src/librustc/infer/error_reporting/mod.rs

   1 //! Error Reporting Code for the inference engine
   2 //!
   3 //! Because of the way inference, and in particular region inference,
   4 //! works, it often happens that errors are not detected until far after
   5 //! the relevant line of code has been type-checked. Therefore, there is
   6 //! an elaborate system to track why a particular constraint in the
   7 //! inference graph arose so that we can explain to the user what gave
   8 //! rise to a particular error.
   9 //!
  10 //! The basis of the system are the "origin" types. An "origin" is the
  11 //! reason that a constraint or inference variable arose. There are
  12 //! different "origin" enums for different kinds of constraints/variables
  13 //! (e.g., `TypeOrigin`, `RegionVariableOrigin`). An origin always has
  14 //! a span, but also more information so that we can generate a meaningful
  15 //! error message.
  16 //!
  17 //! Having a catalog of all the different reasons an error can arise is
  18 //! also useful for other reasons, like cross-referencing FAQs etc, though
  19 //! we are not really taking advantage of this yet.
  20 //!
  21 //! # Region Inference
  22 //!
  23 //! Region inference is particularly tricky because it always succeeds "in
  24 //! the moment" and simply registers a constraint. Then, at the end, we
  25 //! can compute the full graph and report errors, so we need to be able to
  26 //! store and later report what gave rise to the conflicting constraints.
  27 //!
  28 //! # Subtype Trace
  29 //!
  30 //! Determining whether `T1 <: T2` often involves a number of subtypes and
  31 //! subconstraints along the way. A "TypeTrace" is an extended version
  32 //! of an origin that traces the types and other values that were being
  33 //! compared. It is not necessarily comprehensive (in fact, at the time of
  34 //! this writing it only tracks the root values being compared) but I'd
  35 //! like to extend it to include significant "waypoints". For example, if
  36 //! you are comparing `(T1, T2) <: (T3, T4)`, and the problem is that `T2
  37 //! <: T4` fails, I'd like the trace to include enough information to say
  38 //! "in the 2nd element of the tuple". Similarly, failures when comparing
  39 //! arguments or return types in fn types should be able to cite the
  40 //! specific position, etc.
  41 //!
  42 //! # Reality vs plan
  43 //!
  44 //! Of course, there is still a LOT of code in typeck that has yet to be
  45 //! ported to this system, and which relies on string concatenation at the
  46 //! time of error detection.
  47
  48 use super::lexical_region_resolve::RegionResolutionError;
  49 use super::region_constraints::GenericKind;
  50 use super::{InferCtxt, RegionVariableOrigin, SubregionOrigin, TypeTrace, ValuePairs};
  51 use crate::infer::{self, SuppressRegionErrors};
  52
  53 use crate::hir;
  54 use crate::hir::def_id::DefId;
  55 use crate::hir::Node;
  56 use crate::middle::region;
  57 use crate::traits::{ObligationCause, ObligationCauseCode};
  58 use crate::ty::error::TypeError;
  59 use crate::ty::{self, subst::{Subst, SubstsRef}, Region, Ty, TyCtxt, TypeFoldable};
  60 use errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString};
  61 use std::{cmp, fmt};
  62 use syntax_pos::{Pos, Span};
  63
  64 mod note;
  65
  66 mod need_type_info;
  67
  68 pub mod nice_region_error;
  69
  70 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
  71     pub fn note_and_explain_region(
  72         self,
  73         region_scope_tree: &region::ScopeTree,
  74         err: &mut DiagnosticBuilder<'_>,
  75         prefix: &str,
  76         region: ty::Region<'tcx>,
  77         suffix: &str,
  78     ) {
  79         let (description, span) = match *region {
  80             ty::ReScope(scope) => {
  81                 let new_string;
  82                 let unknown_scope = || {
  83                     format!(
  84                         "{}unknown scope: {:?}{}.  Please report a bug.",
  85                         prefix, scope, suffix
  86                     )
  87                 };
  88                 let span = scope.span(self, region_scope_tree);
  89                 let tag = match self.hir().find(scope.node_id(self, region_scope_tree)) {
  90                     Some(Node::Block(_)) => "block",
  91                     Some(Node::Expr(expr)) => match expr.node {
  92                         hir::ExprKind::Call(..) => "call",
  93                         hir::ExprKind::MethodCall(..) => "method call",
  94                         hir::ExprKind::Match(.., hir::MatchSource::IfLetDesugar { .. }) => "if let",
  95                         hir::ExprKind::Match(.., hir::MatchSource::WhileLetDesugar) => "while let",
  96                         hir::ExprKind::Match(.., hir::MatchSource::ForLoopDesugar) => "for",
  97                         hir::ExprKind::Match(..) => "match",
  98                         _ => "expression",
  99                     },
 100                     Some(Node::Stmt(_)) => "statement",
 101                     Some(Node::Item(it)) => Self::item_scope_tag(&it),
 102                     Some(Node::TraitItem(it)) => Self::trait_item_scope_tag(&it),
 103                     Some(Node::ImplItem(it)) => Self::impl_item_scope_tag(&it),
 104                     Some(_) | None => {
 105                         err.span_note(span, &unknown_scope());
 106                         return;
 107                     }
 108                 };
 109                 let scope_decorated_tag = match scope.data {
 110                     region::ScopeData::Node => tag,
 111                     region::ScopeData::CallSite => "scope of call-site for function",
 112                     region::ScopeData::Arguments => "scope of function body",
 113                     region::ScopeData::Destruction => {
 114                         new_string = format!("destruction scope surrounding {}", tag);
 115                         &new_string[..]
 116                     }
 117                     region::ScopeData::Remainder(first_statement_index) => {
 118                         new_string = format!(
 119                             "block suffix following statement {}",
 120                             first_statement_index.index()
 121                         );
 122                         &new_string[..]
 123                     }
 124                 };
 125                 self.explain_span(scope_decorated_tag, span)
 126             }
 127
 128             ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic => {
 129                 self.msg_span_from_free_region(region)
 130             }
 131
 132             ty::ReEmpty => ("the empty lifetime".to_owned(), None),
 133
 134             ty::RePlaceholder(_) => (format!("any other region"), None),
 135
 136             // FIXME(#13998) RePlaceholder should probably print like
 137             // ReFree rather than dumping Debug output on the user.
 138             //
 139             // We shouldn't really be having unification failures with ReVar
 140             // and ReLateBound though.
 141             ty::ReVar(_) | ty::ReLateBound(..) | ty::ReErased => {
 142                 (format!("lifetime {:?}", region), None)
 143             }
 144
 145             // We shouldn't encounter an error message with ReClosureBound.
