]> git.lizzy.rs Git - rust.git/blob - src/librustc/traits/error_reporting.rs
Rollup merge of #61046 - mark-i-m:transcribe-fix, r=petrochenkov
[rust.git] / src / librustc / traits / error_reporting.rs
1 use super::{
2     FulfillmentError,
3     FulfillmentErrorCode,
4     MismatchedProjectionTypes,
5     Obligation,
6     ObligationCause,
7     ObligationCauseCode,
8     OnUnimplementedDirective,
9     OnUnimplementedNote,
10     OutputTypeParameterMismatch,
11     TraitNotObjectSafe,
12     ConstEvalFailure,
13     PredicateObligation,
14     SelectionContext,
15     SelectionError,
16     ObjectSafetyViolation,
17     Overflow,
18 };
19
20 use crate::hir;
21 use crate::hir::Node;
22 use crate::hir::def_id::DefId;
23 use crate::infer::{self, InferCtxt};
24 use crate::infer::type_variable::TypeVariableOrigin;
25 use crate::session::DiagnosticMessageId;
26 use crate::ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
27 use crate::ty::GenericParamDefKind;
28 use crate::ty::error::ExpectedFound;
29 use crate::ty::fast_reject;
30 use crate::ty::fold::TypeFolder;
31 use crate::ty::subst::Subst;
32 use crate::ty::SubtypePredicate;
33 use crate::util::nodemap::{FxHashMap, FxHashSet};
34
35 use errors::{Applicability, DiagnosticBuilder};
36 use std::fmt;
37 use syntax::ast;
38 use syntax::symbol::sym;
39 use syntax_pos::{DUMMY_SP, Span, ExpnInfo, ExpnFormat};
40
41 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
42     pub fn report_fulfillment_errors(&self,
43                                      errors: &[FulfillmentError<'tcx>],
44                                      body_id: Option<hir::BodyId>,
45                                      fallback_has_occurred: bool) {
46         #[derive(Debug)]
47         struct ErrorDescriptor<'tcx> {
48             predicate: ty::Predicate<'tcx>,
49             index: Option<usize>, // None if this is an old error
50         }
51
52         let mut error_map: FxHashMap<_, Vec<_>> =
53             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
54                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
55                     predicate: predicate.clone(),
56                     index: None
57                 }).collect())
58             }).collect();
59
60         for (index, error) in errors.iter().enumerate() {
61             // We want to ignore desugarings here: spans are equivalent even
62             // if one is the result of a desugaring and the other is not.
63             let mut span = error.obligation.cause.span;
64             if let Some(ExpnInfo {
65                 format: ExpnFormat::CompilerDesugaring(_),
66                 def_site: Some(def_span),
67                 ..
68             }) = span.ctxt().outer().expn_info() {
69                 span = def_span;
70             }
71
72             error_map.entry(span).or_default().push(
73                 ErrorDescriptor {
74                     predicate: error.obligation.predicate.clone(),
75                     index: Some(index)
76                 }
77             );
78
79             self.reported_trait_errors.borrow_mut()
80                 .entry(span).or_default()
81                 .push(error.obligation.predicate.clone());
82         }
83
84         // We do this in 2 passes because we want to display errors in order, though
85         // maybe it *is* better to sort errors by span or something.
86         let mut is_suppressed = vec![false; errors.len()];
87         for (_, error_set) in error_map.iter() {
88             // We want to suppress "duplicate" errors with the same span.
89             for error in error_set {
90                 if let Some(index) = error.index {
91                     // Suppress errors that are either:
92                     // 1) strictly implied by another error.
93                     // 2) implied by an error with a smaller index.
94                     for error2 in error_set {
95                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
96                             // Avoid errors being suppressed by already-suppressed
97                             // errors, to prevent all errors from being suppressed
98                             // at once.
99                             continue
100                         }
101
102                         if self.error_implies(&error2.predicate, &error.predicate) &&
103                             !(error2.index >= error.index &&
104                               self.error_implies(&error.predicate, &error2.predicate))
105                         {
106                             info!("skipping {:?} (implied by {:?})", error, error2);
107                             is_suppressed[index] = true;
108                             break
109                         }
110                     }
111                 }
112             }
113         }
114
115         for (error, suppressed) in errors.iter().zip(is_suppressed) {
116             if !suppressed {
117                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
118             }
119         }
120     }
121
122     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
123     // `error` occurring implies that `cond` occurs.
124     fn error_implies(&self,
125                      cond: &ty::Predicate<'tcx>,
126                      error: &ty::Predicate<'tcx>)
127                      -> bool
128     {
129         if cond == error {
130             return true
131         }
132
133         let (cond, error) = match (cond, error) {
134             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
135                 => (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(&self, error: &FulfillmentError<'tcx>,
161                                 body_id: Option<hir::BodyId>,
162                                 fallback_has_occurred: bool) {
163         debug!("report_fulfillment_errors({:?})", error);
164         match error.code {
165             FulfillmentErrorCode::CodeSelectionError(ref e) => {
166                 self.report_selection_error(&error.obligation, e, fallback_has_occurred);
167             }
168             FulfillmentErrorCode::CodeProjectionError(ref e) => {
169                 self.report_projection_error(&error.obligation, e);
170             }
171             FulfillmentErrorCode::CodeAmbiguity => {
172                 self.maybe_report_ambiguity(&error.obligation, body_id);
173             }
174             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
175                 self.report_mismatched_types(&error.obligation.cause,
176                                              expected_found.expected,
177                                              expected_found.found,
178                                              err.clone())
179                     .emit();
180             }
181         }
182     }
183
184     fn report_projection_error(&self,
185                                obligation: &PredicateObligation<'tcx>,
186                                error: &MismatchedProjectionTypes<'tcx>)
187     {
188         let predicate =
189             self.resolve_type_vars_if_possible(&obligation.predicate);
190
191         if predicate.references_error() {
192             return
193         }
194
195         self.probe(|_| {
196             let err_buf;
197             let mut err = &error.err;
198             let mut values = None;
199
200             // try to find the mismatched types to report the error with.
201             //
202             // this can fail if the problem was higher-ranked, in which
203             // cause I have no idea for a good error message.
204             if let ty::Predicate::Projection(ref data) = predicate {
205                 let mut selcx = SelectionContext::new(self);
206                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
207                     obligation.cause.span,
208                     infer::LateBoundRegionConversionTime::HigherRankedType,
209                     data
210                 );
211                 let mut obligations = vec![];
212                 let normalized_ty = super::normalize_projection_type(
213                     &mut selcx,
214                     obligation.param_env,
215                     data.projection_ty,
216                     obligation.cause.clone(),
217                     0,
218                     &mut obligations
219                 );
220                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
221                                         .eq(normalized_ty, data.ty) {
222                     values = Some(infer::ValuePairs::Types(ExpectedFound {
223                         expected: normalized_ty,
224                         found: data.ty,
225                     }));
226                     err_buf = error;
227                     err = &err_buf;
228                 }
229             }
230
231             let msg = format!("type mismatch resolving `{}`", predicate);
232             let error_id = (DiagnosticMessageId::ErrorId(271),
233                             Some(obligation.cause.span), msg);
234             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
235             if fresh {
236                 let mut diag = struct_span_err!(
237                     self.tcx.sess, obligation.cause.span, E0271,
238                     "type mismatch resolving `{}`", predicate
239                 );
240                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
241                 self.note_obligation_cause(&mut diag, obligation);
242                 diag.emit();
243             }
244         });
245     }
246
247     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
248         /// returns the fuzzy category of a given type, or None
249         /// if the type can be equated to any type.
