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
  52 use crate::hir;
  53 use crate::hir::def_id::DefId;
  54 use crate::hir::Node;
  55 use crate::infer::{self, SuppressRegionErrors};
  56 use crate::infer::opaque_types;
  57 use crate::middle::region;
  58 use crate::traits::{
  59     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
  60 };
  61 use crate::ty::error::TypeError;
  62 use crate::ty::{self, subst::{Subst, SubstsRef}, Region, Ty, TyCtxt, TypeFoldable};
  63
  64 use errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString};
  65 use rustc_error_codes::*;
  66 use rustc_target::spec::abi;
  67 use syntax_pos::{Pos, Span};
  68 use std::{cmp, fmt};
  69
  70 mod note;
  71
  72 mod need_type_info;
  73
  74 pub mod nice_region_error;
  75
  76 impl<'tcx> TyCtxt<'tcx> {
  77     pub fn note_and_explain_region(
  78         self,
  79         region_scope_tree: &region::ScopeTree,
  80         err: &mut DiagnosticBuilder<'_>,
  81         prefix: &str,
  82         region: ty::Region<'tcx>,
  83         suffix: &str,
  84     ) {
  85         let (description, span) = match *region {
  86             ty::ReScope(scope) => {
  87                 let new_string;
  88                 let unknown_scope = || {
  89                     format!(
  90                         "{}unknown scope: {:?}{}.  Please report a bug.",
  91                         prefix, scope, suffix
  92                     )
  93                 };
  94                 let span = scope.span(self, region_scope_tree);
  95                 let tag = match self.hir().find(scope.hir_id(region_scope_tree)) {
  96                     Some(Node::Block(_)) => "block",
  97                     Some(Node::Expr(expr)) => match expr.kind {
  98                         hir::ExprKind::Call(..) => "call",
  99                         hir::ExprKind::MethodCall(..) => "method call",
 100                         hir::ExprKind::Match(.., hir::MatchSource::IfLetDesugar { .. }) => "if let",
 101                         hir::ExprKind::Match(.., hir::MatchSource::WhileLetDesugar) => "while let",
 102                         hir::ExprKind::Match(.., hir::MatchSource::ForLoopDesugar) => "for",
 103                         hir::ExprKind::Match(..) => "match",
 104                         _ => "expression",
 105                     },
 106                     Some(Node::Stmt(_)) => "statement",
 107                     Some(Node::Item(it)) => Self::item_scope_tag(&it),
 108                     Some(Node::TraitItem(it)) => Self::trait_item_scope_tag(&it),
 109                     Some(Node::ImplItem(it)) => Self::impl_item_scope_tag(&it),
 110                     Some(_) | None => {
 111                         err.span_note(span, &unknown_scope());
 112                         return;
 113                     }
 114                 };
 115                 let scope_decorated_tag = match scope.data {
 116                     region::ScopeData::Node => tag,
 117                     region::ScopeData::CallSite => "scope of call-site for function",
 118                     region::ScopeData::Arguments => "scope of function body",
 119                     region::ScopeData::Destruction => {
 120                         new_string = format!("destruction scope surrounding {}", tag);
 121                         &new_string[..]
 122                     }
 123                     region::ScopeData::Remainder(first_statement_index) => {
 124                         new_string = format!(
 125                             "block suffix following statement {}",
 126                             first_statement_index.index()
 127                         );
 128                         &new_string[..]
 129                     }
 130                 };
 131                 self.explain_span(scope_decorated_tag, span)
 132             }
 133
 134             ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic => {
 135                 self.msg_span_from_free_region(region)
 136             }
 137
 138             ty::ReEmpty => ("the empty lifetime".to_owned(), None),
 139
 140             ty::RePlaceholder(_) => (format!("any other region"), None),
 141
 142             // FIXME(#13998) RePlaceholder should probably print like
 143             // ReFree rather than dumping Debug output on the user.
 144             //
 145             // We shouldn't really be having unification failures with ReVar
 146             // and ReLateBound though.
 147             ty::ReVar(_) | ty::ReLateBound(..) | ty::ReErased => {
 148                 (format!("lifetime {:?}", region), None)
 149             }
 150
 151             // We shouldn't encounter an error message with ReClosureBound.
 152             ty::ReClosureBound(..) => {
 153                 bug!("encountered unexpected ReClosureBound: {:?}", region,);
 154             }
 155         };
 156
 157         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 158     }
 159
 160     pub fn note_and_explain_free_region(
 161         self,
 162         err: &mut DiagnosticBuilder<'_>,
 163         prefix: &str,
 164         region: ty::Region<'tcx>,
 165         suffix: &str,
 166     ) {
 167         let (description, span) = self.msg_span_from_free_region(region);
 168
 169         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 170     }
 171
 172     fn msg_span_from_free_region(self, region: ty::Region<'tcx>) -> (String, Option<Span>) {
 173         match *region {
 174             ty::ReEarlyBound(_) | ty::ReFree(_) => {
 175                 self.msg_span_from_early_bound_and_free_regions(region)
 176             }
 177             ty::ReStatic => ("the static lifetime".to_owned(), None),
 178             ty::ReEmpty => ("an empty lifetime".to_owned(), None),
 179             _ => bug!("{:?}", region),
 180         }
 181     }
 182
 183     fn msg_span_from_early_bound_and_free_regions(
 184         self,
 185         region: ty::Region<'tcx>,
 186     ) -> (String, Option<Span>) {
 187         let cm = self.sess.source_map();
 188
 189         let scope = region.free_region_binding_scope(self);
 190         let node = self.hir().as_local_hir_id(scope).unwrap_or(hir::DUMMY_HIR_ID);
 191         let tag = match self.hir().find(node) {
 192             Some(Node::Block(_)) | Some(Node::Expr(_)) => "body",
 193             Some(Node::Item(it)) => Self::item_scope_tag(&it),
 194             Some(Node::TraitItem(it)) => Self::trait_item_scope_tag(&it),
 195             Some(Node::ImplItem(it)) => Self::impl_item_scope_tag(&it),
 196             _ => unreachable!(),
 197         };
 198         let (prefix, span) = match *region {
 199             ty::ReEarlyBound(ref br) => {
 200                 let mut sp = cm.def_span(self.hir().span(node));
 201                 if let Some(param) = self.hir()
 202                     .get_generics(scope)
 203                     .and_then(|generics| generics.get_named(br.name))
 204                 {
 205                     sp = param.span;
 206                 }
 207                 (format!("the lifetime `{}` as defined on", br.name), sp)
 208             }
 209             ty::ReFree(ty::FreeRegion {
 210                 bound_region: ty::BoundRegion::BrNamed(_, name),
 211                 ..
 212             }) => {
 213                 let mut sp = cm.def_span(self.hir().span(node));
 214                 if let Some(param) = self.hir()
 215                     .get_generics(scope)
 216                     .and_then(|generics| generics.get_named(name))
 217                 {
 218                     sp = param.span;
 219                 }
 220                 (format!("the lifetime `{}` as defined on", name), sp)
 221             }
 222             ty::ReFree(ref fr) => match fr.bound_region {
 223                 ty::BrAnon(idx) => (
 224                     format!("the anonymous lifetime #{} defined on", idx + 1),
 225                     self.hir().span(node),
 226                 ),
 227                 _ => (
 228                     format!("the lifetime `{}` as defined on", region),
 229                     cm.def_span(self.hir().span(node)),
 230                 ),
 231             },
 232             _ => bug!(),
 233         };
 234         let (msg, opt_span) = self.explain_span(tag, span);
 235         (format!("{} {}", prefix, msg), opt_span)
 236     }
 237
 238     fn emit_msg_span(
 239         err: &mut DiagnosticBuilder<'_>,
 240         prefix: &str,
 241         description: String,
 242         span: Option<Span>,
 243         suffix: &str,
 244     ) {
 245         let message = format!("{}{}{}", prefix, description, suffix);
 246
 247         if let Some(span) = span {
 248             err.span_note(span, &message);
 249         } else {
 250             err.note(&message);
 251         }
 252     }
 253
 254     fn item_scope_tag(item: &hir::Item) -> &'static str {
 255         match item.kind {
 256             hir::ItemKind::Impl(..) => "impl",
 257             hir::ItemKind::Struct(..) => "struct",
 258             hir::ItemKind::Union(..) => "union",
 259             hir::ItemKind::Enum(..) => "enum",
 260             hir::ItemKind::Trait(..) => "trait",
 261             hir::ItemKind::Fn(..) => "function body",
 262             _ => "item",
 263         }
 264     }
 265
 266     fn trait_item_scope_tag(item: &hir::TraitItem) -> &'static str {
 267         match item.kind {
 268             hir::TraitItemKind::Method(..) => "method body",
 269             hir::TraitItemKind::Const(..) | hir::TraitItemKind::Type(..) => "associated item",
 270         }
 271     }
 272
 273     fn impl_item_scope_tag(item: &hir::ImplItem) -> &'static str {
 274         match item.kind {
 275             hir::ImplItemKind::Method(..) => "method body",
 276             hir::ImplItemKind::Const(..)
