2353             }
2354             (0, _, _) => {
2355                 err.note(&format!(
2356                     "{} in the following crates: {}{}",
2357                     msg,
2358                     crate_names.join(", "),
2359                     post,
2360                 ));
2361             }
2362             (_, 0, 0) => {
2363                 let span: MultiSpan = spans.into();
2364                 err.span_note(span, &msg);
2365             }
2366             (_, 1, 1) => {
2367                 let span: MultiSpan = spans.into();
2368                 err.span_note(span, &msg);
2369                 err.note(
2370                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2371                 );
2372             }
2373             _ => {
2374                 let span: MultiSpan = spans.into();
2375                 err.span_note(span, &msg);
2376                 err.note(&format!(
2377                     "and more `impl`s found in the following crates: {}{}",
2378                     crate_names.join(", "),
2379                     post,
2380                 ));
2381             }
2382         }
2383     }
2384
2385     /// Returns `true` if the trait predicate may apply for *some* assignment
2386     /// to the type parameters.
2387     fn predicate_can_apply(
2388         &self,
2389         param_env: ty::ParamEnv<'tcx>,
2390         pred: ty::PolyTraitRef<'tcx>,
2391     ) -> bool {
2392         struct ParamToVarFolder<'a, 'tcx> {
2393             infcx: &'a InferCtxt<'a, 'tcx>,
2394             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2395         }
2396
2397         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2398             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2399                 self.infcx.tcx
2400             }
2401
2402             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2403                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2404                     let infcx = self.infcx;
2405                     *self.var_map.entry(ty).or_insert_with(|| {
2406                         infcx.next_ty_var(TypeVariableOrigin {
2407                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2408                             span: DUMMY_SP,
2409                         })
2410                     })
2411                 } else {
2412                     ty.super_fold_with(self)
2413                 }
2414             }
2415         }
2416
2417         self.probe(|_| {
2418             let mut selcx = SelectionContext::new(self);
2419
2420             let cleaned_pred =
2421                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2422
2423             let cleaned_pred = super::project::normalize(
2424                 &mut selcx,
2425                 param_env,
2426                 ObligationCause::dummy(),
2427                 cleaned_pred,
2428             )
2429             .value;
2430
2431             let obligation = Obligation::new(
2432                 ObligationCause::dummy(),
2433                 param_env,
2434                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
2435             );
2436
2437             self.predicate_may_hold(&obligation)
2438         })
2439     }
2440
2441     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2442         // First, attempt to add note to this error with an async-await-specific
2443         // message, and fall back to regular note otherwise.
2444         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2445             self.note_obligation_cause_code(
2446                 err,
2447                 &obligation.predicate,
2448                 obligation.param_env,
2449                 obligation.cause.code(),
2450                 &mut vec![],
2451                 &mut Default::default(),
2452             );
2453             self.suggest_unsized_bound_if_applicable(err, obligation);
2454         }
2455     }
2456
2457     #[instrument(level = "debug", skip_all)]
2458     fn suggest_unsized_bound_if_applicable(
2459         &self,
2460         err: &mut Diagnostic,
2461         obligation: &PredicateObligation<'tcx>,
2462     ) {
2463         let (
2464             ty::PredicateKind::Trait(pred),
2465             &ObligationCauseCode::BindingObligation(item_def_id, span),
2466         ) = (
2467             obligation.predicate.kind().skip_binder(),
2468             obligation.cause.code().peel_derives(),
2469         )  else {
2470             return;
2471         };
2472         debug!(?pred, ?item_def_id, ?span);
2473
2474         let (Some(node), true) = (
2475             self.tcx.hir().get_if_local(item_def_id),
2476             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2477         ) else {
2478             return;
2479         };
2480         self.maybe_suggest_unsized_generics(err, span, node);
2481     }
2482
2483     #[instrument(level = "debug", skip_all)]
2484     fn maybe_suggest_unsized_generics<'hir>(
2485         &self,
2486         err: &mut Diagnostic,
2487         span: Span,
2488         node: Node<'hir>,
2489     ) {
2490         let Some(generics) = node.generics() else {
2491             return;
2492         };
2493         let sized_trait = self.tcx.lang_items().sized_trait();
2494         debug!(?generics.params);
2495         debug!(?generics.predicates);
2496         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2497             return;
2498         };
2499         let param_def_id = self.tcx.hir().local_def_id(param.hir_id);
2500         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2501         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2502         let explicitly_sized = generics
2503             .bounds_for_param(param_def_id)
2504             .flat_map(|bp| bp.bounds)
2505             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2506         if explicitly_sized {
2507             return;
2508         }
2509         debug!(?param);
2510         match node {
2511             hir::Node::Item(
2512                 item @ hir::Item {
2513                     // Only suggest indirection for uses of type parameters in ADTs.
