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

   1 pub mod on_unimplemented;
   2 pub mod suggestions;
   3
   4 use super::{
   5     EvaluationResult, FulfillmentContext, FulfillmentError, FulfillmentErrorCode,
   6     MismatchedProjectionTypes, Obligation, ObligationCause, ObligationCauseCode,
   7     OnUnimplementedDirective, OnUnimplementedNote, OutputTypeParameterMismatch, Overflow,
   8     PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
   9 };
  10
  11 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  12 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  13 use crate::infer::{self, InferCtxt, TyCtxtInferExt};
  14 use rustc_data_structures::fx::FxHashMap;
  15 use rustc_errors::{
  16     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
  17     MultiSpan, Style,
  18 };
  19 use rustc_hir as hir;
  20 use rustc_hir::def_id::DefId;
  21 use rustc_hir::intravisit::Visitor;
  22 use rustc_hir::GenericParam;
  23 use rustc_hir::Item;
  24 use rustc_hir::Node;
  25 use rustc_infer::infer::error_reporting::same_type_modulo_infer;
  26 use rustc_infer::traits::TraitEngine;
  27 use rustc_middle::traits::select::OverflowError;
  28 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  29 use rustc_middle::ty::error::ExpectedFound;
  30 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  31 use rustc_middle::ty::{
  32     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  33     TypeVisitable,
  34 };
  35 use rustc_span::symbol::{kw, sym};
  36 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  37 use std::fmt;
  38 use std::iter;
  39 use std::ops::ControlFlow;
  40
  41 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  42 use crate::traits::query::normalize::AtExt as _;
  43 use crate::traits::specialize::to_pretty_impl_header;
  44 use on_unimplemented::InferCtxtExt as _;
  45 use suggestions::InferCtxtExt as _;
  46
  47 pub use rustc_infer::traits::error_reporting::*;
  48
  49 // When outputting impl candidates, prefer showing those that are more similar.
  50 //
  51 // We also compare candidates after skipping lifetimes, which has a lower
  52 // priority than exact matches.
  53 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  54 pub enum CandidateSimilarity {
  55     Exact { ignoring_lifetimes: bool },
  56     Fuzzy { ignoring_lifetimes: bool },
  57 }
  58
  59 #[derive(Debug, Clone, Copy)]
  60 pub struct ImplCandidate<'tcx> {
  61     pub trait_ref: ty::TraitRef<'tcx>,
  62     pub similarity: CandidateSimilarity,
  63 }
  64
  65 pub trait InferCtxtExt<'tcx> {
  66     fn report_fulfillment_errors(
  67         &self,
  68         errors: &[FulfillmentError<'tcx>],
  69         body_id: Option<hir::BodyId>,
  70         fallback_has_occurred: bool,
  71     ) -> ErrorGuaranteed;
  72
  73     fn report_overflow_error<T>(
  74         &self,
  75         obligation: &Obligation<'tcx, T>,
  76         suggest_increasing_limit: bool,
  77     ) -> !
  78     where
  79         T: fmt::Display + TypeFoldable<'tcx>;
  80
  81     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
  82
  83     /// The `root_obligation` parameter should be the `root_obligation` field
  84     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
  85     /// then it should be the same as `obligation`.
  86     fn report_selection_error(
  87         &self,
  88         obligation: PredicateObligation<'tcx>,
  89         root_obligation: &PredicateObligation<'tcx>,
  90         error: &SelectionError<'tcx>,
  91         fallback_has_occurred: bool,
  92     );
  93
  94     /// Given some node representing a fn-like thing in the HIR map,
  95     /// returns a span and `ArgKind` information that describes the
  96     /// arguments it expects. This can be supplied to
  97     /// `report_arg_count_mismatch`.
  98     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  99
 100     /// Reports an error when the number of arguments needed by a
 101     /// trait match doesn't match the number that the expression
 102     /// provides.
 103     fn report_arg_count_mismatch(
 104         &self,
 105         span: Span,
 106         found_span: Option<Span>,
 107         expected_args: Vec<ArgKind>,
 108         found_args: Vec<ArgKind>,
 109         is_closure: bool,
 110     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 111
 112     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
 113     /// in that order, and returns the generic type corresponding to the
 114     /// argument of that trait (corresponding to the closure arguments).
 115     fn type_implements_fn_trait(
 116         &self,
 117         param_env: ty::ParamEnv<'tcx>,
 118         ty: ty::Binder<'tcx, Ty<'tcx>>,
 119         constness: ty::BoundConstness,
 120         polarity: ty::ImplPolarity,
 121     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
 122 }
 123
 124 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 125     fn report_fulfillment_errors(
 126         &self,
 127         errors: &[FulfillmentError<'tcx>],
 128         body_id: Option<hir::BodyId>,
 129         fallback_has_occurred: bool,
 130     ) -> ErrorGuaranteed {
 131         #[derive(Debug)]
 132         struct ErrorDescriptor<'tcx> {
 133             predicate: ty::Predicate<'tcx>,
 134             index: Option<usize>, // None if this is an old error
 135         }
 136
 137         let mut error_map: FxHashMap<_, Vec<_>> = self
 138             .reported_trait_errors
 139             .borrow()
 140             .iter()
 141             .map(|(&span, predicates)| {
 142                 (
 143                     span,
 144                     predicates
 145                         .iter()
 146                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 147                         .collect(),
 148                 )
 149             })
 150             .collect();
 151
 152         for (index, error) in errors.iter().enumerate() {
 153             // We want to ignore desugarings here: spans are equivalent even
 154             // if one is the result of a desugaring and the other is not.
 155             let mut span = error.obligation.cause.span;
 156             let expn_data = span.ctxt().outer_expn_data();
 157             if let ExpnKind::Desugaring(_) = expn_data.kind {
 158                 span = expn_data.call_site;
 159             }
 160
 161             error_map.entry(span).or_default().push(ErrorDescriptor {
 162                 predicate: error.obligation.predicate,
 163                 index: Some(index),
 164             });
 165
 166             self.reported_trait_errors
 167                 .borrow_mut()
 168                 .entry(span)
 169                 .or_default()
 170                 .push(error.obligation.predicate);
 171         }
 172
 173         // We do this in 2 passes because we want to display errors in order, though
 174         // maybe it *is* better to sort errors by span or something.
 175         let mut is_suppressed = vec![false; errors.len()];
 176         for (_, error_set) in error_map.iter() {
 177             // We want to suppress "duplicate" errors with the same span.
 178             for error in error_set {
 179                 if let Some(index) = error.index {
 180                     // Suppress errors that are either:
 181                     // 1) strictly implied by another error.
 182                     // 2) implied by an error with a smaller index.
 183                     for error2 in error_set {
 184                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 185                             // Avoid errors being suppressed by already-suppressed
 186                             // errors, to prevent all errors from being suppressed
 187                             // at once.
 188                             continue;
 189                         }
 190
 191                         if self.error_implies(error2.predicate, error.predicate)
 192                             && !(error2.index >= error.index
 193                                 && self.error_implies(error.predicate, error2.predicate))
 194                         {
 195                             info!("skipping {:?} (implied by {:?})", error, error2);
 196                             is_suppressed[index] = true;
 197                             break;
 198                         }
 199                     }
 200                 }
 201             }
 202         }
 203
 204         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 205             if !suppressed {
 206                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 207             }
 208         }
 209
 210         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 211     }
 212
 213     /// Reports that an overflow has occurred and halts compilation. We
 214     /// halt compilation unconditionally because it is important that
 215     /// overflows never be masked -- they basically represent computations
 216     /// whose result could not be truly determined and thus we can't say
 217     /// if the program type checks or not -- and they are unusual
 218     /// occurrences in any case.
 219     fn report_overflow_error<T>(
 220         &self,
 221         obligation: &Obligation<'tcx, T>,
 222         suggest_increasing_limit: bool,
 223     ) -> !
 224     where
 225         T: fmt::Display + TypeFoldable<'tcx>,
 226     {
 227         let predicate = self.resolve_vars_if_possible(obligation.predicate.clone());
 228         let mut err = struct_span_err!(
 229             self.tcx.sess,
 230             obligation.cause.span,
 231             E0275,
 232             "overflow evaluating the requirement `{}`",
 233             predicate
 234         );
 235
 236         if suggest_increasing_limit {
 237             self.suggest_new_overflow_limit(&mut err);
 238         }
 239
 240         self.note_obligation_cause_code(
 241             &mut err,
 242             &obligation.predicate,
 243             obligation.param_env,
 244             obligation.cause.code(),
 245             &mut vec![],
 246             &mut Default::default(),
 247         );
 248
 249         err.emit();
 250         self.tcx.sess.abort_if_errors();
 251         bug!();
 252     }
 253
 254     /// Reports that a cycle was detected which led to overflow and halts
 255     /// compilation. This is equivalent to `report_overflow_error` except
 256     /// that we can give a more helpful error message (and, in particular,
 257     /// we do not suggest increasing the overflow limit, which is not
 258     /// going to help).
 259     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 260         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 261         assert!(!cycle.is_empty());
 262
 263         debug!(?cycle, "report_overflow_error_cycle");
 264
 265         // The 'deepest' obligation is most likely to have a useful
 266         // cause 'backtrace'
 267         self.report_overflow_error(cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(), false);
 268     }
 269
 270     fn report_selection_error(
 271         &self,
 272         mut obligation: PredicateObligation<'tcx>,
 273         root_obligation: &PredicateObligation<'tcx>,
 274         error: &SelectionError<'tcx>,
 275         fallback_has_occurred: bool,
 276     ) {
 277         self.set_tainted_by_errors();
 278         let tcx = self.tcx;
 279         let mut span = obligation.cause.span;
 280
 281         let mut err = match *error {
 282             SelectionError::Ambiguous(ref impls) => {
 283                 let mut err = self.tcx.sess.struct_span_err(
 284                     obligation.cause.span,
 285                     &format!("multiple applicable `impl`s for `{}`", obligation.predicate),
 286                 );
 287                 self.annotate_source_of_ambiguity(&mut err, impls, obligation.predicate);
 288                 err.emit();
 289                 return;
 290             }
 291             SelectionError::Unimplemented => {
 292                 // If this obligation was generated as a result of well-formedness checking, see if we
 293                 // can get a better error message by performing HIR-based well-formedness checking.