 277             | hir::ImplItemKind::OpaqueTy(..)
 278             | hir::ImplItemKind::TyAlias(..) => "associated item",
 279         }
 280     }
 281
 282     fn explain_span(self, heading: &str, span: Span) -> (String, Option<Span>) {
 283         let lo = self.sess.source_map().lookup_char_pos(span.lo());
 284         (
 285             format!("the {} at {}:{}", heading, lo.line, lo.col.to_usize() + 1),
 286             Some(span),
 287         )
 288     }
 289 }
 290
 291 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 292     pub fn report_region_errors(
 293         &self,
 294         region_scope_tree: &region::ScopeTree,
 295         errors: &Vec<RegionResolutionError<'tcx>>,
 296         suppress: SuppressRegionErrors,
 297     ) {
 298         debug!(
 299             "report_region_errors(): {} errors to start, suppress = {:?}",
 300             errors.len(),
 301             suppress
 302         );
 303
 304         if suppress.suppressed() {
 305             return;
 306         }
 307
 308         // try to pre-process the errors, which will group some of them
 309         // together into a `ProcessedErrors` group:
 310         let errors = self.process_errors(errors);
 311
 312         debug!(
 313             "report_region_errors: {} errors after preprocessing",
 314             errors.len()
 315         );
 316
 317         for error in errors {
 318             debug!("report_region_errors: error = {:?}", error);
 319
 320             if !self.try_report_nice_region_error(&error) {
 321                 match error.clone() {
 322                     // These errors could indicate all manner of different
 323                     // problems with many different solutions. Rather
 324                     // than generate a "one size fits all" error, what we
 325                     // attempt to do is go through a number of specific
 326                     // scenarios and try to find the best way to present
 327                     // the error. If all of these fails, we fall back to a rather
 328                     // general bit of code that displays the error information
 329                     RegionResolutionError::ConcreteFailure(origin, sub, sup) => {
 330                         if sub.is_placeholder() || sup.is_placeholder() {
 331                             self.report_placeholder_failure(region_scope_tree, origin, sub, sup)
 332                                 .emit();
 333                         } else {
 334                             self.report_concrete_failure(region_scope_tree, origin, sub, sup)
 335                                 .emit();
 336                         }
 337                     }
 338
 339                     RegionResolutionError::GenericBoundFailure(origin, param_ty, sub) => {
 340                         self.report_generic_bound_failure(
 341                             region_scope_tree,
 342                             origin.span(),
 343                             Some(origin),
 344                             param_ty,
 345                             sub,
 346                         );
 347                     }
 348
 349                     RegionResolutionError::SubSupConflict(
 350                         _,
 351                         var_origin,
 352                         sub_origin,
 353                         sub_r,
 354                         sup_origin,
 355                         sup_r,
 356                     ) => {
 357                         if sub_r.is_placeholder() {
 358                             self.report_placeholder_failure(
 359                                 region_scope_tree,
 360                                 sub_origin,
 361                                 sub_r,
 362                                 sup_r,
 363                             )
 364                                 .emit();
 365                         } else if sup_r.is_placeholder() {
 366                             self.report_placeholder_failure(
 367                                 region_scope_tree,
 368                                 sup_origin,
 369                                 sub_r,
 370                                 sup_r,
 371                             )
 372                                 .emit();
 373                         } else {
 374                             self.report_sub_sup_conflict(
 375                                 region_scope_tree,
 376                                 var_origin,
 377                                 sub_origin,
 378                                 sub_r,
 379                                 sup_origin,
 380                                 sup_r,
 381                             );
 382                         }
 383                     }
 384
 385                     RegionResolutionError::MemberConstraintFailure {
 386                         opaque_type_def_id,
 387                         hidden_ty,
 388                         member_region,
 389                         span: _,
 390                         choice_regions: _,
 391                     } => {
 392                         let hidden_ty = self.resolve_vars_if_possible(&hidden_ty);
 393                         opaque_types::unexpected_hidden_region_diagnostic(
 394                             self.tcx,
 395                             Some(region_scope_tree),
 396                             opaque_type_def_id,
 397                             hidden_ty,
 398                             member_region,
 399                         ).emit();
 400                     }
 401                 }
 402             }
 403         }
 404     }
 405
 406     // This method goes through all the errors and try to group certain types
 407     // of error together, for the purpose of suggesting explicit lifetime
 408     // parameters to the user. This is done so that we can have a more
 409     // complete view of what lifetimes should be the same.
 410     // If the return value is an empty vector, it means that processing
 411     // failed (so the return value of this method should not be used).
 412     //
 413     // The method also attempts to weed out messages that seem like
 414     // duplicates that will be unhelpful to the end-user. But
 415     // obviously it never weeds out ALL errors.
 416     fn process_errors(
 417         &self,
 418         errors: &Vec<RegionResolutionError<'tcx>>,
 419     ) -> Vec<RegionResolutionError<'tcx>> {
 420         debug!("process_errors()");
 421
 422         // We want to avoid reporting generic-bound failures if we can
 423         // avoid it: these have a very high rate of being unhelpful in
 424         // practice. This is because they are basically secondary
 425         // checks that test the state of the region graph after the
 426         // rest of inference is done, and the other kinds of errors
 427         // indicate that the region constraint graph is internally
 428         // inconsistent, so these test results are likely to be
 429         // meaningless.
 430         //
 431         // Therefore, we filter them out of the list unless they are
 432         // the only thing in the list.
 433
 434         let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
 435             RegionResolutionError::GenericBoundFailure(..) => true,
 436             RegionResolutionError::ConcreteFailure(..)
 437                 | RegionResolutionError::SubSupConflict(..)
 438                 | RegionResolutionError::MemberConstraintFailure { .. } => false,
 439         };
 440
 441         let mut errors = if errors.iter().all(|e| is_bound_failure(e)) {
 442             errors.clone()
 443         } else {
 444             errors
 445             .iter()
 446             .filter(|&e| !is_bound_failure(e))
 447             .cloned()
 448             .collect()
 449         };
 450
 451         // sort the errors by span, for better error message stability.
 452         errors.sort_by_key(|u| match *u {
 453             RegionResolutionError::ConcreteFailure(ref sro, _, _) => sro.span(),
 454             RegionResolutionError::GenericBoundFailure(ref sro, _, _) => sro.span(),
 455             RegionResolutionError::SubSupConflict(_, ref rvo, _, _, _, _) => rvo.span(),
 456             RegionResolutionError::MemberConstraintFailure { span, .. } => span,
 457         });
 458         errors
 459     }
 460
 461     /// Adds a note if the types come from similarly named crates
 462     fn check_and_note_conflicting_crates(
 463         &self,
 464         err: &mut DiagnosticBuilder<'_>,
 465         terr: &TypeError<'tcx>,
 466     ) {
 467         use hir::def_id::CrateNum;
 468         use hir::map::DisambiguatedDefPathData;
 469         use ty::print::Printer;
 470         use ty::subst::GenericArg;
 471
 472         struct AbsolutePathPrinter<'tcx> {
 473             tcx: TyCtxt<'tcx>,
 474         }
 475
 476         struct NonTrivialPath;
 477
 478         impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
 479             type Error = NonTrivialPath;
 480
 481             type Path = Vec<String>;
 482             type Region = !;
 483             type Type = !;
 484             type DynExistential = !;
 485             type Const = !;
 486
 487             fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
 488                 self.tcx
 489             }
 490
 491             fn print_region(
 492                 self,
 493                 _region: ty::Region<'_>,
 494             ) -> Result<Self::Region, Self::Error> {
 495                 Err(NonTrivialPath)
 496             }
 497
 498             fn print_type(
 499                 self,
 500                 _ty: Ty<'tcx>,
 501             ) -> Result<Self::Type, Self::Error> {
 502                 Err(NonTrivialPath)
 503             }
 504
 505             fn print_dyn_existential(
 506                 self,
 507                 _predicates: &'tcx ty::List<ty::ExistentialPredicate<'tcx>>,
 508             ) -> Result<Self::DynExistential, Self::Error> {
 509                 Err(NonTrivialPath)
 510             }
 511
 512             fn print_const(
 513                 self,
 514                 _ct: &'tcx ty::Const<'tcx>,
 515             ) -> Result<Self::Const, Self::Error> {
 516                 Err(NonTrivialPath)
 517             }
 518
 519             fn path_crate(
 520                 self,
 521                 cnum: CrateNum,
 522             ) -> Result<Self::Path, Self::Error> {
 523                 Ok(vec![self.tcx.original_crate_name(cnum).to_string()])
 524             }
 525             fn path_qualified(
 526                 self,
 527                 _self_ty: Ty<'tcx>,
 528                 _trait_ref: Option<ty::TraitRef<'tcx>>,
 529             ) -> Result<Self::Path, Self::Error> {
 530                 Err(NonTrivialPath)
 531             }
 532
 533             fn path_append_impl(
 534                 self,
 535                 _print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 536                 _disambiguated_data: &DisambiguatedDefPathData,
 537                 _self_ty: Ty<'tcx>,
 538                 _trait_ref: Option<ty::TraitRef<'tcx>>,
 539             ) -> Result<Self::Path, Self::Error> {
 540                 Err(NonTrivialPath)
 541             }
 542             fn path_append(
 543                 self,
 544                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 545                 disambiguated_data: &DisambiguatedDefPathData,
 546             ) -> Result<Self::Path, Self::Error> {
 547                 let mut path = print_prefix(self)?;
 548                 path.push(disambiguated_data.data.as_symbol().to_string());
 549                 Ok(path)
 550             }
 551             fn path_generic_args(
 552                 self,
 553                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 554                 _args: &[GenericArg<'tcx>],
 555             ) -> Result<Self::Path, Self::Error> {
 556                 print_prefix(self)
 557             }
 558         }
 559
 560         let report_path_match = |err: &mut DiagnosticBuilder<'_>, did1: DefId, did2: DefId| {
 561             // Only external crates, if either is from a local
 562             // module we could have false positives
 563             if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
 564                 let abs_path = |def_id| {
 565                     AbsolutePathPrinter { tcx: self.tcx }
 566                         .print_def_path(def_id, &[])
 567                 };
 568
 569                 // We compare strings because DefPath can be different
 570                 // for imported and non-imported crates
 571                 let same_path = || -> Result<_, NonTrivialPath> {
 572                     Ok(
 573                         self.tcx.def_path_str(did1) == self.tcx.def_path_str(did2) ||
 574                         abs_path(did1)? == abs_path(did2)?