 146             ty::ReClosureBound(..) => {
 147                 bug!("encountered unexpected ReClosureBound: {:?}", region,);
 148             }
 149         };
 150
 151         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 152     }
 153
 154     pub fn note_and_explain_free_region(
 155         self,
 156         err: &mut DiagnosticBuilder<'_>,
 157         prefix: &str,
 158         region: ty::Region<'tcx>,
 159         suffix: &str,
 160     ) {
 161         let (description, span) = self.msg_span_from_free_region(region);
 162
 163         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 164     }
 165
 166     fn msg_span_from_free_region(self, region: ty::Region<'tcx>) -> (String, Option<Span>) {
 167         match *region {
 168             ty::ReEarlyBound(_) | ty::ReFree(_) => {
 169                 self.msg_span_from_early_bound_and_free_regions(region)
 170             }
 171             ty::ReStatic => ("the static lifetime".to_owned(), None),
 172             ty::ReEmpty => ("an empty lifetime".to_owned(), None),
 173             _ => bug!("{:?}", region),
 174         }
 175     }
 176
 177     fn msg_span_from_early_bound_and_free_regions(
 178         self,
 179         region: ty::Region<'tcx>,
 180     ) -> (String, Option<Span>) {
 181         let cm = self.sess.source_map();
 182
 183         let scope = region.free_region_binding_scope(self);
 184         let node = self.hir().as_local_hir_id(scope).unwrap_or(hir::DUMMY_HIR_ID);
 185         let tag = match self.hir().find_by_hir_id(node) {
 186             Some(Node::Block(_)) | Some(Node::Expr(_)) => "body",
 187             Some(Node::Item(it)) => Self::item_scope_tag(&it),
 188             Some(Node::TraitItem(it)) => Self::trait_item_scope_tag(&it),
 189             Some(Node::ImplItem(it)) => Self::impl_item_scope_tag(&it),
 190             _ => unreachable!(),
 191         };
 192         let (prefix, span) = match *region {
 193             ty::ReEarlyBound(ref br) => {
 194                 let mut sp = cm.def_span(self.hir().span_by_hir_id(node));
 195                 if let Some(param) = self.hir()
 196                     .get_generics(scope)
 197                     .and_then(|generics| generics.get_named(&br.name))
 198                 {
 199                     sp = param.span;
 200                 }
 201                 (format!("the lifetime {} as defined on", br.name), sp)
 202             }
 203             ty::ReFree(ty::FreeRegion {
 204                 bound_region: ty::BoundRegion::BrNamed(_, ref name),
 205                 ..
 206             }) => {
 207                 let mut sp = cm.def_span(self.hir().span_by_hir_id(node));
 208                 if let Some(param) = self.hir()
 209                     .get_generics(scope)
 210                     .and_then(|generics| generics.get_named(&name))
 211                 {
 212                     sp = param.span;
 213                 }
 214                 (format!("the lifetime {} as defined on", name), sp)
 215             }
 216             ty::ReFree(ref fr) => match fr.bound_region {
 217                 ty::BrAnon(idx) => (
 218                     format!("the anonymous lifetime #{} defined on", idx + 1),
 219                     self.hir().span_by_hir_id(node),
 220                 ),
 221                 ty::BrFresh(_) => (
 222                     "an anonymous lifetime defined on".to_owned(),
 223                     self.hir().span_by_hir_id(node),
 224                 ),
 225                 _ => (
 226                     format!("the lifetime {} as defined on", region),
 227                     cm.def_span(self.hir().span_by_hir_id(node)),
 228                 ),
 229             },
 230             _ => bug!(),
 231         };
 232         let (msg, opt_span) = self.explain_span(tag, span);
 233         (format!("{} {}", prefix, msg), opt_span)
 234     }
 235
 236     fn emit_msg_span(
 237         err: &mut DiagnosticBuilder<'_>,
 238         prefix: &str,
 239         description: String,
 240         span: Option<Span>,
 241         suffix: &str,
 242     ) {
 243         let message = format!("{}{}{}", prefix, description, suffix);
 244
 245         if let Some(span) = span {
 246             err.span_note(span, &message);
 247         } else {
 248             err.note(&message);
 249         }
 250     }
 251
 252     fn item_scope_tag(item: &hir::Item) -> &'static str {
 253         match item.node {
 254             hir::ItemKind::Impl(..) => "impl",
 255             hir::ItemKind::Struct(..) => "struct",
 256             hir::ItemKind::Union(..) => "union",
 257             hir::ItemKind::Enum(..) => "enum",
 258             hir::ItemKind::Trait(..) => "trait",
 259             hir::ItemKind::Fn(..) => "function body",
 260             _ => "item",
 261         }
 262     }
 263
 264     fn trait_item_scope_tag(item: &hir::TraitItem) -> &'static str {
 265         match item.node {
 266             hir::TraitItemKind::Method(..) => "method body",
 267             hir::TraitItemKind::Const(..) | hir::TraitItemKind::Type(..) => "associated item",
 268         }
 269     }
 270
 271     fn impl_item_scope_tag(item: &hir::ImplItem) -> &'static str {
 272         match item.node {
 273             hir::ImplItemKind::Method(..) => "method body",
 274             hir::ImplItemKind::Const(..)
 275             | hir::ImplItemKind::Existential(..)
 276             | hir::ImplItemKind::Type(..) => "associated item",
 277         }
 278     }
 279
 280     fn explain_span(self, heading: &str, span: Span) -> (String, Option<Span>) {
 281         let lo = self.sess.source_map().lookup_char_pos_adj(span.lo());
 282         (
 283             format!("the {} at {}:{}", heading, lo.line, lo.col.to_usize() + 1),
 284             Some(span),
 285         )
 286     }
 287 }
 288
 289 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 290     pub fn report_region_errors(
 291         &self,
 292         region_scope_tree: &region::ScopeTree,
 293         errors: &Vec<RegionResolutionError<'tcx>>,
 294         suppress: SuppressRegionErrors,
 295     ) {
 296         debug!(
 297             "report_region_errors(): {} errors to start, suppress = {:?}",
 298             errors.len(),
 299             suppress
 300         );
 301
 302         if suppress.suppressed() {
 303             return;
 304         }
 305
 306         // try to pre-process the errors, which will group some of them
 307         // together into a `ProcessedErrors` group:
 308         let errors = self.process_errors(errors);
 309
 310         debug!(
 311             "report_region_errors: {} errors after preprocessing",
 312             errors.len()
 313         );
 314
 315         for error in errors {
 316             debug!("report_region_errors: error = {:?}", error);
 317
 318             if !self.try_report_nice_region_error(&error) {
 319                 match error.clone() {
 320                     // These errors could indicate all manner of different
 321                     // problems with many different solutions. Rather
 322                     // than generate a "one size fits all" error, what we
 323                     // attempt to do is go through a number of specific
 324                     // scenarios and try to find the best way to present
 325                     // the error. If all of these fails, we fall back to a rather
 326                     // general bit of code that displays the error information
 327                     RegionResolutionError::ConcreteFailure(origin, sub, sup) => {
 328                         if sub.is_placeholder() || sup.is_placeholder() {
 329                             self.report_placeholder_failure(region_scope_tree, origin, sub, sup)
 330                                 .emit();
 331                         } else {
 332                             self.report_concrete_failure(region_scope_tree, origin, sub, sup)
 333                                 .emit();
 334                         }
 335                     }
 336
 337                     RegionResolutionError::GenericBoundFailure(origin, param_ty, sub) => {
 338                         self.report_generic_bound_failure(
 339                             region_scope_tree,
 340                             origin.span(),
 341                             Some(origin),
 342                             param_ty,
 343                             sub,
 344                         );
 345                     }
 346
 347                     RegionResolutionError::SubSupConflict(
 348                         _,
 349                         var_origin,
 350                         sub_origin,
 351                         sub_r,
 352                         sup_origin,
 353                         sup_r,
 354                     ) => {
 355                         if sub_r.is_placeholder() {
 356                             self.report_placeholder_failure(
 357                                 region_scope_tree,
 358                                 sub_origin,
 359                                 sub_r,
 360                                 sup_r,
 361                             )
 362                                 .emit();
 363                         } else if sup_r.is_placeholder() {
 364                             self.report_placeholder_failure(
 365                                 region_scope_tree,
 366                                 sup_origin,
 367                                 sub_r,
 368                                 sup_r,
 369                             )
 370                                 .emit();
 371                         } else {
 372                             self.report_sub_sup_conflict(
 373                                 region_scope_tree,
 374                                 var_origin,
 375                                 sub_origin,
 376                                 sub_r,
 377                                 sup_origin,
 378                                 sup_r,
 379                             );
 380                         }
 381                     }
 382                 }
 383             }
 384         }
 385     }
 386
 387     // This method goes through all the errors and try to group certain types
 388     // of error together, for the purpose of suggesting explicit lifetime
 389     // parameters to the user. This is done so that we can have a more
 390     // complete view of what lifetimes should be the same.