2514                     kind:
2515                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2516                     ..
2517                 },
2518             ) => {
2519                 if self.maybe_indirection_for_unsized(err, item, param) {
2520                     return;
2521                 }
2522             }
2523             _ => {}
2524         };
2525         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2526         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param_def_id)
2527         {
2528             (s, " +")
2529         } else {
2530             (span.shrink_to_hi(), ":")
2531         };
2532         err.span_suggestion_verbose(
2533             span,
2534             "consider relaxing the implicit `Sized` restriction",
2535             format!("{} ?Sized", separator),
2536             Applicability::MachineApplicable,
2537         );
2538     }
2539
2540     fn maybe_indirection_for_unsized<'hir>(
2541         &self,
2542         err: &mut Diagnostic,
2543         item: &'hir Item<'hir>,
2544         param: &'hir GenericParam<'hir>,
2545     ) -> bool {
2546         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2547         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2548         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2549         let mut visitor =
2550             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2551         visitor.visit_item(item);
2552         if visitor.invalid_spans.is_empty() {
2553             return false;
2554         }
2555         let mut multispan: MultiSpan = param.span.into();
2556         multispan.push_span_label(
2557             param.span,
2558             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2559         );
2560         for sp in visitor.invalid_spans {
2561             multispan.push_span_label(
2562                 sp,
2563                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2564             );
2565         }
2566         err.span_help(
2567             multispan,
2568             &format!(
2569                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2570                 used through indirection like `&{T}` or `Box<{T}>`",
2571                 T = param.name.ident(),
2572             ),
2573         );
2574         true
2575     }
2576
2577     fn is_recursive_obligation(
2578         &self,
2579         obligated_types: &mut Vec<Ty<'tcx>>,
2580         cause_code: &ObligationCauseCode<'tcx>,
2581     ) -> bool {
2582         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2583             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2584             let self_ty = parent_trait_ref.skip_binder().self_ty();
2585             if obligated_types.iter().any(|ot| ot == &self_ty) {
2586                 return true;
2587             }
2588             if let ty::Adt(def, substs) = self_ty.kind()
2589                 && let [arg] = &substs[..]
2590                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2591                 && let ty::Adt(inner_def, _) = ty.kind()
2592                 && inner_def == def
2593             {
2594                 return true;
2595             }
2596         }
2597         false
2598     }
2599 }
2600
2601 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2602 /// `param: ?Sized` would be a valid constraint.
2603 struct FindTypeParam {
2604     param: rustc_span::Symbol,
2605     invalid_spans: Vec<Span>,
2606     nested: bool,
2607 }
2608
2609 impl<'v> Visitor<'v> for FindTypeParam {
2610     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2611         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2612     }
2613
2614     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2615         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2616         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2617         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2618         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2619         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2620         // in that case should make what happened clear enough.