 575                     )
 576                 };
 577                 if same_path().unwrap_or(false) {
 578                     let crate_name = self.tcx.crate_name(did1.krate);
 579                     err.note(&format!(
 580                         "perhaps two different versions of crate `{}` are being used?",
 581                         crate_name
 582                     ));
 583                 }
 584             }
 585         };
 586         match *terr {
 587             TypeError::Sorts(ref exp_found) => {
 588                 // if they are both "path types", there's a chance of ambiguity
 589                 // due to different versions of the same crate
 590                 if let (&ty::Adt(exp_adt, _), &ty::Adt(found_adt, _))
 591                      = (&exp_found.expected.kind, &exp_found.found.kind)
 592                 {
 593                     report_path_match(err, exp_adt.did, found_adt.did);
 594                 }
 595             }
 596             TypeError::Traits(ref exp_found) => {
 597                 report_path_match(err, exp_found.expected, exp_found.found);
 598             }
 599             _ => (), // FIXME(#22750) handle traits and stuff
 600         }
 601     }
 602
 603     fn note_error_origin(
 604         &self,
 605         err: &mut DiagnosticBuilder<'tcx>,
 606         cause: &ObligationCause<'tcx>,
 607         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
 608     ) {
 609         match cause.code {
 610             ObligationCauseCode::MatchExpressionArmPattern { span, ty } => {
 611                 if ty.is_suggestable() {  // don't show type `_`
 612                     err.span_label(span, format!("this match expression has type `{}`", ty));
 613                 }
 614                 if let Some(ty::error::ExpectedFound { found, .. }) = exp_found {
 615                     if ty.is_box() && ty.boxed_ty() == found {
 616                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 617                             err.span_suggestion(
 618                                 span,
 619                                 "consider dereferencing the boxed value",
 620                                 format!("*{}", snippet),
 621                                 Applicability::MachineApplicable,
 622                             );
 623                         }
 624                     }
 625                 }
 626             }
 627             ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
 628                 source,
 629                 ref prior_arms,
 630                 last_ty,
 631                 discrim_hir_id,
 632                 ..
 633             }) => match source {
 634                 hir::MatchSource::IfLetDesugar { .. } => {
 635                     let msg = "`if let` arms have incompatible types";
 636                     err.span_label(cause.span, msg);
 637                 }
 638                 hir::MatchSource::TryDesugar => {
 639                     if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
 640                         let discrim_expr = self.tcx.hir().expect_expr(discrim_hir_id);
 641                         let discrim_ty = if let hir::ExprKind::Call(_, args) = &discrim_expr.kind {
 642                             let arg_expr = args.first().expect("try desugaring call w/out arg");
 643                             self.in_progress_tables.and_then(|tables| {
 644                                 tables.borrow().expr_ty_opt(arg_expr)
 645                             })
 646                         } else {
 647                             bug!("try desugaring w/out call expr as discriminant");
 648                         };
 649
 650                         match discrim_ty {
 651                             Some(ty) if expected == ty => {
 652                                 let source_map = self.tcx.sess.source_map();
 653                                 err.span_suggestion(
 654                                     source_map.end_point(cause.span),
 655                                     "try removing this `?`",
 656                                     "".to_string(),
 657                                     Applicability::MachineApplicable,
 658                                 );
 659                             },
 660                             _ => {},
 661                         }
 662                     }
 663                 }
 664                 _ => {
 665                     // `last_ty` can be `!`, `expected` will have better info when present.
 666                     let t = self.resolve_vars_if_possible(&match exp_found {
 667                         Some(ty::error::ExpectedFound { expected, .. }) => expected,
 668                         _ => last_ty,
 669                     });
 670                     let msg = "`match` arms have incompatible types";
 671                     err.span_label(cause.span, msg);
 672                     if prior_arms.len() <= 4 {
 673                         for sp in prior_arms {
 674                             err.span_label( *sp, format!("this is found to be of type `{}`", t));
 675                         }
 676                     } else if let Some(sp) = prior_arms.last() {
 677                         err.span_label(
 678                             *sp,
 679                             format!("this and all prior arms are found to be of type `{}`", t),
 680                         );
 681                     }
 682                 }
 683             },
 684             ObligationCauseCode::IfExpression(box IfExpressionCause { then, outer, semicolon }) => {
 685                 err.span_label(then, "expected because of this");
 686                 outer.map(|sp| err.span_label(sp, "if and else have incompatible types"));
 687                 if let Some(sp) = semicolon {
 688                     err.span_suggestion_short(
 689                         sp,
 690                         "consider removing this semicolon",
 691                         String::new(),
 692                         Applicability::MachineApplicable,
 693                     );
 694                 }
 695             }
 696             _ => (),
 697         }
 698     }
 699
 700     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
 701     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
 702     /// populate `other_value` with `other_ty`.
 703     ///
 704     /// ```text
 705     /// Foo<Bar<Qux>>
 706     /// ^^^^--------^ this is highlighted
 707     /// |   |
 708     /// |   this type argument is exactly the same as the other type, not highlighted
 709     /// this is highlighted
 710     /// Bar<Qux>
 711     /// -------- this type is the same as a type argument in the other type, not highlighted
 712     /// ```
 713     fn highlight_outer(
 714         &self,
 715         value: &mut DiagnosticStyledString,
 716         other_value: &mut DiagnosticStyledString,
 717         name: String,
 718         sub: ty::subst::SubstsRef<'tcx>,
 719         pos: usize,
 720         other_ty: Ty<'tcx>,
 721     ) {
 722         // `value` and `other_value` hold two incomplete type representation for display.
 723         // `name` is the path of both types being compared. `sub`
 724         value.push_highlighted(name);
 725         let len = sub.len();
 726         if len > 0 {
 727             value.push_highlighted("<");
 728         }
 729
 730         // Output the lifetimes for the first type
 731         let lifetimes = sub.regions()
 732             .map(|lifetime| {
 733                 let s = lifetime.to_string();
 734                 if s.is_empty() {
 735                     "'_".to_string()
 736                 } else {
 737                     s
 738                 }
 739             })
 740             .collect::<Vec<_>>()
 741             .join(", ");
 742         if !lifetimes.is_empty() {
 743             if sub.regions().count() < len {
 744                 value.push_normal(lifetimes + &", ");
 745             } else {
 746                 value.push_normal(lifetimes);
 747             }
 748         }
 749
 750         // Highlight all the type arguments that aren't at `pos` and compare the type argument at
 751         // `pos` and `other_ty`.
 752         for (i, type_arg) in sub.types().enumerate() {
 753             if i == pos {
 754                 let values = self.cmp(type_arg, other_ty);
 755                 value.0.extend((values.0).0);
 756                 other_value.0.extend((values.1).0);
 757             } else {
 758                 value.push_highlighted(type_arg.to_string());
 759             }
 760
 761             if len > 0 && i != len - 1 {
 762                 value.push_normal(", ");
 763             }
 764         }
 765         if len > 0 {
 766             value.push_highlighted(">");
 767         }
 768     }
 769
 770     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 771     /// as that is the difference to the other type.
