src/librustc/traits/error_reporting.rs

   1 use super::{
   2     ConstEvalFailure, EvaluationResult, FulfillmentError, FulfillmentErrorCode,
   3     MismatchedProjectionTypes, ObjectSafetyViolation, Obligation, ObligationCause,
   4     ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote,
   5     OutputTypeParameterMismatch, Overflow, PredicateObligation, SelectionContext, SelectionError,
   6     TraitNotObjectSafe,
   7 };
   8
   9 use crate::infer::error_reporting::TypeAnnotationNeeded as ErrorCode;
  10 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  11 use crate::infer::{self, InferCtxt};
  12 use crate::mir::interpret::ErrorHandled;
  13 use crate::session::DiagnosticMessageId;
  14 use crate::traits::object_safety_violations;
  15 use crate::ty::error::ExpectedFound;
  16 use crate::ty::fast_reject;
  17 use crate::ty::fold::TypeFolder;
  18 use crate::ty::subst::Subst;
  19 use crate::ty::GenericParamDefKind;
  20 use crate::ty::SubtypePredicate;
  21 use crate::ty::TypeckTables;
  22 use crate::ty::{self, AdtKind, DefIdTree, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable};
  23
  24 use errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, Style};
  25 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  26 use rustc_hir as hir;
  27 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
  28 use rustc_hir::Node;
  29 use rustc_span::source_map::SourceMap;
  30 use rustc_span::symbol::{kw, sym};
  31 use rustc_span::{ExpnKind, MultiSpan, Span, DUMMY_SP};
  32 use std::fmt;
  33 use syntax::ast;
  34
  35 use rustc_error_codes::*;
  36
  37 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
  38     pub fn report_fulfillment_errors(
  39         &self,
  40         errors: &[FulfillmentError<'tcx>],
  41         body_id: Option<hir::BodyId>,
  42         fallback_has_occurred: bool,
  43     ) {
  44         #[derive(Debug)]
  45         struct ErrorDescriptor<'tcx> {
  46             predicate: ty::Predicate<'tcx>,
  47             index: Option<usize>, // None if this is an old error
  48         }
  49
  50         let mut error_map: FxHashMap<_, Vec<_>> = self
  51             .reported_trait_errors
  52             .borrow()
  53             .iter()
  54             .map(|(&span, predicates)| {
  55                 (
  56                     span,
  57                     predicates
  58                         .iter()
  59                         .map(|predicate| ErrorDescriptor {
  60                             predicate: predicate.clone(),
  61                             index: None,
  62                         })
  63                         .collect(),
  64                 )
  65             })
  66             .collect();
  67
  68         for (index, error) in errors.iter().enumerate() {
  69             // We want to ignore desugarings here: spans are equivalent even
  70             // if one is the result of a desugaring and the other is not.
  71             let mut span = error.obligation.cause.span;
  72             let expn_data = span.ctxt().outer_expn_data();
  73             if let ExpnKind::Desugaring(_) = expn_data.kind {
  74                 span = expn_data.call_site;
  75             }
  76
  77             error_map.entry(span).or_default().push(ErrorDescriptor {
  78                 predicate: error.obligation.predicate.clone(),
  79                 index: Some(index),
  80             });
  81
  82             self.reported_trait_errors
  83                 .borrow_mut()
  84                 .entry(span)
  85                 .or_default()
  86                 .push(error.obligation.predicate.clone());
  87         }
  88
  89         // We do this in 2 passes because we want to display errors in order, though
  90         // maybe it *is* better to sort errors by span or something.
  91         let mut is_suppressed = vec![false; errors.len()];
  92         for (_, error_set) in error_map.iter() {
  93             // We want to suppress "duplicate" errors with the same span.
  94             for error in error_set {
  95                 if let Some(index) = error.index {
  96                     // Suppress errors that are either:
  97                     // 1) strictly implied by another error.
  98                     // 2) implied by an error with a smaller index.
  99                     for error2 in error_set {
 100                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 101                             // Avoid errors being suppressed by already-suppressed
 102                             // errors, to prevent all errors from being suppressed
 103                             // at once.
 104                             continue;
 105                         }
 106
 107                         if self.error_implies(&error2.predicate, &error.predicate)
 108                             && !(error2.index >= error.index
 109                                 && self.error_implies(&error.predicate, &error2.predicate))
 110                         {
 111                             info!("skipping {:?} (implied by {:?})", error, error2);
 112                             is_suppressed[index] = true;
 113                             break;
 114                         }
 115                     }
 116                 }
 117             }
 118         }
 119
 120         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 121             if !suppressed {
 122                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 123             }
 124         }
 125     }
 126
 127     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 128     // `error` occurring implies that `cond` occurs.
 129     fn error_implies(&self, cond: &ty::Predicate<'tcx>, error: &ty::Predicate<'tcx>) -> bool {
 130         if cond == error {
 131             return true;
 132         }
 133
 134         let (cond, error) = match (cond, error) {
 135             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error)) => (cond, error),
 136             _ => {
 137                 // FIXME: make this work in other cases too.
 138                 return false;
 139             }
 140         };
 141
 142         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 143             if let ty::Predicate::Trait(implication) = implication {
 144                 let error = error.to_poly_trait_ref();
 145                 let implication = implication.to_poly_trait_ref();
 146                 // FIXME: I'm just not taking associated types at all here.
 147                 // Eventually I'll need to implement param-env-aware
 148                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 149                 let param_env = ty::ParamEnv::empty();
 150                 if self.can_sub(param_env, error, implication).is_ok() {
 151                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 152                     return true;
 153                 }
 154             }
 155         }
 156
 157         false
 158     }
 159
 160     fn report_fulfillment_error(
 161         &self,
 162         error: &FulfillmentError<'tcx>,
 163         body_id: Option<hir::BodyId>,
 164         fallback_has_occurred: bool,
 165     ) {
 166         debug!("report_fulfillment_error({:?})", error);
 167         match error.code {
 168             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
 169                 self.report_selection_error(
 170                     &error.obligation,
 171                     selection_error,
 172                     fallback_has_occurred,
 173                     error.points_at_arg_span,
 174                 );
 175             }
 176             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 177                 self.report_projection_error(&error.obligation, e);
 178             }
 179             FulfillmentErrorCode::CodeAmbiguity => {
 180                 self.maybe_report_ambiguity(&error.obligation, body_id);
 181             }
 182             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 183                 self.report_mismatched_types(
 184                     &error.obligation.cause,
 185                     expected_found.expected,
 186                     expected_found.found,
 187                     err.clone(),
 188                 )
 189                 .emit();
 190             }
 191         }
 192     }
 193
 194     fn report_projection_error(
 195         &self,
 196         obligation: &PredicateObligation<'tcx>,
 197         error: &MismatchedProjectionTypes<'tcx>,
 198     ) {
 199         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 200
 201         if predicate.references_error() {
 202             return;
 203         }
 204
 205         self.probe(|_| {
 206             let err_buf;
 207             let mut err = &error.err;
 208             let mut values = None;
 209
 210             // try to find the mismatched types to report the error with.
 211             //
 212             // this can fail if the problem was higher-ranked, in which
 213             // cause I have no idea for a good error message.
 214             if let ty::Predicate::Projection(ref data) = predicate {
 215                 let mut selcx = SelectionContext::new(self);
 216                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
 217                     obligation.cause.span,
 218                     infer::LateBoundRegionConversionTime::HigherRankedType,
 219                     data,
 220                 );
 221                 let mut obligations = vec![];
 222                 let normalized_ty = super::normalize_projection_type(
 223                     &mut selcx,
 224                     obligation.param_env,
 225                     data.projection_ty,
 226                     obligation.cause.clone(),
 227                     0,
 228                     &mut obligations,
 229                 );
 230
 231                 debug!(
 232                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
 233                     obligation.cause, obligation.param_env
 234                 );
 235
 236                 debug!(
 237                     "report_projection_error normalized_ty={:?} data.ty={:?}",
 238                     normalized_ty, data.ty
 239                 );
 240
 241                 let is_normalized_ty_expected = match &obligation.cause.code {
 242                     ObligationCauseCode::ItemObligation(_)
 243                     | ObligationCauseCode::BindingObligation(_, _)
 244                     | ObligationCauseCode::ObjectCastObligation(_) => false,
 245                     _ => true,
 246                 };
 247
 248                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
 249                     is_normalized_ty_expected,
 250                     normalized_ty,
 251                     data.ty,
 252                 ) {
 253                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
 254                         is_normalized_ty_expected,
 255                         normalized_ty,
 256                         data.ty,
 257                     )));
 258
 259                     err_buf = error;
 260                     err = &err_buf;
 261                 }
 262             }
 263
 264             let msg = format!("type mismatch resolving `{}`", predicate);
 265             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
 266             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 267             if fresh {
 268                 let mut diag = struct_span_err!(
 269                     self.tcx.sess,
 270                     obligation.cause.span,
 271                     E0271,
 272                     "type mismatch resolving `{}`",
 273                     predicate
 274                 );
 275                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 276                 self.note_obligation_cause(&mut diag, obligation);
 277                 diag.emit();
 278             }
 279         });
 280     }
 281
 282     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 283         /// returns the fuzzy category of a given type, or None
 284         /// if the type can be equated to any type.
 285         fn type_category(t: Ty<'_>) -> Option<u32> {
 286             match t.kind {
 287                 ty::Bool => Some(0),
 288                 ty::Char => Some(1),
 289                 ty::Str => Some(2),
 290                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
 291                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
 292                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
 293                 ty::Array(..) | ty::Slice(..) => Some(6),
 294                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
 295                 ty::Dynamic(..) => Some(8),
 296                 ty::Closure(..) => Some(9),
 297                 ty::Tuple(..) => Some(10),
 298                 ty::Projection(..) => Some(11),
 299                 ty::Param(..) => Some(12),
 300                 ty::Opaque(..) => Some(13),
 301                 ty::Never => Some(14),
 302                 ty::Adt(adt, ..) => match adt.adt_kind() {
 303                     AdtKind::Struct => Some(15),
 304                     AdtKind::Union => Some(16),
 305                     AdtKind::Enum => Some(17),
 306                 },
 307                 ty::Generator(..) => Some(18),
 308                 ty::Foreign(..) => Some(19),
 309                 ty::GeneratorWitness(..) => Some(20),
 310                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
 311                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 312             }
 313         }
 314
 315         match (type_category(a), type_category(b)) {
 316             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
 317                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
 318                 _ => cat_a == cat_b,
 319             },
 320             // infer and error can be equated to all types
 321             _ => true,
 322         }
 323     }
 324
 325     fn impl_similar_to(
 326         &self,
 327         trait_ref: ty::PolyTraitRef<'tcx>,
 328         obligation: &PredicateObligation<'tcx>,
 329     ) -> Option<DefId> {
 330         let tcx = self.tcx;
 331         let param_env = obligation.param_env;
 332         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 333         let trait_self_ty = trait_ref.self_ty();
 334
 335         let mut self_match_impls = vec![];
 336         let mut fuzzy_match_impls = vec![];
 337
 338         self.tcx.for_each_relevant_impl(trait_ref.def_id, trait_self_ty, |def_id| {
 339             let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 340             let impl_trait_ref = tcx.impl_trait_ref(def_id).unwrap().subst(tcx, impl_substs);
 341
 342             let impl_self_ty = impl_trait_ref.self_ty();
 343
 344             if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 345                 self_match_impls.push(def_id);
 346
 347                 if trait_ref
 348                     .substs
 349                     .types()
 350                     .skip(1)
 351                     .zip(impl_trait_ref.substs.types().skip(1))
 352                     .all(|(u, v)| self.fuzzy_match_tys(u, v))
 353                 {
 354                     fuzzy_match_impls.push(def_id);
 355                 }
 356             }
 357         });
 358
 359         let impl_def_id = if self_match_impls.len() == 1 {
 360             self_match_impls[0]
 361         } else if fuzzy_match_impls.len() == 1 {
 362             fuzzy_match_impls[0]
 363         } else {
 364             return None;
 365         };
 366
 367         tcx.has_attr(impl_def_id, sym::rustc_on_unimplemented).then_some(impl_def_id)
 368     }
 369
 370     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
 371         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
 372             hir::GeneratorKind::Gen => "a generator",
 373             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
 374             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
 375             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
 376         })
 377     }
 378
 379     /// Used to set on_unimplemented's `ItemContext`
 380     /// to be the enclosing (async) block/function/closure
 381     fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str> {
 382         let hir = &self.tcx.hir();
 383         let node = hir.find(hir_id)?;
 384         if let hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. }) = &node {
 385             self.describe_generator(*body_id).or_else(|| {
 386                 Some(if let hir::FnHeader { asyncness: hir::IsAsync::Async, .. } = sig.header {
 387                     "an async function"
 388                 } else {
 389                     "a function"
 390                 })
 391             })
 392         } else if let hir::Node::Expr(hir::Expr {
 393             kind: hir::ExprKind::Closure(_is_move, _, body_id, _, gen_movability),
 394             ..