 391     // If the return value is an empty vector, it means that processing
 392     // failed (so the return value of this method should not be used).
 393     //
 394     // The method also attempts to weed out messages that seem like
 395     // duplicates that will be unhelpful to the end-user. But
 396     // obviously it never weeds out ALL errors.
 397     fn process_errors(
 398         &self,
 399         errors: &Vec<RegionResolutionError<'tcx>>,
 400     ) -> Vec<RegionResolutionError<'tcx>> {
 401         debug!("process_errors()");
 402
 403         // We want to avoid reporting generic-bound failures if we can
 404         // avoid it: these have a very high rate of being unhelpful in
 405         // practice. This is because they are basically secondary
 406         // checks that test the state of the region graph after the
 407         // rest of inference is done, and the other kinds of errors
 408         // indicate that the region constraint graph is internally
 409         // inconsistent, so these test results are likely to be
 410         // meaningless.
 411         //
 412         // Therefore, we filter them out of the list unless they are
 413         // the only thing in the list.
 414
 415         let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
 416             RegionResolutionError::GenericBoundFailure(..) => true,
 417             RegionResolutionError::ConcreteFailure(..)
 418             | RegionResolutionError::SubSupConflict(..) => false,
 419         };
 420
 421         let mut errors = if errors.iter().all(|e| is_bound_failure(e)) {
 422             errors.clone()
 423         } else {
 424             errors
 425             .iter()
 426             .filter(|&e| !is_bound_failure(e))
 427             .cloned()
 428             .collect()
 429         };
 430
 431         // sort the errors by span, for better error message stability.
 432         errors.sort_by_key(|u| match *u {
 433             RegionResolutionError::ConcreteFailure(ref sro, _, _) => sro.span(),
 434             RegionResolutionError::GenericBoundFailure(ref sro, _, _) => sro.span(),
 435             RegionResolutionError::SubSupConflict(_, ref rvo, _, _, _, _) => rvo.span(),
 436         });
 437         errors
 438     }
 439
 440     /// Adds a note if the types come from similarly named crates
 441     fn check_and_note_conflicting_crates(
 442         &self,
 443         err: &mut DiagnosticBuilder<'_>,
 444         terr: &TypeError<'tcx>,
 445         sp: Span,
 446     ) {
 447         use hir::def_id::CrateNum;
 448         use hir::map::DisambiguatedDefPathData;
 449         use ty::print::Printer;
 450         use ty::subst::Kind;
 451
 452         struct AbsolutePathPrinter<'a, 'gcx, 'tcx> {
 453             tcx: TyCtxt<'a, 'gcx, 'tcx>,
 454         }
 455
 456         struct NonTrivialPath;
 457
 458         impl<'gcx, 'tcx> Printer<'gcx, 'tcx> for AbsolutePathPrinter<'_, 'gcx, 'tcx> {
 459             type Error = NonTrivialPath;
 460
 461             type Path = Vec<String>;
 462             type Region = !;
 463             type Type = !;
 464             type DynExistential = !;
 465
 466             fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx> {
 467                 self.tcx
 468             }
 469
 470             fn print_region(
 471                 self,
 472                 _region: ty::Region<'_>,
 473             ) -> Result<Self::Region, Self::Error> {
 474                 Err(NonTrivialPath)
 475             }
 476
 477             fn print_type(
 478                 self,
 479                 _ty: Ty<'tcx>,
 480             ) -> Result<Self::Type, Self::Error> {
 481                 Err(NonTrivialPath)
 482             }
 483
 484             fn print_dyn_existential(
 485                 self,
 486                 _predicates: &'tcx ty::List<ty::ExistentialPredicate<'tcx>>,
 487             ) -> Result<Self::DynExistential, Self::Error> {
 488                 Err(NonTrivialPath)
 489             }
 490
 491             fn path_crate(
 492                 self,
 493                 cnum: CrateNum,
 494             ) -> Result<Self::Path, Self::Error> {
 495                 Ok(vec![self.tcx.original_crate_name(cnum).to_string()])
 496             }
 497             fn path_qualified(
 498                 self,
 499                 _self_ty: Ty<'tcx>,
 500                 _trait_ref: Option<ty::TraitRef<'tcx>>,
 501             ) -> Result<Self::Path, Self::Error> {
 502                 Err(NonTrivialPath)
 503             }
 504
 505             fn path_append_impl(
 506                 self,
 507                 _print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 508                 _disambiguated_data: &DisambiguatedDefPathData,
 509                 _self_ty: Ty<'tcx>,
 510                 _trait_ref: Option<ty::TraitRef<'tcx>>,
 511             ) -> Result<Self::Path, Self::Error> {
 512                 Err(NonTrivialPath)
 513             }
 514             fn path_append(
 515                 self,
 516                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 517                 disambiguated_data: &DisambiguatedDefPathData,
 518             ) -> Result<Self::Path, Self::Error> {
 519                 let mut path = print_prefix(self)?;
 520                 path.push(disambiguated_data.data.as_interned_str().to_string());
 521                 Ok(path)
 522             }
 523             fn path_generic_args(
 524                 self,
 525                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 526                 _args: &[Kind<'tcx>],
 527             ) -> Result<Self::Path, Self::Error> {
 528                 print_prefix(self)
 529             }
 530         }
 531
 532         let report_path_match = |err: &mut DiagnosticBuilder<'_>, did1: DefId, did2: DefId| {
 533             // Only external crates, if either is from a local
 534             // module we could have false positives
 535             if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
 536                 let abs_path = |def_id| {
 537                     AbsolutePathPrinter { tcx: self.tcx }
 538                         .print_def_path(def_id, &[])
 539                 };
 540
 541                 // We compare strings because DefPath can be different
 542                 // for imported and non-imported crates
 543                 let same_path = || -> Result<_, NonTrivialPath> {
 544                     Ok(
 545                         self.tcx.def_path_str(did1) == self.tcx.def_path_str(did2) ||
 546                         abs_path(did1)? == abs_path(did2)?
 547                     )
 548                 };
 549                 if same_path().unwrap_or(false) {
 550                     let crate_name = self.tcx.crate_name(did1.krate);
 551                     err.span_note(
 552                         sp,
 553                         &format!(
 554                             "Perhaps two different versions \
 555                              of crate `{}` are being used?",
 556                             crate_name
 557                         ),
 558                     );
 559                 }
 560             }
 561         };
 562         match *terr {
 563             TypeError::Sorts(ref exp_found) => {
 564                 // if they are both "path types", there's a chance of ambiguity
 565                 // due to different versions of the same crate
 566                 if let (&ty::Adt(exp_adt, _), &ty::Adt(found_adt, _))
 567                      = (&exp_found.expected.sty, &exp_found.found.sty)
 568                 {
 569                     report_path_match(err, exp_adt.did, found_adt.did);
 570                 }
 571             }
 572             TypeError::Traits(ref exp_found) => {
 573                 report_path_match(err, exp_found.expected, exp_found.found);
 574             }
 575             _ => (), // FIXME(#22750) handle traits and stuff
 576         }
 577     }
 578
 579     fn note_error_origin(
 580         &self,
 581         err: &mut DiagnosticBuilder<'tcx>,
 582         cause: &ObligationCause<'tcx>,
 583         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
 584     ) {
 585         match cause.code {
 586             ObligationCauseCode::MatchExpressionArmPattern { span, ty } => {
 587                 if ty.is_suggestable() {  // don't show type `_`
 588                     err.span_label(span, format!("this match expression has type `{}`", ty));
 589                 }
 590                 if let Some(ty::error::ExpectedFound { found, .. }) = exp_found {
 591                     if ty.is_box() && ty.boxed_ty() == found {
 592                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 593                             err.span_suggestion(
 594                                 span,
 595                                 "consider dereferencing the boxed value",
 596                                 format!("*{}", snippet),
 597                                 Applicability::MachineApplicable,
 598                             );
 599                         }
 600                     }
 601                 }
 602             }
 603             ObligationCauseCode::MatchExpressionArm {
 604                 source,
 605                 ref prior_arms,
 606                 last_ty,
 607                 discrim_hir_id,
 608                 ..