250         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
251             match t.sty {
252                 ty::Bool => Some(0),
253                 ty::Char => Some(1),
254                 ty::Str => Some(2),
255                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
256                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
257                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
258                 ty::Array(..) | ty::Slice(..) => Some(6),
259                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
260                 ty::Dynamic(..) => Some(8),
261                 ty::Closure(..) => Some(9),
262                 ty::Tuple(..) => Some(10),
263                 ty::Projection(..) => Some(11),
264                 ty::Param(..) => Some(12),
265                 ty::Opaque(..) => Some(13),
266                 ty::Never => Some(14),
267                 ty::Adt(adt, ..) => match adt.adt_kind() {
268                     AdtKind::Struct => Some(15),
269                     AdtKind::Union => Some(16),
270                     AdtKind::Enum => Some(17),
271                 },
272                 ty::Generator(..) => Some(18),
273                 ty::Foreign(..) => Some(19),
274                 ty::GeneratorWitness(..) => Some(20),
275                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
276                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
277             }
278         }
279
280         match (type_category(a), type_category(b)) {
281             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
282                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
283                 _ => cat_a == cat_b
284             },
285             // infer and error can be equated to all types
286             _ => true
287         }
288     }
289
290     fn impl_similar_to(&self,
291                        trait_ref: ty::PolyTraitRef<'tcx>,
292                        obligation: &PredicateObligation<'tcx>)
293                        -> Option<DefId>
294     {
295         let tcx = self.tcx;
296         let param_env = obligation.param_env;
297         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
298         let trait_self_ty = trait_ref.self_ty();
299
300         let mut self_match_impls = vec![];
301         let mut fuzzy_match_impls = vec![];
302
303         self.tcx.for_each_relevant_impl(
304             trait_ref.def_id, trait_self_ty, |def_id| {
305                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
306                 let impl_trait_ref = tcx
307                     .impl_trait_ref(def_id)
308                     .unwrap()
309                     .subst(tcx, impl_substs);
310
311                 let impl_self_ty = impl_trait_ref.self_ty();
312
313                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
314                     self_match_impls.push(def_id);
315
316                     if trait_ref.substs.types().skip(1)
317                         .zip(impl_trait_ref.substs.types().skip(1))
318                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
319                     {
320                         fuzzy_match_impls.push(def_id);
321                     }
322                 }
323             });
324
325         let impl_def_id = if self_match_impls.len() == 1 {
326             self_match_impls[0]
327         } else if fuzzy_match_impls.len() == 1 {
328             fuzzy_match_impls[0]
329         } else {
330             return None
331         };
332
333         if tcx.has_attr(impl_def_id, sym::rustc_on_unimplemented) {
334             Some(impl_def_id)
335         } else {
336             None
337         }
338     }
339
340     fn on_unimplemented_note(
341         &self,
342         trait_ref: ty::PolyTraitRef<'tcx>,
343         obligation: &PredicateObligation<'tcx>,
344     ) -> OnUnimplementedNote {
345         let def_id = self.impl_similar_to(trait_ref, obligation)
346             .unwrap_or_else(|| trait_ref.def_id());
347         let trait_ref = *trait_ref.skip_binder();
348
349         let mut flags = vec![];
350         match obligation.cause.code {
351             ObligationCauseCode::BuiltinDerivedObligation(..) |
352             ObligationCauseCode::ImplDerivedObligation(..) => {}
353             _ => {
354                 // this is a "direct", user-specified, rather than derived,
355                 // obligation.
356                 flags.push(("direct".to_owned(), None));
357             }
358         }
359
360         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
361             // FIXME: maybe also have some way of handling methods
362             // from other traits? That would require name resolution,
363             // which we might want to be some sort of hygienic.
364             //
365             // Currently I'm leaving it for what I need for `try`.
366             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
367                 let method = self.tcx.item_name(item);
368                 flags.push(("from_method".to_owned(), None));
369                 flags.push(("from_method".to_owned(), Some(method.to_string())));
370             }
371         }
372         if let Some(t) = self.get_parent_trait_ref(&obligation.cause.code) {
373             flags.push(("parent_trait".to_owned(), Some(t)));
374         }
375
376         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
377             flags.push(("from_desugaring".to_owned(), None));
378             flags.push(("from_desugaring".to_owned(), Some(k.name().to_string())));
379         }
380         let generics = self.tcx.generics_of(def_id);
381         let self_ty = trait_ref.self_ty();
382         // This is also included through the generics list as `Self`,
383         // but the parser won't allow you to use it
384         flags.push(("_Self".to_owned(), Some(self_ty.to_string())));
385         if let Some(def) = self_ty.ty_adt_def() {
386             // We also want to be able to select self's original
387             // signature with no type arguments resolved
388             flags.push(("_Self".to_owned(), Some(self.tcx.type_of(def.did).to_string())));
389         }
390
391         for param in generics.params.iter() {
392             let value = match param.kind {
393                 GenericParamDefKind::Type { .. } |
394                 GenericParamDefKind::Const => {
395                     trait_ref.substs[param.index as usize].to_string()
396                 },
397                 GenericParamDefKind::Lifetime => continue,
398             };
399             let name = param.name.to_string();
400             flags.push((name, Some(value)));
401         }
402
403         if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
404             flags.push(("crate_local".to_owned(), None));
405         }
406
407         // Allow targeting all integers using `{integral}`, even if the exact type was resolved
408         if self_ty.is_integral() {
409             flags.push(("_Self".to_owned(), Some("{integral}".to_owned())));
410         }
411
412         if let ty::Array(aty, len) = self_ty.sty {
413             flags.push(("_Self".to_owned(), Some("[]".to_owned())));
414             flags.push(("_Self".to_owned(), Some(format!("[{}]", aty))));
415             if let Some(def) = aty.ty_adt_def() {
416                 // We also want to be able to select the array's type's original
417                 // signature with no type arguments resolved
418                 flags.push((
419                     "_Self".to_owned(),
420                     Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
421                 ));
422                 let tcx = self.tcx;
423                 if let Some(len) = len.assert_usize(tcx) {
424                     flags.push((
425                         "_Self".to_owned(),
426                         Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
427                     ));
428                 } else {
429                     flags.push((
430                         "_Self".to_owned(),
431                         Some(format!("[{}; _]", self.tcx.type_of(def.did).to_string())),
432                     ));
433                 }
434             }
435         }
436
437         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
438             self.tcx, trait_ref.def_id, def_id
439         ) {
440             command.evaluate(self.tcx, trait_ref, &flags[..])