 294                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 295                     root_obligation.cause.code().peel_derives()
 296                 {
 297                     if let Some(cause) = self
 298                         .tcx
 299                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 300                     {
 301                         obligation.cause = cause.clone();
 302                         span = obligation.cause.span;
 303                     }
 304                 }
 305                 if let ObligationCauseCode::CompareImplMethodObligation {
 306                     impl_item_def_id,
 307                     trait_item_def_id,
 308                 }
 309                 | ObligationCauseCode::CompareImplTypeObligation {
 310                     impl_item_def_id,
 311                     trait_item_def_id,
 312                 } = *obligation.cause.code()
 313                 {
 314                     self.report_extra_impl_obligation(
 315                         span,
 316                         impl_item_def_id,
 317                         trait_item_def_id,
 318                         &format!("`{}`", obligation.predicate),
 319                     )
 320                     .emit();
 321                     return;
 322                 }
 323
 324                 let bound_predicate = obligation.predicate.kind();
 325                 match bound_predicate.skip_binder() {
 326                     ty::PredicateKind::Trait(trait_predicate) => {
 327                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 328                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 329
 330                         trait_predicate.remap_constness_diag(obligation.param_env);
 331                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 332                             == trait_predicate.skip_binder().constness;
 333
 334                         if self.tcx.sess.has_errors().is_some()
 335                             && trait_predicate.references_error()
 336                         {
 337                             return;
 338                         }
 339                         let trait_ref = trait_predicate.to_poly_trait_ref();
 340                         let (post_message, pre_message, type_def) = self
 341                             .get_parent_trait_ref(obligation.cause.code())
 342                             .map(|(t, s)| {
 343                                 (
 344                                     format!(" in `{}`", t),
 345                                     format!("within `{}`, ", t),
 346                                     s.map(|s| (format!("within this `{}`", t), s)),
 347                                 )
 348                             })
 349                             .unwrap_or_default();
 350
 351                         let OnUnimplementedNote {
 352                             message,
 353                             label,
 354                             note,
 355                             enclosing_scope,
 356                             append_const_msg,
 357                         } = self.on_unimplemented_note(trait_ref, &obligation);
 358                         let have_alt_message = message.is_some() || label.is_some();
 359                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 360                         let is_unsize =
 361                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 362                         let (message, note, append_const_msg) = if is_try_conversion {
 363                             (
 364                                 Some(format!(
 365                                     "`?` couldn't convert the error to `{}`",
 366                                     trait_ref.skip_binder().self_ty(),
 367                                 )),
 368                                 Some(
 369                                     "the question mark operation (`?`) implicitly performs a \
 370                                      conversion on the error value using the `From` trait"
 371                                         .to_owned(),
 372                                 ),
 373                                 Some(None),
 374                             )
 375                         } else {
 376                             (message, note, append_const_msg)
 377                         };
 378
 379                         let mut err = struct_span_err!(
 380                             self.tcx.sess,
 381                             span,
 382                             E0277,
 383                             "{}",
 384                             message
 385                                 .and_then(|cannot_do_this| {
 386                                     match (predicate_is_const, append_const_msg) {
 387                                         // do nothing if predicate is not const
 388                                         (false, _) => Some(cannot_do_this),
 389                                         // suggested using default post message
 390                                         (true, Some(None)) => {
 391                                             Some(format!("{cannot_do_this} in const contexts"))
 392                                         }
 393                                         // overridden post message
 394                                         (true, Some(Some(post_message))) => {
 395                                             Some(format!("{cannot_do_this}{post_message}"))
 396                                         }
 397                                         // fallback to generic message
 398                                         (true, None) => None,
 399                                     }
 400                                 })
 401                                 .unwrap_or_else(|| format!(
 402                                     "the trait bound `{}` is not satisfied{}",
 403                                     trait_predicate, post_message,
 404                                 ))
 405                         );
 406
 407                         if is_try_conversion {
 408                             let none_error = self
 409                                 .tcx
 410                                 .get_diagnostic_item(sym::none_error)
 411                                 .map(|def_id| tcx.type_of(def_id));
 412                             let should_convert_option_to_result =
 413                                 Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
 414                             let should_convert_result_to_option =
 415                                 Some(trait_ref.self_ty().skip_binder()) == none_error;
 416                             if should_convert_option_to_result {
 417                                 err.span_suggestion_verbose(
 418                                     span.shrink_to_lo(),
 419                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 420                                      using `Option::ok_or` or `Option::ok_or_else`",
 421                                     ".ok_or_else(|| /* error value */)",
 422                                     Applicability::HasPlaceholders,
 423                                 );
 424                             } else if should_convert_result_to_option {
 425                                 err.span_suggestion_verbose(
 426                                     span.shrink_to_lo(),
 427                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 428                                      using `Result::ok`",
 429                                     ".ok()",
 430                                     Applicability::MachineApplicable,
 431                                 );
 432                             }
 433                             if let Some(ret_span) = self.return_type_span(&obligation) {
 434                                 err.span_label(
 435                                     ret_span,
 436                                     &format!(
 437                                         "expected `{}` because of this",
 438                                         trait_ref.skip_binder().self_ty()
 439                                     ),
 440                                 );
 441                             }
 442                         }
 443
 444                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 445                             && predicate_is_const
 446                         {
 447                             err.note("`~const Drop` was renamed to `~const Destruct`");
 448                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 449                         }
 450
 451                         let explanation = if let ObligationCauseCode::MainFunctionType =
 452                             obligation.cause.code()
 453                         {
 454                             "consider using `()`, or a `Result`".to_owned()
 455                         } else {
 456                             format!(
 457                                 "{}the trait `{}` is not implemented for `{}`",
 458                                 pre_message,
 459                                 trait_predicate.print_modifiers_and_trait_path(),
 460                                 trait_ref.skip_binder().self_ty(),
 461                             )
 462                         };
 463
 464                         if self.suggest_add_reference_to_arg(
 465                             &obligation,
 466                             &mut err,
 467                             trait_predicate,
 468                             have_alt_message,
 469                         ) {
 470                             self.note_obligation_cause(&mut err, &obligation);
 471                             err.emit();
 472                             return;
 473                         }
 474                         if let Some(ref s) = label {
 475                             // If it has a custom `#[rustc_on_unimplemented]`
 476                             // error message, let's display it as the label!
 477                             err.span_label(span, s);
 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);
 535                         }
 536
 537                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 538                         let mut suggested =
 539                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 540                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 541                         suggested |=
 542                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 543                         suggested |= self.suggest_semicolon_removal(
 544                             &obligation,
 545                             &mut err,
 546                             span,
 547                             trait_predicate,
 548                         );
 549                         self.note_version_mismatch(&mut err, &trait_ref);
 550                         self.suggest_remove_await(&obligation, &mut err);
 551                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 552
 553                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 554                             self.suggest_await_before_try(
 555                                 &mut err,
 556                                 &obligation,
 557                                 trait_predicate,
 558                                 span,
 559                             );
 560                         }
 561
 562                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 563                             err.emit();
 564                             return;
 565                         }
 566
 567                         if is_unsize {
 568                             // If the obligation failed due to a missing implementation of the
 569                             // `Unsize` trait, give a pointer to why that might be the case
 570                             err.note(
 571                                 "all implementations of `Unsize` are provided \
 572                                 automatically by the compiler, see \
 573                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 574                                 for more information",
 575                             );
 576                         }
 577
 578                         let is_fn_trait = [
 579                             self.tcx.lang_items().fn_trait(),
 580                             self.tcx.lang_items().fn_mut_trait(),
 581                             self.tcx.lang_items().fn_once_trait(),
 582                         ]
 583                         .contains(&Some(trait_ref.def_id()));
 584                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 585                             *trait_ref.skip_binder().self_ty().kind()
 586                         {
 587                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 588                         } else {
 589                             false
 590                         };
 591                         if is_fn_trait && is_target_feature_fn {
 592                             err.note(
 593                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 594                             );
 595                         }
 596
 597                         // Try to report a help message
 598                         if is_fn_trait
 599                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 600                                 obligation.param_env,
 601                                 trait_ref.self_ty(),
 602                                 trait_predicate.skip_binder().constness,
 603                                 trait_predicate.skip_binder().polarity,
 604                             )
 605                         {
 606                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 607                             // suggestion to add trait bounds for the type, since we only typically implement
 608                             // these traits once.
 609
 610                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 611                             // to implement.
 612                             let selected_kind =
 613                                 ty::ClosureKind::from_def_id(self.tcx, trait_ref.def_id())
 614                                     .expect("expected to map DefId to ClosureKind");
 615                             if !implemented_kind.extends(selected_kind) {
 616                                 err.note(
 617                                     &format!(
 618                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 619                                         trait_ref.skip_binder().self_ty(),
 620                                         implemented_kind,
 621                                         selected_kind
 622                                     )
 623                                 );
 624                             }
 625
 626                             // Note any argument mismatches
 627                             let given_ty = params.skip_binder();
 628                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 629                             if let ty::Tuple(given) = given_ty.kind()
 630                                 && let ty::Tuple(expected) = expected_ty.kind()
 631                             {
 632                                 if expected.len() != given.len() {
 633                                     // Note number of types that were expected and given
 634                                     err.note(
 635                                         &format!(
 636                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 637                                             given.len(),
 638                                             if given.len() == 1 { "" } else { "s" },
 639                                             expected.len(),
 640                                             if expected.len() == 1 { "" } else { "s" },
 641                                         )
 642                                     );
 643                                 } else if !same_type_modulo_infer(given_ty, expected_ty) {
 644                                     // Print type mismatch
 645                                     let (expected_args, given_args) =
 646                                         self.cmp(given_ty, expected_ty);
 647                                     err.note_expected_found(
 648                                         &"a closure with arguments",
 649                                         expected_args,
 650                                         &"a closure with arguments",
 651                                         given_args,
 652                                     );
 653                                 }
 654                             }
 655                         } else if !trait_ref.has_infer_types_or_consts()
 656                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 657                         {
 658                             // If a where-clause may be useful, remind the
 659                             // user that they can add it.
 660                             //
 661                             // don't display an on-unimplemented note, as
 662                             // these notes will often be of the form
 663                             //     "the type `T` can't be frobnicated"
 664                             // which is somewhat confusing.
 665                             self.suggest_restricting_param_bound(
 666                                 &mut err,
 667                                 trait_predicate,
 668                                 None,
 669                                 obligation.cause.body_id,
 670                             );
 671                         } else if !suggested {
 672                             // Can't show anything else useful, try to find similar impls.
 673                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 674                             if !self.report_similar_impl_candidates(
 675                                 impl_candidates,
 676                                 trait_ref,
 677                                 obligation.cause.body_id,
 678                                 &mut err,
 679                             ) {
 680                                 // This is *almost* equivalent to
 681                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 682                                 // trait predicate for that corresponding root obligation. This
 683                                 // lets us get a derived obligation from a type parameter, like
 684                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 685                                 // implementer of `Pattern<'_>`.