 609             } => match source {
 610                 hir::MatchSource::IfLetDesugar { .. } => {
 611                     let msg = "`if let` arms have incompatible types";
 612                     err.span_label(cause.span, msg);
 613                 }
 614                 hir::MatchSource::TryDesugar => {
 615                     if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
 616                         let discrim_expr = self.tcx.hir().expect_expr_by_hir_id(discrim_hir_id);
 617                         let discrim_ty = if let hir::ExprKind::Call(_, args) = &discrim_expr.node {
 618                             let arg_expr = args.first().expect("try desugaring call w/out arg");
 619                             self.in_progress_tables.and_then(|tables| {
 620                                 tables.borrow().expr_ty_opt(arg_expr)
 621                             })
 622                         } else {
 623                             bug!("try desugaring w/out call expr as discriminant");
 624                         };
 625
 626                         match discrim_ty {
 627                             Some(ty) if expected == ty => {
 628                                 let source_map = self.tcx.sess.source_map();
 629                                 err.span_suggestion(
 630                                     source_map.end_point(cause.span),
 631                                     "try removing this `?`",
 632                                     "".to_string(),
 633                                     Applicability::MachineApplicable,
 634                                 );
 635                             },
 636                             _ => {},
 637                         }
 638                     }
 639                 }
 640                 _ => {
 641                     let msg = "`match` arms have incompatible types";
 642                     err.span_label(cause.span, msg);
 643                     if prior_arms.len() <= 4 {
 644                         for sp in prior_arms {
 645                             err.span_label(*sp, format!(
 646                                 "this is found to be of type `{}`",
 647                                 last_ty,
 648                             ));
 649                         }
 650                     } else if let Some(sp) = prior_arms.last() {
 651                         err.span_label(*sp, format!(
 652                             "this and all prior arms are found to be of type `{}`", last_ty,
 653                         ));
 654                     }
 655                 }
 656             },
 657             ObligationCauseCode::IfExpression { then, outer, semicolon } => {
 658                 err.span_label(then, "expected because of this");
 659                 outer.map(|sp| err.span_label(sp, "if and else have incompatible types"));
 660                 if let Some(sp) = semicolon {
 661                     err.span_suggestion_short(
 662                         sp,
 663                         "consider removing this semicolon",
 664                         String::new(),
 665                         Applicability::MachineApplicable,
 666                     );
 667                 }
 668             }
 669             _ => (),
 670         }
 671     }
 672
 673     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
 674     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
 675     /// populate `other_value` with `other_ty`.
 676     ///
 677     /// ```text
 678     /// Foo<Bar<Qux>>
 679     /// ^^^^--------^ this is highlighted
 680     /// |   |
 681     /// |   this type argument is exactly the same as the other type, not highlighted
 682     /// this is highlighted
 683     /// Bar<Qux>
 684     /// -------- this type is the same as a type argument in the other type, not highlighted
 685     /// ```
 686     fn highlight_outer(
 687         &self,
 688         value: &mut DiagnosticStyledString,
 689         other_value: &mut DiagnosticStyledString,
 690         name: String,
 691         sub: ty::subst::SubstsRef<'tcx>,
 692         pos: usize,
 693         other_ty: &Ty<'tcx>,
 694     ) {
 695         // `value` and `other_value` hold two incomplete type representation for display.
 696         // `name` is the path of both types being compared. `sub`
 697         value.push_highlighted(name);
 698         let len = sub.len();
 699         if len > 0 {
 700             value.push_highlighted("<");
 701         }
 702
 703         // Output the lifetimes for the first type
 704         let lifetimes = sub.regions()
 705             .map(|lifetime| {
 706                 let s = lifetime.to_string();
 707                 if s.is_empty() {
 708                     "'_".to_string()
 709                 } else {
 710                     s
 711                 }
 712             })
 713             .collect::<Vec<_>>()
 714             .join(", ");
 715         if !lifetimes.is_empty() {
 716             if sub.regions().count() < len {
 717                 value.push_normal(lifetimes + &", ");
 718             } else {
 719                 value.push_normal(lifetimes);
 720             }
 721         }
 722
 723         // Highlight all the type arguments that aren't at `pos` and compare the type argument at
 724         // `pos` and `other_ty`.
 725         for (i, type_arg) in sub.types().enumerate() {
 726             if i == pos {
 727                 let values = self.cmp(type_arg, other_ty);
 728                 value.0.extend((values.0).0);
 729                 other_value.0.extend((values.1).0);
 730             } else {
 731                 value.push_highlighted(type_arg.to_string());
 732             }
 733
 734             if len > 0 && i != len - 1 {
 735                 value.push_normal(", ");
 736             }
 737             //self.push_comma(&mut value, &mut other_value, len, i);
 738         }
 739         if len > 0 {
 740             value.push_highlighted(">");
 741         }
 742     }
 743
 744     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 745     /// as that is the difference to the other type.
 746     ///
 747     /// For the following code:
 748     ///
 749     /// ```norun
 750     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 751     /// ```
 752     ///
 753     /// The type error output will behave in the following way:
 754     ///
 755     /// ```text
 756     /// Foo<Bar<Qux>>
 757     /// ^^^^--------^ this is highlighted
 758     /// |   |
 759     /// |   this type argument is exactly the same as the other type, not highlighted
 760     /// this is highlighted
 761     /// Bar<Qux>
 762     /// -------- this type is the same as a type argument in the other type, not highlighted
 763     /// ```
 764     fn cmp_type_arg(
 765         &self,
 766         mut t1_out: &mut DiagnosticStyledString,
 767         mut t2_out: &mut DiagnosticStyledString,
 768         path: String,
 769         sub: ty::subst::SubstsRef<'tcx>,
 770         other_path: String,
 771         other_ty: &Ty<'tcx>,
 772     ) -> Option<()> {
 773         for (i, ta) in sub.types().enumerate() {
 774             if &ta == other_ty {
 775                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 776                 return Some(());
 777             }
 778             if let &ty::Adt(def, _) = &ta.sty {
 779                 let path_ = self.tcx.def_path_str(def.did.clone());
 780                 if path_ == other_path {
 781                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 782                     return Some(());
 783                 }
 784             }
 785         }
 786         None
 787     }
 788
 789     /// Adds a `,` to the type representation only if it is appropriate.
 790     fn push_comma(
 791         &self,
 792         value: &mut DiagnosticStyledString,
 793         other_value: &mut DiagnosticStyledString,
 794         len: usize,
 795         pos: usize,
 796     ) {
 797         if len > 0 && pos != len - 1 {
 798             value.push_normal(", ");
 799             other_value.push_normal(", ");
 800         }
 801     }
 802
 803     /// For generic types with parameters with defaults, remove the parameters corresponding to
 804     /// the defaults. This repeats a lot of the logic found in `ty::print::pretty`.