 772     ///
 773     /// For the following code:
 774     ///
 775     /// ```norun
 776     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 777     /// ```
 778     ///
 779     /// The type error output will behave in the following way:
 780     ///
 781     /// ```text
 782     /// Foo<Bar<Qux>>
 783     /// ^^^^--------^ this is highlighted
 784     /// |   |
 785     /// |   this type argument is exactly the same as the other type, not highlighted
 786     /// this is highlighted
 787     /// Bar<Qux>
 788     /// -------- this type is the same as a type argument in the other type, not highlighted
 789     /// ```
 790     fn cmp_type_arg(
 791         &self,
 792         mut t1_out: &mut DiagnosticStyledString,
 793         mut t2_out: &mut DiagnosticStyledString,
 794         path: String,
 795         sub: ty::subst::SubstsRef<'tcx>,
 796         other_path: String,
 797         other_ty: Ty<'tcx>,
 798     ) -> Option<()> {
 799         for (i, ta) in sub.types().enumerate() {
 800             if ta == other_ty {
 801                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 802                 return Some(());
 803             }
 804             if let &ty::Adt(def, _) = &ta.kind {
 805                 let path_ = self.tcx.def_path_str(def.did.clone());
 806                 if path_ == other_path {
 807                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 808                     return Some(());
 809                 }
 810             }
 811         }
 812         None
 813     }
 814
 815     /// Adds a `,` to the type representation only if it is appropriate.
 816     fn push_comma(
 817         &self,
 818         value: &mut DiagnosticStyledString,
 819         other_value: &mut DiagnosticStyledString,
 820         len: usize,
 821         pos: usize,
 822     ) {
 823         if len > 0 && pos != len - 1 {
 824             value.push_normal(", ");
 825             other_value.push_normal(", ");
 826         }
 827     }
 828
 829     /// For generic types with parameters with defaults, remove the parameters corresponding to
 830     /// the defaults. This repeats a lot of the logic found in `ty::print::pretty`.
 831     fn strip_generic_default_params(
 832         &self,
 833         def_id: DefId,
 834         substs: ty::subst::SubstsRef<'tcx>,
 835     ) -> SubstsRef<'tcx> {
 836         let generics = self.tcx.generics_of(def_id);
 837         let mut num_supplied_defaults = 0;
 838         let mut type_params = generics.params.iter().rev().filter_map(|param| match param.kind {
 839             ty::GenericParamDefKind::Lifetime => None,
 840             ty::GenericParamDefKind::Type { has_default, .. } => Some((param.def_id, has_default)),
 841             ty::GenericParamDefKind::Const => None, // FIXME(const_generics:defaults)
 842         }).peekable();
 843         let has_default = {
 844             let has_default = type_params.peek().map(|(_, has_default)| has_default);
 845             *has_default.unwrap_or(&false)
 846         };
 847         if has_default {
 848             let types = substs.types().rev();
 849             for ((def_id, has_default), actual) in type_params.zip(types) {
 850                 if !has_default {
 851                     break;
 852                 }
 853                 if self.tcx.type_of(def_id).subst(self.tcx, substs) != actual {
 854                     break;
 855                 }
 856                 num_supplied_defaults += 1;
 857             }
 858         }
 859         let len = generics.params.len();
 860         let mut generics = generics.clone();
 861         generics.params.truncate(len - num_supplied_defaults);
 862         substs.truncate_to(self.tcx, &generics)
 863     }
 864
 865     /// Given two `fn` signatures highlight only sub-parts that are different.
 866     fn cmp_fn_sig(
 867         &self,
 868         sig1: &ty::PolyFnSig<'tcx>,
 869         sig2: &ty::PolyFnSig<'tcx>,
 870     ) -> (DiagnosticStyledString, DiagnosticStyledString) {
 871         let get_lifetimes = |sig| {
 872             use crate::hir::def::Namespace;
 873             let mut s = String::new();
 874             let (_, (sig, reg)) = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS)
 875                 .name_all_regions(sig)
 876                 .unwrap();
 877             let lts: Vec<String> = reg.into_iter().map(|(_, kind)| kind.to_string()).collect();
 878             (if lts.is_empty() {
 879                 String::new()
 880             } else {
 881                 format!("for<{}> ", lts.join(", "))
 882             }, sig)
 883         };
 884
 885         let (lt1, sig1) = get_lifetimes(sig1);
 886         let (lt2, sig2) = get_lifetimes(sig2);
 887
 888         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 889         let mut values = (
 890             DiagnosticStyledString::normal("".to_string()),
 891             DiagnosticStyledString::normal("".to_string()),
 892         );
 893
 894         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 895         // ^^^^^^
 896         values.0.push(sig1.unsafety.prefix_str(), sig1.unsafety != sig2.unsafety);
 897         values.1.push(sig2.unsafety.prefix_str(), sig1.unsafety != sig2.unsafety);
 898
 899         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 900         //        ^^^^^^^^^^
 901         if sig1.abi != abi::Abi::Rust {
 902             values.0.push(format!("extern {} ", sig1.abi), sig1.abi != sig2.abi);
 903         }
 904         if sig2.abi != abi::Abi::Rust {
 905             values.1.push(format!("extern {} ", sig2.abi), sig1.abi != sig2.abi);
 906         }
 907
 908         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 909         //                   ^^^^^^^^
 910         let lifetime_diff = lt1 != lt2;
 911         values.0.push(lt1, lifetime_diff);
 912         values.1.push(lt2, lifetime_diff);
 913
 914         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 915         //                           ^^^
 916         values.0.push_normal("fn(");
 917         values.1.push_normal("fn(");
 918
 919         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 920         //                              ^^^^^
 921         let len1 = sig1.inputs().len();
 922         let len2 = sig2.inputs().len();
 923         if len1 == len2 {
 924             for (i, (l, r)) in sig1.inputs().iter().zip(sig2.inputs().iter()).enumerate() {
 925                 let (x1, x2) = self.cmp(l, r);
 926                 (values.0).0.extend(x1.0);
 927                 (values.1).0.extend(x2.0);
 928                 self.push_comma(&mut values.0, &mut values.1, len1, i);
 929             }
 930         } else {
 931             for (i, l) in sig1.inputs().iter().enumerate() {
 932                 values.0.push_highlighted(l.to_string());
 933                 if i != len1 - 1 {
 934                     values.0.push_highlighted(", ");
 935                 }
 936             }
 937             for (i, r) in sig2.inputs().iter().enumerate() {
 938                 values.1.push_highlighted(r.to_string());
 939                 if i != len2 - 1 {
 940                     values.1.push_highlighted(", ");
 941                 }
 942             }
 943         }
 944
 945         if sig1.c_variadic {
 946             if len1 > 0 {
 947                 values.0.push_normal(", ");
 948             }
 949             values.0.push("...", !sig2.c_variadic);
 950         }
 951         if sig2.c_variadic {
 952             if len2 > 0 {
 953                 values.1.push_normal(", ");
 954             }
 955             values.1.push("...", !sig1.c_variadic);
 956         }
 957
 958         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 959         //                                   ^
 960         values.0.push_normal(")");
 961         values.1.push_normal(")");
 962
 963         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 964         //                                     ^^^^^^^^
 965         let output1 = sig1.output();
 966         let output2 = sig2.output();
 967         let (x1, x2) = self.cmp(output1, output2);
 968         if !output1.is_unit() {
 969             values.0.push_normal(" -> ");
 970             (values.0).0.extend(x1.0);
 971         }
 972         if !output2.is_unit() {
 973             values.1.push_normal(" -> ");
 974             (values.1).0.extend(x2.0);
 975         }
 976         values
 977     }
 978
 979     /// Compares two given types, eliding parts that are the same between them and highlighting
 980     /// relevant differences, and return two representation of those types for highlighted printing.
 981     fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> (DiagnosticStyledString, DiagnosticStyledString) {
 982         debug!("cmp(t1={}, t1.kind={:?}, t2={}, t2.kind={:?})", t1, t1.kind, t2, t2.kind);
 983
 984         // helper functions
 985         fn equals<'tcx>(a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 986             match (&a.kind, &b.kind) {
 987                 (a, b) if *a == *b => true,
 988                 (&ty::Int(_), &ty::Infer(ty::InferTy::IntVar(_)))
 989                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Int(_))
 990                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Infer(ty::InferTy::IntVar(_)))
 991                 | (&ty::Float(_), &ty::Infer(ty::InferTy::FloatVar(_)))
 992                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Float(_))
 993                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Infer(ty::InferTy::FloatVar(_))) => {
 994                     true
 995                 }
 996                 _ => false,
 997             }
 998         }
 999
1000         fn push_ty_ref<'tcx>(
1001             r: &ty::Region<'tcx>,
1002             ty: Ty<'tcx>,
1003             mutbl: hir::Mutability,
1004             s: &mut DiagnosticStyledString,
1005         ) {
1006             let mut r = r.to_string();
1007             if r == "'_" {
1008                 r.clear();
1009             } else {
1010                 r.push(' ');
1011             }
1012             s.push_highlighted(format!("&{}{}", r, mutbl.prefix_str()));
1013             s.push_normal(ty.to_string());
1014         }
1015
1016         // process starts here
1017         match (&t1.kind, &t2.kind) {
1018             (&ty::Adt(def1, sub1), &ty::Adt(def2, sub2)) => {
1019                 let sub_no_defaults_1 = self.strip_generic_default_params(def1.did, sub1);
1020                 let sub_no_defaults_2 = self.strip_generic_default_params(def2.did, sub2);
1021                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1022                 let path1 = self.tcx.def_path_str(def1.did.clone());
1023                 let path2 = self.tcx.def_path_str(def2.did.clone());
1024                 if def1.did == def2.did {
1025                     // Easy case. Replace same types with `_` to shorten the output and highlight
1026                     // the differing ones.