 686                                 let mut code = obligation.cause.code();
 687                                 let mut trait_pred = trait_predicate;
 688                                 let mut peeled = false;
 689                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
 690                                     code = parent_code;
 691                                     if let Some(parent_trait_pred) = parent_trait_pred {
 692                                         trait_pred = parent_trait_pred;
 693                                         peeled = true;
 694                                     }
 695                                 }
 696                                 let def_id = trait_pred.def_id();
 697                                 // Mention *all* the `impl`s for the *top most* obligation, the
 698                                 // user might have meant to use one of them, if any found. We skip
 699                                 // auto-traits or fundamental traits that might not be exactly what
 700                                 // the user might expect to be presented with. Instead this is
 701                                 // useful for less general traits.
 702                                 if peeled
 703                                     && !self.tcx.trait_is_auto(def_id)
 704                                     && !self.tcx.lang_items().items().contains(&Some(def_id))
 705                                 {
 706                                     let trait_ref = trait_pred.to_poly_trait_ref();
 707                                     let impl_candidates =
 708                                         self.find_similar_impl_candidates(trait_ref);
 709                                     self.report_similar_impl_candidates(
 710                                         impl_candidates,
 711                                         trait_ref,
 712                                         obligation.cause.body_id,
 713                                         &mut err,
 714                                     );
 715                                 }
 716                             }
 717                         }
 718
 719                         // Changing mutability doesn't make a difference to whether we have
 720                         // an `Unsize` impl (Fixes ICE in #71036)
 721                         if !is_unsize {
 722                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
 723                         }
 724
 725                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 726                         // implemented, and fallback has occurred, then it could be due to a
 727                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 728                         // note informing about the change in behaviour.
 729                         if trait_predicate.skip_binder().self_ty().is_never()
 730                             && fallback_has_occurred
 731                         {
 732                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 733                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 734                                     self.tcx.mk_unit(),
 735                                     &trait_pred.trait_ref.substs[1..],
 736                                 );
 737                                 trait_pred
 738                             });
 739                             let unit_obligation = obligation.with(predicate.to_predicate(tcx));
 740                             if self.predicate_may_hold(&unit_obligation) {
 741                                 err.note(
 742                                     "this error might have been caused by changes to \
 743                                     Rust's type-inference algorithm (see issue #48950 \
 744                                     <https://github.com/rust-lang/rust/issues/48950> \
 745                                     for more information)",
 746                                 );
 747                                 err.help("did you intend to use the type `()` here instead?");
 748                             }
 749                         }
 750
 751                         // Return early if the trait is Debug or Display and the invocation
 752                         // originates within a standard library macro, because the output
 753                         // is otherwise overwhelming and unhelpful (see #85844 for an
 754                         // example).
 755
 756                         let in_std_macro =
 757                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
 758                                 Some(macro_def_id) => {
 759                                     let crate_name = tcx.crate_name(macro_def_id.krate);
 760                                     crate_name == sym::std || crate_name == sym::core
 761                                 }
 762                                 None => false,
 763                             };
 764
 765                         if in_std_macro
 766                             && matches!(
 767                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
 768                                 Some(sym::Debug | sym::Display)
 769                             )
 770                         {
 771                             err.emit();
 772                             return;
 773                         }
 774
 775                         err
 776                     }
 777
 778                     ty::PredicateKind::Subtype(predicate) => {
 779                         // Errors for Subtype predicates show up as
 780                         // `FulfillmentErrorCode::CodeSubtypeError`,
 781                         // not selection error.
 782                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 783                     }
 784
 785                     ty::PredicateKind::Coerce(predicate) => {
 786                         // Errors for Coerce predicates show up as
 787                         // `FulfillmentErrorCode::CodeSubtypeError`,
 788                         // not selection error.
 789                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
 790                     }
 791
 792                     ty::PredicateKind::RegionOutlives(..)
 793                     | ty::PredicateKind::Projection(..)
 794                     | ty::PredicateKind::TypeOutlives(..) => {
 795                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
 796                         struct_span_err!(
 797                             self.tcx.sess,
 798                             span,
 799                             E0280,
 800                             "the requirement `{}` is not satisfied",
 801                             predicate
 802                         )
 803                     }
 804
 805                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
 806                         let violations = self.tcx.object_safety_violations(trait_def_id);
 807                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 808                     }
 809
 810                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 811                         let found_kind = self.closure_kind(closure_substs).unwrap();
 812                         let closure_span = self.tcx.def_span(closure_def_id);
 813                         let mut err = struct_span_err!(
 814                             self.tcx.sess,
 815                             closure_span,
 816                             E0525,
 817                             "expected a closure that implements the `{}` trait, \
 818                              but this closure only implements `{}`",
 819                             kind,
 820                             found_kind
 821                         );
 822
 823                         err.span_label(
 824                             closure_span,
 825                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 826                         );
 827                         err.span_label(
 828                             obligation.cause.span,
 829                             format!("the requirement to implement `{}` derives from here", kind),
 830                         );
 831
 832                         // Additional context information explaining why the closure only implements
 833                         // a particular trait.
 834                         if let Some(typeck_results) = self.in_progress_typeck_results {
 835                             let hir_id = self
 836                                 .tcx
 837                                 .hir()
 838                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
 839                             let typeck_results = typeck_results.borrow();
 840                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
 841                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
 842                                     err.span_label(
 843                                         *span,
 844                                         format!(
 845                                             "closure is `FnOnce` because it moves the \
 846                                          variable `{}` out of its environment",
 847                                             ty::place_to_string_for_capture(tcx, place)
 848                                         ),
 849                                     );
 850                                 }
 851                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
 852                                     err.span_label(
 853                                         *span,
 854                                         format!(
 855                                             "closure is `FnMut` because it mutates the \
 856                                          variable `{}` here",
 857                                             ty::place_to_string_for_capture(tcx, place)
 858                                         ),
 859                                     );
 860                                 }
 861                                 _ => {}
 862                             }
 863                         }
 864
 865                         err.emit();
 866                         return;
 867                     }
 868
 869                     ty::PredicateKind::WellFormed(ty) => {
 870                         if !self.tcx.sess.opts.unstable_opts.chalk {
 871                             // WF predicates cannot themselves make
 872                             // errors. They can only block due to
 873                             // ambiguity; otherwise, they always
 874                             // degenerate into other obligations
 875                             // (which may fail).
 876                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 877                         } else {
 878                             // FIXME: we'll need a better message which takes into account
 879                             // which bounds actually failed to hold.
 880                             self.tcx.sess.struct_span_err(
 881                                 span,
 882                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 883                             )
 884                         }
 885                     }
 886
 887                     ty::PredicateKind::ConstEvaluatable(..) => {
 888                         // Errors for `ConstEvaluatable` predicates show up as
 889                         // `SelectionError::ConstEvalFailure`,
 890                         // not `Unimplemented`.
 891                         span_bug!(
 892                             span,
 893                             "const-evaluatable requirement gave wrong error: `{:?}`",
 894                             obligation
 895                         )
 896                     }
 897
 898                     ty::PredicateKind::ConstEquate(..) => {
 899                         // Errors for `ConstEquate` predicates show up as
 900                         // `SelectionError::ConstEvalFailure`,
 901                         // not `Unimplemented`.
 902                         span_bug!(
 903                             span,
 904                             "const-equate requirement gave wrong error: `{:?}`",
 905                             obligation
 906                         )
 907                     }
 908
 909                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
 910                         span,
 911                         "TypeWellFormedFromEnv predicate should only exist in the environment"
 912                     ),
 913                 }
 914             }
 915
 916             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
 917                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
 918                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
 919
 920                 if expected_trait_ref.self_ty().references_error() {
 921                     return;
 922                 }
 923
 924                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
 925                     return;
 926                 };
 927
 928                 let found_did = match *found_trait_ty.kind() {
 929                     ty::Closure(did, _)
 930                     | ty::Foreign(did)
 931                     | ty::FnDef(did, _)
 932                     | ty::Generator(did, ..) => Some(did),
 933                     ty::Adt(def, _) => Some(def.did()),
 934                     _ => None,
 935                 };
 936
 937                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
 938
 939                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 940                     // We check closures twice, with obligations flowing in different directions,
 941                     // but we want to complain about them only once.
 942                     return;
 943                 }
 944
 945                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 946
 947                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
 948                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 949                     _ => vec![ArgKind::empty()],
 950                 };
 951
 952                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 953                 let expected = match expected_ty.kind() {
 954                     ty::Tuple(ref tys) => {
 955                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
 956                     }
 957                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 958                 };
 959
 960                 if found.len() == expected.len() {
 961                     self.report_closure_arg_mismatch(
 962                         span,
 963                         found_span,
 964                         found_trait_ref,
 965                         expected_trait_ref,
 966                     )
 967                 } else {
 968                     let (closure_span, found) = found_did
 969                         .and_then(|did| {
 970                             let node = self.tcx.hir().get_if_local(did)?;
 971                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 972                             Some((Some(found_span), found))
 973                         })
 974                         .unwrap_or((found_span, found));
 975
 976                     self.report_arg_count_mismatch(
 977                         span,
 978                         closure_span,
 979                         expected,
 980                         found,
 981                         found_trait_ty.is_closure(),
 982                     )
 983                 }
 984             }
 985
 986             TraitNotObjectSafe(did) => {
 987                 let violations = self.tcx.object_safety_violations(did);
 988                 report_object_safety_error(self.tcx, span, did, violations)
 989             }
 990
 991             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
 992                 bug!(
 993                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
 994                 )
 995             }
 996             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
 997                 if !self.tcx.features().generic_const_exprs {
 998                     let mut err = self.tcx.sess.struct_span_err(
 999                         span,
1000                         "constant expression depends on a generic parameter",
1001                     );
1002                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1003                     // issue. However, this is currently not actually possible
1004                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1005                     //
1006                     // Note that with `feature(generic_const_exprs)` this case should not
1007                     // be reachable.
1008                     err.note("this may fail depending on what value the parameter takes");
1009                     err.emit();
1010                     return;
1011                 }
1012
1013                 match obligation.predicate.kind().skip_binder() {
1014                     ty::PredicateKind::ConstEvaluatable(uv) => {
1015                         let mut err =
1016                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1017                         let const_span = self.tcx.def_span(uv.def.did);
1018                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1019                             Ok(snippet) => err.help(&format!(
1020                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1021                                 snippet
1022                             )),
1023                             _ => err.help("consider adding a `where` bound using this expression"),
1024                         };
1025                         err
1026                     }
1027                     _ => {
1028                         span_bug!(
1029                             span,
1030                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1031                         )
1032                     }
1033                 }
1034             }
1035
1036             // Already reported in the query.
1037             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1038                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1039                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1040                 return;
1041             }
1042             // Already reported.
1043             Overflow(OverflowError::Error(_)) => {
1044                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1045                 return;
1046             }
1047             Overflow(_) => {
1048                 bug!("overflow should be handled before the `report_selection_error` path");
1049             }
1050             SelectionError::ErrorReporting => {
1051                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1052             }
1053         };
1054
1055         self.note_obligation_cause(&mut err, &obligation);
1056         self.point_at_returns_when_relevant(&mut err, &obligation);
1057
1058         err.emit();
1059     }
1060
1061     /// Given some node representing a fn-like thing in the HIR map,
1062     /// returns a span and `ArgKind` information that describes the
1063     /// arguments it expects. This can be supplied to
1064     /// `report_arg_count_mismatch`.