 805     fn strip_generic_default_params(
 806         &self,
 807         def_id: DefId,
 808         substs: ty::subst::SubstsRef<'tcx>,
 809     ) -> SubstsRef<'tcx> {
 810         let generics = self.tcx.generics_of(def_id);
 811         let mut num_supplied_defaults = 0;
 812         let mut type_params = generics.params.iter().rev().filter_map(|param| match param.kind {
 813             ty::GenericParamDefKind::Lifetime => None,
 814             ty::GenericParamDefKind::Type { has_default, .. } => Some((param.def_id, has_default)),
 815             ty::GenericParamDefKind::Const => None, // FIXME(const_generics:defaults)
 816         }).peekable();
 817         let has_default = {
 818             let has_default = type_params.peek().map(|(_, has_default)| has_default);
 819             *has_default.unwrap_or(&false)
 820         };
 821         if has_default {
 822             let types = substs.types().rev();
 823             for ((def_id, has_default), actual) in type_params.zip(types) {
 824                 if !has_default {
 825                     break;
 826                 }
 827                 if self.tcx.type_of(def_id).subst(self.tcx, substs) != actual {
 828                     break;
 829                 }
 830                 num_supplied_defaults += 1;
 831             }
 832         }
 833         let len = generics.params.len();
 834         let mut generics = generics.clone();
 835         generics.params.truncate(len - num_supplied_defaults);
 836         substs.truncate_to(self.tcx, &generics)
 837     }
 838
 839     /// Compares two given types, eliding parts that are the same between them and highlighting
 840     /// relevant differences, and return two representation of those types for highlighted printing.
 841     fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> (DiagnosticStyledString, DiagnosticStyledString) {
 842         fn equals<'tcx>(a: &Ty<'tcx>, b: &Ty<'tcx>) -> bool {
 843             match (&a.sty, &b.sty) {
 844                 (a, b) if *a == *b => true,
 845                 (&ty::Int(_), &ty::Infer(ty::InferTy::IntVar(_)))
 846                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Int(_))
 847                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Infer(ty::InferTy::IntVar(_)))
 848                 | (&ty::Float(_), &ty::Infer(ty::InferTy::FloatVar(_)))
 849                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Float(_))
 850                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Infer(ty::InferTy::FloatVar(_))) => {
 851                     true
 852                 }
 853                 _ => false,
 854             }
 855         }
 856
 857         fn push_ty_ref<'tcx>(
 858             r: &ty::Region<'tcx>,
 859             ty: Ty<'tcx>,
 860             mutbl: hir::Mutability,
 861             s: &mut DiagnosticStyledString,
 862         ) {
 863             let mut r = r.to_string();
 864             if r == "'_" {
 865                 r.clear();
 866             } else {
 867                 r.push(' ');
 868             }
 869             s.push_highlighted(format!(
 870                 "&{}{}",
 871                 r,
 872                 if mutbl == hir::MutMutable { "mut " } else { "" }
 873             ));
 874             s.push_normal(ty.to_string());
 875         }
 876
 877         match (&t1.sty, &t2.sty) {
 878             (&ty::Adt(def1, sub1), &ty::Adt(def2, sub2)) => {
 879                 let sub_no_defaults_1 = self.strip_generic_default_params(def1.did, sub1);
 880                 let sub_no_defaults_2 = self.strip_generic_default_params(def2.did, sub2);
 881                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 882                 let path1 = self.tcx.def_path_str(def1.did.clone());
 883                 let path2 = self.tcx.def_path_str(def2.did.clone());
 884                 if def1.did == def2.did {
 885                     // Easy case. Replace same types with `_` to shorten the output and highlight
 886                     // the differing ones.
 887                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
 888                     //     Foo<Bar, _>
 889                     //     Foo<Quz, _>
 890                     //         ---  ^ type argument elided
 891                     //         |
 892                     //         highlighted in output
 893                     values.0.push_normal(path1);
 894                     values.1.push_normal(path2);
 895
 896                     // Avoid printing out default generic parameters that are common to both
 897                     // types.
 898                     let len1 = sub_no_defaults_1.len();
 899                     let len2 = sub_no_defaults_2.len();
 900                     let common_len = cmp::min(len1, len2);
 901                     let remainder1: Vec<_> = sub1.types().skip(common_len).collect();
 902                     let remainder2: Vec<_> = sub2.types().skip(common_len).collect();
 903                     let common_default_params = remainder1
 904                         .iter()
 905                         .rev()
 906                         .zip(remainder2.iter().rev())
 907                         .filter(|(a, b)| a == b)
 908                         .count();
 909                     let len = sub1.len() - common_default_params;
 910
 911                     // Only draw `<...>` if there're lifetime/type arguments.
 912                     if len > 0 {
 913                         values.0.push_normal("<");
 914                         values.1.push_normal("<");
 915                     }
 916
 917                     fn lifetime_display(lifetime: Region<'_>) -> String {
 918                         let s = lifetime.to_string();
 919                         if s.is_empty() {
 920                             "'_".to_string()
 921                         } else {
 922                             s
 923                         }
 924                     }
 925                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
 926                     // all diagnostics that use this output
 927                     //
 928                     //     Foo<'x, '_, Bar>
 929                     //     Foo<'y, '_, Qux>
 930                     //         ^^  ^^  --- type arguments are not elided
 931                     //         |   |
 932                     //         |   elided as they were the same
 933                     //         not elided, they were different, but irrelevant
 934                     let lifetimes = sub1.regions().zip(sub2.regions());
 935                     for (i, lifetimes) in lifetimes.enumerate() {
 936                         let l1 = lifetime_display(lifetimes.0);
 937                         let l2 = lifetime_display(lifetimes.1);
 938                         if l1 == l2 {
 939                             values.0.push_normal("'_");
 940                             values.1.push_normal("'_");
 941                         } else {
 942                             values.0.push_highlighted(l1);
 943                             values.1.push_highlighted(l2);
 944                         }
 945                         self.push_comma(&mut values.0, &mut values.1, len, i);
 946                     }
 947
 948                     // We're comparing two types with the same path, so we compare the type
 949                     // arguments for both. If they are the same, do not highlight and elide from the
 950                     // output.
 951                     //     Foo<_, Bar>
 952                     //     Foo<_, Qux>
 953                     //         ^ elided type as this type argument was the same in both sides
 954                     let type_arguments = sub1.types().zip(sub2.types());
 955                     let regions_len = sub1.regions().count();
 956                     for (i, (ta1, ta2)) in type_arguments.take(len).enumerate() {
 957                         let i = i + regions_len;
 958                         if ta1 == ta2 {
 959                             values.0.push_normal("_");
 960                             values.1.push_normal("_");
 961                         } else {
 962                             let (x1, x2) = self.cmp(ta1, ta2);
 963                             (values.0).0.extend(x1.0);
 964                             (values.1).0.extend(x2.0);
 965                         }
 966                         self.push_comma(&mut values.0, &mut values.1, len, i);
 967                     }
 968
 969                     // Close the type argument bracket.
 970                     // Only draw `<...>` if there're lifetime/type arguments.
 971                     if len > 0 {
 972                         values.0.push_normal(">");
 973                         values.1.push_normal(">");
 974                     }
 975                     values
 976                 } else {
 977                     // Check for case:
 978                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
 979                     //     Foo<Bar<Qux>
 980                     //         ------- this type argument is exactly the same as the other type
 981                     //     Bar<Qux>
 982                     if self.cmp_type_arg(
 983                         &mut values.0,
 984                         &mut values.1,
 985                         path1.clone(),
 986                         sub_no_defaults_1,
 987                         path2.clone(),
 988                         &t2,
 989                     ).is_some()
 990                     {
 991                         return values;
 992                     }
 993                     // Check for case:
 994                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
 995                     //     Bar<Qux>
 996                     //     Foo<Bar<Qux>>
 997                     //         ------- this type argument is exactly the same as the other type
 998                     if self.cmp_type_arg(
 999                         &mut values.1,
1000                         &mut values.0,
1001                         path2,
1002                         sub_no_defaults_2,
1003                         path1,
1004                         &t1,
1005                     ).is_some()
1006                     {
1007                         return values;
1008                     }
1009
1010                     // We couldn't find anything in common, highlight everything.