441         } else {
442             OnUnimplementedNote::empty()
443         }
444     }
445
446     fn find_similar_impl_candidates(&self,
447                                     trait_ref: ty::PolyTraitRef<'tcx>)
448                                     -> Vec<ty::TraitRef<'tcx>>
449     {
450         let simp = fast_reject::simplify_type(self.tcx,
451                                               trait_ref.skip_binder().self_ty(),
452                                               true);
453         let all_impls = self.tcx.all_impls(trait_ref.def_id());
454
455         match simp {
456             Some(simp) => all_impls.iter().filter_map(|&def_id| {
457                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
458                 let imp_simp = fast_reject::simplify_type(self.tcx,
459                                                           imp.self_ty(),
460                                                           true);
461                 if let Some(imp_simp) = imp_simp {
462                     if simp != imp_simp {
463                         return None
464                     }
465                 }
466
467                 Some(imp)
468             }).collect(),
469             None => all_impls.iter().map(|&def_id|
470                 self.tcx.impl_trait_ref(def_id).unwrap()
471             ).collect()
472         }
473     }
474
475     fn report_similar_impl_candidates(&self,
476                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
477                                       err: &mut DiagnosticBuilder<'_>)
478     {
479         if impl_candidates.is_empty() {
480             return;
481         }
482
483         let len = impl_candidates.len();
484         let end = if impl_candidates.len() <= 5 {
485             impl_candidates.len()
486         } else {
487             4
488         };
489
490         let normalize = |candidate| self.tcx.global_tcx().infer_ctxt().enter(|ref infcx| {
491             let normalized = infcx
492                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
493                 .normalize(candidate)
494                 .ok();
495             match normalized {
496                 Some(normalized) => format!("\n  {:?}", normalized.value),
497                 None => format!("\n  {:?}", candidate),
498             }
499         });
500
501         // Sort impl candidates so that ordering is consistent for UI tests.
502         let mut normalized_impl_candidates = impl_candidates
503             .iter()
504             .map(normalize)
505             .collect::<Vec<String>>();
506
507         // Sort before taking the `..end` range,
508         // because the ordering of `impl_candidates` may not be deterministic:
509         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
510         normalized_impl_candidates.sort();
511
512         err.help(&format!("the following implementations were found:{}{}",
513                           normalized_impl_candidates[..end].join(""),
514                           if len > 5 {
515                               format!("\nand {} others", len - 4)
516                           } else {
517                               String::new()
518                           }
519                           ));
520     }
521
522     /// Reports that an overflow has occurred and halts compilation. We
523     /// halt compilation unconditionally because it is important that
524     /// overflows never be masked -- they basically represent computations
525     /// whose result could not be truly determined and thus we can't say
526     /// if the program type checks or not -- and they are unusual
527     /// occurrences in any case.
528     pub fn report_overflow_error<T>(&self,
529                                     obligation: &Obligation<'tcx, T>,
530                                     suggest_increasing_limit: bool) -> !
531         where T: fmt::Display + TypeFoldable<'tcx>
532     {
533         let predicate =
534             self.resolve_type_vars_if_possible(&obligation.predicate);
535         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
536                                        "overflow evaluating the requirement `{}`",
537                                        predicate);
538
539         if suggest_increasing_limit {
540             self.suggest_new_overflow_limit(&mut err);
541         }
542
543         self.note_obligation_cause(&mut err, obligation);
544
545         err.emit();
546         self.tcx.sess.abort_if_errors();
547         bug!();
548     }
549
550     /// Reports that a cycle was detected which led to overflow and halts
551     /// compilation. This is equivalent to `report_overflow_error` except
552     /// that we can give a more helpful error message (and, in particular,
553     /// we do not suggest increasing the overflow limit, which is not
554     /// going to help).
555     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
556         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
557         assert!(cycle.len() > 0);
558
559         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
560
561         self.report_overflow_error(&cycle[0], false);
562     }
563
564     pub fn report_extra_impl_obligation(&self,
565                                         error_span: Span,
566                                         item_name: ast::Name,
567                                         _impl_item_def_id: DefId,
568                                         trait_item_def_id: DefId,
569                                         requirement: &dyn fmt::Display)
570                                         -> DiagnosticBuilder<'tcx>
571     {
572         let msg = "impl has stricter requirements than trait";
573         let sp = self.tcx.sess.source_map().def_span(error_span);
574
575         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
576
577         if let Some(trait_item_span) = self.tcx.hir().span_if_local(trait_item_def_id) {
578             let span = self.tcx.sess.source_map().def_span(trait_item_span);
579             err.span_label(span, format!("definition of `{}` from trait", item_name));
580         }
581
582         err.span_label(sp, format!("impl has extra requirement {}", requirement));
583
584         err
585     }
586
587
588     /// Gets the parent trait chain start
589     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
590         match code {
591             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
592                 let parent_trait_ref = self.resolve_type_vars_if_possible(
593                     &data.parent_trait_ref);
594                 match self.get_parent_trait_ref(&data.parent_code) {
595                     Some(t) => Some(t),
596                     None => Some(parent_trait_ref.skip_binder().self_ty().to_string()),
597                 }
598             }
599             _ => None,
600         }
601     }
602
603     pub fn report_selection_error(
604         &self,
605         obligation: &PredicateObligation<'tcx>,
606         error: &SelectionError<'tcx>,
607         fallback_has_occurred: bool,
608     ) {
609         let span = obligation.cause.span;
610
611         let mut err = match *error {
612             SelectionError::Unimplemented => {
613                 if let ObligationCauseCode::CompareImplMethodObligation {
614                     item_name, impl_item_def_id, trait_item_def_id,
615                 } = obligation.cause.code {
616                     self.report_extra_impl_obligation(
617                         span,
618                         item_name,
619                         impl_item_def_id,
620                         trait_item_def_id,
621                         &format!("`{}`", obligation.predicate))
622                         .emit();
623                     return;
624                 }
625                 match obligation.predicate {
626                     ty::Predicate::Trait(ref trait_predicate) => {
627                         let trait_predicate =
628                             self.resolve_type_vars_if_possible(trait_predicate);
629
630                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
631                             return;
632                         }
633                         let trait_ref = trait_predicate.to_poly_trait_ref();
634                         let (post_message, pre_message) =
635                             self.get_parent_trait_ref(&obligation.cause.code)
636                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
637                             .unwrap_or_default();
638
639                         let OnUnimplementedNote { message, label, note }
640                             = self.on_unimplemented_note(trait_ref, obligation);
641                         let have_alt_message = message.is_some() || label.is_some();
642                         let is_try = self.tcx.sess.source_map().span_to_snippet(span)
643                             .map(|s| &s == "?")
644                             .unwrap_or(false);
645                         let is_from = format!("{}", trait_ref).starts_with("std::convert::From<");
646                         let (message, note) = if is_try && is_from {
647                             (Some(format!(
648                                 "`?` couldn't convert the error to `{}`",
649                                 trait_ref.self_ty(),
650                             )), Some(
651                                 "the question mark operation (`?`) implicitly performs a \
652                                  conversion on the error value using the `From` trait".to_owned()
653                             ))
654                         } else {
655                             (message, note)
656                         };
657
658                         let mut err = struct_span_err!(
659                             self.tcx.sess,
660                             span,
661                             E0277,
662                             "{}",
663                             message.unwrap_or_else(||
664                                 format!("the trait bound `{}` is not satisfied{}",
665                                          trait_ref.to_predicate(), post_message)
666                             ));
667
668                         let explanation =
669                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
670                                 "consider using `()`, or a `Result`".to_owned()
671                             } else {
672                                 format!("{}the trait `{}` is not implemented for `{}`",
673                                         pre_message,
674                                         trait_ref,
675                                         trait_ref.self_ty())
676                             };
677
678                         if let Some(ref s) = label {
679                             // If it has a custom "#[rustc_on_unimplemented]"
680                             // error message, let's display it as the label!