1027                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
1028                     //     Foo<Bar, _>
1029                     //     Foo<Quz, _>
1030                     //         ---  ^ type argument elided
1031                     //         |
1032                     //         highlighted in output
1033                     values.0.push_normal(path1);
1034                     values.1.push_normal(path2);
1035
1036                     // Avoid printing out default generic parameters that are common to both
1037                     // types.
1038                     let len1 = sub_no_defaults_1.len();
1039                     let len2 = sub_no_defaults_2.len();
1040                     let common_len = cmp::min(len1, len2);
1041                     let remainder1: Vec<_> = sub1.types().skip(common_len).collect();
1042                     let remainder2: Vec<_> = sub2.types().skip(common_len).collect();
1043                     let common_default_params = remainder1
1044                         .iter()
1045                         .rev()
1046                         .zip(remainder2.iter().rev())
1047                         .filter(|(a, b)| a == b)
1048                         .count();
1049                     let len = sub1.len() - common_default_params;
1050                     let consts_offset = len - sub1.consts().count();
1051
1052                     // Only draw `<...>` if there're lifetime/type arguments.
1053                     if len > 0 {
1054                         values.0.push_normal("<");
1055                         values.1.push_normal("<");
1056                     }
1057
1058                     fn lifetime_display(lifetime: Region<'_>) -> String {
1059                         let s = lifetime.to_string();
1060                         if s.is_empty() {
1061                             "'_".to_string()
1062                         } else {
1063                             s
1064                         }
1065                     }
1066                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
1067                     // all diagnostics that use this output
1068                     //
1069                     //     Foo<'x, '_, Bar>
1070                     //     Foo<'y, '_, Qux>
1071                     //         ^^  ^^  --- type arguments are not elided
1072                     //         |   |
1073                     //         |   elided as they were the same
1074                     //         not elided, they were different, but irrelevant
1075                     let lifetimes = sub1.regions().zip(sub2.regions());
1076                     for (i, lifetimes) in lifetimes.enumerate() {
1077                         let l1 = lifetime_display(lifetimes.0);
1078                         let l2 = lifetime_display(lifetimes.1);
1079                         if lifetimes.0 == lifetimes.1 {
1080                             values.0.push_normal("'_");
1081                             values.1.push_normal("'_");
1082                         } else {
1083                             values.0.push_highlighted(l1);
1084                             values.1.push_highlighted(l2);
1085                         }
1086                         self.push_comma(&mut values.0, &mut values.1, len, i);
1087                     }
1088
1089                     // We're comparing two types with the same path, so we compare the type
1090                     // arguments for both. If they are the same, do not highlight and elide from the
1091                     // output.
1092                     //     Foo<_, Bar>
1093                     //     Foo<_, Qux>
1094                     //         ^ elided type as this type argument was the same in both sides
1095                     let type_arguments = sub1.types().zip(sub2.types());
1096                     let regions_len = sub1.regions().count();
1097                     let num_display_types = consts_offset - regions_len;
1098                     for (i, (ta1, ta2)) in type_arguments.take(num_display_types).enumerate() {
1099                         let i = i + regions_len;
1100                         if ta1 == ta2 {
1101                             values.0.push_normal("_");
1102                             values.1.push_normal("_");
1103                         } else {
1104                             let (x1, x2) = self.cmp(ta1, ta2);
1105                             (values.0).0.extend(x1.0);
1106                             (values.1).0.extend(x2.0);
1107                         }
1108                         self.push_comma(&mut values.0, &mut values.1, len, i);
1109                     }
1110
1111                     // Do the same for const arguments, if they are equal, do not highlight and
1112                     // elide them from the output.
1113                     let const_arguments = sub1.consts().zip(sub2.consts());
1114                     for (i, (ca1, ca2)) in const_arguments.enumerate() {
1115                         let i = i + consts_offset;
1116                         if ca1 == ca2 {
1117                             values.0.push_normal("_");
1118                             values.1.push_normal("_");
1119                         } else {
1120                             values.0.push_highlighted(ca1.to_string());
1121                             values.1.push_highlighted(ca2.to_string());
1122                         }
1123                         self.push_comma(&mut values.0, &mut values.1, len, i);
1124                     }
1125
1126                     // Close the type argument bracket.
1127                     // Only draw `<...>` if there're lifetime/type arguments.
1128                     if len > 0 {
1129                         values.0.push_normal(">");
1130                         values.1.push_normal(">");
1131                     }
1132                     values
1133                 } else {
1134                     // Check for case:
1135                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
1136                     //     Foo<Bar<Qux>
1137                     //         ------- this type argument is exactly the same as the other type
1138                     //     Bar<Qux>
1139                     if self.cmp_type_arg(
1140                         &mut values.0,
1141                         &mut values.1,
1142                         path1.clone(),
1143                         sub_no_defaults_1,
1144                         path2.clone(),
1145                         &t2,
1146                     ).is_some()
1147                     {
1148                         return values;
1149                     }
1150                     // Check for case:
1151                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
1152                     //     Bar<Qux>
1153                     //     Foo<Bar<Qux>>
1154                     //         ------- this type argument is exactly the same as the other type
1155                     if self.cmp_type_arg(
1156                         &mut values.1,
1157                         &mut values.0,
1158                         path2,
1159                         sub_no_defaults_2,
1160                         path1,
1161                         &t1,
1162                     ).is_some()
1163                     {
1164                         return values;
1165                     }
1166
1167                     // We can't find anything in common, highlight relevant part of type path.
1168                     //     let x: foo::bar::Baz<Qux> = y:<foo::bar::Bar<Zar>>();
1169                     //     foo::bar::Baz<Qux>
1170                     //     foo::bar::Bar<Zar>
1171                     //               -------- this part of the path is different
1172
1173                     let t1_str = t1.to_string();
1174                     let t2_str = t2.to_string();
1175                     let min_len = t1_str.len().min(t2_str.len());
1176
1177                     const SEPARATOR: &str = "::";
1178                     let separator_len = SEPARATOR.len();
1179                     let split_idx: usize =
1180                         t1_str.split(SEPARATOR)
1181                             .zip(t2_str.split(SEPARATOR))
1182                             .take_while(|(mod1_str, mod2_str)| mod1_str == mod2_str)
1183                             .map(|(mod_str, _)| mod_str.len() + separator_len)
1184                             .sum();
1185
1186                     debug!("cmp: separator_len={}, split_idx={}, min_len={}",
1187                         separator_len, split_idx, min_len
1188                     );
1189
1190                     if split_idx >= min_len {
1191                         // paths are identical, highlight everything
1192                         (
1193                             DiagnosticStyledString::highlighted(t1_str),
1194                             DiagnosticStyledString::highlighted(t2_str)
1195                         )
1196                     } else {
1197                         let (common, uniq1) = t1_str.split_at(split_idx);
1198                         let (_, uniq2) = t2_str.split_at(split_idx);
1199                         debug!("cmp: common={}, uniq1={}, uniq2={}", common, uniq1, uniq2);
1200
1201                         values.0.push_normal(common);
1202                         values.0.push_highlighted(uniq1);
1203                         values.1.push_normal(common);
1204                         values.1.push_highlighted(uniq2);
1205
1206                         values
1207                     }
1208                 }
1209             }
1210
1211             // When finding T != &T, highlight only the borrow
1212             (&ty::Ref(r1, ref_ty1, mutbl1), _) if equals(&ref_ty1, &t2) => {
1213                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1214                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
1215                 values.1.push_normal(t2.to_string());
1216                 values
1217             }
1218             (_, &ty::Ref(r2, ref_ty2, mutbl2)) if equals(&t1, &ref_ty2) => {
1219                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1220                 values.0.push_normal(t1.to_string());
1221                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
1222                 values
1223             }
1224
1225             // When encountering &T != &mut T, highlight only the borrow
1226             (&ty::Ref(r1, ref_ty1, mutbl1), &ty::Ref(r2, ref_ty2, mutbl2))
1227                 if equals(&ref_ty1, &ref_ty2) =>
1228             {
1229                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1230                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
1231                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
1232                 values
1233             }
1234
1235             // When encountering tuples of the same size, highlight only the differing types
1236             (&ty::Tuple(substs1), &ty::Tuple(substs2)) if substs1.len() == substs2.len() => {
1237                 let mut values = (
1238                     DiagnosticStyledString::normal("("),
1239                     DiagnosticStyledString::normal("("),
1240                 );
1241                 let len = substs1.len();
1242                 for (i, (left, right)) in substs1.types().zip(substs2.types()).enumerate() {
1243                     let (x1, x2) = self.cmp(left, right);
1244                     (values.0).0.extend(x1.0);
1245                     (values.1).0.extend(x2.0);
1246                     self.push_comma(&mut values.0, &mut values.1, len, i);
1247                 }
1248                 if len == 1 { // Keep the output for single element tuples as `(ty,)`.