1065     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
1066         let sm = self.tcx.sess.source_map();
1067         let hir = self.tcx.hir();
1068         Some(match node {
1069             Node::Expr(&hir::Expr {
1070                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, .. }),
1071                 ..
1072             }) => (
1073                 fn_decl_span,
1074                 hir.body(body)
1075                     .params
1076                     .iter()
1077                     .map(|arg| {
1078                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
1079                             *arg.pat
1080                         {
1081                             Some(ArgKind::Tuple(
1082                                 Some(span),
1083                                 args.iter()
1084                                     .map(|pat| {
1085                                         sm.span_to_snippet(pat.span)
1086                                             .ok()
1087                                             .map(|snippet| (snippet, "_".to_owned()))
1088                                     })
1089                                     .collect::<Option<Vec<_>>>()?,
1090                             ))
1091                         } else {
1092                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
1093                             Some(ArgKind::Arg(name, "_".to_owned()))
1094                         }
1095                     })
1096                     .collect::<Option<Vec<ArgKind>>>()?,
1097             ),
1098             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
1099             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
1100             | Node::TraitItem(&hir::TraitItem {
1101                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
1102             }) => (
1103                 sig.span,
1104                 sig.decl
1105                     .inputs
1106                     .iter()
1107                     .map(|arg| match arg.kind {
1108                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1109                             Some(arg.span),
1110                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
1111                         ),
1112                         _ => ArgKind::empty(),
1113                     })
1114                     .collect::<Vec<ArgKind>>(),
1115             ),
1116             Node::Ctor(ref variant_data) => {
1117                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
1118                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1119             }
1120             _ => panic!("non-FnLike node found: {:?}", node),
1121         })
1122     }
1123
1124     /// Reports an error when the number of arguments needed by a
1125     /// trait match doesn't match the number that the expression
1126     /// provides.
1127     fn report_arg_count_mismatch(
1128         &self,
1129         span: Span,
1130         found_span: Option<Span>,
1131         expected_args: Vec<ArgKind>,
1132         found_args: Vec<ArgKind>,
1133         is_closure: bool,
1134     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
1135         let kind = if is_closure { "closure" } else { "function" };
1136
1137         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1138             let arg_length = arguments.len();
1139             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
1140             match (arg_length, arguments.get(0)) {
1141                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1142                     format!("a single {}-tuple as argument", fields.len())
1143                 }
1144                 _ => format!(
1145                     "{} {}argument{}",
1146                     arg_length,
1147                     if distinct && arg_length > 1 { "distinct " } else { "" },
1148                     pluralize!(arg_length)
1149                 ),
1150             }
1151         };
1152
1153         let expected_str = args_str(&expected_args, &found_args);
1154         let found_str = args_str(&found_args, &expected_args);
1155
1156         let mut err = struct_span_err!(
1157             self.tcx.sess,
1158             span,
1159             E0593,
1160             "{} is expected to take {}, but it takes {}",
1161             kind,
1162             expected_str,
1163             found_str,
1164         );
1165
1166         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1167
1168         if let Some(found_span) = found_span {
1169             err.span_label(found_span, format!("takes {}", found_str));
1170
1171             // move |_| { ... }
1172             // ^^^^^^^^-- def_span
1173             //
1174             // move |_| { ... }
1175             // ^^^^^-- prefix
1176             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1177             // move |_| { ... }
1178             //      ^^^-- pipe_span
1179             let pipe_span =
1180                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
1181
1182             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1183             // found arguments is empty (assume the user just wants to ignore args in this case).
1184             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1185             if found_args.is_empty() && is_closure {
1186                 let underscores = vec!["_"; expected_args.len()].join(", ");
1187                 err.span_suggestion_verbose(
1188                     pipe_span,
1189                     &format!(
1190                         "consider changing the closure to take and ignore the expected argument{}",
1191                         pluralize!(expected_args.len())
1192                     ),
1193                     format!("|{}|", underscores),
1194                     Applicability::MachineApplicable,
1195                 );
1196             }
1197
1198             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1199                 if fields.len() == expected_args.len() {
1200                     let sugg = fields
1201                         .iter()
1202                         .map(|(name, _)| name.to_owned())
1203                         .collect::<Vec<String>>()
1204                         .join(", ");
1205                     err.span_suggestion_verbose(
1206                         found_span,
1207                         "change the closure to take multiple arguments instead of a single tuple",
1208                         format!("|{}|", sugg),
1209                         Applicability::MachineApplicable,
1210                     );
1211                 }
1212             }
1213             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
1214                 && fields.len() == found_args.len()
1215                 && is_closure
1216             {
1217                 let sugg = format!(
1218                     "|({}){}|",
1219                     found_args
1220                         .iter()
1221                         .map(|arg| match arg {
1222                             ArgKind::Arg(name, _) => name.to_owned(),
1223                             _ => "_".to_owned(),
1224                         })
1225                         .collect::<Vec<String>>()
1226                         .join(", "),
1227                     // add type annotations if available
1228                     if found_args.iter().any(|arg| match arg {
1229                         ArgKind::Arg(_, ty) => ty != "_",
1230                         _ => false,
1231                     }) {
1232                         format!(
1233                             ": ({})",
1234                             fields
1235                                 .iter()
1236                                 .map(|(_, ty)| ty.to_owned())
1237                                 .collect::<Vec<String>>()
1238                                 .join(", ")
1239                         )
1240                     } else {
1241                         String::new()
1242                     },
1243                 );
1244                 err.span_suggestion_verbose(
1245                     found_span,
1246                     "change the closure to accept a tuple instead of individual arguments",
1247                     sugg,
1248                     Applicability::MachineApplicable,
1249                 );
1250             }
1251         }
1252
1253         err
1254     }
1255
1256     fn type_implements_fn_trait(
1257         &self,
1258         param_env: ty::ParamEnv<'tcx>,
1259         ty: ty::Binder<'tcx, Ty<'tcx>>,
1260         constness: ty::BoundConstness,
1261         polarity: ty::ImplPolarity,
1262     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
1263         self.commit_if_ok(|_| {
1264             for trait_def_id in [
1265                 self.tcx.lang_items().fn_trait(),
1266                 self.tcx.lang_items().fn_mut_trait(),
1267                 self.tcx.lang_items().fn_once_trait(),
1268             ] {
1269                 let Some(trait_def_id) = trait_def_id else { continue };
1270                 // Make a fresh inference variable so we can determine what the substitutions
1271                 // of the trait are.
1272                 let var = self.next_ty_var(TypeVariableOrigin {
1273                     span: DUMMY_SP,
1274                     kind: TypeVariableOriginKind::MiscVariable,
1275                 });
1276                 let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
1277                 let obligation = Obligation::new(
1278                     ObligationCause::dummy(),
1279                     param_env,
1280                     ty.rebind(ty::TraitPredicate {
1281                         trait_ref: ty::TraitRef::new(trait_def_id, substs),
1282                         constness,
1283                         polarity,
1284                     })
1285                     .to_predicate(self.tcx),
1286                 );
1287                 let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
1288                 fulfill_cx.register_predicate_obligation(self, obligation);
1289                 if fulfill_cx.select_all_or_error(self).is_empty() {
1290                     return Ok((
1291                         ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
1292                             .expect("expected to map DefId to ClosureKind"),
1293                         ty.rebind(self.resolve_vars_if_possible(var)),
1294                     ));
1295                 }
1296             }
1297
1298             Err(())
1299         })
1300     }
1301 }
1302
1303 trait InferCtxtPrivExt<'hir, 'tcx> {
1304     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1305     // `error` occurring implies that `cond` occurs.
1306     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1307
1308     fn report_fulfillment_error(
1309         &self,
1310         error: &FulfillmentError<'tcx>,
1311         body_id: Option<hir::BodyId>,
1312         fallback_has_occurred: bool,
1313     );
1314
1315     fn report_projection_error(
1316         &self,
1317         obligation: &PredicateObligation<'tcx>,
1318         error: &MismatchedProjectionTypes<'tcx>,
1319     );
1320
1321     fn fuzzy_match_tys(
1322         &self,
1323         a: Ty<'tcx>,
1324         b: Ty<'tcx>,
1325         ignoring_lifetimes: bool,
1326     ) -> Option<CandidateSimilarity>;
1327
1328     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1329
1330     fn find_similar_impl_candidates(
1331         &self,
1332         trait_ref: ty::PolyTraitRef<'tcx>,
1333     ) -> Vec<ImplCandidate<'tcx>>;
1334
1335     fn report_similar_impl_candidates(
1336         &self,
1337         impl_candidates: Vec<ImplCandidate<'tcx>>,
1338         trait_ref: ty::PolyTraitRef<'tcx>,
1339         body_id: hir::HirId,
1340         err: &mut Diagnostic,
1341     ) -> bool;
1342
1343     /// Gets the parent trait chain start
1344     fn get_parent_trait_ref(
1345         &self,
1346         code: &ObligationCauseCode<'tcx>,
1347     ) -> Option<(String, Option<Span>)>;
1348
1349     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1350     /// with the same path as `trait_ref`, a help message about
1351     /// a probable version mismatch is added to `err`
1352     fn note_version_mismatch(
1353         &self,
1354         err: &mut Diagnostic,
1355         trait_ref: &ty::PolyTraitRef<'tcx>,
1356     ) -> bool;
1357
1358     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1359     /// `trait_ref`.
1360     ///
1361     /// For this to work, `new_self_ty` must have no escaping bound variables.
1362     fn mk_trait_obligation_with_new_self_ty(
1363         &self,
1364         param_env: ty::ParamEnv<'tcx>,
1365         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1366     ) -> PredicateObligation<'tcx>;
1367
1368     fn maybe_report_ambiguity(
1369         &self,
1370         obligation: &PredicateObligation<'tcx>,
1371         body_id: Option<hir::BodyId>,
1372     );
1373
1374     fn predicate_can_apply(
1375         &self,
1376         param_env: ty::ParamEnv<'tcx>,
1377         pred: ty::PolyTraitRef<'tcx>,
1378     ) -> bool;
1379
1380     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1381
1382     fn suggest_unsized_bound_if_applicable(
1383         &self,
1384         err: &mut Diagnostic,
1385         obligation: &PredicateObligation<'tcx>,
1386     );
1387
1388     fn annotate_source_of_ambiguity(
1389         &self,
1390         err: &mut Diagnostic,
1391         impls: &[DefId],
1392         predicate: ty::Predicate<'tcx>,
1393     );
1394
1395     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'hir>);
1396
1397     fn maybe_indirection_for_unsized(
1398         &self,
1399         err: &mut Diagnostic,
1400         item: &'hir Item<'hir>,
1401         param: &'hir GenericParam<'hir>,
1402     ) -> bool;
1403
1404     fn is_recursive_obligation(
1405         &self,
1406         obligated_types: &mut Vec<Ty<'tcx>>,
1407         cause_code: &ObligationCauseCode<'tcx>,
1408     ) -> bool;
1409 }
1410
1411 impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
1412     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1413     // `error` occurring implies that `cond` occurs.