 395         }) = &node
 396         {
 397             self.describe_generator(*body_id).or_else(|| {
 398                 Some(if gen_movability.is_some() { "an async closure" } else { "a closure" })
 399             })
 400         } else if let hir::Node::Expr(hir::Expr { .. }) = &node {
 401             let parent_hid = hir.get_parent_node(hir_id);
 402             if parent_hid != hir_id {
 403                 return self.describe_enclosure(parent_hid);
 404             } else {
 405                 None
 406             }
 407         } else {
 408             None
 409         }
 410     }
 411
 412     fn on_unimplemented_note(
 413         &self,
 414         trait_ref: ty::PolyTraitRef<'tcx>,
 415         obligation: &PredicateObligation<'tcx>,
 416     ) -> OnUnimplementedNote {
 417         let def_id =
 418             self.impl_similar_to(trait_ref, obligation).unwrap_or_else(|| trait_ref.def_id());
 419         let trait_ref = *trait_ref.skip_binder();
 420
 421         let mut flags = vec![];
 422         flags.push((
 423             sym::item_context,
 424             self.describe_enclosure(obligation.cause.body_id).map(|s| s.to_owned()),
 425         ));
 426
 427         match obligation.cause.code {
 428             ObligationCauseCode::BuiltinDerivedObligation(..)
 429             | ObligationCauseCode::ImplDerivedObligation(..) => {}
 430             _ => {
 431                 // this is a "direct", user-specified, rather than derived,
 432                 // obligation.
 433                 flags.push((sym::direct, None));
 434             }
 435         }
 436
 437         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 438             // FIXME: maybe also have some way of handling methods
 439             // from other traits? That would require name resolution,
 440             // which we might want to be some sort of hygienic.
 441             //
 442             // Currently I'm leaving it for what I need for `try`.
 443             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 444                 let method = self.tcx.item_name(item);
 445                 flags.push((sym::from_method, None));
 446                 flags.push((sym::from_method, Some(method.to_string())));
 447             }
 448         }
 449         if let Some((t, _)) = self.get_parent_trait_ref(&obligation.cause.code) {
 450             flags.push((sym::parent_trait, Some(t)));
 451         }
 452
 453         if let Some(k) = obligation.cause.span.desugaring_kind() {
 454             flags.push((sym::from_desugaring, None));
 455             flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
 456         }
 457         let generics = self.tcx.generics_of(def_id);
 458         let self_ty = trait_ref.self_ty();
 459         // This is also included through the generics list as `Self`,
 460         // but the parser won't allow you to use it
 461         flags.push((sym::_Self, Some(self_ty.to_string())));
 462         if let Some(def) = self_ty.ty_adt_def() {
 463             // We also want to be able to select self's original
 464             // signature with no type arguments resolved
 465             flags.push((sym::_Self, Some(self.tcx.type_of(def.did).to_string())));
 466         }
 467
 468         for param in generics.params.iter() {
 469             let value = match param.kind {
 470                 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => {
 471                     trait_ref.substs[param.index as usize].to_string()
 472                 }
 473                 GenericParamDefKind::Lifetime => continue,
 474             };
 475             let name = param.name;
 476             flags.push((name, Some(value)));
 477         }
 478
 479         if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
 480             flags.push((sym::crate_local, None));
 481         }
 482
 483         // Allow targeting all integers using `{integral}`, even if the exact type was resolved
 484         if self_ty.is_integral() {
 485             flags.push((sym::_Self, Some("{integral}".to_owned())));
 486         }
 487
 488         if let ty::Array(aty, len) = self_ty.kind {
 489             flags.push((sym::_Self, Some("[]".to_owned())));
 490             flags.push((sym::_Self, Some(format!("[{}]", aty))));
 491             if let Some(def) = aty.ty_adt_def() {
 492                 // We also want to be able to select the array's type's original
 493                 // signature with no type arguments resolved
 494                 flags.push((
 495                     sym::_Self,
 496                     Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
 497                 ));
 498                 let tcx = self.tcx;
 499                 if let Some(len) = len.try_eval_usize(tcx, ty::ParamEnv::empty()) {
 500                     flags.push((
 501                         sym::_Self,
 502                         Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
 503                     ));
 504                 } else {
 505                     flags.push((
 506                         sym::_Self,
 507                         Some(format!("[{}; _]", self.tcx.type_of(def.did).to_string())),
 508                     ));
 509                 }
 510             }
 511         }
 512
 513         if let Ok(Some(command)) =
 514             OnUnimplementedDirective::of_item(self.tcx, trait_ref.def_id, def_id)
 515         {
 516             command.evaluate(self.tcx, trait_ref, &flags[..])
 517         } else {
 518             OnUnimplementedNote::default()
 519         }
 520     }
 521
 522     fn find_similar_impl_candidates(
 523         &self,
 524         trait_ref: ty::PolyTraitRef<'tcx>,
 525     ) -> Vec<ty::TraitRef<'tcx>> {
 526         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
 527         let all_impls = self.tcx.all_impls(trait_ref.def_id());
 528
 529         match simp {
 530             Some(simp) => all_impls
 531                 .iter()
 532                 .filter_map(|&def_id| {
 533                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 534                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
 535                     if let Some(imp_simp) = imp_simp {
 536                         if simp != imp_simp {
 537                             return None;
 538                         }
 539                     }
 540
 541                     Some(imp)
 542                 })
 543                 .collect(),
 544             None => {
 545                 all_impls.iter().map(|&def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect()
 546             }
 547         }
 548     }
 549
 550     fn report_similar_impl_candidates(
 551         &self,
 552         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 553         err: &mut DiagnosticBuilder<'_>,
 554     ) {
 555         if impl_candidates.is_empty() {
 556             return;
 557         }
 558
 559         let len = impl_candidates.len();
 560         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
 561
 562         let normalize = |candidate| {
 563             self.tcx.infer_ctxt().enter(|ref infcx| {
 564                 let normalized = infcx
 565                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 566                     .normalize(candidate)
 567                     .ok();
 568                 match normalized {
 569                     Some(normalized) => format!("\n  {:?}", normalized.value),
 570                     None => format!("\n  {:?}", candidate),
 571                 }
 572             })
 573         };
 574
 575         // Sort impl candidates so that ordering is consistent for UI tests.
 576         let mut normalized_impl_candidates =
 577             impl_candidates.iter().map(normalize).collect::<Vec<String>>();
 578
 579         // Sort before taking the `..end` range,
 580         // because the ordering of `impl_candidates` may not be deterministic:
 581         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
 582         normalized_impl_candidates.sort();
 583
 584         err.help(&format!(
 585             "the following implementations were found:{}{}",
 586             normalized_impl_candidates[..end].join(""),
 587             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
 588         ));
 589     }
 590
 591     /// Reports that an overflow has occurred and halts compilation. We
 592     /// halt compilation unconditionally because it is important that
 593     /// overflows never be masked -- they basically represent computations
 594     /// whose result could not be truly determined and thus we can't say
 595     /// if the program type checks or not -- and they are unusual
 596     /// occurrences in any case.
 597     pub fn report_overflow_error<T>(
 598         &self,
 599         obligation: &Obligation<'tcx, T>,
 600         suggest_increasing_limit: bool,
 601     ) -> !
 602     where
 603         T: fmt::Display + TypeFoldable<'tcx>,
 604     {
 605         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 606         let mut err = struct_span_err!(
 607             self.tcx.sess,
 608             obligation.cause.span,
 609             E0275,
 610             "overflow evaluating the requirement `{}`",
 611             predicate
 612         );
 613
 614         if suggest_increasing_limit {
 615             self.suggest_new_overflow_limit(&mut err);
 616         }
 617
 618         self.note_obligation_cause_code(
 619             &mut err,
 620             &obligation.predicate,
 621             &obligation.cause.code,
 622             &mut vec![],
 623         );
 624
 625         err.emit();
 626         self.tcx.sess.abort_if_errors();
 627         bug!();
 628     }
 629
 630     /// Reports that a cycle was detected which led to overflow and halts
 631     /// compilation. This is equivalent to `report_overflow_error` except
 632     /// that we can give a more helpful error message (and, in particular,
 633     /// we do not suggest increasing the overflow limit, which is not
 634     /// going to help).
 635     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 636         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 637         assert!(cycle.len() > 0);
 638
 639         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 640
 641         self.report_overflow_error(&cycle[0], false);
 642     }
 643
 644     pub fn report_extra_impl_obligation(
 645         &self,
 646         error_span: Span,
 647         item_name: ast::Name,
 648         _impl_item_def_id: DefId,
 649         trait_item_def_id: DefId,
 650         requirement: &dyn fmt::Display,
 651     ) -> DiagnosticBuilder<'tcx> {
 652         let msg = "impl has stricter requirements than trait";
 653         let sp = self.tcx.sess.source_map().def_span(error_span);
 654
 655         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 656
 657         if let Some(trait_item_span) = self.tcx.hir().span_if_local(trait_item_def_id) {
 658             let span = self.tcx.sess.source_map().def_span(trait_item_span);
 659             err.span_label(span, format!("definition of `{}` from trait", item_name));
 660         }
 661
 662         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 663
 664         err
 665     }
 666
 667     /// Gets the parent trait chain start
 668     fn get_parent_trait_ref(
 669         &self,
 670         code: &ObligationCauseCode<'tcx>,
 671     ) -> Option<(String, Option<Span>)> {
 672         match code {
 673             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 674                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
 675                 match self.get_parent_trait_ref(&data.parent_code) {
 676                     Some(t) => Some(t),
 677                     None => {
 678                         let ty = parent_trait_ref.skip_binder().self_ty();
 679                         let span = if let ty::Closure(def_id, _)
 680                         | ty::Opaque(def_id, _)
 681                         | ty::Generator(def_id, ..)
 682                         | ty::Foreign(def_id) = ty.kind
 683                         {
 684                             Some(self.tcx.def_span(def_id))
 685                         } else {
 686                             None
 687                         };
 688                         Some((ty.to_string(), span))
 689                     }
 690                 }
 691             }
 692             _ => None,
 693         }
 694     }
 695
 696     pub fn report_selection_error(
 697         &self,
 698         obligation: &PredicateObligation<'tcx>,
 699         error: &SelectionError<'tcx>,
 700         fallback_has_occurred: bool,
 701         points_at_arg: bool,
 702     ) {
 703         let tcx = self.tcx;
 704         let span = obligation.cause.span;
 705
 706         let mut err = match *error {
 707             SelectionError::Unimplemented => {
 708                 if let ObligationCauseCode::CompareImplMethodObligation {
 709                     item_name,
 710                     impl_item_def_id,
 711                     trait_item_def_id,
 712                 }
 713                 | ObligationCauseCode::CompareImplTypeObligation {
 714                     item_name,
 715                     impl_item_def_id,
 716                     trait_item_def_id,
 717                 } = obligation.cause.code
 718                 {
 719                     self.report_extra_impl_obligation(
 720                         span,
 721                         item_name,
 722                         impl_item_def_id,
 723                         trait_item_def_id,
 724                         &format!("`{}`", obligation.predicate),
 725                     )
 726                     .emit();
 727                     return;
 728                 }
 729                 match obligation.predicate {
 730                     ty::Predicate::Trait(ref trait_predicate) => {
 731                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 732
 733                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 734                             return;
 735                         }
 736                         let trait_ref = trait_predicate.to_poly_trait_ref();
 737                         let (post_message, pre_message, type_def) = self
 738                             .get_parent_trait_ref(&obligation.cause.code)
 739                             .map(|(t, s)| {
 740                                 (
 741                                     format!(" in `{}`", t),
 742                                     format!("within `{}`, ", t),
 743                                     s.map(|s| (format!("within this `{}`", t), s)),
 744                                 )
 745                             })
 746                             .unwrap_or_default();
 747
 748                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
 749                             self.on_unimplemented_note(trait_ref, obligation);
 750                         let have_alt_message = message.is_some() || label.is_some();
 751                         let is_try = self
 752                             .tcx
 753                             .sess
 754                             .source_map()
 755                             .span_to_snippet(span)
 756                             .map(|s| &s == "?")