1249                     values.0.push_normal(",");
1250                     values.1.push_normal(",");
1251                 }
1252                 values.0.push_normal(")");
1253                 values.1.push_normal(")");
1254                 values
1255             }
1256
1257             (ty::FnDef(did1, substs1), ty::FnDef(did2, substs2)) => {
1258                 let sig1 = self.tcx.fn_sig(*did1).subst(self.tcx, substs1);
1259                 let sig2 = self.tcx.fn_sig(*did2).subst(self.tcx, substs2);
1260                 let mut values = self.cmp_fn_sig(&sig1, &sig2);
1261                 let path1 = format!(" {{{}}}", self.tcx.def_path_str_with_substs(*did1, substs1));
1262                 let path2 = format!(" {{{}}}", self.tcx.def_path_str_with_substs(*did2, substs2));
1263                 let same_path = path1 == path2;
1264                 values.0.push(path1, !same_path);
1265                 values.1.push(path2, !same_path);
1266                 values
1267             }
1268
1269             (ty::FnDef(did1, substs1), ty::FnPtr(sig2)) => {
1270                 let sig1 = self.tcx.fn_sig(*did1).subst(self.tcx, substs1);
1271                 let mut values = self.cmp_fn_sig(&sig1, sig2);
1272                 values.0.push_normal(format!(
1273                     " {{{}}}",
1274                     self.tcx.def_path_str_with_substs(*did1, substs1)),
1275                 );
1276                 values
1277             }
1278
1279             (ty::FnPtr(sig1), ty::FnDef(did2, substs2)) => {
1280                 let sig2 = self.tcx.fn_sig(*did2).subst(self.tcx, substs2);
1281                 let mut values = self.cmp_fn_sig(sig1, &sig2);
1282                 values.1.push_normal(format!(
1283                     " {{{}}}",
1284                     self.tcx.def_path_str_with_substs(*did2, substs2)),
1285                 );
1286                 values
1287             }
1288
1289             (ty::FnPtr(sig1), ty::FnPtr(sig2)) => {
1290                 self.cmp_fn_sig(sig1, sig2)
1291             }
1292
1293             _ => {
1294                 if t1 == t2 {
1295                     // The two types are the same, elide and don't highlight.
1296                     (
1297                         DiagnosticStyledString::normal("_"),
1298                         DiagnosticStyledString::normal("_"),
1299                     )
1300                 } else {
1301                     // We couldn't find anything in common, highlight everything.
1302                     (
1303                         DiagnosticStyledString::highlighted(t1.to_string()),
1304                         DiagnosticStyledString::highlighted(t2.to_string()),
1305                     )
1306                 }
1307             }
1308         }
1309     }
1310
1311     pub fn note_type_err(
1312         &self,
1313         diag: &mut DiagnosticBuilder<'tcx>,
1314         cause: &ObligationCause<'tcx>,
1315         secondary_span: Option<(Span, String)>,
1316         mut values: Option<ValuePairs<'tcx>>,
1317         terr: &TypeError<'tcx>,
1318     ) {
1319         // For some types of errors, expected-found does not make
1320         // sense, so just ignore the values we were given.
1321         match terr {
1322             TypeError::CyclicTy(_) => {
1323                 values = None;
1324             }
1325             _ => {}
1326         }
1327
1328         debug!("note_type_err(diag={:?})", diag);
1329         let (expected_found, exp_found, is_simple_error) = match values {
1330             None => (None, None, false),
1331             Some(values) => {
1332                 let (is_simple_error, exp_found) = match values {
1333                     ValuePairs::Types(exp_found) => {
1334                         let is_simple_err = exp_found.expected.is_simple_text()
1335                             && exp_found.found.is_simple_text();
1336
1337                         (is_simple_err, Some(exp_found))
1338                     }
1339                     _ => (false, None),
1340                 };
1341                 let vals = match self.values_str(&values) {
1342                     Some((expected, found)) => Some((expected, found)),
1343                     None => {
1344                         // Derived error. Cancel the emitter.
1345                         diag.cancel();
1346                         return;
1347                     }
1348                 };
1349                 (vals, exp_found, is_simple_error)
1350             }
1351         };
1352
1353         let span = cause.span(self.tcx);
1354
1355         // Ignore msg for object safe coercion
1356         // since E0038 message will be printed
1357         match terr {
1358             TypeError::ObjectUnsafeCoercion(_) => {}
1359             _ => {
1360                 diag.span_label(span, terr.to_string());
1361                 if let Some((sp, msg)) = secondary_span {
1362                     diag.span_label(sp, msg);
1363                 }
1364             }
1365         };
1366
1367         if let Some((expected, found)) = expected_found {
1368             let expected_label = exp_found.map_or("type".into(), |ef| ef.expected.prefix_string());
1369             let found_label = exp_found.map_or("type".into(), |ef| ef.found.prefix_string());
1370             match (&terr, expected == found) {
1371                 (TypeError::Sorts(values), extra) => {
1372                     let sort_string = |ty: Ty<'tcx>| match (extra, &ty.kind) {
1373                         (true, ty::Opaque(def_id, _)) => format!(
1374                             " (opaque type at {})",
1375                             self.tcx.sess.source_map()
1376                                 .mk_substr_filename(self.tcx.def_span(*def_id)),
1377                         ),
1378                         (true, _) => format!(" ({})", ty.sort_string(self.tcx)),
1379                         (false, _) => "".to_string(),
1380                     };
1381                     if !(values.expected.is_simple_text() && values.found.is_simple_text()) || (
1382                         exp_found.map_or(false, |ef| {
1383                             // This happens when the type error is a subset of the expectation,
1384                             // like when you have two references but one is `usize` and the other
1385                             // is `f32`. In those cases we still want to show the `note`. If the
1386                             // value from `ef` is `Infer(_)`, then we ignore it.
1387                             if !ef.expected.is_ty_infer() {
1388                                 ef.expected != values.expected
1389                             } else if !ef.found.is_ty_infer() {
1390                                 ef.found != values.found
1391                             } else {
1392                                 false
1393                             }
1394                         })
1395                     ) {
1396                         diag.note_expected_found_extra(
1397                             &expected_label,
1398                             expected,
1399                             &found_label,
1400                             found,
1401                             &sort_string(values.expected),
1402                             &sort_string(values.found),
1403                         );
1404                     }
1405                 }
1406                 (TypeError::ObjectUnsafeCoercion(_), _) => {
1407                     diag.note_unsuccessfull_coercion(found, expected);
1408                 }
1409                 (_, _) => {
1410                     debug!(
1411                         "note_type_err: exp_found={:?}, expected={:?} found={:?}",
1412                         exp_found, expected, found
1413                     );
1414                     if !is_simple_error || terr.must_include_note() {
1415                         diag.note_expected_found(&expected_label, expected, &found_label, found);
1416                     }
1417                 }
1418             }
1419         }
1420         if let Some(exp_found) = exp_found {
1421             self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
1422         }
1423
1424         // In some (most?) cases cause.body_id points to actual body, but in some cases
1425         // it's a actual definition. According to the comments (e.g. in
1426         // librustc_typeck/check/compare_method.rs:compare_predicate_entailment) the latter
1427         // is relied upon by some other code. This might (or might not) need cleanup.
1428         let body_owner_def_id = self.tcx.hir().opt_local_def_id(cause.body_id)
1429             .unwrap_or_else(|| {
1430                 self.tcx.hir().body_owner_def_id(hir::BodyId { hir_id: cause.body_id })
1431             });
1432         self.check_and_note_conflicting_crates(diag, terr);
1433         self.tcx.note_and_explain_type_err(diag, terr, span, body_owner_def_id);
1434
1435         // It reads better to have the error origin as the final
1436         // thing.
1437         self.note_error_origin(diag, &cause, exp_found);
1438     }
1439
1440     /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
1441     /// suggests it.