2621         match ty.kind {
2622             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2623             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2624                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2625             {
2626                 if !self.nested {
2627                     debug!(?ty, "FindTypeParam::visit_ty");
2628                     self.invalid_spans.push(ty.span);
2629                 }
2630             }
2631             hir::TyKind::Path(_) => {
2632                 let prev = self.nested;
2633                 self.nested = true;
2634                 hir::intravisit::walk_ty(self, ty);
2635                 self.nested = prev;
2636             }
2637             _ => {
2638                 hir::intravisit::walk_ty(self, ty);
2639             }
2640         }
2641     }
2642 }
2643
2644 pub fn recursive_type_with_infinite_size_error<'tcx>(
2645     tcx: TyCtxt<'tcx>,
2646     type_def_id: DefId,
2647     spans: Vec<(Span, Option<hir::HirId>)>,
2648 ) {
2649     assert!(type_def_id.is_local());
2650     let span = tcx.def_span(type_def_id);
2651     let path = tcx.def_path_str(type_def_id);
2652     let mut err =
2653         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
2654     err.span_label(span, "recursive type has infinite size");
2655     for &(span, _) in &spans {
2656         err.span_label(span, "recursive without indirection");
2657     }
2658     let msg = format!(
2659         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
2660         path,
2661     );
2662     if spans.len() <= 4 {
2663         // FIXME(compiler-errors): This suggestion might be erroneous if Box is shadowed
2664         err.multipart_suggestion(
2665             &msg,
2666             spans
2667                 .into_iter()
2668                 .flat_map(|(span, field_id)| {
2669                     if let Some(generic_span) = get_option_generic_from_field_id(tcx, field_id) {
2670                         // If we match an `Option` and can grab the span of the Option's generic, then
2671                         // suggest boxing the generic arg for a non-null niche optimization.
2672                         vec![
2673                             (generic_span.shrink_to_lo(), "Box<".to_string()),
2674                             (generic_span.shrink_to_hi(), ">".to_string()),
2675                         ]
2676                     } else {
2677                         vec![
2678                             (span.shrink_to_lo(), "Box<".to_string()),
2679                             (span.shrink_to_hi(), ">".to_string()),
2680                         ]
2681                     }
2682                 })
2683                 .collect(),
2684             Applicability::HasPlaceholders,
2685         );
2686     } else {
2687         err.help(&msg);
2688     }
2689     err.emit();
2690 }
2691
2692 /// Extract the span for the generic type `T` of `Option<T>` in a field definition
2693 fn get_option_generic_from_field_id(tcx: TyCtxt<'_>, field_id: Option<hir::HirId>) -> Option<Span> {
2694     let node = tcx.hir().find(field_id?);
2695
2696     // Expect a field from our field_id
2697     let Some(hir::Node::Field(field_def)) = node
2698         else { bug!("Expected HirId corresponding to FieldDef, found: {:?}", node) };
2699
2700     // Match a type that is a simple QPath with no Self
2701     let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = &field_def.ty.kind
2702         else { return None };
2703
2704     // Check if the path we're checking resolves to Option
2705     let hir::def::Res::Def(_, did) = path.res
2706         else { return None };
2707
2708     // Bail if this path doesn't describe `::core::option::Option`
2709     if !tcx.is_diagnostic_item(sym::Option, did) {
2710         return None;
2711     }
2712
2713     // Match a single generic arg in the 0th path segment
2714     let generic_arg = path.segments.last()?.args?.args.get(0)?;
2715
2716     // Take the span out of the type, if it's a type
2717     if let hir::GenericArg::Type(generic_ty) = generic_arg { Some(generic_ty.span) } else { None }
2718 }
2719
2720 /// Summarizes information
2721 #[derive(Clone)]
2722 pub enum ArgKind {
2723     /// An argument of non-tuple type. Parameters are (name, ty)
2724     Arg(String, String),
2725
2726     /// An argument of tuple type. For a "found" argument, the span is
2727     /// the location in the source of the pattern. For an "expected"
2728     /// argument, it will be None. The vector is a list of (name, ty)
2729     /// strings for the components of the tuple.
2730     Tuple(Option<Span>, Vec<(String, String)>),
2731 }
2732
2733 impl ArgKind {
2734     fn empty() -> ArgKind {
2735         ArgKind::Arg("_".to_owned(), "_".to_owned())
2736     }
2737
2738     /// Creates an `ArgKind` from the expected type of an
2739     /// argument. It has no name (`_`) and an optional source span.
2740     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2741         match t.kind() {
2742             ty::Tuple(tys) => ArgKind::Tuple(
2743                 span,
2744                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2745             ),
2746             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2747         }
2748     }
2749 }
2750
2751 struct HasNumericInferVisitor;
2752
2753 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2754     type BreakTy = ();
2755
2756     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2757         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2758             ControlFlow::Break(())
2759         } else {
2760             ControlFlow::CONTINUE
2761         }
2762     }
2763 }