681                             err.span_label(span, s.as_str());
682                             err.help(&explanation);
683                         } else {
684                             err.span_label(span, explanation);
685                         }
686                         if let Some(ref s) = note {
687                             // If it has a custom "#[rustc_on_unimplemented]" note, let's display it
688                             err.note(s.as_str());
689                         }
690
691                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
692                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
693                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
694
695                         // Try to report a help message
696                         if !trait_ref.has_infer_types() &&
697                             self.predicate_can_apply(obligation.param_env, trait_ref) {
698                             // If a where-clause may be useful, remind the
699                             // user that they can add it.
700                             //
701                             // don't display an on-unimplemented note, as
702                             // these notes will often be of the form
703                             //     "the type `T` can't be frobnicated"
704                             // which is somewhat confusing.
705                             err.help(&format!("consider adding a `where {}` bound",
706                                               trait_ref.to_predicate()));
707                         } else if !have_alt_message {
708                             // Can't show anything else useful, try to find similar impls.
709                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
710                             self.report_similar_impl_candidates(impl_candidates, &mut err);
711                         }
712
713                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
714                         // implemented, and fallback has occurred, then it could be due to a
715                         // variable that used to fallback to `()` now falling back to `!`. Issue a
716                         // note informing about the change in behaviour.
717                         if trait_predicate.skip_binder().self_ty().is_never()
718                             && fallback_has_occurred
719                         {
720                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
721                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
722                                     self.tcx.mk_unit(),
723                                     &trait_pred.trait_ref.substs[1..],
724                                 );
725                                 trait_pred
726                             });
727                             let unit_obligation = Obligation {
728                                 predicate: ty::Predicate::Trait(predicate),
729                                 .. obligation.clone()
730                             };
731                             if self.predicate_may_hold(&unit_obligation) {
732                                 err.note("the trait is implemented for `()`. \
733                                          Possibly this error has been caused by changes to \
734                                          Rust's type-inference algorithm \
735                                          (see: https://github.com/rust-lang/rust/issues/48950 \
736                                          for more info). Consider whether you meant to use the \
737                                          type `()` here instead.");
738                             }
739                         }
740
741                         err
742                     }
743
744                     ty::Predicate::Subtype(ref predicate) => {
745                         // Errors for Subtype predicates show up as
746                         // `FulfillmentErrorCode::CodeSubtypeError`,
747                         // not selection error.
748                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
749                     }
750
751                     ty::Predicate::RegionOutlives(ref predicate) => {
752                         let predicate = self.resolve_type_vars_if_possible(predicate);
753                         let err = self.region_outlives_predicate(&obligation.cause,
754                                                                  &predicate).err().unwrap();
755                         struct_span_err!(
756                             self.tcx.sess, span, E0279,
757                             "the requirement `{}` is not satisfied (`{}`)",
758                             predicate, err,
759                         )
760                     }
761
762                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
763                         let predicate =
764                             self.resolve_type_vars_if_possible(&obligation.predicate);
765                         struct_span_err!(self.tcx.sess, span, E0280,
766                             "the requirement `{}` is not satisfied",
767                             predicate)
768                     }
769
770                     ty::Predicate::ObjectSafe(trait_def_id) => {
771                         let violations = self.tcx.global_tcx()
772                             .object_safety_violations(trait_def_id);
773                         if let Some(err) = self.tcx.report_object_safety_error(
774                             span,
775                             trait_def_id,
776                             violations,
777                         ) {
778                             err
779                         } else {
780                             return;
781                         }
782                     }
783
784                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
785                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
786                         let closure_span = self.tcx.sess.source_map()
787                             .def_span(self.tcx.hir().span_if_local(closure_def_id).unwrap());
788                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id).unwrap();
789                         let mut err = struct_span_err!(
790                             self.tcx.sess, closure_span, E0525,
791                             "expected a closure that implements the `{}` trait, \
792                              but this closure only implements `{}`",
793                             kind,
794                             found_kind);
795
796                         err.span_label(
797                             closure_span,
798                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
799                         err.span_label(
800                             obligation.cause.span,
801                             format!("the requirement to implement `{}` derives from here", kind));
802
803                         // Additional context information explaining why the closure only implements
804                         // a particular trait.
805                         if let Some(tables) = self.in_progress_tables {
806                             let tables = tables.borrow();
807                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
808                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
809                                     err.span_label(*span, format!(
810                                         "closure is `FnOnce` because it moves the \
811                                          variable `{}` out of its environment", name));
812                                 },
813                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
814                                     err.span_label(*span, format!(
815                                         "closure is `FnMut` because it mutates the \
816                                          variable `{}` here", name));
817                                 },
818                                 _ => {}
819                             }
820                         }
821
822                         err.emit();
823                         return;
824                     }
825
826                     ty::Predicate::WellFormed(ty) => {
827                         if !self.tcx.sess.opts.debugging_opts.chalk {
828                             // WF predicates cannot themselves make
829                             // errors. They can only block due to
830                             // ambiguity; otherwise, they always
831                             // degenerate into other obligations
832                             // (which may fail).
833                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
834                         } else {
835                             // FIXME: we'll need a better message which takes into account
836                             // which bounds actually failed to hold.
837                             self.tcx.sess.struct_span_err(
838                                 span,
839                                 &format!("the type `{}` is not well-formed (chalk)", ty)
840                             )
841                         }
842                     }
843
844                     ty::Predicate::ConstEvaluatable(..) => {
845                         // Errors for `ConstEvaluatable` predicates show up as
846                         // `SelectionError::ConstEvalFailure`,
847                         // not `Unimplemented`.