 757                             .unwrap_or(false);
 758                         let is_from = format!("{}", trait_ref.print_only_trait_path())
 759                             .starts_with("std::convert::From<");
 760                         let (message, note) = if is_try && is_from {
 761                             (
 762                                 Some(format!(
 763                                     "`?` couldn't convert the error to `{}`",
 764                                     trait_ref.self_ty(),
 765                                 )),
 766                                 Some(
 767                                     "the question mark operation (`?`) implicitly performs a \
 768                                  conversion on the error value using the `From` trait"
 769                                         .to_owned(),
 770                                 ),
 771                             )
 772                         } else {
 773                             (message, note)
 774                         };
 775
 776                         let mut err = struct_span_err!(
 777                             self.tcx.sess,
 778                             span,
 779                             E0277,
 780                             "{}",
 781                             message.unwrap_or_else(|| format!(
 782                                 "the trait bound `{}` is not satisfied{}",
 783                                 trait_ref.to_predicate(),
 784                                 post_message,
 785                             ))
 786                         );
 787
 788                         let explanation =
 789                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 790                                 "consider using `()`, or a `Result`".to_owned()
 791                             } else {
 792                                 format!(
 793                                     "{}the trait `{}` is not implemented for `{}`",
 794                                     pre_message,
 795                                     trait_ref.print_only_trait_path(),
 796                                     trait_ref.self_ty(),
 797                                 )
 798                             };
 799
 800                         if self.suggest_add_reference_to_arg(
 801                             &obligation,
 802                             &mut err,
 803                             &trait_ref,
 804                             points_at_arg,
 805                             have_alt_message,
 806                         ) {
 807                             self.note_obligation_cause(&mut err, obligation);
 808                             err.emit();
 809                             return;
 810                         }
 811                         if let Some(ref s) = label {
 812                             // If it has a custom `#[rustc_on_unimplemented]`
 813                             // error message, let's display it as the label!
 814                             err.span_label(span, s.as_str());
 815                             err.help(&explanation);
 816                         } else {
 817                             err.span_label(span, explanation);
 818                         }
 819                         if let Some((msg, span)) = type_def {
 820                             err.span_label(span, &msg);
 821                         }
 822                         if let Some(ref s) = note {
 823                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 824                             err.note(s.as_str());
 825                         }
 826                         if let Some(ref s) = enclosing_scope {
 827                             let enclosing_scope_span = tcx.def_span(
 828                                 tcx.hir()
 829                                     .opt_local_def_id(obligation.cause.body_id)
 830                                     .unwrap_or_else(|| {
 831                                         tcx.hir().body_owner_def_id(hir::BodyId {
 832                                             hir_id: obligation.cause.body_id,
 833                                         })
 834                                     }),
 835                             );
 836
 837                             err.span_label(enclosing_scope_span, s.as_str());
 838                         }
 839
 840                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 841                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 842                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 843                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 844                         self.note_version_mismatch(&mut err, &trait_ref);
 845
 846                         // Try to report a help message
 847                         if !trait_ref.has_infer_types()
 848                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 849                         {
 850                             // If a where-clause may be useful, remind the
 851                             // user that they can add it.
 852                             //
 853                             // don't display an on-unimplemented note, as
 854                             // these notes will often be of the form
 855                             //     "the type `T` can't be frobnicated"
 856                             // which is somewhat confusing.
 857                             self.suggest_restricting_param_bound(
 858                                 &mut err,
 859                                 &trait_ref,
 860                                 obligation.cause.body_id,
 861                             );
 862                         } else {
 863                             if !have_alt_message {
 864                                 // Can't show anything else useful, try to find similar impls.
 865                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 866                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 867                             }
 868                             self.suggest_change_mut(
 869                                 &obligation,
 870                                 &mut err,
 871                                 &trait_ref,
 872                                 points_at_arg,
 873                             );
 874                         }
 875
 876                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 877                         // implemented, and fallback has occurred, then it could be due to a
 878                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 879                         // note informing about the change in behaviour.
 880                         if trait_predicate.skip_binder().self_ty().is_never()
 881                             && fallback_has_occurred
 882                         {
 883                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 884                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 885                                     self.tcx.mk_unit(),
 886                                     &trait_pred.trait_ref.substs[1..],
 887                                 );
 888                                 trait_pred
 889                             });
 890                             let unit_obligation = Obligation {
 891                                 predicate: ty::Predicate::Trait(predicate),
 892                                 ..obligation.clone()
 893                             };
 894                             if self.predicate_may_hold(&unit_obligation) {
 895                                 err.note(
 896                                     "the trait is implemented for `()`. \
 897                                          Possibly this error has been caused by changes to \
 898                                          Rust's type-inference algorithm \
 899                                          (see: https://github.com/rust-lang/rust/issues/48950 \
 900                                          for more info). Consider whether you meant to use the \
 901                                          type `()` here instead.",
 902                                 );
 903                             }
 904                         }
 905
 906                         err
 907                     }
 908
 909                     ty::Predicate::Subtype(ref predicate) => {
 910                         // Errors for Subtype predicates show up as
 911                         // `FulfillmentErrorCode::CodeSubtypeError`,
 912                         // not selection error.
 913                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 914                     }
 915
 916                     ty::Predicate::RegionOutlives(ref predicate) => {
 917                         let predicate = self.resolve_vars_if_possible(predicate);
 918                         let err = self
 919                             .region_outlives_predicate(&obligation.cause, &predicate)
 920                             .err()
 921                             .unwrap();
 922                         struct_span_err!(
 923                             self.tcx.sess,
 924                             span,
 925                             E0279,
 926                             "the requirement `{}` is not satisfied (`{}`)",
 927                             predicate,
 928                             err,
 929                         )
 930                     }
 931
 932                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 933                         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 934                         struct_span_err!(
 935                             self.tcx.sess,
 936                             span,
 937                             E0280,
 938                             "the requirement `{}` is not satisfied",
 939                             predicate
 940                         )
 941                     }
 942
 943                     ty::Predicate::ObjectSafe(trait_def_id) => {
 944                         let violations = object_safety_violations(self.tcx, trait_def_id);
 945                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 946                     }
 947
 948                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 949                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 950                         let closure_span = self
 951                             .tcx
 952                             .sess
 953                             .source_map()
 954                             .def_span(self.tcx.hir().span_if_local(closure_def_id).unwrap());
 955                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id).unwrap();
 956                         let mut err = struct_span_err!(
 957                             self.tcx.sess,
 958                             closure_span,
 959                             E0525,
 960                             "expected a closure that implements the `{}` trait, \
 961                              but this closure only implements `{}`",
 962                             kind,
 963                             found_kind
 964                         );
 965
 966                         err.span_label(
 967                             closure_span,
 968                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 969                         );
 970                         err.span_label(
 971                             obligation.cause.span,
 972                             format!("the requirement to implement `{}` derives from here", kind),
 973                         );
 974
 975                         // Additional context information explaining why the closure only implements
 976                         // a particular trait.
 977                         if let Some(tables) = self.in_progress_tables {
 978                             let tables = tables.borrow();
 979                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 980                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 981                                     err.span_label(
 982                                         *span,
 983                                         format!(
 984                                             "closure is `FnOnce` because it moves the \
 985                                          variable `{}` out of its environment",
 986                                             name
 987                                         ),
 988                                     );
 989                                 }
 990                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 991                                     err.span_label(
 992                                         *span,
 993                                         format!(
 994                                             "closure is `FnMut` because it mutates the \
 995                                          variable `{}` here",
 996                                             name
 997                                         ),
 998                                     );
 999                                 }
1000                                 _ => {}
1001                             }
1002                         }
1003
1004                         err.emit();
1005                         return;
1006                     }
1007
1008                     ty::Predicate::WellFormed(ty) => {
1009                         if !self.tcx.sess.opts.debugging_opts.chalk {
1010                             // WF predicates cannot themselves make
1011                             // errors. They can only block due to
1012                             // ambiguity; otherwise, they always
1013                             // degenerate into other obligations
1014                             // (which may fail).
1015                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
1016                         } else {
1017                             // FIXME: we'll need a better message which takes into account
1018                             // which bounds actually failed to hold.
1019                             self.tcx.sess.struct_span_err(
1020                                 span,
1021                                 &format!("the type `{}` is not well-formed (chalk)", ty),
1022                             )
1023                         }
1024                     }
1025
1026                     ty::Predicate::ConstEvaluatable(..) => {
1027                         // Errors for `ConstEvaluatable` predicates show up as
1028                         // `SelectionError::ConstEvalFailure`,
1029                         // not `Unimplemented`.
1030                         span_bug!(
1031                             span,
1032                             "const-evaluatable requirement gave wrong error: `{:?}`",
1033                             obligation
1034                         )
1035                     }
1036                 }
1037             }
1038
1039             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
1040                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
1041                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
1042
1043                 if expected_trait_ref.self_ty().references_error() {
1044                     return;
1045                 }
1046
1047                 let found_trait_ty = found_trait_ref.self_ty();
1048
1049                 let found_did = match found_trait_ty.kind {
1050                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
1051                     ty::Adt(def, _) => Some(def.did),
1052                     _ => None,
1053                 };
1054
1055                 let found_span = found_did
1056                     .and_then(|did| self.tcx.hir().span_if_local(did))
1057                     .map(|sp| self.tcx.sess.source_map().def_span(sp)); // the sp could be an fn def
1058
1059                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1060                     // We check closures twice, with obligations flowing in different directions,
1061                     // but we want to complain about them only once.
1062                     return;
1063                 }
1064
1065                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1066
1067                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
1068                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1069                     _ => vec![ArgKind::empty()],
1070                 };
1071
1072                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1073                 let expected = match expected_ty.kind {
1074                     ty::Tuple(ref tys) => tys
1075                         .iter()
1076                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
1077                         .collect(),
1078                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
1079                 };
1080
1081                 if found.len() == expected.len() {
1082                     self.report_closure_arg_mismatch(
1083                         span,
1084                         found_span,
1085                         found_trait_ref,
1086                         expected_trait_ref,
1087                     )
1088                 } else {
1089                     let (closure_span, found) = found_did
1090                         .and_then(|did| self.tcx.hir().get_if_local(did))
1091                         .map(|node| {
1092                             let (found_span, found) = self.get_fn_like_arguments(node);
1093                             (Some(found_span), found)
1094                         })
1095                         .unwrap_or((found_span, found));
1096
1097                     self.report_arg_count_mismatch(
1098                         span,
1099                         closure_span,
1100                         expected,
1101                         found,
1102                         found_trait_ty.is_closure(),
1103                     )
1104                 }
1105             }
1106
1107             TraitNotObjectSafe(did) => {
1108                 let violations = object_safety_violations(self.tcx, did);
1109                 report_object_safety_error(self.tcx, span, did, violations)
1110             }
1111
1112             // already reported in the query
1113             ConstEvalFailure(err) => {
1114                 if let ErrorHandled::TooGeneric = err {
1115                     // Silence this error, as it can be produced during intermediate steps
1116                     // when a constant is not yet able to be evaluated (but will be later).
1117                     return;
1118                 }
1119                 self.tcx.sess.delay_span_bug(
1120                     span,
1121                     &format!("constant in type had an ignored error: {:?}", err),
1122                 );
1123                 return;
1124             }
1125
1126             Overflow => {
1127                 bug!("overflow should be handled before the `report_selection_error` path");
1128             }
1129         };
1130
1131         self.note_obligation_cause(&mut err, obligation);
1132
1133         err.emit();
1134     }
1135
1136     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1137     /// with the same path as `trait_ref`, a help message about
1138     /// a probable version mismatch is added to `err`
1139     fn note_version_mismatch(
1140         &self,
1141         err: &mut DiagnosticBuilder<'_>,
1142         trait_ref: &ty::PolyTraitRef<'tcx>,
1143     ) {
1144         let get_trait_impl = |trait_def_id| {
1145             let mut trait_impl = None;
1146             self.tcx.for_each_relevant_impl(trait_def_id, trait_ref.self_ty(), |impl_def_id| {
1147                 if trait_impl.is_none() {
1148                     trait_impl = Some(impl_def_id);
1149                 }
1150             });
1151             trait_impl
1152         };
1153         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1154         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
1155         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1156             .iter()
1157             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1158             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1159             .collect();
1160         for trait_with_same_path in traits_with_same_path {
1161             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1162                 let impl_span = self.tcx.def_span(impl_def_id);
1163                 err.span_help(impl_span, "trait impl with same name found");
1164                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1165                 let crate_msg = format!(
1166                     "Perhaps two different versions of crate `{}` are being used?",
1167                     trait_crate
1168                 );
1169                 err.note(&crate_msg);
1170             }
1171         }
1172     }
1173     fn suggest_restricting_param_bound(
1174         &self,
1175         mut err: &mut DiagnosticBuilder<'_>,
1176         trait_ref: &ty::PolyTraitRef<'_>,
1177         body_id: hir::HirId,
1178     ) {
1179         let self_ty = trait_ref.self_ty();
1180         let (param_ty, projection) = match &self_ty.kind {
1181             ty::Param(_) => (true, None),
1182             ty::Projection(projection) => (false, Some(projection)),
1183             _ => return,
1184         };
1185
1186         let suggest_restriction =
1187             |generics: &hir::Generics<'_>, msg, err: &mut DiagnosticBuilder<'_>| {
1188                 let span = generics.where_clause.span_for_predicates_or_empty_place();
1189                 if !span.from_expansion() && span.desugaring_kind().is_none() {
1190                     err.span_suggestion(
1191                         generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi(),
1192                         &format!("consider further restricting {}", msg),
1193                         format!(
1194                             "{} {} ",
1195                             if !generics.where_clause.predicates.is_empty() {
1196                                 ","
1197                             } else {
1198                                 " where"
1199                             },
1200                             trait_ref.to_predicate(),
1201                         ),
1202                         Applicability::MachineApplicable,
1203                     );
1204                 }
1205             };
1206
1207         // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
1208         //        don't suggest `T: Sized + ?Sized`.