1442     fn suggest_as_ref_where_appropriate(
1443         &self,
1444         span: Span,
1445         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1446         diag: &mut DiagnosticBuilder<'tcx>,
1447     ) {
1448         match (&exp_found.expected.kind, &exp_found.found.kind) {
1449             (ty::Adt(exp_def, exp_substs), ty::Ref(_, found_ty, _)) => {
1450                 if let ty::Adt(found_def, found_substs) = found_ty.kind {
1451                     let path_str = format!("{:?}", exp_def);
1452                     if exp_def == &found_def {
1453                         let opt_msg = "you can convert from `&Option<T>` to `Option<&T>` using \
1454                                        `.as_ref()`";
1455                         let result_msg = "you can convert from `&Result<T, E>` to \
1456                                           `Result<&T, &E>` using `.as_ref()`";
1457                         let have_as_ref = &[
1458                             ("std::option::Option", opt_msg),
1459                             ("core::option::Option", opt_msg),
1460                             ("std::result::Result", result_msg),
1461                             ("core::result::Result", result_msg),
1462                         ];
1463                         if let Some(msg) = have_as_ref.iter()
1464                             .filter_map(|(path, msg)| if &path_str == path {
1465                                 Some(msg)
1466                             } else {
1467                                 None
1468                             }).next()
1469                         {
1470                             let mut show_suggestion = true;
1471                             for (exp_ty, found_ty) in exp_substs.types().zip(found_substs.types()) {
1472                                 match exp_ty.kind {
1473                                     ty::Ref(_, exp_ty, _) => {
1474                                         match (&exp_ty.kind, &found_ty.kind) {
1475                                             (_, ty::Param(_)) |
1476                                             (_, ty::Infer(_)) |
1477                                             (ty::Param(_), _) |
1478                                             (ty::Infer(_), _) => {}
1479                                             _ if ty::TyS::same_type(exp_ty, found_ty) => {}
1480                                             _ => show_suggestion = false,
1481                                         };
1482                                     }
1483                                     ty::Param(_) | ty::Infer(_) => {}
1484                                     _ => show_suggestion = false,
1485                                 }
1486                             }
1487                             if let (Ok(snippet), true) = (
1488                                 self.tcx.sess.source_map().span_to_snippet(span),
1489                                 show_suggestion,
1490                             ) {
1491                                 diag.span_suggestion(
1492                                     span,
1493                                     msg,
1494                                     format!("{}.as_ref()", snippet),
1495                                     Applicability::MachineApplicable,
1496                                 );
1497                             }
1498                         }
1499                     }
1500                 }
1501             }
1502             _ => {}
1503         }
1504     }
1505
1506     pub fn report_and_explain_type_error(
1507         &self,
1508         trace: TypeTrace<'tcx>,
1509         terr: &TypeError<'tcx>,
1510     ) -> DiagnosticBuilder<'tcx> {
1511         debug!(
1512             "report_and_explain_type_error(trace={:?}, terr={:?})",
1513             trace, terr
1514         );
1515
1516         let span = trace.cause.span(self.tcx);
1517         let failure_code = trace.cause.as_failure_code(terr);
1518         let mut diag = match failure_code {
1519             FailureCode::Error0038(did) => {
1520                 let violations = self.tcx.object_safety_violations(did);
1521                 self.tcx.report_object_safety_error(span, did, violations)
1522             }
1523             FailureCode::Error0317(failure_str) => {
1524                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
1525             }
1526             FailureCode::Error0580(failure_str) => {
1527                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
1528             }
1529             FailureCode::Error0308(failure_str) => {
1530                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
1531             }
1532             FailureCode::Error0644(failure_str) => {
1533                 struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
1534             }
1535         };
1536         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
1537         diag
1538     }
1539
1540     fn values_str(
1541         &self,
1542         values: &ValuePairs<'tcx>,
1543     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1544         match *values {
1545             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
1546             infer::Regions(ref exp_found) => self.expected_found_str(exp_found),
1547             infer::Consts(ref exp_found) => self.expected_found_str(exp_found),
1548             infer::TraitRefs(ref exp_found) => {
1549                 let pretty_exp_found = ty::error::ExpectedFound {
1550                     expected: exp_found.expected.print_only_trait_path(),
1551                     found: exp_found.found.print_only_trait_path()
1552                 };
1553                 self.expected_found_str(&pretty_exp_found)
1554             },
1555             infer::PolyTraitRefs(ref exp_found) => {
1556                 let pretty_exp_found = ty::error::ExpectedFound {
1557                     expected: exp_found.expected.print_only_trait_path(),
1558                     found: exp_found.found.print_only_trait_path()
1559                 };
1560                 self.expected_found_str(&pretty_exp_found)
1561             },
1562         }
1563     }
1564
1565     fn expected_found_str_ty(
1566         &self,
1567         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1568     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1569         let exp_found = self.resolve_vars_if_possible(exp_found);
1570         if exp_found.references_error() {
1571             return None;
1572         }
1573
1574         Some(self.cmp(exp_found.expected, exp_found.found))
1575     }
1576
1577     /// Returns a string of the form "expected `{}`, found `{}`".
1578     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
1579         &self,
1580         exp_found: &ty::error::ExpectedFound<T>,
1581     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1582         let exp_found = self.resolve_vars_if_possible(exp_found);
1583         if exp_found.references_error() {
1584             return None;
1585         }
1586
1587         Some((
1588             DiagnosticStyledString::highlighted(exp_found.expected.to_string()),
1589             DiagnosticStyledString::highlighted(exp_found.found.to_string()),
1590         ))
1591     }
1592
1593     pub fn report_generic_bound_failure(
1594         &self,
1595         region_scope_tree: &region::ScopeTree,
1596         span: Span,
1597         origin: Option<SubregionOrigin<'tcx>>,
1598         bound_kind: GenericKind<'tcx>,
1599         sub: Region<'tcx>,
1600     ) {
1601         self.construct_generic_bound_failure(region_scope_tree, span, origin, bound_kind, sub)
1602             .emit()
1603     }
1604
1605     pub fn construct_generic_bound_failure(
1606         &self,
1607         region_scope_tree: &region::ScopeTree,
1608         span: Span,
1609         origin: Option<SubregionOrigin<'tcx>>,
1610         bound_kind: GenericKind<'tcx>,
1611         sub: Region<'tcx>,
1612     ) -> DiagnosticBuilder<'a> {
1613         // Attempt to obtain the span of the parameter so we can
1614         // suggest adding an explicit lifetime bound to it.
1615         let type_param_span = match (self.in_progress_tables, bound_kind) {
1616             (Some(ref table), GenericKind::Param(ref param)) => {
1617                 let table = table.borrow();
1618                 table.local_id_root.and_then(|did| {
1619                     let generics = self.tcx.generics_of(did);
1620                     // Account for the case where `did` corresponds to `Self`, which doesn't have
1621                     // the expected type argument.
1622                     if !(generics.has_self && param.index == 0) {
1623                         let type_param = generics.type_param(param, self.tcx);
1624                         let hir = &self.tcx.hir();
1625                         hir.as_local_hir_id(type_param.def_id).map(|id| {
1626                             // Get the `hir::Param` to verify whether it already has any bounds.
1627                             // We do this to avoid suggesting code that ends up as `T: 'a'b`,
1628                             // instead we suggest `T: 'a + 'b` in that case.
1629                             let mut has_bounds = false;
1630                             if let Node::GenericParam(param) = hir.get(id) {
1631                                 has_bounds = !param.bounds.is_empty();
1632                             }
1633                             let sp = hir.span(id);
1634                             // `sp` only covers `T`, change it so that it covers
1635                             // `T:` when appropriate
1636                             let is_impl_trait = bound_kind.to_string().starts_with("impl ");
1637                             let sp = if has_bounds && !is_impl_trait {
1638                                 sp.to(self.tcx
1639                                     .sess
1640                                     .source_map()
1641                                     .next_point(self.tcx.sess.source_map().next_point(sp)))
1642                             } else {
1643                                 sp
1644                             };
1645                             (sp, has_bounds, is_impl_trait)
1646                         })
1647                     } else {
1648                         None
1649                     }
1650                 })
1651             }
1652             _ => None,
1653         };
1654
1655         let labeled_user_string = match bound_kind {
1656             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
1657             GenericKind::Projection(ref p) => format!("the associated type `{}`", p),
1658         };
1659
1660         if let Some(SubregionOrigin::CompareImplMethodObligation {
1661             span,
1662             item_name,
1663             impl_item_def_id,
1664             trait_item_def_id,
1665         }) = origin
1666         {
1667             return self.report_extra_impl_obligation(
1668                 span,
1669                 item_name,
1670                 impl_item_def_id,
1671                 trait_item_def_id,
1672                 &format!("`{}: {}`", bound_kind, sub),
1673             );
1674         }
1675
1676         fn binding_suggestion<'tcx, S: fmt::Display>(
1677             err: &mut DiagnosticBuilder<'tcx>,
1678             type_param_span: Option<(Span, bool, bool)>,
1679             bound_kind: GenericKind<'tcx>,
1680             sub: S,
1681         ) {
1682             let consider = format!(
1683                 "consider adding an explicit lifetime bound {}",
1684                 if type_param_span.map(|(_, _, is_impl_trait)| is_impl_trait).unwrap_or(false) {
1685                     format!(" `{}` to `{}`...", sub, bound_kind)
1686                 } else {
1687                     format!("`{}: {}`...", bound_kind, sub)
1688                 },
1689             );
1690             if let Some((sp, has_lifetimes, is_impl_trait)) = type_param_span {
1691                 let suggestion = if is_impl_trait {
1692                     format!("{} + {}", bound_kind, sub)
1693                 } else {
1694                     let tail = if has_lifetimes { " + " } else { "" };
1695                     format!("{}: {}{}", bound_kind, sub, tail)
1696                 };
1697                 err.span_suggestion_short(
1698                     sp,
1699                     &consider,
1700                     suggestion,
1701                     Applicability::MaybeIncorrect, // Issue #41966
1702                 );
1703             } else {
1704                 err.help(&consider);
1705             }
1706         }
1707
1708         let mut err = match *sub {
1709             ty::ReEarlyBound(_)
1710             | ty::ReFree(ty::FreeRegion {
1711                 bound_region: ty::BrNamed(..),
1712                 ..
1713             }) => {
1714                 // Does the required lifetime have a nice name we can print?