1011                     //     let x: Bar<Qux> = y::<Foo<Zar>>();
1012                     (
1013                         DiagnosticStyledString::highlighted(t1.to_string()),
1014                         DiagnosticStyledString::highlighted(t2.to_string()),
1015                     )
1016                 }
1017             }
1018
1019             // When finding T != &T, highlight only the borrow
1020             (&ty::Ref(r1, ref_ty1, mutbl1), _) if equals(&ref_ty1, &t2) => {
1021                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1022                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
1023                 values.1.push_normal(t2.to_string());
1024                 values
1025             }
1026             (_, &ty::Ref(r2, ref_ty2, mutbl2)) if equals(&t1, &ref_ty2) => {
1027                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1028                 values.0.push_normal(t1.to_string());
1029                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
1030                 values
1031             }
1032
1033             // When encountering &T != &mut T, highlight only the borrow
1034             (&ty::Ref(r1, ref_ty1, mutbl1), &ty::Ref(r2, ref_ty2, mutbl2))
1035                 if equals(&ref_ty1, &ref_ty2) =>
1036             {
1037                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1038                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
1039                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
1040                 values
1041             }
1042
1043             _ => {
1044                 if t1 == t2 {
1045                     // The two types are the same, elide and don't highlight.
1046                     (
1047                         DiagnosticStyledString::normal("_"),
1048                         DiagnosticStyledString::normal("_"),
1049                     )
1050                 } else {
1051                     // We couldn't find anything in common, highlight everything.
1052                     (
1053                         DiagnosticStyledString::highlighted(t1.to_string()),
1054                         DiagnosticStyledString::highlighted(t2.to_string()),
1055                     )
1056                 }
1057             }
1058         }
1059     }
1060
1061     pub fn note_type_err(
1062         &self,
1063         diag: &mut DiagnosticBuilder<'tcx>,
1064         cause: &ObligationCause<'tcx>,
1065         secondary_span: Option<(Span, String)>,
1066         mut values: Option<ValuePairs<'tcx>>,
1067         terr: &TypeError<'tcx>,
1068     ) {
1069         // For some types of errors, expected-found does not make
1070         // sense, so just ignore the values we were given.
1071         match terr {
1072             TypeError::CyclicTy(_) => {
1073                 values = None;
1074             }
1075             _ => {}
1076         }
1077
1078         let (expected_found, exp_found, is_simple_error) = match values {
1079             None => (None, None, false),
1080             Some(values) => {
1081                 let (is_simple_error, exp_found) = match values {
1082                     ValuePairs::Types(exp_found) => {
1083                         let is_simple_err =
1084                             exp_found.expected.is_primitive() && exp_found.found.is_primitive();
1085
1086                         (is_simple_err, Some(exp_found))
1087                     }
1088                     _ => (false, None),
1089                 };
1090                 let vals = match self.values_str(&values) {
1091                     Some((expected, found)) => Some((expected, found)),
1092                     None => {
1093                         // Derived error. Cancel the emitter.
1094                         self.tcx.sess.diagnostic().cancel(diag);
1095                         return;
1096                     }
1097                 };
1098                 (vals, exp_found, is_simple_error)
1099             }
1100         };
1101
1102         let span = cause.span(&self.tcx);
1103
1104         diag.span_label(span, terr.to_string());
1105         if let Some((sp, msg)) = secondary_span {
1106             diag.span_label(sp, msg);
1107         }
1108
1109         if let Some((expected, found)) = expected_found {
1110             match (terr, is_simple_error, expected == found) {
1111                 (&TypeError::Sorts(ref values), false, true) => {
1112                     diag.note_expected_found_extra(
1113                         &"type",
1114                         expected,
1115                         found,
1116                         &format!(" ({})", values.expected.sort_string(self.tcx)),
1117                         &format!(" ({})", values.found.sort_string(self.tcx)),
1118                     );
1119                 }
1120                 (_, false, _) => {
1121                     if let Some(exp_found) = exp_found {
1122                         let (def_id, ret_ty) = match exp_found.found.sty {
1123                             ty::FnDef(def, _) => {
1124                                 (Some(def), Some(self.tcx.fn_sig(def).output()))
1125                             }
1126                             _ => (None, None),
1127                         };
1128
1129                         let exp_is_struct = match exp_found.expected.sty {
1130                             ty::Adt(def, _) => def.is_struct(),
1131                             _ => false,
1132                         };
1133
1134                         if let (Some(def_id), Some(ret_ty)) = (def_id, ret_ty) {
1135                             if exp_is_struct && &exp_found.expected == ret_ty.skip_binder() {
1136                                 let message = format!(
1137                                     "did you mean `{}(/* fields */)`?",
1138                                     self.tcx.def_path_str(def_id)
1139                                 );
1140                                 diag.span_label(span, message);
1141                             }
1142                         }
1143                         self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
1144                     }
1145
1146                     diag.note_expected_found(&"type", expected, found);
1147                 }
1148                 _ => (),
1149             }
1150         }
1151
1152         self.check_and_note_conflicting_crates(diag, terr, span);
1153         self.tcx.note_and_explain_type_err(diag, terr, span);
1154
1155         // It reads better to have the error origin as the final
1156         // thing.
1157         self.note_error_origin(diag, &cause, exp_found);
1158     }
1159
1160     /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
1161     /// suggest it.
1162     fn suggest_as_ref_where_appropriate(
1163         &self,
1164         span: Span,
1165         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1166         diag: &mut DiagnosticBuilder<'tcx>,
1167     ) {
1168         match (&exp_found.expected.sty, &exp_found.found.sty) {
1169             (ty::Adt(exp_def, exp_substs), ty::Ref(_, found_ty, _)) => {
1170                 if let ty::Adt(found_def, found_substs) = found_ty.sty {
1171                     let path_str = format!("{:?}", exp_def);
1172                     if exp_def == &found_def {
1173                         let opt_msg = "you can convert from `&Option<T>` to `Option<&T>` using \
1174                                        `.as_ref()`";
1175                         let result_msg = "you can convert from `&Result<T, E>` to \
1176                                           `Result<&T, &E>` using `.as_ref()`";
1177                         let have_as_ref = &[
1178                             ("std::option::Option", opt_msg),
1179                             ("core::option::Option", opt_msg),
1180                             ("std::result::Result", result_msg),
1181                             ("core::result::Result", result_msg),
1182                         ];
1183                         if let Some(msg) = have_as_ref.iter()
1184                             .filter_map(|(path, msg)| if &path_str == path {
1185                                 Some(msg)
1186                             } else {
1187                                 None
1188                             }).next()
1189                         {
1190                             let mut show_suggestion = true;
1191                             for (exp_ty, found_ty) in exp_substs.types().zip(found_substs.types()) {
1192                                 match exp_ty.sty {
1193                                     ty::Ref(_, exp_ty, _) => {
1194                                         match (&exp_ty.sty, &found_ty.sty) {
1195                                             (_, ty::Param(_)) |
1196                                             (_, ty::Infer(_)) |
1197                                             (ty::Param(_), _) |
1198                                             (ty::Infer(_), _) => {}
1199                                             _ if ty::TyS::same_type(exp_ty, found_ty) => {}
1200                                             _ => show_suggestion = false,
1201                                         };
1202                                     }
1203                                     ty::Param(_) | ty::Infer(_) => {}
1204                                     _ => show_suggestion = false,
1205                                 }
1206                             }
1207                             if let (Ok(snippet), true) = (
1208                                 self.tcx.sess.source_map().span_to_snippet(span),
1209                                 show_suggestion,
1210                             ) {
1211                                 diag.span_suggestion(
1212                                     span,
1213                                     msg,
1214                                     format!("{}.as_ref()", snippet),
1215                                     Applicability::MachineApplicable,
1216                                 );
1217                             }
1218                         }
1219                     }
1220                 }
1221             }
1222             _ => {}
1223         }
1224     }
1225
1226     pub fn report_and_explain_type_error(
1227         &self,
1228         trace: TypeTrace<'tcx>,
1229         terr: &TypeError<'tcx>,
1230     ) -> DiagnosticBuilder<'tcx> {
1231         debug!(
1232             "report_and_explain_type_error(trace={:?}, terr={:?})",
1233             trace, terr
1234         );
1235
1236         let span = trace.cause.span(&self.tcx);
1237         let failure_code = trace.cause.as_failure_code(terr);
1238         let mut diag = match failure_code {
1239             FailureCode::Error0317(failure_str) => {
1240                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
1241             }
1242             FailureCode::Error0580(failure_str) => {
1243                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
1244             }
1245             FailureCode::Error0308(failure_str) => {
1246                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
1247             }
1248             FailureCode::Error0644(failure_str) => {
1249                 struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
1250             }
1251         };
1252         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
1253         diag
1254     }
1255
1256     fn values_str(
1257         &self,
1258         values: &ValuePairs<'tcx>,
1259     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1260         match *values {
1261             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
1262             infer::Regions(ref exp_found) => self.expected_found_str(exp_found),
1263             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
1264             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
1265         }
1266     }
1267
1268     fn expected_found_str_ty(
1269         &self,
1270         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1271     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1272         let exp_found = self.resolve_type_vars_if_possible(exp_found);
1273         if exp_found.references_error() {
1274             return None;
1275         }
1276
1277         Some(self.cmp(exp_found.expected, exp_found.found))
1278     }
1279
1280     /// Returns a string of the form "expected `{}`, found `{}`".