1209         let mut hir_id = body_id;
1210         while let Some(node) = self.tcx.hir().find(hir_id) {
1211             match node {
1212                 hir::Node::TraitItem(hir::TraitItem {
1213                     generics,
1214                     kind: hir::TraitItemKind::Method(..),
1215                     ..
1216                 }) if param_ty && self_ty == self.tcx.types.self_param => {
1217                     // Restricting `Self` for a single method.
1218                     suggest_restriction(&generics, "`Self`", err);
1219                     return;
1220                 }
1221
1222                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, generics, _), .. })
1223                 | hir::Node::TraitItem(hir::TraitItem {
1224                     generics,
1225                     kind: hir::TraitItemKind::Method(..),
1226                     ..
1227                 })
1228                 | hir::Node::ImplItem(hir::ImplItem {
1229                     generics,
1230                     kind: hir::ImplItemKind::Method(..),
1231                     ..
1232                 })
1233                 | hir::Node::Item(hir::Item {
1234                     kind: hir::ItemKind::Trait(_, _, generics, _, _),
1235                     ..
1236                 })
1237                 | hir::Node::Item(hir::Item {
1238                     kind: hir::ItemKind::Impl(_, _, _, generics, ..),
1239                     ..
1240                 }) if projection.is_some() => {
1241                     // Missing associated type bound.
1242                     suggest_restriction(&generics, "the associated type", err);
1243                     return;
1244                 }
1245
1246                 hir::Node::Item(hir::Item {
1247                     kind: hir::ItemKind::Struct(_, generics),
1248                     span,
1249                     ..
1250                 })
1251                 | hir::Node::Item(hir::Item {
1252                     kind: hir::ItemKind::Enum(_, generics), span, ..
1253                 })
1254                 | hir::Node::Item(hir::Item {
1255                     kind: hir::ItemKind::Union(_, generics),
1256                     span,
1257                     ..
1258                 })
1259                 | hir::Node::Item(hir::Item {
1260                     kind: hir::ItemKind::Trait(_, _, generics, ..),
1261                     span,
1262                     ..
1263                 })
1264                 | hir::Node::Item(hir::Item {
1265                     kind: hir::ItemKind::Impl(_, _, _, generics, ..),
1266                     span,
1267                     ..
1268                 })
1269                 | hir::Node::Item(hir::Item {
1270                     kind: hir::ItemKind::Fn(_, generics, _),
1271                     span,
1272                     ..
1273                 })
1274                 | hir::Node::Item(hir::Item {
1275                     kind: hir::ItemKind::TyAlias(_, generics),
1276                     span,
1277                     ..
1278                 })
1279                 | hir::Node::Item(hir::Item {
1280                     kind: hir::ItemKind::TraitAlias(generics, _),
1281                     span,
1282                     ..
1283                 })
1284                 | hir::Node::Item(hir::Item {
1285                     kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
1286                     span,
1287                     ..
1288                 })
1289                 | hir::Node::TraitItem(hir::TraitItem { generics, span, .. })
1290                 | hir::Node::ImplItem(hir::ImplItem { generics, span, .. })
1291                     if param_ty =>
1292                 {
1293                     // Missing generic type parameter bound.
1294                     let param_name = self_ty.to_string();
1295                     let constraint = trait_ref.print_only_trait_path().to_string();
1296                     if suggest_constraining_type_param(
1297                         generics,
1298                         &mut err,
1299                         &param_name,
1300                         &constraint,
1301                         self.tcx.sess.source_map(),
1302                         *span,
1303                     ) {
1304                         return;
1305                     }
1306                 }
1307
1308                 hir::Node::Crate => return,
1309
1310                 _ => {}
1311             }
1312
1313             hir_id = self.tcx.hir().get_parent_item(hir_id);
1314         }
1315     }
1316
1317     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
1318     /// suggestion to borrow the initializer in order to use have a slice instead.
1319     fn suggest_borrow_on_unsized_slice(
1320         &self,
1321         code: &ObligationCauseCode<'tcx>,
1322         err: &mut DiagnosticBuilder<'tcx>,
1323     ) {
1324         if let &ObligationCauseCode::VariableType(hir_id) = code {
1325             let parent_node = self.tcx.hir().get_parent_node(hir_id);
1326             if let Some(Node::Local(ref local)) = self.tcx.hir().find(parent_node) {
1327                 if let Some(ref expr) = local.init {
1328                     if let hir::ExprKind::Index(_, _) = expr.kind {
1329                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
1330                             err.span_suggestion(
1331                                 expr.span,
1332                                 "consider borrowing here",
1333                                 format!("&{}", snippet),
1334                                 Applicability::MachineApplicable,
1335                             );
1336                         }
1337                     }
1338                 }
1339             }
1340         }
1341     }
1342
1343     fn mk_obligation_for_def_id(
1344         &self,
1345         def_id: DefId,
1346         output_ty: Ty<'tcx>,
1347         cause: ObligationCause<'tcx>,
1348         param_env: ty::ParamEnv<'tcx>,
1349     ) -> PredicateObligation<'tcx> {
1350         let new_trait_ref =
1351             ty::TraitRef { def_id, substs: self.tcx.mk_substs_trait(output_ty, &[]) };
1352         Obligation::new(cause, param_env, new_trait_ref.to_predicate())
1353     }
1354
1355     /// Given a closure's `DefId`, return the given name of the closure.
1356     ///
1357     /// This doesn't account for reassignments, but it's only used for suggestions.
1358     fn get_closure_name(
1359         &self,
1360         def_id: DefId,
1361         err: &mut DiagnosticBuilder<'_>,
1362         msg: &str,
1363     ) -> Option<String> {
1364         let get_name =
1365             |err: &mut DiagnosticBuilder<'_>, kind: &hir::PatKind<'_>| -> Option<String> {
1366                 // Get the local name of this closure. This can be inaccurate because
1367                 // of the possibility of reassignment, but this should be good enough.
1368                 match &kind {
1369                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, name, None) => {
1370                         Some(format!("{}", name))
1371                     }
1372                     _ => {
1373                         err.note(&msg);
1374                         None
1375                     }
1376                 }
1377             };
1378
1379         let hir = self.tcx.hir();
1380         let hir_id = hir.as_local_hir_id(def_id)?;
1381         let parent_node = hir.get_parent_node(hir_id);
1382         match hir.find(parent_node) {
1383             Some(hir::Node::Stmt(hir::Stmt { kind: hir::StmtKind::Local(local), .. })) => {
1384                 get_name(err, &local.pat.kind)
1385             }
1386             // Different to previous arm because one is `&hir::Local` and the other
1387             // is `P<hir::Local>`.
1388             Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
1389             _ => return None,
1390         }
1391     }
1392
1393     /// We tried to apply the bound to an `fn` or closure. Check whether calling it would
1394     /// evaluate to a type that *would* satisfy the trait binding. If it would, suggest calling
1395     /// it: `bar(foo)` → `bar(foo())`. This case is *very* likely to be hit if `foo` is `async`.
1396     fn suggest_fn_call(
1397         &self,
1398         obligation: &PredicateObligation<'tcx>,
1399         err: &mut DiagnosticBuilder<'_>,
1400         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1401         points_at_arg: bool,
1402     ) {
1403         let self_ty = trait_ref.self_ty();
1404         let (def_id, output_ty, callable) = match self_ty.kind {
1405             ty::Closure(def_id, substs) => {
1406                 (def_id, self.closure_sig(def_id, substs).output(), "closure")
1407             }
1408             ty::FnDef(def_id, _) => (def_id, self_ty.fn_sig(self.tcx).output(), "function"),
1409             _ => return,
1410         };
1411         let msg = format!("use parentheses to call the {}", callable);
1412
1413         let obligation = self.mk_obligation_for_def_id(
1414             trait_ref.def_id(),
1415             output_ty.skip_binder(),
1416             obligation.cause.clone(),
1417             obligation.param_env,
1418         );
1419
1420         match self.evaluate_obligation(&obligation) {
1421             Ok(EvaluationResult::EvaluatedToOk)
1422             | Ok(EvaluationResult::EvaluatedToOkModuloRegions)
1423             | Ok(EvaluationResult::EvaluatedToAmbig) => {}
1424             _ => return,
1425         }
1426         let hir = self.tcx.hir();
1427         // Get the name of the callable and the arguments to be used in the suggestion.
1428         let snippet = match hir.get_if_local(def_id) {
1429             Some(hir::Node::Expr(hir::Expr {
1430                 kind: hir::ExprKind::Closure(_, decl, _, span, ..),
1431                 ..
1432             })) => {
1433                 err.span_label(*span, "consider calling this closure");
1434                 let name = match self.get_closure_name(def_id, err, &msg) {
1435                     Some(name) => name,
1436                     None => return,
1437                 };
1438                 let args = decl.inputs.iter().map(|_| "_").collect::<Vec<_>>().join(", ");
1439                 format!("{}({})", name, args)
1440             }
1441             Some(hir::Node::Item(hir::Item {
1442                 ident,
1443                 kind: hir::ItemKind::Fn(.., body_id),
1444                 ..
1445             })) => {
1446                 err.span_label(ident.span, "consider calling this function");
1447                 let body = hir.body(*body_id);
1448                 let args = body
1449                     .params
1450                     .iter()
1451                     .map(|arg| match &arg.pat.kind {
1452                         hir::PatKind::Binding(_, _, ident, None)
1453                         // FIXME: provide a better suggestion when encountering `SelfLower`, it
1454                         // should suggest a method call.
1455                         if ident.name != kw::SelfLower => ident.to_string(),
1456                         _ => "_".to_string(),
1457                     })
1458                     .collect::<Vec<_>>()
1459                     .join(", ");
1460                 format!("{}({})", ident, args)
1461             }
1462             _ => return,
1463         };
1464         if points_at_arg {
1465             // When the obligation error has been ensured to have been caused by
1466             // an argument, the `obligation.cause.span` points at the expression
1467             // of the argument, so we can provide a suggestion. This is signaled
1468             // by `points_at_arg`. Otherwise, we give a more general note.
1469             err.span_suggestion(
1470                 obligation.cause.span,
1471                 &msg,
1472                 snippet,
1473                 Applicability::HasPlaceholders,
1474             );
1475         } else {
1476             err.help(&format!("{}: `{}`", msg, snippet));
1477         }
1478     }
1479
1480     fn suggest_add_reference_to_arg(
1481         &self,
1482         obligation: &PredicateObligation<'tcx>,
1483         err: &mut DiagnosticBuilder<'tcx>,
1484         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1485         points_at_arg: bool,
1486         has_custom_message: bool,
1487     ) -> bool {
1488         if !points_at_arg {
1489             return false;
1490         }
1491
1492         let span = obligation.cause.span;
1493         let param_env = obligation.param_env;
1494         let trait_ref = trait_ref.skip_binder();
1495
1496         if let ObligationCauseCode::ImplDerivedObligation(obligation) = &obligation.cause.code {
1497             // Try to apply the original trait binding obligation by borrowing.
1498             let self_ty = trait_ref.self_ty();
1499             let found = self_ty.to_string();
1500             let new_self_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, self_ty);
1501             let substs = self.tcx.mk_substs_trait(new_self_ty, &[]);
1502             let new_trait_ref = ty::TraitRef::new(obligation.parent_trait_ref.def_id(), substs);
1503             let new_obligation =
1504                 Obligation::new(ObligationCause::dummy(), param_env, new_trait_ref.to_predicate());
1505             if self.predicate_must_hold_modulo_regions(&new_obligation) {
1506                 if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1507                     // We have a very specific type of error, where just borrowing this argument
1508                     // might solve the problem. In cases like this, the important part is the
1509                     // original type obligation, not the last one that failed, which is arbitrary.
1510                     // Because of this, we modify the error to refer to the original obligation and
1511                     // return early in the caller.
1512                     let msg = format!(
1513                         "the trait bound `{}: {}` is not satisfied",
1514                         found,
1515                         obligation.parent_trait_ref.skip_binder().print_only_trait_path(),
1516                     );
1517                     if has_custom_message {
1518                         err.note(&msg);
1519                     } else {
1520                         err.message = vec![(msg, Style::NoStyle)];
1521                     }
1522                     if snippet.starts_with('&') {
1523                         // This is already a literal borrow and the obligation is failing
1524                         // somewhere else in the obligation chain. Do not suggest non-sense.