1414     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1415         if cond == error {
1416             return true;
1417         }
1418
1419         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1420         let bound_error = error.kind();
1421         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1422             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
1423                 (cond, bound_error.rebind(error))
1424             }
1425             _ => {
1426                 // FIXME: make this work in other cases too.
1427                 return false;
1428             }
1429         };
1430
1431         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1432             let bound_predicate = obligation.predicate.kind();
1433             if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
1434                 let error = error.to_poly_trait_ref();
1435                 let implication = bound_predicate.rebind(implication.trait_ref);
1436                 // FIXME: I'm just not taking associated types at all here.
1437                 // Eventually I'll need to implement param-env-aware
1438                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1439                 let param_env = ty::ParamEnv::empty();
1440                 if self.can_sub(param_env, error, implication).is_ok() {
1441                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1442                     return true;
1443                 }
1444             }
1445         }
1446
1447         false
1448     }
1449
1450     #[instrument(skip(self), level = "debug")]
1451     fn report_fulfillment_error(
1452         &self,
1453         error: &FulfillmentError<'tcx>,
1454         body_id: Option<hir::BodyId>,
1455         fallback_has_occurred: bool,
1456     ) {
1457         match error.code {
1458             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1459                 self.report_selection_error(
1460                     error.obligation.clone(),
1461                     &error.root_obligation,
1462                     selection_error,
1463                     fallback_has_occurred,
1464                 );
1465             }
1466             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1467                 self.report_projection_error(&error.obligation, e);
1468             }
1469             FulfillmentErrorCode::CodeAmbiguity => {
1470                 self.maybe_report_ambiguity(&error.obligation, body_id);
1471             }
1472             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1473                 self.report_mismatched_types(
1474                     &error.obligation.cause,
1475                     expected_found.expected,
1476                     expected_found.found,
1477                     err.clone(),
1478                 )
1479                 .emit();
1480             }
1481             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1482                 self.report_mismatched_consts(
1483                     &error.obligation.cause,
1484                     expected_found.expected,
1485                     expected_found.found,
1486                     err.clone(),
1487                 )
1488                 .emit();
1489             }
1490         }
1491     }
1492
1493     #[instrument(level = "debug", skip_all)]
1494     fn report_projection_error(
1495         &self,
1496         obligation: &PredicateObligation<'tcx>,
1497         error: &MismatchedProjectionTypes<'tcx>,
1498     ) {
1499         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1500
1501         if predicate.references_error() {
1502             return;
1503         }
1504
1505         self.probe(|_| {
1506             let err_buf;
1507             let mut err = &error.err;
1508             let mut values = None;
1509
1510             // try to find the mismatched types to report the error with.
1511             //
1512             // this can fail if the problem was higher-ranked, in which
1513             // cause I have no idea for a good error message.
1514             let bound_predicate = predicate.kind();
1515             if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
1516                 let mut selcx = SelectionContext::new(self);
1517                 let data = self.replace_bound_vars_with_fresh_vars(
1518                     obligation.cause.span,
1519                     infer::LateBoundRegionConversionTime::HigherRankedType,
1520                     bound_predicate.rebind(data),
1521                 );
1522                 let mut obligations = vec![];
1523                 let normalized_ty = super::normalize_projection_type(
1524                     &mut selcx,
1525                     obligation.param_env,
1526                     data.projection_ty,
1527                     obligation.cause.clone(),
1528                     0,
1529                     &mut obligations,
1530                 );
1531
1532                 debug!(?obligation.cause, ?obligation.param_env);
1533
1534                 debug!(?normalized_ty, data.ty = ?data.term);
1535
1536                 let is_normalized_ty_expected = !matches!(
1537                     obligation.cause.code().peel_derives(),
1538                     ObligationCauseCode::ItemObligation(_)
1539                         | ObligationCauseCode::BindingObligation(_, _)
1540                         | ObligationCauseCode::ObjectCastObligation(..)
1541                         | ObligationCauseCode::OpaqueType
1542                 );
1543                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1544                     is_normalized_ty_expected,
1545                     normalized_ty,
1546                     data.term,
1547                 ) {
1548                     values = Some(infer::ValuePairs::Terms(ExpectedFound::new(
1549                         is_normalized_ty_expected,
1550                         normalized_ty,
1551                         data.term,
1552                     )));
1553                     err_buf = error;
1554                     err = &err_buf;
1555                 }
1556             }
1557
1558             let mut diag = struct_span_err!(
1559                 self.tcx.sess,
1560                 obligation.cause.span,
1561                 E0271,
1562                 "type mismatch resolving `{}`",
1563                 predicate
1564             );
1565             let secondary_span = match predicate.kind().skip_binder() {
1566                 ty::PredicateKind::Projection(proj) => self
1567                     .tcx
1568                     .opt_associated_item(proj.projection_ty.item_def_id)
1569                     .and_then(|trait_assoc_item| {
1570                         self.tcx
1571                             .trait_of_item(proj.projection_ty.item_def_id)
1572                             .map(|id| (trait_assoc_item, id))
1573                     })
1574                     .and_then(|(trait_assoc_item, id)| {
1575                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1576                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1577                             self.tcx
1578                                 .associated_items(did)
1579                                 .in_definition_order()
1580                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1581                         })
1582                     })
1583                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1584                         Some(
1585                             hir::Node::TraitItem(hir::TraitItem {
1586                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1587                                 ..
1588                             })
1589                             | hir::Node::ImplItem(hir::ImplItem {
1590                                 kind: hir::ImplItemKind::TyAlias(ty),
1591                                 ..
1592                             }),
1593                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1594                         _ => None,
1595                     }),
1596                 _ => None,
1597             };
1598             self.note_type_err(
1599                 &mut diag,
1600                 &obligation.cause,
1601                 secondary_span,
1602                 values,
1603                 err,
1604                 true,
1605                 false,
1606             );
1607             self.note_obligation_cause(&mut diag, obligation);
1608             diag.emit();
1609         });
1610     }
1611
1612     fn fuzzy_match_tys(
1613         &self,
1614         mut a: Ty<'tcx>,
1615         mut b: Ty<'tcx>,
1616         ignoring_lifetimes: bool,
1617     ) -> Option<CandidateSimilarity> {
1618         /// returns the fuzzy category of a given type, or None
1619         /// if the type can be equated to any type.
1620         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1621             match t.kind() {
1622                 ty::Bool => Some(0),
1623                 ty::Char => Some(1),
1624                 ty::Str => Some(2),
1625                 ty::Adt(def, _) if tcx.is_diagnostic_item(sym::String, def.did()) => Some(2),
1626                 ty::Int(..)
1627                 | ty::Uint(..)
1628                 | ty::Float(..)
1629                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1630                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1631                 ty::Array(..) | ty::Slice(..) => Some(6),
1632                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1633                 ty::Dynamic(..) => Some(8),
1634                 ty::Closure(..) => Some(9),
1635                 ty::Tuple(..) => Some(10),
1636                 ty::Param(..) => Some(11),
1637                 ty::Projection(..) => Some(12),
1638                 ty::Opaque(..) => Some(13),
1639                 ty::Never => Some(14),
1640                 ty::Adt(..) => Some(15),
1641                 ty::Generator(..) => Some(16),
1642                 ty::Foreign(..) => Some(17),
1643                 ty::GeneratorWitness(..) => Some(18),
1644                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1645             }
1646         }
1647
1648         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1649             loop {
1650                 match t.kind() {
1651                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1652                         t = *inner
1653                     }
1654                     _ => break t,
1655                 }
1656             }
1657         };
1658
1659         if !ignoring_lifetimes {
1660             a = strip_references(a);
1661             b = strip_references(b);
1662         }
1663
1664         let cat_a = type_category(self.tcx, a)?;
1665         let cat_b = type_category(self.tcx, b)?;
1666         if a == b {
1667             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1668         } else if cat_a == cat_b {
1669             match (a.kind(), b.kind()) {
1670                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1671                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1672                 // Matching on references results in a lot of unhelpful
1673                 // suggestions, so let's just not do that for now.
1674                 //
1675                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1676                 // to prioritize that impl.
1677                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1678                     self.fuzzy_match_tys(a, b, true).is_some()
1679                 }
1680                 _ => true,
1681             }
1682             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1683         } else if ignoring_lifetimes {
1684             None
1685         } else {
1686             self.fuzzy_match_tys(a, b, true)
1687         }
1688     }
1689
1690     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1691         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1692             hir::GeneratorKind::Gen => "a generator",
1693             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1694             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1695             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1696         })
1697     }
1698
1699     fn find_similar_impl_candidates(
1700         &self,
1701         trait_ref: ty::PolyTraitRef<'tcx>,
1702     ) -> Vec<ImplCandidate<'tcx>> {
1703         self.tcx
1704             .all_impls(trait_ref.def_id())
1705             .filter_map(|def_id| {
1706                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative {
1707                     return None;
1708                 }
1709
1710                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1711
1712                 self.fuzzy_match_tys(trait_ref.skip_binder().self_ty(), imp.self_ty(), false)
1713                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1714             })
1715             .collect()
1716     }
1717
1718     fn report_similar_impl_candidates(
1719         &self,
1720         impl_candidates: Vec<ImplCandidate<'tcx>>,
1721         trait_ref: ty::PolyTraitRef<'tcx>,
1722         body_id: hir::HirId,
1723         err: &mut Diagnostic,
1724     ) -> bool {
1725         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1726             candidates.sort();
1727             candidates.dedup();
1728             let len = candidates.len();
1729             if candidates.len() == 0 {
1730                 return false;
1731             }
1732             if candidates.len() == 1 {
1733                 err.highlighted_help(vec![
1734                     (
1735                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1736                         Style::NoStyle,
1737                     ),
1738                     ("is".to_string(), Style::Highlight),
1739                     (" implemented for `".to_string(), Style::NoStyle),
1740                     (candidates[0].self_ty().to_string(), Style::Highlight),
1741                     ("`".to_string(), Style::NoStyle),
1742                 ]);
1743                 return true;
1744             }
1745             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1746             // Check if the trait is the same in all cases. If so, we'll only show the type.