1281     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
1282         &self,
1283         exp_found: &ty::error::ExpectedFound<T>,
1284     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1285         let exp_found = self.resolve_type_vars_if_possible(exp_found);
1286         if exp_found.references_error() {
1287             return None;
1288         }
1289
1290         Some((
1291             DiagnosticStyledString::highlighted(exp_found.expected.to_string()),
1292             DiagnosticStyledString::highlighted(exp_found.found.to_string()),
1293         ))
1294     }
1295
1296     pub fn report_generic_bound_failure(
1297         &self,
1298         region_scope_tree: &region::ScopeTree,
1299         span: Span,
1300         origin: Option<SubregionOrigin<'tcx>>,
1301         bound_kind: GenericKind<'tcx>,
1302         sub: Region<'tcx>,
1303     ) {
1304         self.construct_generic_bound_failure(region_scope_tree, span, origin, bound_kind, sub)
1305             .emit()
1306     }
1307
1308     pub fn construct_generic_bound_failure(
1309         &self,
1310         region_scope_tree: &region::ScopeTree,
1311         span: Span,
1312         origin: Option<SubregionOrigin<'tcx>>,
1313         bound_kind: GenericKind<'tcx>,
1314         sub: Region<'tcx>,
1315     ) -> DiagnosticBuilder<'a> {
1316         // Attempt to obtain the span of the parameter so we can
1317         // suggest adding an explicit lifetime bound to it.
1318         let type_param_span = match (self.in_progress_tables, bound_kind) {
1319             (Some(ref table), GenericKind::Param(ref param)) => {
1320                 let table = table.borrow();
1321                 table.local_id_root.and_then(|did| {
1322                     let generics = self.tcx.generics_of(did);
1323                     // Account for the case where `did` corresponds to `Self`, which doesn't have
1324                     // the expected type argument.
1325                     if !param.is_self() {
1326                         let type_param = generics.type_param(param, self.tcx);
1327                         let hir = &self.tcx.hir();
1328                         hir.as_local_node_id(type_param.def_id).map(|id| {
1329                             // Get the `hir::Param` to verify whether it already has any bounds.
1330                             // We do this to avoid suggesting code that ends up as `T: 'a'b`,
1331                             // instead we suggest `T: 'a + 'b` in that case.
1332                             let mut has_bounds = false;
1333                             if let Node::GenericParam(ref param) = hir.get(id) {
1334                                 has_bounds = !param.bounds.is_empty();
1335                             }
1336                             let sp = hir.span(id);
1337                             // `sp` only covers `T`, change it so that it covers
1338                             // `T:` when appropriate
1339                             let is_impl_trait = bound_kind.to_string().starts_with("impl ");
1340                             let sp = if has_bounds && !is_impl_trait {
1341                                 sp.to(self.tcx
1342                                     .sess
1343                                     .source_map()
1344                                     .next_point(self.tcx.sess.source_map().next_point(sp)))
1345                             } else {
1346                                 sp
1347                             };
1348                             (sp, has_bounds, is_impl_trait)
1349                         })
1350                     } else {
1351                         None
1352                     }
1353                 })
1354             }
1355             _ => None,
1356         };
1357
1358         let labeled_user_string = match bound_kind {
1359             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
1360             GenericKind::Projection(ref p) => format!("the associated type `{}`", p),
1361         };
1362
1363         if let Some(SubregionOrigin::CompareImplMethodObligation {
1364             span,
1365             item_name,
1366             impl_item_def_id,
1367             trait_item_def_id,
1368         }) = origin
1369         {
1370             return self.report_extra_impl_obligation(
1371                 span,
1372                 item_name,
1373                 impl_item_def_id,
1374                 trait_item_def_id,
1375                 &format!("`{}: {}`", bound_kind, sub),
1376             );
1377         }
1378
1379         fn binding_suggestion<'tcx, S: fmt::Display>(
1380             err: &mut DiagnosticBuilder<'tcx>,
1381             type_param_span: Option<(Span, bool, bool)>,
1382             bound_kind: GenericKind<'tcx>,
1383             sub: S,
1384         ) {
1385             let consider = format!(
1386                 "consider adding an explicit lifetime bound {}",
1387                 if type_param_span.map(|(_, _, is_impl_trait)| is_impl_trait).unwrap_or(false) {
1388                     format!(" `{}` to `{}`...", sub, bound_kind)
1389                 } else {
1390                     format!("`{}: {}`...", bound_kind, sub)
1391                 },
1392             );
1393             if let Some((sp, has_lifetimes, is_impl_trait)) = type_param_span {
1394                 let suggestion = if is_impl_trait {
1395                     format!("{} + {}", bound_kind, sub)
1396                 } else {
1397                     let tail = if has_lifetimes { " + " } else { "" };
1398                     format!("{}: {}{}", bound_kind, sub, tail)
1399                 };
1400                 err.span_suggestion_short(
1401                     sp,
1402                     &consider,
1403                     suggestion,
1404                     Applicability::MaybeIncorrect, // Issue #41966
1405                 );
1406             } else {
1407                 err.help(&consider);
1408             }
1409         }
1410
1411         let mut err = match *sub {
1412             ty::ReEarlyBound(_)
1413             | ty::ReFree(ty::FreeRegion {
1414                 bound_region: ty::BrNamed(..),
1415                 ..
1416             }) => {
1417                 // Does the required lifetime have a nice name we can print?
1418                 let mut err = struct_span_err!(
1419                     self.tcx.sess,
1420                     span,
1421                     E0309,
1422                     "{} may not live long enough",
1423                     labeled_user_string
1424                 );
1425                 binding_suggestion(&mut err, type_param_span, bound_kind, sub);
1426                 err
1427             }
1428
1429             ty::ReStatic => {
1430                 // Does the required lifetime have a nice name we can print?
1431                 let mut err = struct_span_err!(
1432                     self.tcx.sess,
1433                     span,
1434                     E0310,
1435                     "{} may not live long enough",
1436                     labeled_user_string
1437                 );
1438                 binding_suggestion(&mut err, type_param_span, bound_kind, "'static");
1439                 err
1440             }
1441
1442             _ => {
1443                 // If not, be less specific.
