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

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