1747             let mut traits: Vec<_> =
1748                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1749             traits.sort();
1750             traits.dedup();
1751
1752             let mut candidates: Vec<String> = candidates
1753                 .into_iter()
1754                 .map(|c| {
1755                     if traits.len() == 1 {
1756                         format!("\n  {}", c.self_ty())
1757                     } else {
1758                         format!("\n  {}", c)
1759                     }
1760                 })
1761                 .collect();
1762
1763             candidates.sort();
1764             candidates.dedup();
1765             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1766             err.help(&format!(
1767                 "the following other types implement trait `{}`:{}{}",
1768                 trait_ref.print_only_trait_path(),
1769                 candidates[..end].join(""),
1770                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1771             ));
1772             true
1773         };
1774
1775         let def_id = trait_ref.def_id();
1776         if impl_candidates.is_empty() {
1777             if self.tcx.trait_is_auto(def_id)
1778                 || self.tcx.lang_items().items().contains(&Some(def_id))
1779                 || self.tcx.get_diagnostic_name(def_id).is_some()
1780             {
1781                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1782                 return false;
1783             }
1784             let normalized_impl_candidates: Vec<_> = self
1785                 .tcx
1786                 .all_impls(def_id)
1787                 // Ignore automatically derived impls and `!Trait` impls.
1788                 .filter(|&def_id| {
1789                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1790                         || self.tcx.is_builtin_derive(def_id)
1791                 })
1792                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1793                 .filter(|trait_ref| {
1794                     let self_ty = trait_ref.self_ty();
1795                     // Avoid mentioning type parameters.
1796                     if let ty::Param(_) = self_ty.kind() {
1797                         false
1798                     }
1799                     // Avoid mentioning types that are private to another crate
1800                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
1801                         // FIXME(compiler-errors): This could be generalized, both to
1802                         // be more granular, and probably look past other `#[fundamental]`
1803                         // types, too.
1804                         self.tcx
1805                             .visibility(def.did())
1806                             .is_accessible_from(body_id.owner.to_def_id(), self.tcx)
1807                     } else {
1808                         true
1809                     }
1810                 })
1811                 .collect();
1812             return report(normalized_impl_candidates, err);
1813         }
1814
1815         let normalize = |candidate| {
1816             self.tcx.infer_ctxt().enter(|ref infcx| {
1817                 let normalized = infcx
1818                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1819                     .normalize(candidate)
1820                     .ok();
1821                 match normalized {
1822                     Some(normalized) => normalized.value,
1823                     None => candidate,
1824                 }
1825             })
1826         };
1827
1828         // Sort impl candidates so that ordering is consistent for UI tests.
1829         // because the ordering of `impl_candidates` may not be deterministic:
1830         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1831         //
1832         // Prefer more similar candidates first, then sort lexicographically
1833         // by their normalized string representation.
1834         let mut normalized_impl_candidates_and_similarities = impl_candidates
1835             .into_iter()
1836             .map(|ImplCandidate { trait_ref, similarity }| {
1837                 let normalized = normalize(trait_ref);
1838                 (similarity, normalized)
1839             })
1840             .collect::<Vec<_>>();
1841         normalized_impl_candidates_and_similarities.sort();
1842         normalized_impl_candidates_and_similarities.dedup();
1843
1844         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
1845             .into_iter()
1846             .map(|(_, normalized)| normalized)
1847             .collect::<Vec<_>>();
1848
1849         report(normalized_impl_candidates, err)
1850     }
1851
1852     /// Gets the parent trait chain start
1853     fn get_parent_trait_ref(
1854         &self,
1855         code: &ObligationCauseCode<'tcx>,
1856     ) -> Option<(String, Option<Span>)> {
1857         match code {
1858             ObligationCauseCode::BuiltinDerivedObligation(data) => {
1859                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
1860                 match self.get_parent_trait_ref(&data.parent_code) {
1861                     Some(t) => Some(t),
1862                     None => {
1863                         let ty = parent_trait_ref.skip_binder().self_ty();
1864                         let span = TyCategory::from_ty(self.tcx, ty)
1865                             .map(|(_, def_id)| self.tcx.def_span(def_id));
1866                         Some((ty.to_string(), span))
1867                     }
1868                 }
1869             }
1870             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
1871                 self.get_parent_trait_ref(&parent_code)
1872             }
1873             _ => None,
1874         }
1875     }
1876
1877     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1878     /// with the same path as `trait_ref`, a help message about
1879     /// a probable version mismatch is added to `err`
1880     fn note_version_mismatch(
1881         &self,
1882         err: &mut Diagnostic,
1883         trait_ref: &ty::PolyTraitRef<'tcx>,
1884     ) -> bool {
1885         let get_trait_impl = |trait_def_id| {
1886             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
1887         };
1888         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1889         let traits_with_same_path: std::collections::BTreeSet<_> = self
1890             .tcx
1891             .all_traits()
1892             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
1893             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
1894             .collect();
1895         let mut suggested = false;
1896         for trait_with_same_path in traits_with_same_path {
1897             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
1898                 let impl_span = self.tcx.def_span(impl_def_id);
1899                 err.span_help(impl_span, "trait impl with same name found");
1900                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1901                 let crate_msg = format!(
1902                     "perhaps two different versions of crate `{}` are being used?",
1903                     trait_crate
1904                 );
1905                 err.note(&crate_msg);
1906                 suggested = true;
1907             }
1908         }
1909         suggested
1910     }
1911
1912     fn mk_trait_obligation_with_new_self_ty(
1913         &self,
1914         param_env: ty::ParamEnv<'tcx>,
1915         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1916     ) -> PredicateObligation<'tcx> {
1917         let trait_pred = trait_ref_and_ty.map_bound_ref(|(tr, new_self_ty)| ty::TraitPredicate {
1918             trait_ref: ty::TraitRef {
1919                 substs: self.tcx.mk_substs_trait(*new_self_ty, &tr.trait_ref.substs[1..]),
1920                 ..tr.trait_ref
1921             },
1922             ..*tr
1923         });
1924
1925         Obligation::new(ObligationCause::dummy(), param_env, trait_pred.to_predicate(self.tcx))
1926     }
1927
1928     #[instrument(skip(self), level = "debug")]
1929     fn maybe_report_ambiguity(
1930         &self,
1931         obligation: &PredicateObligation<'tcx>,
1932         body_id: Option<hir::BodyId>,
1933     ) {
1934         // Unable to successfully determine, probably means
1935         // insufficient type information, but could mean
1936         // ambiguous impls. The latter *ought* to be a
1937         // coherence violation, so we don't report it here.
1938
1939         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1940         let span = obligation.cause.span;
1941
1942         debug!(?predicate, obligation.cause.code = tracing::field::debug(&obligation.cause.code()));
1943
1944         // Ambiguity errors are often caused as fallout from earlier errors.
1945         // We ignore them if this `infcx` is tainted in some cases below.
1946
1947         let bound_predicate = predicate.kind();
1948         let mut err = match bound_predicate.skip_binder() {
1949             ty::PredicateKind::Trait(data) => {
1950                 let trait_ref = bound_predicate.rebind(data.trait_ref);
1951                 debug!(?trait_ref);
1952
1953                 if predicate.references_error() {
1954                     return;
1955                 }
1956
1957                 // This is kind of a hack: it frequently happens that some earlier
1958                 // error prevents types from being fully inferred, and then we get
1959                 // a bunch of uninteresting errors saying something like "<generic
1960                 // #0> doesn't implement Sized".  It may even be true that we
1961                 // could just skip over all checks where the self-ty is an
1962                 // inference variable, but I was afraid that there might be an
1963                 // inference variable created, registered as an obligation, and
1964                 // then never forced by writeback, and hence by skipping here we'd
1965                 // be ignoring the fact that we don't KNOW the type works
1966                 // out. Though even that would probably be harmless, given that
1967                 // we're only talking about builtin traits, which are known to be
1968                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1969                 // avoid inundating the user with unnecessary errors, but we now
1970                 // check upstream for type errors and don't add the obligations to
1971                 // begin with in those cases.
1972                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
1973                     if !self.is_tainted_by_errors() {
1974                         self.emit_inference_failure_err(
1975                             body_id,
1976                             span,
1977                             trait_ref.self_ty().skip_binder().into(),
1978                             ErrorCode::E0282,
1979                             false,
1980                         )
1981                         .emit();
1982                     }
1983                     return;
1984                 }
1985
1986                 // Typically, this ambiguity should only happen if
1987                 // there are unresolved type inference variables
1988                 // (otherwise it would suggest a coherence
1989                 // failure). But given #21974 that is not necessarily
1990                 // the case -- we can have multiple where clauses that
1991                 // are only distinguished by a region, which results
1992                 // in an ambiguity even when all types are fully
1993                 // known, since we don't dispatch based on region
1994                 // relationships.
1995
1996                 // Pick the first substitution that still contains inference variables as the one
1997                 // we're going to emit an error for. If there are none (see above), fall back to
1998                 // a more general error.
1999                 let subst = data.trait_ref.substs.iter().find(|s| s.has_infer_types_or_consts());
2000
2001                 let mut err = if let Some(subst) = subst {
2002                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2003                 } else {
2004                     struct_span_err!(
2005                         self.tcx.sess,
2006                         span,
2007                         E0283,
2008                         "type annotations needed: cannot satisfy `{}`",
2009                         predicate,
2010                     )
2011                 };
2012
2013                 let obligation = Obligation::new(
2014                     obligation.cause.clone(),
2015                     obligation.param_env,
2016                     trait_ref.to_poly_trait_predicate(),
2017                 );
2018                 let mut selcx = SelectionContext::with_query_mode(
2019                     &self,
2020                     crate::traits::TraitQueryMode::Standard,
2021                 );
2022                 match selcx.select_from_obligation(&obligation) {
2023                     Err(SelectionError::Ambiguous(impls)) if impls.len() > 1 => {
2024                         self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2025                     }
2026                     _ => {
2027                         if self.is_tainted_by_errors() {
2028                             err.cancel();
2029                             return;
2030                         }
2031                         err.note(&format!("cannot satisfy `{}`", predicate));
2032                     }
2033                 }
2034
2035                 if let ObligationCauseCode::ItemObligation(def_id) = *obligation.cause.code() {
2036                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2037                 } else if let (
2038                     Ok(ref snippet),
2039                     &ObligationCauseCode::BindingObligation(def_id, _),
2040                 ) =
2041                     (self.tcx.sess.source_map().span_to_snippet(span), obligation.cause.code())
2042                 {
2043                     let generics = self.tcx.generics_of(def_id);
2044                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2045                         && !snippet.ends_with('>')
2046                         && !generics.has_impl_trait()
2047                         && !self.tcx.fn_trait_kind_from_lang_item(def_id).is_some()
2048                     {
2049                         // FIXME: To avoid spurious suggestions in functions where type arguments
2050                         // where already supplied, we check the snippet to make sure it doesn't
2051                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2052                         // instead. Otherwise we would produce the following output:
2053                         //
2054                         // error[E0283]: type annotations needed
2055                         //   --> $DIR/issue-54954.rs:3:24
2056                         //    |
2057                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2058                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2059                         //    |                        |
2060                         //    |                        cannot infer type
2061                         //    |                        help: consider specifying the type argument
2062                         //    |                        in the function call:
2063                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2064                         // ...