1444                 let mut err = struct_span_err!(
1445                     self.tcx.sess,
1446                     span,
1447                     E0311,
1448                     "{} may not live long enough",
1449                     labeled_user_string
1450                 );
1451                 err.help(&format!(
1452                     "consider adding an explicit lifetime bound for `{}`",
1453                     bound_kind
1454                 ));
1455                 self.tcx.note_and_explain_region(
1456                     region_scope_tree,
1457                     &mut err,
1458                     &format!("{} must be valid for ", labeled_user_string),
1459                     sub,
1460                     "...",
1461                 );
1462                 err
1463             }
1464         };
1465
1466         if let Some(origin) = origin {
1467             self.note_region_origin(&mut err, &origin);
1468         }
1469         err
1470     }
1471
1472     fn report_sub_sup_conflict(
1473         &self,
1474         region_scope_tree: &region::ScopeTree,
1475         var_origin: RegionVariableOrigin,
1476         sub_origin: SubregionOrigin<'tcx>,
1477         sub_region: Region<'tcx>,
1478         sup_origin: SubregionOrigin<'tcx>,
1479         sup_region: Region<'tcx>,
1480     ) {
1481         let mut err = self.report_inference_failure(var_origin);
1482
1483         self.tcx.note_and_explain_region(
1484             region_scope_tree,
1485             &mut err,
1486             "first, the lifetime cannot outlive ",
1487             sup_region,
1488             "...",
1489         );
1490
1491         match (&sup_origin, &sub_origin) {
1492             (&infer::Subtype(ref sup_trace), &infer::Subtype(ref sub_trace)) => {
1493                 debug!("report_sub_sup_conflict: var_origin={:?}", var_origin);
1494                 debug!("report_sub_sup_conflict: sub_region={:?}", sub_region);
1495                 debug!("report_sub_sup_conflict: sub_origin={:?}", sub_origin);
1496                 debug!("report_sub_sup_conflict: sup_region={:?}", sup_region);
1497                 debug!("report_sub_sup_conflict: sup_origin={:?}", sup_origin);
1498                 debug!("report_sub_sup_conflict: sup_trace={:?}", sup_trace);
1499                 debug!("report_sub_sup_conflict: sub_trace={:?}", sub_trace);
1500                 debug!("report_sub_sup_conflict: sup_trace.values={:?}", sup_trace.values);
1501                 debug!("report_sub_sup_conflict: sub_trace.values={:?}", sub_trace.values);
1502
1503                 if let (Some((sup_expected, sup_found)), Some((sub_expected, sub_found))) = (
1504                     self.values_str(&sup_trace.values),
1505                     self.values_str(&sub_trace.values),
1506                 ) {
1507                     if sub_expected == sup_expected && sub_found == sup_found {
1508                         self.tcx.note_and_explain_region(
1509                             region_scope_tree,
1510                             &mut err,
1511                             "...but the lifetime must also be valid for ",
1512                             sub_region,
1513                             "...",
1514                         );
1515                         err.note(&format!(
1516                             "...so that the {}:\nexpected {}\n   found {}",
1517                             sup_trace.cause.as_requirement_str(),
1518                             sup_expected.content(),
1519                             sup_found.content()
1520                         ));
1521                         err.emit();
1522                         return;
1523                     }
1524                 }
1525             }
1526             _ => {}
1527         }
1528
1529         self.note_region_origin(&mut err, &sup_origin);
1530
1531         self.tcx.note_and_explain_region(
1532             region_scope_tree,
1533             &mut err,
1534             "but, the lifetime must be valid for ",
1535             sub_region,
1536             "...",
1537         );
1538
1539         self.note_region_origin(&mut err, &sub_origin);
1540         err.emit();
1541     }
1542 }
1543
1544 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1545     fn report_inference_failure(
1546         &self,
1547         var_origin: RegionVariableOrigin,
1548     ) -> DiagnosticBuilder<'tcx> {
1549         let br_string = |br: ty::BoundRegion| {
1550             let mut s = match br {
1551                 ty::BrNamed(_, name) => name.to_string(),
1552                 _ => String::new(),
1553             };
1554             if !s.is_empty() {
1555                 s.push_str(" ");
1556             }
1557             s
1558         };
1559         let var_description = match var_origin {
1560             infer::MiscVariable(_) => String::new(),
1561             infer::PatternRegion(_) => " for pattern".to_string(),
1562             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
1563             infer::Autoref(_) => " for autoref".to_string(),
1564             infer::Coercion(_) => " for automatic coercion".to_string(),
1565             infer::LateBoundRegion(_, br, infer::FnCall) => {
1566                 format!(" for lifetime parameter {}in function call", br_string(br))
1567             }
1568             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
1569                 format!(" for lifetime parameter {}in generic type", br_string(br))
1570             }
1571             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
1572                 " for lifetime parameter {}in trait containing associated type `{}`",
1573                 br_string(br),
1574                 self.tcx.associated_item(def_id).ident
1575             ),
1576             infer::EarlyBoundRegion(_, name) => format!(" for lifetime parameter `{}`", name),
1577             infer::BoundRegionInCoherence(name) => {
1578                 format!(" for lifetime parameter `{}` in coherence check", name)
1579             }
1580             infer::UpvarRegion(ref upvar_id, _) => {
1581                 let var_name = self.tcx.hir().name_by_hir_id(upvar_id.var_path.hir_id);
1582                 format!(" for capture of `{}` by closure", var_name)
1583             }
1584             infer::NLL(..) => bug!("NLL variable found in lexical phase"),
1585         };
1586
1587         struct_span_err!(
1588             self.tcx.sess,
1589             var_origin.span(),
1590             E0495,
1591             "cannot infer an appropriate lifetime{} \
1592              due to conflicting requirements",
1593             var_description
1594         )
1595     }
1596 }
1597
1598 enum FailureCode {
1599     Error0317(&'static str),
1600     Error0580(&'static str),
1601     Error0308(&'static str),
1602     Error0644(&'static str),
1603 }
1604
1605 impl<'tcx> ObligationCause<'tcx> {
1606     fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode {
1607         use self::FailureCode::*;
1608         use crate::traits::ObligationCauseCode::*;
1609         match self.code {
1610             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
1611             MatchExpressionArm { source, .. } => Error0308(match source {
1612                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have incompatible types",
1613                 hir::MatchSource::TryDesugar => {
1614                     "try expression alternatives have incompatible types"
1615                 }
1616                 _ => "match arms have incompatible types",
1617             }),
1618             IfExpression { .. } => Error0308("if and else have incompatible types"),
1619             IfExpressionWithNoElse => Error0317("if may be missing an else clause"),
1620             MainFunctionType => Error0580("main function has wrong type"),
1621             StartFunctionType => Error0308("start function has wrong type"),
1622             IntrinsicType => Error0308("intrinsic has wrong type"),
1623             MethodReceiver => Error0308("mismatched method receiver"),
1624
1625             // In the case where we have no more specific thing to
1626             // say, also take a look at the error code, maybe we can
1627             // tailor to that.
1628             _ => match terr {
1629                 TypeError::CyclicTy(ty) if ty.is_closure() || ty.is_generator() => {
1630                     Error0644("closure/generator type that references itself")
1631                 }
1632                 _ => Error0308("mismatched types"),
1633             },
1634         }
1635     }
1636
1637     fn as_requirement_str(&self) -> &'static str {
1638         use crate::traits::ObligationCauseCode::*;
1639         match self.code {
1640             CompareImplMethodObligation { .. } => "method type is compatible with trait",
1641             ExprAssignable => "expression is assignable",
1642             MatchExpressionArm { source, .. } => match source {
1643                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have compatible types",
1644                 _ => "match arms have compatible types",
1645             },
1646             IfExpression { .. } => "if and else have compatible types",
1647             IfExpressionWithNoElse => "if missing an else returns ()",
1648             MainFunctionType => "`main` function has the correct type",
1649             StartFunctionType => "`start` function has the correct type",
1650             IntrinsicType => "intrinsic has the correct type",
1651             MethodReceiver => "method receiver has the correct type",
1652             _ => "types are compatible",
1653         }
1654     }
1655 }