1525                         return false;
1526                     }
1527                     err.span_label(
1528                         span,
1529                         &format!(
1530                             "expected an implementor of trait `{}`",
1531                             obligation.parent_trait_ref.skip_binder().print_only_trait_path(),
1532                         ),
1533                     );
1534                     err.span_suggestion(
1535                         span,
1536                         "consider borrowing here",
1537                         format!("&{}", snippet),
1538                         Applicability::MaybeIncorrect,
1539                     );
1540                     return true;
1541                 }
1542             }
1543         }
1544         false
1545     }
1546
1547     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
1548     /// suggest removing these references until we reach a type that implements the trait.
1549     fn suggest_remove_reference(
1550         &self,
1551         obligation: &PredicateObligation<'tcx>,
1552         err: &mut DiagnosticBuilder<'tcx>,
1553         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1554     ) {
1555         let trait_ref = trait_ref.skip_binder();
1556         let span = obligation.cause.span;
1557
1558         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1559             let refs_number =
1560                 snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
1561             if let Some('\'') =
1562                 snippet.chars().filter(|c| !c.is_whitespace()).skip(refs_number).next()
1563             {
1564                 // Do not suggest removal of borrow from type arguments.
1565                 return;
1566             }
1567
1568             let mut trait_type = trait_ref.self_ty();
1569
1570             for refs_remaining in 0..refs_number {
1571                 if let ty::Ref(_, t_type, _) = trait_type.kind {
1572                     trait_type = t_type;
1573
1574                     let new_obligation = self.mk_obligation_for_def_id(
1575                         trait_ref.def_id,
1576                         trait_type,
1577                         ObligationCause::dummy(),
1578                         obligation.param_env,
1579                     );
1580
1581                     if self.predicate_may_hold(&new_obligation) {
1582                         let sp = self
1583                             .tcx
1584                             .sess
1585                             .source_map()
1586                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1587
1588                         let remove_refs = refs_remaining + 1;
1589                         let format_str =
1590                             format!("consider removing {} leading `&`-references", remove_refs);
1591
1592                         err.span_suggestion_short(
1593                             sp,
1594                             &format_str,
1595                             String::new(),
1596                             Applicability::MachineApplicable,
1597                         );
1598                         break;
1599                     }
1600                 } else {
1601                     break;
1602                 }
1603             }
1604         }
1605     }
1606
1607     /// Check if the trait bound is implemented for a different mutability and note it in the
1608     /// final error.
1609     fn suggest_change_mut(
1610         &self,
1611         obligation: &PredicateObligation<'tcx>,
1612         err: &mut DiagnosticBuilder<'tcx>,
1613         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1614         points_at_arg: bool,
1615     ) {
1616         let span = obligation.cause.span;
1617         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1618             let refs_number =
1619                 snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
1620             if let Some('\'') =
1621                 snippet.chars().filter(|c| !c.is_whitespace()).skip(refs_number).next()
1622             {
1623                 // Do not suggest removal of borrow from type arguments.
1624                 return;
1625             }
1626             let trait_ref = self.resolve_vars_if_possible(trait_ref);
1627             if trait_ref.has_infer_types() {
1628                 // Do not ICE while trying to find if a reborrow would succeed on a trait with
1629                 // unresolved bindings.
1630                 return;
1631             }
1632
1633             if let ty::Ref(region, t_type, mutability) = trait_ref.skip_binder().self_ty().kind {
1634                 let trait_type = match mutability {
1635                     hir::Mutability::Mut => self.tcx.mk_imm_ref(region, t_type),
1636                     hir::Mutability::Not => self.tcx.mk_mut_ref(region, t_type),
1637                 };
1638
1639                 let new_obligation = self.mk_obligation_for_def_id(
1640                     trait_ref.skip_binder().def_id,
1641                     trait_type,
1642                     ObligationCause::dummy(),
1643                     obligation.param_env,
1644                 );
1645
1646                 if self.evaluate_obligation_no_overflow(&new_obligation).must_apply_modulo_regions()
1647                 {
1648                     let sp = self
1649                         .tcx
1650                         .sess
1651                         .source_map()
1652                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1653                     if points_at_arg && mutability == hir::Mutability::Not && refs_number > 0 {
1654                         err.span_suggestion(
1655                             sp,
1656                             "consider changing this borrow's mutability",
1657                             "&mut ".to_string(),
1658                             Applicability::MachineApplicable,
1659                         );
1660                     } else {
1661                         err.note(&format!(
1662                             "`{}` is implemented for `{:?}`, but not for `{:?}`",
1663                             trait_ref.print_only_trait_path(),
1664                             trait_type,
1665                             trait_ref.skip_binder().self_ty(),
1666                         ));
1667                     }
1668                 }
1669             }
1670         }
1671     }
1672
1673     fn suggest_semicolon_removal(
1674         &self,
1675         obligation: &PredicateObligation<'tcx>,
1676         err: &mut DiagnosticBuilder<'tcx>,
1677         span: Span,
1678         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1679     ) {
1680         let hir = self.tcx.hir();
1681         let parent_node = hir.get_parent_node(obligation.cause.body_id);
1682         let node = hir.find(parent_node);
1683         if let Some(hir::Node::Item(hir::Item {
1684             kind: hir::ItemKind::Fn(sig, _, body_id), ..
1685         })) = node
1686         {
1687             let body = hir.body(*body_id);
1688             if let hir::ExprKind::Block(blk, _) = &body.value.kind {
1689                 if sig.decl.output.span().overlaps(span)
1690                     && blk.expr.is_none()
1691                     && "()" == &trait_ref.self_ty().to_string()
1692                 {
1693                     // FIXME(estebank): When encountering a method with a trait
1694                     // bound not satisfied in the return type with a body that has
1695                     // no return, suggest removal of semicolon on last statement.
1696                     // Once that is added, close #54771.
1697                     if let Some(ref stmt) = blk.stmts.last() {
1698                         let sp = self.tcx.sess.source_map().end_point(stmt.span);
1699                         err.span_label(sp, "consider removing this semicolon");
1700                     }
1701                 }
1702             }
1703         }
1704     }
1705
1706     /// Given some node representing a fn-like thing in the HIR map,
1707     /// returns a span and `ArgKind` information that describes the
1708     /// arguments it expects. This can be supplied to
1709     /// `report_arg_count_mismatch`.
1710     pub fn get_fn_like_arguments(&self, node: Node<'_>) -> (Span, Vec<ArgKind>) {
1711         match node {
1712             Node::Expr(&hir::Expr {
1713                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
1714                 ..
1715             }) => (
1716                 self.tcx.sess.source_map().def_span(span),
1717                 self.tcx
1718                     .hir()
1719                     .body(id)
1720                     .params
1721                     .iter()
1722                     .map(|arg| {
1723                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
1724                             *arg.pat
1725                         {
1726                             ArgKind::Tuple(
1727                                 Some(span),
1728                                 args.iter()
1729                                     .map(|pat| {
1730                                         let snippet = self
1731                                             .tcx
1732                                             .sess
1733                                             .source_map()
1734                                             .span_to_snippet(pat.span)
1735                                             .unwrap();
1736                                         (snippet, "_".to_owned())
1737                                     })
1738                                     .collect::<Vec<_>>(),
1739                             )
1740                         } else {
1741                             let name =
1742                                 self.tcx.sess.source_map().span_to_snippet(arg.pat.span).unwrap();
1743                             ArgKind::Arg(name, "_".to_owned())
1744                         }
1745                     })
1746                     .collect::<Vec<ArgKind>>(),
1747             ),
1748             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
1749             | Node::ImplItem(&hir::ImplItem {
1750                 span,
1751                 kind: hir::ImplItemKind::Method(ref sig, _),
1752                 ..
1753             })
1754             | Node::TraitItem(&hir::TraitItem {
1755                 span,
1756                 kind: hir::TraitItemKind::Method(ref sig, _),
1757                 ..
1758             }) => (
1759                 self.tcx.sess.source_map().def_span(span),
1760                 sig.decl
1761                     .inputs
1762                     .iter()
1763                     .map(|arg| match arg.clone().kind {
1764                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1765                             Some(arg.span),
1766                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
1767                         ),
1768                         _ => ArgKind::empty(),
1769                     })
1770                     .collect::<Vec<ArgKind>>(),
1771             ),
1772             Node::Ctor(ref variant_data) => {
1773                 let span = variant_data
1774                     .ctor_hir_id()
1775                     .map(|hir_id| self.tcx.hir().span(hir_id))
1776                     .unwrap_or(DUMMY_SP);
1777                 let span = self.tcx.sess.source_map().def_span(span);
1778
1779                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1780             }
1781             _ => panic!("non-FnLike node found: {:?}", node),
1782         }
1783     }
1784
1785     /// Reports an error when the number of arguments needed by a
1786     /// trait match doesn't match the number that the expression
1787     /// provides.
1788     pub fn report_arg_count_mismatch(
1789         &self,
1790         span: Span,
1791         found_span: Option<Span>,
1792         expected_args: Vec<ArgKind>,
1793         found_args: Vec<ArgKind>,
1794         is_closure: bool,
1795     ) -> DiagnosticBuilder<'tcx> {
1796         let kind = if is_closure { "closure" } else { "function" };
1797
1798         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1799             let arg_length = arguments.len();
1800             let distinct = match &other[..] {
1801                 &[ArgKind::Tuple(..)] => true,
1802                 _ => false,
1803             };
1804             match (arg_length, arguments.get(0)) {
1805                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1806                     format!("a single {}-tuple as argument", fields.len())
1807                 }
1808                 _ => format!(
1809                     "{} {}argument{}",
1810                     arg_length,
1811                     if distinct && arg_length > 1 { "distinct " } else { "" },
1812                     pluralize!(arg_length)
1813                 ),
1814             }
1815         };
1816
1817         let expected_str = args_str(&expected_args, &found_args);
1818         let found_str = args_str(&found_args, &expected_args);
1819
1820         let mut err = struct_span_err!(
1821             self.tcx.sess,
1822             span,
1823             E0593,
1824             "{} is expected to take {}, but it takes {}",
1825             kind,
1826             expected_str,
1827             found_str,
1828         );
1829
1830         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1831
1832         if let Some(found_span) = found_span {
1833             err.span_label(found_span, format!("takes {}", found_str));
1834
1835             // move |_| { ... }
1836             // ^^^^^^^^-- def_span
1837             //
1838             // move |_| { ... }
1839             // ^^^^^-- prefix
1840             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1841             // move |_| { ... }
1842             //      ^^^-- pipe_span
1843             let pipe_span =
1844                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
1845
1846             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1847             // found arguments is empty (assume the user just wants to ignore args in this case).