1715                 let mut err = struct_span_err!(
1716                     self.tcx.sess,
1717                     span,
1718                     E0309,
1719                     "{} may not live long enough",
1720                     labeled_user_string
1721                 );
1722                 binding_suggestion(&mut err, type_param_span, bound_kind, sub);
1723                 err
1724             }
1725
1726             ty::ReStatic => {
1727                 // Does the required lifetime have a nice name we can print?
1728                 let mut err = struct_span_err!(
1729                     self.tcx.sess,
1730                     span,
1731                     E0310,
1732                     "{} may not live long enough",
1733                     labeled_user_string
1734                 );
1735                 binding_suggestion(&mut err, type_param_span, bound_kind, "'static");
1736                 err
1737             }
1738
1739             _ => {
1740                 // If not, be less specific.
1741                 let mut err = struct_span_err!(
1742                     self.tcx.sess,
1743                     span,
1744                     E0311,
1745                     "{} may not live long enough",
1746                     labeled_user_string
1747                 );
1748                 err.help(&format!(
1749                     "consider adding an explicit lifetime bound for `{}`",
1750                     bound_kind
1751                 ));
1752                 self.tcx.note_and_explain_region(
1753                     region_scope_tree,
1754                     &mut err,
1755                     &format!("{} must be valid for ", labeled_user_string),
1756                     sub,
1757                     "...",
1758                 );
1759                 err
1760             }
1761         };
1762
1763         if let Some(origin) = origin {
1764             self.note_region_origin(&mut err, &origin);
1765         }
1766         err
1767     }
1768
1769     fn report_sub_sup_conflict(
1770         &self,
1771         region_scope_tree: &region::ScopeTree,
1772         var_origin: RegionVariableOrigin,
1773         sub_origin: SubregionOrigin<'tcx>,
1774         sub_region: Region<'tcx>,
1775         sup_origin: SubregionOrigin<'tcx>,
1776         sup_region: Region<'tcx>,
1777     ) {
1778         let mut err = self.report_inference_failure(var_origin);
1779
1780         self.tcx.note_and_explain_region(
1781             region_scope_tree,
1782             &mut err,
1783             "first, the lifetime cannot outlive ",
1784             sup_region,
1785             "...",
1786         );
1787
1788         match (&sup_origin, &sub_origin) {
1789             (&infer::Subtype(ref sup_trace), &infer::Subtype(ref sub_trace)) => {
1790                 debug!("report_sub_sup_conflict: var_origin={:?}", var_origin);
1791                 debug!("report_sub_sup_conflict: sub_region={:?}", sub_region);
1792                 debug!("report_sub_sup_conflict: sub_origin={:?}", sub_origin);
1793                 debug!("report_sub_sup_conflict: sup_region={:?}", sup_region);
1794                 debug!("report_sub_sup_conflict: sup_origin={:?}", sup_origin);
1795                 debug!("report_sub_sup_conflict: sup_trace={:?}", sup_trace);
1796                 debug!("report_sub_sup_conflict: sub_trace={:?}", sub_trace);
1797                 debug!("report_sub_sup_conflict: sup_trace.values={:?}", sup_trace.values);
1798                 debug!("report_sub_sup_conflict: sub_trace.values={:?}", sub_trace.values);
1799
1800                 if let (Some((sup_expected, sup_found)), Some((sub_expected, sub_found))) = (
1801                     self.values_str(&sup_trace.values),
1802                     self.values_str(&sub_trace.values),
1803                 ) {
1804                     if sub_expected == sup_expected && sub_found == sup_found {
1805                         self.tcx.note_and_explain_region(
1806                             region_scope_tree,
1807                             &mut err,
1808                             "...but the lifetime must also be valid for ",
1809                             sub_region,
1810                             "...",
1811                         );
1812                         err.note(&format!(
1813                             "...so that the {}:\nexpected {}\n   found {}",
1814                             sup_trace.cause.as_requirement_str(),
1815                             sup_expected.content(),
1816                             sup_found.content()
1817                         ));
1818                         err.emit();
1819                         return;
1820                     }
1821                 }
1822             }
1823             _ => {}
1824         }
1825
1826         self.note_region_origin(&mut err, &sup_origin);
1827
1828         self.tcx.note_and_explain_region(
1829             region_scope_tree,
1830             &mut err,
1831             "but, the lifetime must be valid for ",
1832             sub_region,
1833             "...",
1834         );
1835
1836         self.note_region_origin(&mut err, &sub_origin);
1837         err.emit();
1838     }
1839 }
1840
1841 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
1842     fn report_inference_failure(
1843         &self,
1844         var_origin: RegionVariableOrigin,
1845     ) -> DiagnosticBuilder<'tcx> {
1846         let br_string = |br: ty::BoundRegion| {
1847             let mut s = match br {
1848                 ty::BrNamed(_, name) => name.to_string(),
1849                 _ => String::new(),
1850             };
1851             if !s.is_empty() {
1852                 s.push_str(" ");
1853             }
1854             s
1855         };
1856         let var_description = match var_origin {
1857             infer::MiscVariable(_) => String::new(),
1858             infer::PatternRegion(_) => " for pattern".to_string(),
1859             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
1860             infer::Autoref(_) => " for autoref".to_string(),
1861             infer::Coercion(_) => " for automatic coercion".to_string(),
1862             infer::LateBoundRegion(_, br, infer::FnCall) => {
1863                 format!(" for lifetime parameter {}in function call", br_string(br))
1864             }
1865             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
1866                 format!(" for lifetime parameter {}in generic type", br_string(br))
1867             }
1868             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
1869                 " for lifetime parameter {}in trait containing associated type `{}`",
1870                 br_string(br),
1871                 self.tcx.associated_item(def_id).ident
1872             ),
1873             infer::EarlyBoundRegion(_, name) => format!(" for lifetime parameter `{}`", name),
1874             infer::BoundRegionInCoherence(name) => {
1875                 format!(" for lifetime parameter `{}` in coherence check", name)
1876             }
1877             infer::UpvarRegion(ref upvar_id, _) => {
1878                 let var_name = self.tcx.hir().name(upvar_id.var_path.hir_id);
1879                 format!(" for capture of `{}` by closure", var_name)
1880             }
1881             infer::NLL(..) => bug!("NLL variable found in lexical phase"),
1882         };
1883
1884         struct_span_err!(
1885             self.tcx.sess,
1886             var_origin.span(),
1887             E0495,
1888             "cannot infer an appropriate lifetime{} \
1889              due to conflicting requirements",
1890             var_description
1891         )
1892     }
1893 }
1894
1895 enum FailureCode {
1896     Error0038(DefId),
1897     Error0317(&'static str),
1898     Error0580(&'static str),
1899     Error0308(&'static str),
1900     Error0644(&'static str),
1901 }
1902
1903 impl<'tcx> ObligationCause<'tcx> {
1904     fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode {
1905         use self::FailureCode::*;
1906         use crate::traits::ObligationCauseCode::*;
1907         match self.code {
1908             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
1909             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) =>
1910                 Error0308(match source {
1911                     hir::MatchSource::IfLetDesugar { .. } =>
1912                         "`if let` arms have incompatible types",
1913                     hir::MatchSource::TryDesugar => {
1914                         "try expression alternatives have incompatible types"
1915                     }
1916                     _ => "match arms have incompatible types",
1917                 }),
1918             IfExpression { .. } => Error0308("if and else have incompatible types"),
1919             IfExpressionWithNoElse => Error0317("if may be missing an else clause"),
1920             MainFunctionType => Error0580("main function has wrong type"),
1921             StartFunctionType => Error0308("start function has wrong type"),
1922             IntrinsicType => Error0308("intrinsic has wrong type"),
1923             MethodReceiver => Error0308("mismatched `self` parameter type"),
1924
1925             // In the case where we have no more specific thing to
1926             // say, also take a look at the error code, maybe we can
1927             // tailor to that.
1928             _ => match terr {
1929                 TypeError::CyclicTy(ty) if ty.is_closure() || ty.is_generator() => {
1930                     Error0644("closure/generator type that references itself")
1931                 }
1932                 TypeError::IntrinsicCast => {
1933                     Error0308("cannot coerce intrinsics to function pointers")
1934                 }
1935                 TypeError::ObjectUnsafeCoercion(did) => Error0038(did.clone()),
1936                 _ => Error0308("mismatched types"),
1937             },
1938         }
1939     }
1940
1941     fn as_requirement_str(&self) -> &'static str {
1942         use crate::traits::ObligationCauseCode::*;
1943         match self.code {
1944             CompareImplMethodObligation { .. } => "method type is compatible with trait",
1945             ExprAssignable => "expression is assignable",
1946             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => match source {
1947                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have compatible types",
1948                 _ => "match arms have compatible types",
1949             },
1950             IfExpression { .. } => "if and else have incompatible types",
1951             IfExpressionWithNoElse => "if missing an else returns ()",
1952             MainFunctionType => "`main` function has the correct type",
1953             StartFunctionType => "`start` function has the correct type",
1954             IntrinsicType => "intrinsic has the correct type",
1955             MethodReceiver => "method receiver has the correct type",
1956             _ => "types are compatible",
1957         }
1958     }
1959 }