848                         span_bug!(span,
849                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
850                     }
851                 }
852             }
853
854             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
855                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
856                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
857
858                 if expected_trait_ref.self_ty().references_error() {
859                     return;
860                 }
861
862                 let found_trait_ty = found_trait_ref.self_ty();
863
864                 let found_did = match found_trait_ty.sty {
865                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
866                     ty::Adt(def, _) => Some(def.did),
867                     _ => None,
868                 };
869
870                 let found_span = found_did.and_then(|did|
871                     self.tcx.hir().span_if_local(did)
872                 ).map(|sp| self.tcx.sess.source_map().def_span(sp)); // the sp could be an fn def
873
874                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
875                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
876                     _ => vec![ArgKind::empty()],
877                 };
878
879                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
880                 let expected = match expected_ty.sty {
881                     ty::Tuple(ref tys) => tys.iter()
882                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span))).collect(),
883                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
884                 };
885
886                 if found.len() == expected.len() {
887                     self.report_closure_arg_mismatch(span,
888                                                      found_span,
889                                                      found_trait_ref,
890                                                      expected_trait_ref)
891                 } else {
892                     let (closure_span, found) = found_did
893                         .and_then(|did| self.tcx.hir().get_if_local(did))
894                         .map(|node| {
895                             let (found_span, found) = self.get_fn_like_arguments(node);
896                             (Some(found_span), found)
897                         }).unwrap_or((found_span, found));
898
899                     self.report_arg_count_mismatch(span,
900                                                    closure_span,
901                                                    expected,
902                                                    found,
903                                                    found_trait_ty.is_closure())
904                 }
905             }
906
907             TraitNotObjectSafe(did) => {
908                 let violations = self.tcx.global_tcx().object_safety_violations(did);
909                 if let Some(err) = self.tcx.report_object_safety_error(span, did, violations) {
910                     err
911                 } else {
912                     return;
913                 }
914             }
915
916             // already reported in the query
917             ConstEvalFailure(_) => {
918                 self.tcx.sess.delay_span_bug(span, "constant in type had an ignored error");
919                 return;
920             }
921
922             Overflow => {
923                 bug!("overflow should be handled before the `report_selection_error` path");
924             }
925         };
926         self.note_obligation_cause(&mut err, obligation);
927         err.emit();
928     }
929
930     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
931     /// suggestion to borrow the initializer in order to use have a slice instead.
932     fn suggest_borrow_on_unsized_slice(
933         &self,
934         code: &ObligationCauseCode<'tcx>,
935         err: &mut DiagnosticBuilder<'tcx>,
936     ) {
937         if let &ObligationCauseCode::VariableType(node_id) = code {
938             let parent_node = self.tcx.hir().get_parent_node(node_id);
939             if let Some(Node::Local(ref local)) = self.tcx.hir().find(parent_node) {
940                 if let Some(ref expr) = local.init {
941                     if let hir::ExprKind::Index(_, _) = expr.node {
942                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
943                             err.span_suggestion(
944                                 expr.span,
945                                 "consider borrowing here",
946                                 format!("&{}", snippet),
947                                 Applicability::MachineApplicable
948                             );
949                         }
950                     }
951                 }
952             }
953         }
954     }
955
956     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
957     /// suggest removing these references until we reach a type that implements the trait.
958     fn suggest_remove_reference(
959         &self,
960         obligation: &PredicateObligation<'tcx>,
961         err: &mut DiagnosticBuilder<'tcx>,
962         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
963     ) {
964         let trait_ref = trait_ref.skip_binder();
965         let span = obligation.cause.span;
966
967         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
968             let refs_number = snippet.chars()
969                 .filter(|c| !c.is_whitespace())
970                 .take_while(|c| *c == '&')
971                 .count();
972
973             let mut trait_type = trait_ref.self_ty();
974
975             for refs_remaining in 0..refs_number {
976                 if let ty::Ref(_, t_type, _) = trait_type.sty {
977                     trait_type = t_type;
978
979                     let substs = self.tcx.mk_substs_trait(trait_type, &[]);
980                     let new_trait_ref = ty::TraitRef::new(trait_ref.def_id, substs);
981                     let new_obligation = Obligation::new(ObligationCause::dummy(),
982                                                          obligation.param_env,
983                                                          new_trait_ref.to_predicate());
984
985                     if self.predicate_may_hold(&new_obligation) {
986                         let sp = self.tcx.sess.source_map()
987                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
988
989                         let remove_refs = refs_remaining + 1;
990                         let format_str = format!("consider removing {} leading `&`-references",
991                                                  remove_refs);
992
993                         err.span_suggestion_short(
994                             sp, &format_str, String::new(), Applicability::MachineApplicable
995                         );
996                         break;
997                     }
998                 } else {
999                     break;
1000                 }
1001             }
1002         }
1003     }
1004
1005     fn suggest_semicolon_removal(
1006         &self,
1007         obligation: &PredicateObligation<'tcx>,
1008         err: &mut DiagnosticBuilder<'tcx>,
1009         span: Span,
1010         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1011     ) {
1012         let hir = self.tcx.hir();
1013         let parent_node = hir.get_parent_node(
1014             hir.hir_to_node_id(obligation.cause.body_id),
1015         );
1016         let node = hir.find(parent_node);
1017         if let Some(hir::Node::Item(hir::Item {
1018             node: hir::ItemKind::Fn(decl, _, _, body_id),
1019             ..
1020         })) = node {
1021             let body = hir.body(*body_id);
1022             if let hir::ExprKind::Block(blk, _) = &body.value.node {
1023                 if decl.output.span().overlaps(span) && blk.expr.is_none() &&
1024                     "()" == &trait_ref.self_ty().to_string()
1025                 {
1026                     // FIXME(estebank): When encountering a method with a trait
1027                     // bound not satisfied in the return type with a body that has
1028                     // no return, suggest removal of semicolon on last statement.
1029                     // Once that is added, close #54771.
1030                     if let Some(ref stmt) = blk.stmts.last() {
1031                         let sp = self.tcx.sess.source_map().end_point(stmt.span);
1032                         err.span_label(sp, "consider removing this semicolon");
1033                     }
1034                 }
1035             }
1036         }
1037     }
1038
1039     /// Given some node representing a fn-like thing in the HIR map,
1040     /// returns a span and `ArgKind` information that describes the
1041     /// arguments it expects. This can be supplied to
1042     /// `report_arg_count_mismatch`.
1043     pub fn get_fn_like_arguments(&self, node: Node<'_>) -> (Span, Vec<ArgKind>) {
1044         match node {
1045             Node::Expr(&hir::Expr {
1046                 node: hir::ExprKind::Closure(_, ref _decl, id, span, _),
1047                 ..
1048             }) => {
1049                 (self.tcx.sess.source_map().def_span(span), self.tcx.hir().body(id).arguments.iter()
1050                     .map(|arg| {
1051                         if let hir::Pat {
1052                             node: hir::PatKind::Tuple(args, _),
1053                             span,
1054                             ..
1055                         } = arg.pat.clone().into_inner() {
1056                             ArgKind::Tuple(
1057                                 Some(span),
1058                                 args.iter().map(|pat| {
1059                                     let snippet = self.tcx.sess.source_map()
1060                                         .span_to_snippet(pat.span).unwrap();
1061                                     (snippet, "_".to_owned())
1062                                 }).collect::<Vec<_>>(),
1063                             )
1064                         } else {
1065                             let name = self.tcx.sess.source_map()
1066                                 .span_to_snippet(arg.pat.span).unwrap();
1067                             ArgKind::Arg(name, "_".to_owned())
1068                         }
1069                     })
1070                     .collect::<Vec<ArgKind>>())
1071             }
1072             Node::Item(&hir::Item {
1073                 span,
1074                 node: hir::ItemKind::Fn(ref decl, ..),
1075                 ..
1076             }) |
1077             Node::ImplItem(&hir::ImplItem {
1078                 span,
1079                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
1080                 ..
1081             }) |
1082             Node::TraitItem(&hir::TraitItem {
1083                 span,
1084                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
1085                 ..