1848             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1849             if found_args.is_empty() && is_closure {
1850                 let underscores = vec!["_"; expected_args.len()].join(", ");
1851                 err.span_suggestion(
1852                     pipe_span,
1853                     &format!(
1854                         "consider changing the closure to take and ignore the expected argument{}",
1855                         if expected_args.len() < 2 { "" } else { "s" }
1856                     ),
1857                     format!("|{}|", underscores),
1858                     Applicability::MachineApplicable,
1859                 );
1860             }
1861
1862             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1863                 if fields.len() == expected_args.len() {
1864                     let sugg = fields
1865                         .iter()
1866                         .map(|(name, _)| name.to_owned())
1867                         .collect::<Vec<String>>()
1868                         .join(", ");
1869                     err.span_suggestion(
1870                         found_span,
1871                         "change the closure to take multiple arguments instead of a single tuple",
1872                         format!("|{}|", sugg),
1873                         Applicability::MachineApplicable,
1874                     );
1875                 }
1876             }
1877             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1878                 if fields.len() == found_args.len() && is_closure {
1879                     let sugg = format!(
1880                         "|({}){}|",
1881                         found_args
1882                             .iter()
1883                             .map(|arg| match arg {
1884                                 ArgKind::Arg(name, _) => name.to_owned(),
1885                                 _ => "_".to_owned(),
1886                             })
1887                             .collect::<Vec<String>>()
1888                             .join(", "),
1889                         // add type annotations if available
1890                         if found_args.iter().any(|arg| match arg {
1891                             ArgKind::Arg(_, ty) => ty != "_",
1892                             _ => false,
1893                         }) {
1894                             format!(
1895                                 ": ({})",
1896                                 fields
1897                                     .iter()
1898                                     .map(|(_, ty)| ty.to_owned())
1899                                     .collect::<Vec<String>>()
1900                                     .join(", ")
1901                             )
1902                         } else {
1903                             String::new()
1904                         },
1905                     );
1906                     err.span_suggestion(
1907                         found_span,
1908                         "change the closure to accept a tuple instead of individual arguments",
1909                         sugg,
1910                         Applicability::MachineApplicable,
1911                     );
1912                 }
1913             }
1914         }
1915
1916         err
1917     }
1918
1919     fn report_closure_arg_mismatch(
1920         &self,
1921         span: Span,
1922         found_span: Option<Span>,
1923         expected_ref: ty::PolyTraitRef<'tcx>,
1924         found: ty::PolyTraitRef<'tcx>,
1925     ) -> DiagnosticBuilder<'tcx> {
1926         fn build_fn_sig_string<'tcx>(tcx: TyCtxt<'tcx>, trait_ref: &ty::TraitRef<'tcx>) -> String {
1927             let inputs = trait_ref.substs.type_at(1);
1928             let sig = if let ty::Tuple(inputs) = inputs.kind {
1929                 tcx.mk_fn_sig(
1930                     inputs.iter().map(|k| k.expect_ty()),
1931                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1932                     false,
1933                     hir::Unsafety::Normal,
1934                     ::rustc_target::spec::abi::Abi::Rust,
1935                 )
1936             } else {
1937                 tcx.mk_fn_sig(
1938                     ::std::iter::once(inputs),
1939                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1940                     false,
1941                     hir::Unsafety::Normal,
1942                     ::rustc_target::spec::abi::Abi::Rust,
1943                 )
1944             };
1945             ty::Binder::bind(sig).to_string()
1946         }
1947
1948         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1949         let mut err = struct_span_err!(
1950             self.tcx.sess,
1951             span,
1952             E0631,
1953             "type mismatch in {} arguments",
1954             if argument_is_closure { "closure" } else { "function" }
1955         );
1956
1957         let found_str = format!(
1958             "expected signature of `{}`",
1959             build_fn_sig_string(self.tcx, found.skip_binder())
1960         );
1961         err.span_label(span, found_str);
1962
1963         let found_span = found_span.unwrap_or(span);
1964         let expected_str = format!(
1965             "found signature of `{}`",
1966             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1967         );
1968         err.span_label(found_span, expected_str);
1969
1970         err
1971     }
1972 }
1973
1974 pub fn recursive_type_with_infinite_size_error(
1975     tcx: TyCtxt<'tcx>,
1976     type_def_id: DefId,
1977 ) -> DiagnosticBuilder<'tcx> {
1978     assert!(type_def_id.is_local());
1979     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1980     let span = tcx.sess.source_map().def_span(span);
1981     let mut err = struct_span_err!(
1982         tcx.sess,
1983         span,
1984         E0072,
1985         "recursive type `{}` has infinite size",
1986         tcx.def_path_str(type_def_id)
1987     );
1988     err.span_label(span, "recursive type has infinite size");
1989     err.help(&format!(
1990         "insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1991                            at some point to make `{}` representable",
1992         tcx.def_path_str(type_def_id)
1993     ));
1994     err
1995 }
1996
1997 pub fn report_object_safety_error(
1998     tcx: TyCtxt<'tcx>,
1999     span: Span,
2000     trait_def_id: DefId,
2001     violations: Vec<ObjectSafetyViolation>,
2002 ) -> DiagnosticBuilder<'tcx> {
2003     let trait_str = tcx.def_path_str(trait_def_id);
2004     let span = tcx.sess.source_map().def_span(span);
2005     let mut err = struct_span_err!(
2006         tcx.sess,
2007         span,
2008         E0038,
2009         "the trait `{}` cannot be made into an object",
2010         trait_str
2011     );
2012     err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
2013
2014     let mut reported_violations = FxHashSet::default();
2015     for violation in violations {
2016         if reported_violations.insert(violation.clone()) {
2017             match violation.span() {
2018                 Some(span) => err.span_label(span, violation.error_msg()),
2019                 None => err.note(&violation.error_msg()),
2020             };
2021         }
2022     }
2023
2024     if tcx.sess.trait_methods_not_found.borrow().contains(&span) {
2025         // Avoid emitting error caused by non-existing method (#58734)
2026         err.cancel();
2027     }
2028
2029     err
2030 }
2031
2032 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
2033     fn maybe_report_ambiguity(
2034         &self,
2035         obligation: &PredicateObligation<'tcx>,
2036         body_id: Option<hir::BodyId>,
2037     ) {
2038         // Unable to successfully determine, probably means
2039         // insufficient type information, but could mean
2040         // ambiguous impls. The latter *ought* to be a
2041         // coherence violation, so we don't report it here.
2042
2043         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
2044         let span = obligation.cause.span;
2045
2046         debug!(
2047             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
2048             predicate, obligation, body_id, obligation.cause.code,
2049         );
2050
2051         // Ambiguity errors are often caused as fallout from earlier
2052         // errors. So just ignore them if this infcx is tainted.
2053         if self.is_tainted_by_errors() {
2054             return;
2055         }
2056
2057         let mut err = match predicate {
2058             ty::Predicate::Trait(ref data) => {
2059                 let trait_ref = data.to_poly_trait_ref();
2060                 let self_ty = trait_ref.self_ty();
2061                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
2062
2063                 if predicate.references_error() {
2064                     return;
2065                 }
2066                 // Typically, this ambiguity should only happen if
2067                 // there are unresolved type inference variables
2068                 // (otherwise it would suggest a coherence
2069                 // failure). But given #21974 that is not necessarily
2070                 // the case -- we can have multiple where clauses that
2071                 // are only distinguished by a region, which results
2072                 // in an ambiguity even when all types are fully
2073                 // known, since we don't dispatch based on region
2074                 // relationships.
2075
2076                 // This is kind of a hack: it frequently happens that some earlier
2077                 // error prevents types from being fully inferred, and then we get
2078                 // a bunch of uninteresting errors saying something like "<generic
2079                 // #0> doesn't implement Sized".  It may even be true that we
2080                 // could just skip over all checks where the self-ty is an
2081                 // inference variable, but I was afraid that there might be an
2082                 // inference variable created, registered as an obligation, and
2083                 // then never forced by writeback, and hence by skipping here we'd
2084                 // be ignoring the fact that we don't KNOW the type works
2085                 // out. Though even that would probably be harmless, given that
2086                 // we're only talking about builtin traits, which are known to be
2087                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2088                 // avoid inundating the user with unnecessary errors, but we now
2089                 // check upstream for type errors and dont add the obligations to
2090                 // begin with in those cases.
2091                 if self
2092                     .tcx
2093                     .lang_items()
2094                     .sized_trait()
2095                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
2096                 {
2097                     self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
2098                     return;
2099                 }
2100                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
2101                 err.note(&format!("cannot resolve `{}`", predicate));
2102                 if let (Ok(ref snippet), ObligationCauseCode::BindingObligation(ref def_id, _)) =
2103                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
2104                 {
2105                     let generics = self.tcx.generics_of(*def_id);
2106                     if !generics.params.is_empty() && !snippet.ends_with('>') {
2107                         // FIXME: To avoid spurious suggestions in functions where type arguments
2108                         // where already supplied, we check the snippet to make sure it doesn't
2109                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2110                         // instead. Otherwise we would produce the following output:
2111                         //
2112                         // error[E0283]: type annotations needed
2113                         //   --> $DIR/issue-54954.rs:3:24
2114                         //    |
2115                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2116                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2117                         //    |                        |
2118                         //    |                        cannot infer type
2119                         //    |                        help: consider specifying the type argument
2120                         //    |                        in the function call:
2121                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2122                         // ...
2123                         // LL |     const fn const_val<T: Sized>() -> usize {
2124                         //    |              --------- - required by this bound in `Tt::const_val`
2125                         //    |
2126                         //    = note: cannot resolve `_: Tt`
2127
2128                         err.span_suggestion(
2129                             span,
2130                             &format!(
2131                                 "consider specifying the type argument{} in the function call",
2132                                 if generics.params.len() > 1 { "s" } else { "" },
2133                             ),
2134                             format!(
2135                                 "{}::<{}>",
2136                                 snippet,
2137                                 generics
2138                                     .params
2139                                     .iter()
2140                                     .map(|p| p.name.to_string())
2141                                     .collect::<Vec<String>>()
2142                                     .join(", ")
2143                             ),
2144                             Applicability::HasPlaceholders,
2145                         );
2146                     }
2147                 }
2148                 err
2149             }
2150
2151             ty::Predicate::WellFormed(ty) => {
2152                 // Same hacky approach as above to avoid deluging user
2153                 // with error messages.
2154                 if ty.references_error() || self.tcx.sess.has_errors() {
2155                     return;
2156                 }
2157                 self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
2158             }
2159
2160             ty::Predicate::Subtype(ref data) => {
2161                 if data.references_error() || self.tcx.sess.has_errors() {
2162                     // no need to overload user in such cases
2163                     return;
2164                 }
2165                 let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
2166                 // both must be type variables, or the other would've been instantiated
2167                 assert!(a.is_ty_var() && b.is_ty_var());
2168                 self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
2169             }
2170             ty::Predicate::Projection(ref data) => {
2171                 let trait_ref = data.to_poly_trait_ref(self.tcx);
2172                 let self_ty = trait_ref.self_ty();
2173                 if predicate.references_error() {
2174                     return;
2175                 }
2176                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
2177                 err.note(&format!("cannot resolve `{}`", predicate));
2178                 err
2179             }
2180
2181             _ => {
2182                 if self.tcx.sess.has_errors() {
2183                     return;
2184                 }
2185                 let mut err = struct_span_err!(
2186                     self.tcx.sess,
2187                     span,
2188                     E0284,
2189                     "type annotations needed: cannot resolve `{}`",
2190                     predicate,
2191                 );
2192                 err.span_label(span, &format!("cannot resolve `{}`", predicate));
2193                 err
2194             }
2195         };
2196         self.note_obligation_cause(&mut err, obligation);
2197         err.emit();
2198     }
2199
2200     /// Returns `true` if the trait predicate may apply for *some* assignment
2201     /// to the type parameters.
2202     fn predicate_can_apply(
2203         &self,
2204         param_env: ty::ParamEnv<'tcx>,
2205         pred: ty::PolyTraitRef<'tcx>,
2206     ) -> bool {
2207         struct ParamToVarFolder<'a, 'tcx> {
2208             infcx: &'a InferCtxt<'a, 'tcx>,
2209             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2210         }
2211
2212         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2213             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2214                 self.infcx.tcx
2215             }
2216
2217             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2218                 if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
2219                     let infcx = self.infcx;
2220                     self.var_map.entry(ty).or_insert_with(|| {
2221                         infcx.next_ty_var(TypeVariableOrigin {
2222                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2223                             span: DUMMY_SP,
2224                         })
2225                     })
2226                 } else {
2227                     ty.super_fold_with(self)
2228                 }
2229             }
2230         }
2231
2232         self.probe(|_| {
2233             let mut selcx = SelectionContext::new(self);
2234
2235             let cleaned_pred =
2236                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2237
2238             let cleaned_pred = super::project::normalize(
2239                 &mut selcx,
2240                 param_env,
2241                 ObligationCause::dummy(),
2242                 &cleaned_pred,
2243             )
2244             .value;
2245
2246             let obligation =
2247                 Obligation::new(ObligationCause::dummy(), param_env, cleaned_pred.to_predicate());
2248
2249             self.predicate_may_hold(&obligation)
2250         })
2251     }
2252
2253     fn note_obligation_cause(
2254         &self,
2255         err: &mut DiagnosticBuilder<'_>,
2256         obligation: &PredicateObligation<'tcx>,
2257     ) {
2258         // First, attempt to add note to this error with an async-await-specific
2259         // message, and fall back to regular note otherwise.
2260         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2261             self.note_obligation_cause_code(
2262                 err,
2263                 &obligation.predicate,
2264                 &obligation.cause.code,
2265                 &mut vec![],
2266             );
2267         }
2268     }
2269
2270     /// Adds an async-await specific note to the diagnostic when the future does not implement
2271     /// an auto trait because of a captured type.
2272     ///
2273     /// ```ignore (diagnostic)
2274     /// note: future does not implement `Qux` as this value is used across an await
2275     ///   --> $DIR/issue-64130-3-other.rs:17:5
2276     ///    |
2277     /// LL |     let x = Foo;
2278     ///    |         - has type `Foo`
2279     /// LL |     baz().await;
2280     ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
2281     /// LL | }
2282     ///    | - `x` is later dropped here
2283     /// ```
2284     ///
2285     /// When the diagnostic does not implement `Send` or `Sync` specifically, then the diagnostic
2286     /// is "replaced" with a different message and a more specific error.
2287     ///
2288     /// ```ignore (diagnostic)
2289     /// error: future cannot be sent between threads safely
2290     ///   --> $DIR/issue-64130-2-send.rs:21:5
2291     ///    |
2292     /// LL | fn is_send<T: Send>(t: T) { }
2293     ///    |    -------    ---- required by this bound in `is_send`
2294     /// ...