2065                         // LL |     const fn const_val<T: Sized>() -> usize {
2066                         //    |                        - required by this bound in `Tt::const_val`
2067                         //    |
2068                         //    = note: cannot satisfy `_: Tt`
2069
2070                         // Clear any more general suggestions in favor of our specific one
2071                         err.clear_suggestions();
2072
2073                         err.span_suggestion_verbose(
2074                             span.shrink_to_hi(),
2075                             &format!(
2076                                 "consider specifying the type argument{} in the function call",
2077                                 pluralize!(generics.params.len()),
2078                             ),
2079                             format!(
2080                                 "::<{}>",
2081                                 generics
2082                                     .params
2083                                     .iter()
2084                                     .map(|p| p.name.to_string())
2085                                     .collect::<Vec<String>>()
2086                                     .join(", ")
2087                             ),
2088                             Applicability::HasPlaceholders,
2089                         );
2090                     }
2091                 }
2092
2093                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2094                     (body_id, subst.map(|subst| subst.unpack()))
2095                 {
2096                     struct FindExprBySpan<'hir> {
2097                         span: Span,
2098                         result: Option<&'hir hir::Expr<'hir>>,
2099                     }
2100
2101                     impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
2102                         fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2103                             if self.span == ex.span {
2104                                 self.result = Some(ex);
2105                             } else {
2106                                 hir::intravisit::walk_expr(self, ex);
2107                             }
2108                         }
2109                     }
2110
2111                     let mut expr_finder = FindExprBySpan { span, result: None };
2112
2113                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2114
2115                     if let Some(hir::Expr {
2116                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2117                     ) = expr_finder.result
2118                         && let [
2119                             ..,
2120                             trait_path_segment @ hir::PathSegment {
2121                                 res: Some(rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id)),
2122                                 ..
2123                             },
2124                             hir::PathSegment {
2125                                 ident: assoc_item_name,
2126                                 res: Some(rustc_hir::def::Res::Def(_, item_id)),
2127                                 ..
2128                             }
2129                         ] = path.segments
2130                         && data.trait_ref.def_id == *trait_id
2131                         && self.tcx.trait_of_item(item_id) == Some(*trait_id)
2132                         && !self.is_tainted_by_errors()
2133                     {
2134                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2135                             ty::AssocKind::Const => ("refer to the", "constant"),
2136                             ty::AssocKind::Fn => ("call", "function"),
2137                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2138                         };
2139
2140                         // Replace the more general E0283 with a more specific error
2141                         err.cancel();
2142                         err = self.tcx.sess.struct_span_err_with_code(
2143                             span,
2144                             &format!(
2145                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2146                              ),
2147                             rustc_errors::error_code!(E0790),
2148                         );
2149
2150                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2151                             && let Some(hir::Node::Item(hir::Item { ident: trait_name, kind: hir::ItemKind::Trait(_, _, _, _, trait_item_refs), .. })) = self.tcx.hir().find_by_def_id(local_def_id)
2152                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2153                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2154                         }
2155
2156                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2157
2158                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2159
2160                         if trait_impls.blanket_impls().is_empty()
2161                             && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
2162                             && let Some(impl_def_id) = impl_ty.def() {
2163                             let message = if trait_impls.non_blanket_impls().len() == 1 {
2164                                 "use the fully-qualified path to the only available implementation".to_string()
2165                             } else {
2166                                 format!(
2167                                     "use a fully-qualified path to a specific available implementation ({} found)",
2168                                     trait_impls.non_blanket_impls().len()
2169                                 )
2170                             };
2171
2172                             err.multipart_suggestion(
2173                                 message,
2174                                 vec![
2175                                     (trait_path_segment.ident.span.shrink_to_lo(), format!("<{} as ", self.tcx.def_path(impl_def_id).to_string_no_crate_verbose())),
2176                                     (trait_path_segment.ident.span.shrink_to_hi(), format!(">"))
2177                                 ],
2178                                 Applicability::MaybeIncorrect
2179                             );
2180                         }
2181                     }
2182                 };
2183
2184                 err
2185             }
2186
2187             ty::PredicateKind::WellFormed(arg) => {
2188                 // Same hacky approach as above to avoid deluging user
2189                 // with error messages.
2190                 if arg.references_error()
2191                     || self.tcx.sess.has_errors().is_some()
2192                     || self.is_tainted_by_errors()
2193                 {
2194                     return;
2195                 }
2196
2197                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2198             }
2199
2200             ty::PredicateKind::Subtype(data) => {
2201                 if data.references_error()
2202                     || self.tcx.sess.has_errors().is_some()
2203                     || self.is_tainted_by_errors()
2204                 {
2205                     // no need to overload user in such cases
2206                     return;
2207                 }
2208                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2209                 // both must be type variables, or the other would've been instantiated
2210                 assert!(a.is_ty_var() && b.is_ty_var());
2211                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2212             }
2213             ty::PredicateKind::Projection(data) => {
2214                 if predicate.references_error() || self.is_tainted_by_errors() {
2215                     return;
2216                 }
2217                 let subst = data
2218                     .projection_ty
2219                     .substs
2220                     .iter()
2221                     .chain(Some(data.term.into_arg()))
2222                     .find(|g| g.has_infer_types_or_consts());
2223                 if let Some(subst) = subst {
2224                     let mut err = self.emit_inference_failure_err(
2225                         body_id,
2226                         span,
2227                         subst,
2228                         ErrorCode::E0284,
2229                         true,
2230                     );
2231                     err.note(&format!("cannot satisfy `{}`", predicate));
2232                     err
2233                 } else {
2234                     // If we can't find a substitution, just print a generic error
2235                     let mut err = struct_span_err!(
2236                         self.tcx.sess,
2237                         span,
2238                         E0284,
2239                         "type annotations needed: cannot satisfy `{}`",
2240                         predicate,
2241                     );
2242                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2243                     err
2244                 }
2245             }
2246
2247             ty::PredicateKind::ConstEvaluatable(data) => {
2248                 if predicate.references_error() || self.is_tainted_by_errors() {
2249                     return;
2250                 }
2251                 let subst = data.substs.iter().find(|g| g.has_infer_types_or_consts());
2252                 if let Some(subst) = subst {
2253                     let err = self.emit_inference_failure_err(
2254                         body_id,
2255                         span,
2256                         subst,
2257                         ErrorCode::E0284,
2258                         true,
2259                     );
2260                     err
2261                 } else {
2262                     // If we can't find a substitution, just print a generic error
2263                     let mut err = struct_span_err!(
2264                         self.tcx.sess,
2265                         span,
2266                         E0284,
2267                         "type annotations needed: cannot satisfy `{}`",
2268                         predicate,
2269                     );
2270                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2271                     err
2272                 }
2273             }
2274             _ => {
2275                 if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
2276                     return;
2277                 }
2278                 let mut err = struct_span_err!(
2279                     self.tcx.sess,
2280                     span,
2281                     E0284,
2282                     "type annotations needed: cannot satisfy `{}`",
2283                     predicate,
2284                 );
2285                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2286                 err
2287             }
2288         };
2289         self.note_obligation_cause(&mut err, obligation);
2290         err.emit();
2291     }
2292
2293     fn annotate_source_of_ambiguity(
2294         &self,
2295         err: &mut Diagnostic,
2296         impls: &[DefId],
2297         predicate: ty::Predicate<'tcx>,
2298     ) {
2299         let mut spans = vec![];
2300         let mut crates = vec![];
2301         let mut post = vec![];
2302         for def_id in impls {
2303             match self.tcx.span_of_impl(*def_id) {
2304                 Ok(span) => spans.push(span),
2305                 Err(name) => {
2306                     crates.push(name);
2307                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2308                         post.push(header);
2309                     }
2310                 }
2311             }
2312         }
2313         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2314         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2315         crate_names.sort();
2316         crate_names.dedup();
2317         post.sort();
2318         post.dedup();
2319
2320         if self.is_tainted_by_errors()
2321             && (crate_names.len() == 1
2322                 && spans.len() == 0
2323                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2324                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2325         {
2326             // Avoid complaining about other inference issues for expressions like
2327             // `42 >> 1`, where the types are still `{integer}`, but we want to
2328             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2329             // NOTE(eddyb) this was `.cancel()`, but `err`
2330             // is borrowed, so we can't fully defuse it.
2331             err.downgrade_to_delayed_bug();
2332             return;
2333         }
2334         let post = if post.len() > 4 {
2335             format!(
2336                 ":\n{}\nand {} more",
2337                 post.iter().map(|p| format!("- {}", p)).take(4).collect::<Vec<_>>().join("\n"),
2338                 post.len() - 4,
2339             )
2340         } else if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2341             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2342         } else if post.len() == 1 {
2343             format!(": `{}`", post[0])
2344         } else {
2345             String::new()
2346         };
2347
2348         match (spans.len(), crates.len(), crate_names.len()) {
2349             (0, 0, 0) => {
2350                 err.note(&format!("cannot satisfy `{}`", predicate));
2351             }
2352             (0, _, 1) => {
2353                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2354             }
2355             (0, _, _) => {
2356                 err.note(&format!(
2357                     "{} in the following crates: {}{}",
2358                     msg,
2359                     crate_names.join(", "),
2360                     post,
2361                 ));
2362             }
2363             (_, 0, 0) => {
2364                 let span: MultiSpan = spans.into();
2365                 err.span_note(span, &msg);
2366             }
2367             (_, 1, 1) => {
2368                 let span: MultiSpan = spans.into();
2369                 err.span_note(span, &msg);
2370                 err.note(
2371                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2372                 );
2373             }
2374             _ => {
2375                 let span: MultiSpan = spans.into();
2376                 err.span_note(span, &msg);
2377                 err.note(&format!(
2378                     "and more `impl`s found in the following crates: {}{}",
2379                     crate_names.join(", "),
2380                     post,
2381                 ));
2382             }
2383         }
2384     }
2385
2386     /// Returns `true` if the trait predicate may apply for *some* assignment
2387     /// to the type parameters.