1086             }) => {
1087                 (self.tcx.sess.source_map().def_span(span), decl.inputs.iter()
1088                         .map(|arg| match arg.clone().node {
1089                     hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1090                         Some(arg.span),
1091                         vec![("_".to_owned(), "_".to_owned()); tys.len()]
1092                     ),
1093                     _ => ArgKind::empty()
1094                 }).collect::<Vec<ArgKind>>())
1095             }
1096             Node::Ctor(ref variant_data) => {
1097                 let span = variant_data.ctor_hir_id()
1098                     .map(|hir_id| self.tcx.hir().span_by_hir_id(hir_id))
1099                     .unwrap_or(DUMMY_SP);
1100                 let span = self.tcx.sess.source_map().def_span(span);
1101
1102                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1103             }
1104             _ => panic!("non-FnLike node found: {:?}", node),
1105         }
1106     }
1107
1108     /// Reports an error when the number of arguments needed by a
1109     /// trait match doesn't match the number that the expression
1110     /// provides.
1111     pub fn report_arg_count_mismatch(
1112         &self,
1113         span: Span,
1114         found_span: Option<Span>,
1115         expected_args: Vec<ArgKind>,
1116         found_args: Vec<ArgKind>,
1117         is_closure: bool,
1118     ) -> DiagnosticBuilder<'tcx> {
1119         let kind = if is_closure { "closure" } else { "function" };
1120
1121         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1122             let arg_length = arguments.len();
1123             let distinct = match &other[..] {
1124                 &[ArgKind::Tuple(..)] => true,
1125                 _ => false,
1126             };
1127             match (arg_length, arguments.get(0)) {
1128                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1129                     format!("a single {}-tuple as argument", fields.len())
1130                 }
1131                 _ => format!("{} {}argument{}",
1132                              arg_length,
1133                              if distinct && arg_length > 1 { "distinct " } else { "" },
1134                              if arg_length == 1 { "" } else { "s" }),
1135             }
1136         };
1137
1138         let expected_str = args_str(&expected_args, &found_args);
1139         let found_str = args_str(&found_args, &expected_args);
1140
1141         let mut err = struct_span_err!(
1142             self.tcx.sess,
1143             span,
1144             E0593,
1145             "{} is expected to take {}, but it takes {}",
1146             kind,
1147             expected_str,
1148             found_str,
1149         );
1150
1151         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1152
1153         if let Some(found_span) = found_span {
1154             err.span_label(found_span, format!("takes {}", found_str));
1155
1156             // move |_| { ... }
1157             // ^^^^^^^^-- def_span
1158             //
1159             // move |_| { ... }
1160             // ^^^^^-- prefix
1161             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1162             // move |_| { ... }
1163             //      ^^^-- pipe_span
1164             let pipe_span = if let Some(span) = found_span.trim_start(prefix_span) {
1165                 span
1166             } else {
1167                 found_span
1168             };
1169
1170             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1171             // found arguments is empty (assume the user just wants to ignore args in this case).
1172             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1173             if found_args.is_empty() && is_closure {
1174                 let underscores = vec!["_"; expected_args.len()].join(", ");
1175                 err.span_suggestion(
1176                     pipe_span,
1177                     &format!(
1178                         "consider changing the closure to take and ignore the expected argument{}",
1179                         if expected_args.len() < 2 {
1180                             ""
1181                         } else {
1182                             "s"
1183                         }
1184                     ),
1185                     format!("|{}|", underscores),
1186                     Applicability::MachineApplicable,
1187                 );
1188             }
1189
1190             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1191                 if fields.len() == expected_args.len() {
1192                     let sugg = fields.iter()
1193                         .map(|(name, _)| name.to_owned())
1194                         .collect::<Vec<String>>()
1195                         .join(", ");
1196                     err.span_suggestion(
1197                         found_span,
1198                         "change the closure to take multiple arguments instead of a single tuple",
1199                         format!("|{}|", sugg),
1200                         Applicability::MachineApplicable,
1201                     );
1202                 }
1203             }
1204             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1205                 if fields.len() == found_args.len() && is_closure {
1206                     let sugg = format!(
1207                         "|({}){}|",
1208                         found_args.iter()
1209                             .map(|arg| match arg {
1210                                 ArgKind::Arg(name, _) => name.to_owned(),
1211                                 _ => "_".to_owned(),
1212                             })
1213                             .collect::<Vec<String>>()
1214                             .join(", "),
1215                         // add type annotations if available
1216                         if found_args.iter().any(|arg| match arg {
1217                             ArgKind::Arg(_, ty) => ty != "_",
1218                             _ => false,
1219                         }) {
1220                             format!(": ({})",
1221                                     fields.iter()
1222                                         .map(|(_, ty)| ty.to_owned())
1223                                         .collect::<Vec<String>>()
1224                                         .join(", "))
1225                         } else {
1226                             String::new()
1227                         },
1228                     );
1229                     err.span_suggestion(
1230                         found_span,
1231                         "change the closure to accept a tuple instead of individual arguments",
1232                         sugg,
1233                         Applicability::MachineApplicable,
1234                     );
1235                 }
1236             }
1237         }
1238
1239         err
1240     }
1241
1242     fn report_closure_arg_mismatch(&self,
1243                            span: Span,
1244                            found_span: Option<Span>,
1245                            expected_ref: ty::PolyTraitRef<'tcx>,
1246                            found: ty::PolyTraitRef<'tcx>)
1247         -> DiagnosticBuilder<'tcx>
1248     {
1249         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
1250                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
1251             let inputs = trait_ref.substs.type_at(1);
1252             let sig = if let ty::Tuple(inputs) = inputs.sty {
1253                 tcx.mk_fn_sig(
1254                     inputs.iter().map(|k| k.expect_ty()),
1255                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1256                     false,
1257                     hir::Unsafety::Normal,
1258                     ::rustc_target::spec::abi::Abi::Rust
1259                 )
1260             } else {
1261                 tcx.mk_fn_sig(
1262                     ::std::iter::once(inputs),
1263                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1264                     false,
1265                     hir::Unsafety::Normal,
1266                     ::rustc_target::spec::abi::Abi::Rust
1267                 )
1268             };
1269             ty::Binder::bind(sig).to_string()
1270         }
1271
1272         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1273         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1274                                        "type mismatch in {} arguments",
1275                                        if argument_is_closure { "closure" } else { "function" });
1276
1277         let found_str = format!(
1278             "expected signature of `{}`",
1279             build_fn_sig_string(self.tcx, found.skip_binder())
1280         );
1281         err.span_label(span, found_str);
1282
1283         let found_span = found_span.unwrap_or(span);
1284         let expected_str = format!(
1285             "found signature of `{}`",
1286             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1287         );
1288         err.span_label(found_span, expected_str);
1289
1290         err
1291     }
1292 }
1293
1294 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1295     pub fn recursive_type_with_infinite_size_error(self,
1296                                                    type_def_id: DefId)
1297                                                    -> DiagnosticBuilder<'tcx>
1298     {
1299         assert!(type_def_id.is_local());
1300         let span = self.hir().span_if_local(type_def_id).unwrap();
1301         let span = self.sess.source_map().def_span(span);
1302         let mut err = struct_span_err!(self.sess, span, E0072,
1303                                        "recursive type `{}` has infinite size",
1304                                        self.def_path_str(type_def_id));
1305         err.span_label(span, "recursive type has infinite size");
1306         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1307                            at some point to make `{}` representable",
1308                           self.def_path_str(type_def_id)));
1309         err
1310     }
1311
1312     pub fn report_object_safety_error(
1313         self,
1314         span: Span,
1315         trait_def_id: DefId,
1316         violations: Vec<ObjectSafetyViolation>,
1317     ) -> Option<DiagnosticBuilder<'tcx>> {
1318         if self.sess.trait_methods_not_found.borrow().contains(&span) {
1319             // Avoid emitting error caused by non-existing method (#58734)
1320             return None;
1321         }
1322         let trait_str = self.def_path_str(trait_def_id);
1323         let span = self.sess.source_map().def_span(span);
1324         let mut err = struct_span_err!(
1325             self.sess, span, E0038,
1326             "the trait `{}` cannot be made into an object",
1327             trait_str);
1328         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1329
1330         let mut reported_violations = FxHashSet::default();
1331         for violation in violations {
1332             if reported_violations.insert(violation.clone()) {
1333                 err.note(&violation.error_msg());
1334             }
1335         }
1336         Some(err)
1337     }
1338 }
1339
1340 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1341     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1342                               body_id: Option<hir::BodyId>) {
1343         // Unable to successfully determine, probably means
1344         // insufficient type information, but could mean
1345         // ambiguous impls. The latter *ought* to be a
1346         // coherence violation, so we don't report it here.