2295     /// LL |     is_send(bar());
2296     ///    |     ^^^^^^^ future returned by `bar` is not send
2297     ///    |
2298     ///    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not
2299     ///            implemented for `Foo`
2300     /// note: future is not send as this value is used across an await
2301     ///   --> $DIR/issue-64130-2-send.rs:15:5
2302     ///    |
2303     /// LL |     let x = Foo;
2304     ///    |         - has type `Foo`
2305     /// LL |     baz().await;
2306     ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
2307     /// LL | }
2308     ///    | - `x` is later dropped here
2309     /// ```
2310     ///
2311     /// Returns `true` if an async-await specific note was added to the diagnostic.
2312     fn maybe_note_obligation_cause_for_async_await(
2313         &self,
2314         err: &mut DiagnosticBuilder<'_>,
2315         obligation: &PredicateObligation<'tcx>,
2316     ) -> bool {
2317         debug!(
2318             "maybe_note_obligation_cause_for_async_await: obligation.predicate={:?} \
2319                 obligation.cause.span={:?}",
2320             obligation.predicate, obligation.cause.span
2321         );
2322         let source_map = self.tcx.sess.source_map();
2323
2324         // Attempt to detect an async-await error by looking at the obligation causes, looking
2325         // for a generator to be present.
2326         //
2327         // When a future does not implement a trait because of a captured type in one of the
2328         // generators somewhere in the call stack, then the result is a chain of obligations.
2329         //
2330         // Given a `async fn` A that calls a `async fn` B which captures a non-send type and that
2331         // future is passed as an argument to a function C which requires a `Send` type, then the
2332         // chain looks something like this:
2333         //
2334         // - `BuiltinDerivedObligation` with a generator witness (B)
2335         // - `BuiltinDerivedObligation` with a generator (B)
2336         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (B)
2337         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
2338         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
2339         // - `BuiltinDerivedObligation` with a generator witness (A)
2340         // - `BuiltinDerivedObligation` with a generator (A)
2341         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (A)
2342         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
2343         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
2344         // - `BindingObligation` with `impl_send (Send requirement)
2345         //
2346         // The first obligation in the chain is the most useful and has the generator that captured
2347         // the type. The last generator has information about where the bound was introduced. At
2348         // least one generator should be present for this diagnostic to be modified.
2349         let (mut trait_ref, mut target_ty) = match obligation.predicate {
2350             ty::Predicate::Trait(p) => {
2351                 (Some(p.skip_binder().trait_ref), Some(p.skip_binder().self_ty()))
2352             }
2353             _ => (None, None),
2354         };
2355         let mut generator = None;
2356         let mut last_generator = None;
2357         let mut next_code = Some(&obligation.cause.code);
2358         while let Some(code) = next_code {
2359             debug!("maybe_note_obligation_cause_for_async_await: code={:?}", code);
2360             match code {
2361                 ObligationCauseCode::BuiltinDerivedObligation(derived_obligation)
2362                 | ObligationCauseCode::ImplDerivedObligation(derived_obligation) => {
2363                     let ty = derived_obligation.parent_trait_ref.self_ty();
2364                     debug!(
2365                         "maybe_note_obligation_cause_for_async_await: \
2366                             parent_trait_ref={:?} self_ty.kind={:?}",
2367                         derived_obligation.parent_trait_ref, ty.kind
2368                     );
2369
2370                     match ty.kind {
2371                         ty::Generator(did, ..) => {
2372                             generator = generator.or(Some(did));
2373                             last_generator = Some(did);
2374                         }
2375                         ty::GeneratorWitness(..) => {}
2376                         _ if generator.is_none() => {
2377                             trait_ref = Some(*derived_obligation.parent_trait_ref.skip_binder());
2378                             target_ty = Some(ty);
2379                         }
2380                         _ => {}
2381                     }
2382
2383                     next_code = Some(derived_obligation.parent_code.as_ref());
2384                 }
2385                 _ => break,
2386             }
2387         }
2388
2389         // Only continue if a generator was found.
2390         debug!(
2391             "maybe_note_obligation_cause_for_async_await: generator={:?} trait_ref={:?} \
2392                 target_ty={:?}",
2393             generator, trait_ref, target_ty
2394         );
2395         let (generator_did, trait_ref, target_ty) = match (generator, trait_ref, target_ty) {
2396             (Some(generator_did), Some(trait_ref), Some(target_ty)) => {
2397                 (generator_did, trait_ref, target_ty)
2398             }
2399             _ => return false,
2400         };
2401
2402         let span = self.tcx.def_span(generator_did);
2403
2404         // Do not ICE on closure typeck (#66868).
2405         if self.tcx.hir().as_local_hir_id(generator_did).is_none() {
2406             return false;
2407         }
2408
2409         // Get the tables from the infcx if the generator is the function we are
2410         // currently type-checking; otherwise, get them by performing a query.
2411         // This is needed to avoid cycles.
2412         let in_progress_tables = self.in_progress_tables.map(|t| t.borrow());
2413         let generator_did_root = self.tcx.closure_base_def_id(generator_did);
2414         debug!(
2415             "maybe_note_obligation_cause_for_async_await: generator_did={:?} \
2416              generator_did_root={:?} in_progress_tables.local_id_root={:?} span={:?}",
2417             generator_did,
2418             generator_did_root,
2419             in_progress_tables.as_ref().map(|t| t.local_id_root),
2420             span
2421         );
2422         let query_tables;
2423         let tables: &TypeckTables<'tcx> = match &in_progress_tables {
2424             Some(t) if t.local_id_root == Some(generator_did_root) => t,
2425             _ => {
2426                 query_tables = self.tcx.typeck_tables_of(generator_did);
2427                 &query_tables
2428             }
2429         };
2430
2431         // Look for a type inside the generator interior that matches the target type to get
2432         // a span.
2433         let target_ty_erased = self.tcx.erase_regions(&target_ty);
2434         let target_span = tables
2435             .generator_interior_types
2436             .iter()
2437             .find(|ty::GeneratorInteriorTypeCause { ty, .. }| {
2438                 // Careful: the regions for types that appear in the
2439                 // generator interior are not generally known, so we
2440                 // want to erase them when comparing (and anyway,
2441                 // `Send` and other bounds are generally unaffected by
2442                 // the choice of region).  When erasing regions, we
2443                 // also have to erase late-bound regions. This is
2444                 // because the types that appear in the generator
2445                 // interior generally contain "bound regions" to
2446                 // represent regions that are part of the suspended
2447                 // generator frame. Bound regions are preserved by
2448                 // `erase_regions` and so we must also call
2449                 // `erase_late_bound_regions`.
2450                 let ty_erased = self.tcx.erase_late_bound_regions(&ty::Binder::bind(*ty));
2451                 let ty_erased = self.tcx.erase_regions(&ty_erased);
2452                 let eq = ty::TyS::same_type(ty_erased, target_ty_erased);
2453                 debug!(
2454                     "maybe_note_obligation_cause_for_async_await: ty_erased={:?} \
2455                         target_ty_erased={:?} eq={:?}",
2456                     ty_erased, target_ty_erased, eq
2457                 );
2458                 eq
2459             })
2460             .map(|ty::GeneratorInteriorTypeCause { span, scope_span, .. }| {
2461                 (span, source_map.span_to_snippet(*span), scope_span)
2462             });
2463         debug!(
2464             "maybe_note_obligation_cause_for_async_await: target_ty={:?} \
2465                 generator_interior_types={:?} target_span={:?}",
2466             target_ty, tables.generator_interior_types, target_span
2467         );
2468         if let Some((target_span, Ok(snippet), scope_span)) = target_span {
2469             self.note_obligation_cause_for_async_await(
2470                 err,
2471                 *target_span,
2472                 scope_span,
2473                 snippet,
2474                 generator_did,
2475                 last_generator,
2476                 trait_ref,
2477                 target_ty,
2478                 tables,
2479                 obligation,
2480                 next_code,
2481             );
2482             true
2483         } else {
2484             false
2485         }
2486     }
2487
2488     /// Unconditionally adds the diagnostic note described in
2489     /// `maybe_note_obligation_cause_for_async_await`'s documentation comment.
2490     fn note_obligation_cause_for_async_await(
2491         &self,
2492         err: &mut DiagnosticBuilder<'_>,
2493         target_span: Span,
2494         scope_span: &Option<Span>,
2495         snippet: String,
2496         first_generator: DefId,
2497         last_generator: Option<DefId>,
2498         trait_ref: ty::TraitRef<'_>,
2499         target_ty: Ty<'tcx>,
2500         tables: &ty::TypeckTables<'_>,
2501         obligation: &PredicateObligation<'tcx>,
2502         next_code: Option<&ObligationCauseCode<'tcx>>,
2503     ) {
2504         let source_map = self.tcx.sess.source_map();
2505
2506         let is_async_fn = self
2507             .tcx
2508             .parent(first_generator)
2509             .map(|parent_did| self.tcx.asyncness(parent_did))
2510             .map(|parent_asyncness| parent_asyncness == hir::IsAsync::Async)
2511             .unwrap_or(false);
2512         let is_async_move = self
2513             .tcx
2514             .hir()
2515             .as_local_hir_id(first_generator)
2516             .and_then(|hir_id| self.tcx.hir().maybe_body_owned_by(hir_id))
2517             .map(|body_id| self.tcx.hir().body(body_id))
2518             .and_then(|body| body.generator_kind())
2519             .map(|generator_kind| match generator_kind {
2520                 hir::GeneratorKind::Async(..) => true,
2521                 _ => false,
2522             })
2523             .unwrap_or(false);
2524         let await_or_yield = if is_async_fn || is_async_move { "await" } else { "yield" };
2525
2526         // Special case the primary error message when send or sync is the trait that was
2527         // not implemented.
2528         let is_send = self.tcx.is_diagnostic_item(sym::send_trait, trait_ref.def_id);
2529         let is_sync = self.tcx.is_diagnostic_item(sym::sync_trait, trait_ref.def_id);
2530         let trait_explanation = if is_send || is_sync {
2531             let (trait_name, trait_verb) =
2532                 if is_send { ("`Send`", "sent") } else { ("`Sync`", "shared") };
2533
2534             err.clear_code();
2535             err.set_primary_message(format!(
2536                 "future cannot be {} between threads safely",
2537                 trait_verb
2538             ));
2539
2540             let original_span = err.span.primary_span().unwrap();
2541             let mut span = MultiSpan::from_span(original_span);
2542
2543             let message = if let Some(name) = last_generator
2544                 .and_then(|generator_did| self.tcx.parent(generator_did))
2545                 .and_then(|parent_did| self.tcx.hir().as_local_hir_id(parent_did))
2546                 .and_then(|parent_hir_id| self.tcx.hir().opt_name(parent_hir_id))
2547             {
2548                 format!("future returned by `{}` is not {}", name, trait_name)
2549             } else {
2550                 format!("future is not {}", trait_name)
2551             };
2552
2553             span.push_span_label(original_span, message);
2554             err.set_span(span);
2555
2556             format!("is not {}", trait_name)
2557         } else {
2558             format!("does not implement `{}`", trait_ref.print_only_trait_path())
2559         };
2560
2561         // Look at the last interior type to get a span for the `.await`.
2562         let await_span = tables.generator_interior_types.iter().map(|i| i.span).last().unwrap();
2563         let mut span = MultiSpan::from_span(await_span);
2564         span.push_span_label(
2565             await_span,
2566             format!("{} occurs here, with `{}` maybe used later", await_or_yield, snippet),
2567         );
2568
2569         span.push_span_label(target_span, format!("has type `{}`", target_ty));
2570
2571         // If available, use the scope span to annotate the drop location.
2572         if let Some(scope_span) = scope_span {
2573             span.push_span_label(
2574                 source_map.end_point(*scope_span),
2575                 format!("`{}` is later dropped here", snippet),
2576             );
2577         }
2578
2579         err.span_note(
2580             span,
2581             &format!(
2582                 "future {} as this value is used across an {}",
2583                 trait_explanation, await_or_yield,
2584             ),
2585         );
2586
2587         // Add a note for the item obligation that remains - normally a note pointing to the
2588         // bound that introduced the obligation (e.g. `T: Send`).