2388     fn predicate_can_apply(
2389         &self,
2390         param_env: ty::ParamEnv<'tcx>,
2391         pred: ty::PolyTraitRef<'tcx>,
2392     ) -> bool {
2393         struct ParamToVarFolder<'a, 'tcx> {
2394             infcx: &'a InferCtxt<'a, 'tcx>,
2395             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2396         }
2397
2398         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2399             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2400                 self.infcx.tcx
2401             }
2402
2403             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2404                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2405                     let infcx = self.infcx;
2406                     *self.var_map.entry(ty).or_insert_with(|| {
2407                         infcx.next_ty_var(TypeVariableOrigin {
2408                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2409                             span: DUMMY_SP,
2410                         })
2411                     })
2412                 } else {
2413                     ty.super_fold_with(self)
2414                 }
2415             }
2416         }
2417
2418         self.probe(|_| {
2419             let mut selcx = SelectionContext::new(self);
2420
2421             let cleaned_pred =
2422                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2423
2424             let cleaned_pred = super::project::normalize(
2425                 &mut selcx,
2426                 param_env,
2427                 ObligationCause::dummy(),
2428                 cleaned_pred,
2429             )
2430             .value;
2431
2432             let obligation = Obligation::new(
2433                 ObligationCause::dummy(),
2434                 param_env,
2435                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
2436             );
2437
2438             self.predicate_may_hold(&obligation)
2439         })
2440     }
2441
2442     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2443         // First, attempt to add note to this error with an async-await-specific
2444         // message, and fall back to regular note otherwise.
2445         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2446             self.note_obligation_cause_code(
2447                 err,
2448                 &obligation.predicate,
2449                 obligation.param_env,
2450                 obligation.cause.code(),
2451                 &mut vec![],
2452                 &mut Default::default(),
2453             );
2454             self.suggest_unsized_bound_if_applicable(err, obligation);
2455         }
2456     }
2457
2458     #[instrument(level = "debug", skip_all)]
2459     fn suggest_unsized_bound_if_applicable(
2460         &self,
2461         err: &mut Diagnostic,
2462         obligation: &PredicateObligation<'tcx>,
2463     ) {
2464         let (
2465             ty::PredicateKind::Trait(pred),
2466             &ObligationCauseCode::BindingObligation(item_def_id, span),
2467         ) = (
2468             obligation.predicate.kind().skip_binder(),
2469             obligation.cause.code().peel_derives(),
2470         )  else {
2471             return;
2472         };
2473         debug!(?pred, ?item_def_id, ?span);
2474
2475         let (Some(node), true) = (
2476             self.tcx.hir().get_if_local(item_def_id),
2477             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2478         ) else {
2479             return;
2480         };
2481         self.maybe_suggest_unsized_generics(err, span, node);
2482     }
2483
2484     #[instrument(level = "debug", skip_all)]
2485     fn maybe_suggest_unsized_generics<'hir>(
2486         &self,
2487         err: &mut Diagnostic,
2488         span: Span,
2489         node: Node<'hir>,
2490     ) {
2491         let Some(generics) = node.generics() else {
2492             return;
2493         };
2494         let sized_trait = self.tcx.lang_items().sized_trait();
2495         debug!(?generics.params);
2496         debug!(?generics.predicates);
2497         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2498             return;
2499         };
2500         let param_def_id = self.tcx.hir().local_def_id(param.hir_id);
2501         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2502         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2503         let explicitly_sized = generics
2504             .bounds_for_param(param_def_id)
2505             .flat_map(|bp| bp.bounds)
2506             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2507         if explicitly_sized {
2508             return;
2509         }
2510         debug!(?param);
2511         match node {
2512             hir::Node::Item(
2513                 item @ hir::Item {
2514                     // Only suggest indirection for uses of type parameters in ADTs.
2515                     kind:
2516                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2517                     ..
2518                 },
2519             ) => {
2520                 if self.maybe_indirection_for_unsized(err, item, param) {
2521                     return;
2522                 }
2523             }
2524             _ => {}
2525         };
2526         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2527         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param_def_id)
2528         {
2529             (s, " +")
2530         } else {
2531             (span.shrink_to_hi(), ":")
2532         };
2533         err.span_suggestion_verbose(
2534             span,
2535             "consider relaxing the implicit `Sized` restriction",
2536             format!("{} ?Sized", separator),
2537             Applicability::MachineApplicable,
2538         );
2539     }
2540
2541     fn maybe_indirection_for_unsized<'hir>(
2542         &self,
2543         err: &mut Diagnostic,
2544         item: &'hir Item<'hir>,
2545         param: &'hir GenericParam<'hir>,
2546     ) -> bool {
2547         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2548         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2549         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2550         let mut visitor =
2551             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2552         visitor.visit_item(item);
2553         if visitor.invalid_spans.is_empty() {
2554             return false;
2555         }
2556         let mut multispan: MultiSpan = param.span.into();
2557         multispan.push_span_label(
2558             param.span,
2559             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2560         );
2561         for sp in visitor.invalid_spans {
2562             multispan.push_span_label(
2563                 sp,
2564                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2565             );
2566         }
2567         err.span_help(
2568             multispan,
2569             &format!(
2570                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2571                 used through indirection like `&{T}` or `Box<{T}>`",
2572                 T = param.name.ident(),
2573             ),
2574         );
2575         true
2576     }
2577
2578     fn is_recursive_obligation(
2579         &self,
2580         obligated_types: &mut Vec<Ty<'tcx>>,
2581         cause_code: &ObligationCauseCode<'tcx>,
2582     ) -> bool {
2583         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2584             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2585             let self_ty = parent_trait_ref.skip_binder().self_ty();
2586             if obligated_types.iter().any(|ot| ot == &self_ty) {
2587                 return true;
2588             }
2589             if let ty::Adt(def, substs) = self_ty.kind()
2590                 && let [arg] = &substs[..]
2591                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2592                 && let ty::Adt(inner_def, _) = ty.kind()
2593                 && inner_def == def
2594             {
2595                 return true;
2596             }
2597         }
2598         false
2599     }
2600 }
2601
2602 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2603 /// `param: ?Sized` would be a valid constraint.
2604 struct FindTypeParam {
2605     param: rustc_span::Symbol,
2606     invalid_spans: Vec<Span>,
2607     nested: bool,
2608 }
2609
2610 impl<'v> Visitor<'v> for FindTypeParam {
2611     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2612         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2613     }
2614
2615     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2616         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2617         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2618         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2619         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2620         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2621         // in that case should make what happened clear enough.
2622         match ty.kind {
2623             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2624             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2625                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2626             {
2627                 if !self.nested {
2628                     debug!(?ty, "FindTypeParam::visit_ty");
2629                     self.invalid_spans.push(ty.span);
2630                 }
2631             }
2632             hir::TyKind::Path(_) => {
2633                 let prev = self.nested;
2634                 self.nested = true;
2635                 hir::intravisit::walk_ty(self, ty);
2636                 self.nested = prev;
2637             }
2638             _ => {
2639                 hir::intravisit::walk_ty(self, ty);
2640             }
2641         }
2642     }
2643 }
2644
2645 pub fn recursive_type_with_infinite_size_error<'tcx>(
2646     tcx: TyCtxt<'tcx>,
2647     type_def_id: DefId,
2648     spans: Vec<(Span, Option<hir::HirId>)>,
2649 ) {
2650     assert!(type_def_id.is_local());
2651     let span = tcx.def_span(type_def_id);
2652     let path = tcx.def_path_str(type_def_id);
2653     let mut err =
2654         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
2655     err.span_label(span, "recursive type has infinite size");
2656     for &(span, _) in &spans {
2657         err.span_label(span, "recursive without indirection");
2658     }
2659     let msg = format!(
2660         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
2661         path,
2662     );
2663     if spans.len() <= 4 {
2664         // FIXME(compiler-errors): This suggestion might be erroneous if Box is shadowed
2665         err.multipart_suggestion(
2666             &msg,
2667             spans
2668                 .into_iter()
2669                 .flat_map(|(span, field_id)| {
2670                     if let Some(generic_span) = get_option_generic_from_field_id(tcx, field_id) {
2671                         // If we match an `Option` and can grab the span of the Option's generic, then
2672                         // suggest boxing the generic arg for a non-null niche optimization.
2673                         vec![
2674                             (generic_span.shrink_to_lo(), "Box<".to_string()),
2675                             (generic_span.shrink_to_hi(), ">".to_string()),
2676                         ]
2677                     } else {
2678                         vec![
2679                             (span.shrink_to_lo(), "Box<".to_string()),
2680                             (span.shrink_to_hi(), ">".to_string()),
2681                         ]
2682                     }
2683                 })
2684                 .collect(),
2685             Applicability::HasPlaceholders,
2686         );
2687     } else {
2688         err.help(&msg);
2689     }
2690     err.emit();
2691 }
2692
2693 /// Extract the span for the generic type `T` of `Option<T>` in a field definition
2694 fn get_option_generic_from_field_id(tcx: TyCtxt<'_>, field_id: Option<hir::HirId>) -> Option<Span> {
2695     let node = tcx.hir().find(field_id?);
2696
2697     // Expect a field from our field_id
2698     let Some(hir::Node::Field(field_def)) = node
2699         else { bug!("Expected HirId corresponding to FieldDef, found: {:?}", node) };
2700
2701     // Match a type that is a simple QPath with no Self
2702     let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = &field_def.ty.kind
2703         else { return None };
2704
2705     // Check if the path we're checking resolves to Option
2706     let hir::def::Res::Def(_, did) = path.res
2707         else { return None };
2708
2709     // Bail if this path doesn't describe `::core::option::Option`
2710     if !tcx.is_diagnostic_item(sym::Option, did) {
2711         return None;
2712     }
2713
2714     // Match a single generic arg in the 0th path segment
2715     let generic_arg = path.segments.last()?.args?.args.get(0)?;
2716
2717     // Take the span out of the type, if it's a type
2718     if let hir::GenericArg::Type(generic_ty) = generic_arg { Some(generic_ty.span) } else { None }
2719 }
2720
2721 /// Summarizes information
2722 #[derive(Clone)]
2723 pub enum ArgKind {
2724     /// An argument of non-tuple type. Parameters are (name, ty)
2725     Arg(String, String),
2726
2727     /// An argument of tuple type. For a "found" argument, the span is
2728     /// the location in the source of the pattern. For an "expected"
2729     /// argument, it will be None. The vector is a list of (name, ty)
2730     /// strings for the components of the tuple.
2731     Tuple(Option<Span>, Vec<(String, String)>),
2732 }
2733
2734 impl ArgKind {
2735     fn empty() -> ArgKind {
2736         ArgKind::Arg("_".to_owned(), "_".to_owned())
2737     }
2738
2739     /// Creates an `ArgKind` from the expected type of an
2740     /// argument. It has no name (`_`) and an optional source span.
2741     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2742         match t.kind() {
2743             ty::Tuple(tys) => ArgKind::Tuple(
2744                 span,
2745                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2746             ),
2747             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2748         }
2749     }
2750 }
2751
2752 struct HasNumericInferVisitor;
2753
2754 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2755     type BreakTy = ();
2756
2757     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2758         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2759             ControlFlow::Break(())
2760         } else {
2761             ControlFlow::CONTINUE
2762         }
2763     }
2764 }