1347
1348         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1349         let span = obligation.cause.span;
1350
1351         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1352                predicate,
1353                obligation);
1354
1355         // Ambiguity errors are often caused as fallout from earlier
1356         // errors. So just ignore them if this infcx is tainted.
1357         if self.is_tainted_by_errors() {
1358             return;
1359         }
1360
1361         match predicate {
1362             ty::Predicate::Trait(ref data) => {
1363                 let trait_ref = data.to_poly_trait_ref();
1364                 let self_ty = trait_ref.self_ty();
1365                 if predicate.references_error() {
1366                     return;
1367                 }
1368                 // Typically, this ambiguity should only happen if
1369                 // there are unresolved type inference variables
1370                 // (otherwise it would suggest a coherence
1371                 // failure). But given #21974 that is not necessarily
1372                 // the case -- we can have multiple where clauses that
1373                 // are only distinguished by a region, which results
1374                 // in an ambiguity even when all types are fully
1375                 // known, since we don't dispatch based on region
1376                 // relationships.
1377
1378                 // This is kind of a hack: it frequently happens that some earlier
1379                 // error prevents types from being fully inferred, and then we get
1380                 // a bunch of uninteresting errors saying something like "<generic
1381                 // #0> doesn't implement Sized".  It may even be true that we
1382                 // could just skip over all checks where the self-ty is an
1383                 // inference variable, but I was afraid that there might be an
1384                 // inference variable created, registered as an obligation, and
1385                 // then never forced by writeback, and hence by skipping here we'd
1386                 // be ignoring the fact that we don't KNOW the type works
1387                 // out. Though even that would probably be harmless, given that
1388                 // we're only talking about builtin traits, which are known to be
1389                 // inhabited. But in any case I just threw in this check for
1390                 // has_errors() to be sure that compilation isn't happening
1391                 // anyway. In that case, why inundate the user.
1392                 if !self.tcx.sess.has_errors() {
1393                     if
1394                         self.tcx.lang_items().sized_trait()
1395                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1396                     {
1397                         self.need_type_info_err(body_id, span, self_ty).emit();
1398                     } else {
1399                         let mut err = struct_span_err!(self.tcx.sess,
1400                                                        span, E0283,
1401                                                        "type annotations required: \
1402                                                         cannot resolve `{}`",
1403                                                        predicate);
1404                         self.note_obligation_cause(&mut err, obligation);
1405                         err.emit();
1406                     }
1407                 }
1408             }
1409
1410             ty::Predicate::WellFormed(ty) => {
1411                 // Same hacky approach as above to avoid deluging user
1412                 // with error messages.
1413                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1414                     self.need_type_info_err(body_id, span, ty).emit();
1415                 }
1416             }
1417
1418             ty::Predicate::Subtype(ref data) => {
1419                 if data.references_error() || self.tcx.sess.has_errors() {
1420                     // no need to overload user in such cases
1421                 } else {
1422                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1423                     // both must be type variables, or the other would've been instantiated
1424                     assert!(a.is_ty_var() && b.is_ty_var());
1425                     self.need_type_info_err(body_id,
1426                                             obligation.cause.span,
1427                                             a).emit();
1428                 }
1429             }
1430
1431             _ => {
1432                 if !self.tcx.sess.has_errors() {
1433                     let mut err = struct_span_err!(self.tcx.sess,
1434                                                    obligation.cause.span, E0284,
1435                                                    "type annotations required: \
1436                                                     cannot resolve `{}`",
1437                                                    predicate);
1438                     self.note_obligation_cause(&mut err, obligation);
1439                     err.emit();
1440                 }
1441             }
1442         }
1443     }
1444
1445     /// Returns `true` if the trait predicate may apply for *some* assignment
1446     /// to the type parameters.
1447     fn predicate_can_apply(&self,
1448                            param_env: ty::ParamEnv<'tcx>,
1449                            pred: ty::PolyTraitRef<'tcx>)
1450                            -> bool {
1451         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1452             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1453             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1454         }
1455
1456         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1457             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1458
1459             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1460                 if let ty::Param(ty::ParamTy {name, .. }) = ty.sty {
1461                     let infcx = self.infcx;
1462                     self.var_map.entry(ty).or_insert_with(||
1463                         infcx.next_ty_var(
1464                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1465                 } else {
1466                     ty.super_fold_with(self)
1467                 }
1468             }
1469         }
1470
1471         self.probe(|_| {
1472             let mut selcx = SelectionContext::new(self);
1473
1474             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1475                 infcx: self,
1476                 var_map: Default::default()
1477             });
1478
1479             let cleaned_pred = super::project::normalize(
1480                 &mut selcx,
1481                 param_env,
1482                 ObligationCause::dummy(),
1483                 &cleaned_pred
1484             ).value;
1485
1486             let obligation = Obligation::new(
1487                 ObligationCause::dummy(),
1488                 param_env,
1489                 cleaned_pred.to_predicate()
1490             );
1491
1492             self.predicate_may_hold(&obligation)
1493         })
1494     }
1495
1496     fn note_obligation_cause<T>(&self,
1497                                 err: &mut DiagnosticBuilder<'_>,
1498                                 obligation: &Obligation<'tcx, T>)
1499         where T: fmt::Display
1500     {
1501         self.note_obligation_cause_code(err,
1502                                         &obligation.predicate,
1503                                         &obligation.cause.code,
1504                                         &mut vec![]);
1505     }
1506
1507     fn note_obligation_cause_code<T>(&self,
1508                                      err: &mut DiagnosticBuilder<'_>,
1509                                      predicate: &T,
1510                                      cause_code: &ObligationCauseCode<'tcx>,
1511                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1512         where T: fmt::Display
1513     {
1514         let tcx = self.tcx;
1515         match *cause_code {
1516             ObligationCauseCode::ExprAssignable |
1517             ObligationCauseCode::MatchExpressionArm { .. } |
1518             ObligationCauseCode::MatchExpressionArmPattern { .. } |
1519             ObligationCauseCode::IfExpression { .. } |
1520             ObligationCauseCode::IfExpressionWithNoElse |
1521             ObligationCauseCode::MainFunctionType |
1522             ObligationCauseCode::StartFunctionType |
1523             ObligationCauseCode::IntrinsicType |
1524             ObligationCauseCode::MethodReceiver |
1525             ObligationCauseCode::ReturnNoExpression |
1526             ObligationCauseCode::MiscObligation => {}
1527             ObligationCauseCode::SliceOrArrayElem => {
1528                 err.note("slice and array elements must have `Sized` type");
1529             }
1530             ObligationCauseCode::TupleElem => {
1531                 err.note("only the last element of a tuple may have a dynamically sized type");
1532             }