2589         debug!("note_obligation_cause_for_async_await: next_code={:?}", next_code);
2590         self.note_obligation_cause_code(
2591             err,
2592             &obligation.predicate,
2593             next_code.unwrap(),
2594             &mut Vec::new(),
2595         );
2596     }
2597
2598     fn note_obligation_cause_code<T>(
2599         &self,
2600         err: &mut DiagnosticBuilder<'_>,
2601         predicate: &T,
2602         cause_code: &ObligationCauseCode<'tcx>,
2603         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
2604     ) where
2605         T: fmt::Display,
2606     {
2607         let tcx = self.tcx;
2608         match *cause_code {
2609             ObligationCauseCode::ExprAssignable
2610             | ObligationCauseCode::MatchExpressionArm { .. }
2611             | ObligationCauseCode::Pattern { .. }
2612             | ObligationCauseCode::IfExpression { .. }
2613             | ObligationCauseCode::IfExpressionWithNoElse
2614             | ObligationCauseCode::MainFunctionType
2615             | ObligationCauseCode::StartFunctionType
2616             | ObligationCauseCode::IntrinsicType
2617             | ObligationCauseCode::MethodReceiver
2618             | ObligationCauseCode::ReturnNoExpression
2619             | ObligationCauseCode::MiscObligation => {}
2620             ObligationCauseCode::SliceOrArrayElem => {
2621                 err.note("slice and array elements must have `Sized` type");
2622             }
2623             ObligationCauseCode::TupleElem => {
2624                 err.note("only the last element of a tuple may have a dynamically sized type");
2625             }
2626             ObligationCauseCode::ProjectionWf(data) => {
2627                 err.note(&format!("required so that the projection `{}` is well-formed", data,));
2628             }
2629             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
2630                 err.note(&format!(
2631                     "required so that reference `{}` does not outlive its referent",
2632                     ref_ty,
2633                 ));
2634             }
2635             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
2636                 err.note(&format!(
2637                     "required so that the lifetime bound of `{}` for `{}` is satisfied",
2638                     region, object_ty,
2639                 ));
2640             }
2641             ObligationCauseCode::ItemObligation(item_def_id) => {
2642                 let item_name = tcx.def_path_str(item_def_id);
2643                 let msg = format!("required by `{}`", item_name);
2644
2645                 if let Some(sp) = tcx.hir().span_if_local(item_def_id) {
2646                     let sp = tcx.sess.source_map().def_span(sp);
2647                     err.span_label(sp, &msg);
2648                 } else {
2649                     err.note(&msg);
2650                 }
2651             }
2652             ObligationCauseCode::BindingObligation(item_def_id, span) => {
2653                 let item_name = tcx.def_path_str(item_def_id);
2654                 let msg = format!("required by this bound in `{}`", item_name);
2655                 if let Some(ident) = tcx.opt_item_name(item_def_id) {
2656                     err.span_label(ident.span, "");
2657                 }
2658                 if span != DUMMY_SP {
2659                     err.span_label(span, &msg);
2660                 } else {
2661                     err.note(&msg);
2662                 }
2663             }
2664             ObligationCauseCode::ObjectCastObligation(object_ty) => {
2665                 err.note(&format!(
2666                     "required for the cast to the object type `{}`",
2667                     self.ty_to_string(object_ty)
2668                 ));
2669             }
2670             ObligationCauseCode::Coercion { source: _, target } => {
2671                 err.note(&format!("required by cast to type `{}`", self.ty_to_string(target)));
2672             }
2673             ObligationCauseCode::RepeatVec(suggest_const_in_array_repeat_expressions) => {
2674                 err.note(
2675                     "the `Copy` trait is required because the \
2676                           repeated element will be copied",
2677                 );
2678                 if suggest_const_in_array_repeat_expressions {
2679                     err.note(
2680                         "this array initializer can be evaluated at compile-time, for more \
2681                               information, see issue \
2682                               https://github.com/rust-lang/rust/issues/49147",
2683                     );
2684                     if tcx.sess.opts.unstable_features.is_nightly_build() {
2685                         err.help(
2686                             "add `#![feature(const_in_array_repeat_expressions)]` to the \
2687                                   crate attributes to enable",
2688                         );
2689                     }
2690                 }
2691             }
2692             ObligationCauseCode::VariableType(_) => {
2693                 err.note("all local variables must have a statically known size");
2694                 if !self.tcx.features().unsized_locals {
2695                     err.help("unsized locals are gated as an unstable feature");
2696                 }
2697             }
2698             ObligationCauseCode::SizedArgumentType => {
2699                 err.note("all function arguments must have a statically known size");
2700                 if !self.tcx.features().unsized_locals {
2701                     err.help("unsized locals are gated as an unstable feature");
2702                 }
2703             }
2704             ObligationCauseCode::SizedReturnType => {
2705                 err.note(
2706                     "the return type of a function must have a \
2707                           statically known size",
2708                 );
2709             }
2710             ObligationCauseCode::SizedYieldType => {
2711                 err.note(
2712                     "the yield type of a generator must have a \
2713                           statically known size",
2714                 );
2715             }
2716             ObligationCauseCode::AssignmentLhsSized => {
2717                 err.note("the left-hand-side of an assignment must have a statically known size");
2718             }
2719             ObligationCauseCode::TupleInitializerSized => {
2720                 err.note("tuples must have a statically known size to be initialized");
2721             }
2722             ObligationCauseCode::StructInitializerSized => {
2723                 err.note("structs must have a statically known size to be initialized");
2724             }
2725             ObligationCauseCode::FieldSized { adt_kind: ref item, last } => match *item {
2726                 AdtKind::Struct => {
2727                     if last {
2728                         err.note(
2729                             "the last field of a packed struct may only have a \
2730                                       dynamically sized type if it does not need drop to be run",
2731                         );
2732                     } else {
2733                         err.note(
2734                             "only the last field of a struct may have a dynamically \
2735                                       sized type",
2736                         );
2737                     }
2738                 }
2739                 AdtKind::Union => {
2740                     err.note("no field of a union may have a dynamically sized type");
2741                 }
2742                 AdtKind::Enum => {
2743                     err.note("no field of an enum variant may have a dynamically sized type");
2744                 }
2745             },
2746             ObligationCauseCode::ConstSized => {
2747                 err.note("constant expressions must have a statically known size");
2748             }
2749             ObligationCauseCode::ConstPatternStructural => {
2750                 err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
2751             }
2752             ObligationCauseCode::SharedStatic => {
2753                 err.note("shared static variables must have a type that implements `Sync`");
2754             }
2755             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
2756                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2757                 let ty = parent_trait_ref.skip_binder().self_ty();
2758                 err.note(&format!("required because it appears within the type `{}`", ty));
2759                 obligated_types.push(ty);
2760
2761                 let parent_predicate = parent_trait_ref.to_predicate();
2762                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
2763                     self.note_obligation_cause_code(
2764                         err,
2765                         &parent_predicate,
2766                         &data.parent_code,
2767                         obligated_types,
2768                     );
2769                 }
2770             }
2771             ObligationCauseCode::ImplDerivedObligation(ref data) => {
2772                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2773                 err.note(&format!(
2774                     "required because of the requirements on the impl of `{}` for `{}`",
2775                     parent_trait_ref.print_only_trait_path(),
2776                     parent_trait_ref.skip_binder().self_ty()
2777                 ));
2778                 let parent_predicate = parent_trait_ref.to_predicate();
2779                 self.note_obligation_cause_code(
2780                     err,
2781                     &parent_predicate,
2782                     &data.parent_code,
2783                     obligated_types,
2784                 );
2785             }
2786             ObligationCauseCode::CompareImplMethodObligation { .. } => {
2787                 err.note(&format!(
2788                     "the requirement `{}` appears on the impl method \
2789                               but not on the corresponding trait method",
2790                     predicate
2791                 ));
2792             }
2793             ObligationCauseCode::CompareImplTypeObligation { .. } => {
2794                 err.note(&format!(
2795                     "the requirement `{}` appears on the associated impl type\
2796                      but not on the corresponding associated trait type",
2797                     predicate
2798                 ));
2799             }
2800             ObligationCauseCode::ReturnType
2801             | ObligationCauseCode::ReturnValue(_)
2802             | ObligationCauseCode::BlockTailExpression(_) => (),
2803             ObligationCauseCode::TrivialBound => {
2804                 err.help("see issue #48214");
2805                 if tcx.sess.opts.unstable_features.is_nightly_build() {
2806                     err.help(
2807                         "add `#![feature(trivial_bounds)]` to the \
2808                               crate attributes to enable",
2809                     );
2810                 }
2811             }
2812             ObligationCauseCode::AssocTypeBound(ref data) => {
2813                 err.span_label(data.original, "associated type defined here");
2814                 if let Some(sp) = data.impl_span {
2815                     err.span_label(sp, "in this `impl` item");
2816                 }
2817                 for sp in &data.bounds {
2818                     err.span_label(*sp, "restricted in this bound");
2819                 }
2820             }
2821         }
2822     }
2823
2824     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>) {
2825         let current_limit = self.tcx.sess.recursion_limit.get();
2826         let suggested_limit = current_limit * 2;
2827         err.help(&format!(
2828             "consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
2829             suggested_limit
2830         ));
2831     }
2832
2833     fn is_recursive_obligation(
2834         &self,
2835         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
2836         cause_code: &ObligationCauseCode<'tcx>,
2837     ) -> bool {
2838         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2839             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2840
2841             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
2842                 return true;
2843             }
2844         }
2845         false
2846     }
2847 }
2848
2849 /// Summarizes information
2850 #[derive(Clone)]
2851 pub enum ArgKind {
2852     /// An argument of non-tuple type. Parameters are (name, ty)
2853     Arg(String, String),
2854
2855     /// An argument of tuple type. For a "found" argument, the span is
2856     /// the locationo in the source of the pattern. For a "expected"
2857     /// argument, it will be None. The vector is a list of (name, ty)
2858     /// strings for the components of the tuple.
2859     Tuple(Option<Span>, Vec<(String, String)>),
2860 }
2861
2862 impl ArgKind {
2863     fn empty() -> ArgKind {
2864         ArgKind::Arg("_".to_owned(), "_".to_owned())
2865     }
2866
2867     /// Creates an `ArgKind` from the expected type of an
2868     /// argument. It has no name (`_`) and an optional source span.
2869     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2870         match t.kind {
2871             ty::Tuple(ref tys) => ArgKind::Tuple(
2872                 span,
2873                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2874             ),
2875             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2876         }
2877     }
2878 }
2879
2880 /// Suggest restricting a type param with a new bound.
2881 pub fn suggest_constraining_type_param(
2882     generics: &hir::Generics<'_>,
2883     err: &mut DiagnosticBuilder<'_>,
2884     param_name: &str,
2885     constraint: &str,
2886     source_map: &SourceMap,
2887     span: Span,
2888 ) -> bool {
2889     let restrict_msg = "consider further restricting this bound";
2890     if let Some(param) =
2891         generics.params.iter().filter(|p| p.name.ident().as_str() == param_name).next()
2892     {
2893         if param_name.starts_with("impl ") {
2894             // `impl Trait` in argument:
2895             // `fn foo(x: impl Trait) {}` → `fn foo(t: impl Trait + Trait2) {}`
2896             err.span_suggestion(
2897                 param.span,
2898                 restrict_msg,
2899                 // `impl CurrentTrait + MissingTrait`
2900                 format!("{} + {}", param_name, constraint),
2901                 Applicability::MachineApplicable,
2902             );
2903         } else if generics.where_clause.predicates.is_empty() && param.bounds.is_empty() {
2904             // If there are no bounds whatsoever, suggest adding a constraint
2905             // to the type parameter:
2906             // `fn foo<T>(t: T) {}` → `fn foo<T: Trait>(t: T) {}`
2907             err.span_suggestion(
2908                 param.span,
2909                 "consider restricting this bound",
2910                 format!("{}: {}", param_name, constraint),
2911                 Applicability::MachineApplicable,
2912             );
2913         } else if !generics.where_clause.predicates.is_empty() {
2914             // There is a `where` clause, so suggest expanding it:
2915             // `fn foo<T>(t: T) where T: Debug {}` →
2916             // `fn foo<T>(t: T) where T: Debug, T: Trait {}`
2917             err.span_suggestion(
2918                 generics.where_clause.span().unwrap().shrink_to_hi(),
2919                 &format!("consider further restricting type parameter `{}`", param_name),
2920                 format!(", {}: {}", param_name, constraint),
2921                 Applicability::MachineApplicable,
2922             );
2923         } else {
2924             // If there is no `where` clause lean towards constraining to the
2925             // type parameter:
2926             // `fn foo<X: Bar, T>(t: T, x: X) {}` → `fn foo<T: Trait>(t: T) {}`
2927             // `fn foo<T: Bar>(t: T) {}` → `fn foo<T: Bar + Trait>(t: T) {}`
2928             let sp = param.span.with_hi(span.hi());
2929             let span = source_map.span_through_char(sp, ':');
2930             if sp != param.span && sp != span {
2931                 // Only suggest if we have high certainty that the span
2932                 // covers the colon in `foo<T: Trait>`.
2933                 err.span_suggestion(
2934                     span,
2935                     restrict_msg,
2936                     format!("{}: {} + ", param_name, constraint),
2937                     Applicability::MachineApplicable,
2938                 );
2939             } else {
2940                 err.span_label(
2941                     param.span,
2942                     &format!("consider adding a `where {}: {}` bound", param_name, constraint),
2943                 );
2944             }
2945         }
2946         return true;
2947     }
2948     false
2949 }