1533             ObligationCauseCode::ProjectionWf(data) => {
1534                 err.note(&format!("required so that the projection `{}` is well-formed",
1535                                   data));
1536             }
1537             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1538                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1539                                   ref_ty));
1540             }
1541             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1542                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1543                                    is satisfied",
1544                                   region, object_ty));
1545             }
1546             ObligationCauseCode::ItemObligation(item_def_id) => {
1547                 let item_name = tcx.def_path_str(item_def_id);
1548                 let msg = format!("required by `{}`", item_name);
1549
1550                 if let Some(sp) = tcx.hir().span_if_local(item_def_id) {
1551                     let sp = tcx.sess.source_map().def_span(sp);
1552                     err.span_note(sp, &msg);
1553                 } else {
1554                     err.note(&msg);
1555                 }
1556             }
1557             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1558                 err.note(&format!("required for the cast to the object type `{}`",
1559                                   self.ty_to_string(object_ty)));
1560             }
1561             ObligationCauseCode::RepeatVec => {
1562                 err.note("the `Copy` trait is required because the \
1563                           repeated element will be copied");
1564             }
1565             ObligationCauseCode::VariableType(_) => {
1566                 err.note("all local variables must have a statically known size");
1567                 if !self.tcx.features().unsized_locals {
1568                     err.help("unsized locals are gated as an unstable feature");
1569                 }
1570             }
1571             ObligationCauseCode::SizedArgumentType => {
1572                 err.note("all function arguments must have a statically known size");
1573                 if !self.tcx.features().unsized_locals {
1574                     err.help("unsized locals are gated as an unstable feature");
1575                 }
1576             }
1577             ObligationCauseCode::SizedReturnType => {
1578                 err.note("the return type of a function must have a \
1579                           statically known size");
1580             }
1581             ObligationCauseCode::SizedYieldType => {
1582                 err.note("the yield type of a generator must have a \
1583                           statically known size");
1584             }
1585             ObligationCauseCode::AssignmentLhsSized => {
1586                 err.note("the left-hand-side of an assignment must have a statically known size");
1587             }
1588             ObligationCauseCode::TupleInitializerSized => {
1589                 err.note("tuples must have a statically known size to be initialized");
1590             }
1591             ObligationCauseCode::StructInitializerSized => {
1592                 err.note("structs must have a statically known size to be initialized");
1593             }
1594             ObligationCauseCode::FieldSized { adt_kind: ref item, last } => {
1595                 match *item {
1596                     AdtKind::Struct => {
1597                         if last {
1598                             err.note("the last field of a packed struct may only have a \
1599                                       dynamically sized type if it does not need drop to be run");
1600                         } else {
1601                             err.note("only the last field of a struct may have a dynamically \
1602                                       sized type");
1603                         }
1604                     }
1605                     AdtKind::Union => {
1606                         err.note("no field of a union may have a dynamically sized type");
1607                     }
1608                     AdtKind::Enum => {
1609                         err.note("no field of an enum variant may have a dynamically sized type");
1610                     }
1611                 }
1612             }
1613             ObligationCauseCode::ConstSized => {
1614                 err.note("constant expressions must have a statically known size");
1615             }
1616             ObligationCauseCode::SharedStatic => {
1617                 err.note("shared static variables must have a type that implements `Sync`");
1618             }
1619             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1620                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1621                 let ty = parent_trait_ref.skip_binder().self_ty();
1622                 err.note(&format!("required because it appears within the type `{}`", ty));
1623                 obligated_types.push(ty);
1624
1625                 let parent_predicate = parent_trait_ref.to_predicate();
1626                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1627                     self.note_obligation_cause_code(err,
1628                                                     &parent_predicate,
1629                                                     &data.parent_code,
1630                                                     obligated_types);
1631                 }
1632             }
1633             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1634                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1635                 err.note(
1636                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1637                              parent_trait_ref,
1638                              parent_trait_ref.skip_binder().self_ty()));
1639                 let parent_predicate = parent_trait_ref.to_predicate();
1640                 self.note_obligation_cause_code(err,
1641                                                 &parent_predicate,
1642                                                 &data.parent_code,
1643                                                 obligated_types);
1644             }
1645             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1646                 err.note(
1647                     &format!("the requirement `{}` appears on the impl method \
1648                               but not on the corresponding trait method",
1649                              predicate));
1650             }
1651             ObligationCauseCode::ReturnType(_) |
1652             ObligationCauseCode::BlockTailExpression(_) => (),
1653             ObligationCauseCode::TrivialBound => {
1654                 err.help("see issue #48214");
1655                 if tcx.sess.opts.unstable_features.is_nightly_build() {
1656                     err.help("add #![feature(trivial_bounds)] to the \
1657                               crate attributes to enable",
1658                     );
1659                 }
1660             }
1661         }
1662     }
1663
1664     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>) {
1665         let current_limit = self.tcx.sess.recursion_limit.get();
1666         let suggested_limit = current_limit * 2;
1667         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1668                           suggested_limit));
1669     }
1670
1671     fn is_recursive_obligation(&self,
1672                                obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1673                                cause_code: &ObligationCauseCode<'tcx>) -> bool {
1674         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1675             let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1676
1677             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1678                 return true;
1679             }
1680         }
1681         false
1682     }
1683 }
1684
1685 /// Summarizes information
1686 #[derive(Clone)]
1687 pub enum ArgKind {
1688     /// An argument of non-tuple type. Parameters are (name, ty)
1689     Arg(String, String),
1690
1691     /// An argument of tuple type. For a "found" argument, the span is
1692     /// the locationo in the source of the pattern. For a "expected"
1693     /// argument, it will be None. The vector is a list of (name, ty)
1694     /// strings for the components of the tuple.
1695     Tuple(Option<Span>, Vec<(String, String)>),
1696 }
1697
1698 impl ArgKind {
1699     fn empty() -> ArgKind {
1700         ArgKind::Arg("_".to_owned(), "_".to_owned())
1701     }
1702
1703     /// Creates an `ArgKind` from the expected type of an
1704     /// argument. It has no name (`_`) and an optional source span.
1705     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1706         match t.sty {
1707             ty::Tuple(ref tys) => ArgKind::Tuple(
1708                 span,
1709                 tys.iter()
1710                    .map(|ty| ("_".to_owned(), ty.to_string()))
1711                    .collect::<Vec<_>>()
1712             ),
1713             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1714         }
1715     }
1716 }