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 pub use need_type_info::TypeAnnotationNeeded;
  74
  75 pub mod nice_region_error;
  76
  77 impl<'tcx> TyCtxt<'tcx> {
  78     pub fn note_and_explain_region(
  79         self,
  80         region_scope_tree: &region::ScopeTree,
  81         err: &mut DiagnosticBuilder<'_>,
  82         prefix: &str,
  83         region: ty::Region<'tcx>,
  84         suffix: &str,
  85     ) {
  86         let (description, span) = match *region {
  87             ty::ReScope(scope) => {
  88                 let new_string;
  89                 let unknown_scope = || {
  90                     format!(
  91                         "{}unknown scope: {:?}{}.  Please report a bug.",
  92                         prefix, scope, suffix
  93                     )
  94                 };
  95                 let span = scope.span(self, region_scope_tree);
  96                 let tag = match self.hir().find(scope.hir_id(region_scope_tree)) {
  97                     Some(Node::Block(_)) => "block",
  98                     Some(Node::Expr(expr)) => match expr.kind {
  99                         hir::ExprKind::Call(..) => "call",
 100                         hir::ExprKind::MethodCall(..) => "method call",
 101                         hir::ExprKind::Match(.., hir::MatchSource::IfLetDesugar { .. }) => "if let",
 102                         hir::ExprKind::Match(.., hir::MatchSource::WhileLetDesugar) => "while let",
 103                         hir::ExprKind::Match(.., hir::MatchSource::ForLoopDesugar) => "for",
 104                         hir::ExprKind::Match(..) => "match",
 105                         _ => "expression",
 106                     },
 107                     Some(Node::Stmt(_)) => "statement",
 108                     Some(Node::Item(it)) => Self::item_scope_tag(&it),
 109                     Some(Node::TraitItem(it)) => Self::trait_item_scope_tag(&it),
 110                     Some(Node::ImplItem(it)) => Self::impl_item_scope_tag(&it),
 111                     Some(_) | None => {
 112                         err.span_note(span, &unknown_scope());
 113                         return;
 114                     }
 115                 };
 116                 let scope_decorated_tag = match scope.data {
 117                     region::ScopeData::Node => tag,
 118                     region::ScopeData::CallSite => "scope of call-site for function",
 119                     region::ScopeData::Arguments => "scope of function body",
 120                     region::ScopeData::Destruction => {
 121                         new_string = format!("destruction scope surrounding {}", tag);
 122                         &new_string[..]
 123                     }
 124                     region::ScopeData::Remainder(first_statement_index) => {
 125                         new_string = format!(
 126                             "block suffix following statement {}",
 127                             first_statement_index.index()
 128                         );
 129                         &new_string[..]
 130                     }
 131                 };
 132                 self.explain_span(scope_decorated_tag, span)
 133             }
 134
 135             ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic => {
 136                 self.msg_span_from_free_region(region)
 137             }
 138
 139             ty::ReEmpty => ("the empty lifetime".to_owned(), None),
 140
 141             ty::RePlaceholder(_) => (format!("any other region"), None),
 142
 143             // FIXME(#13998) RePlaceholder should probably print like
 144             // ReFree rather than dumping Debug output on the user.
 145             //
 146             // We shouldn't really be having unification failures with ReVar
 147             // and ReLateBound though.
 148             ty::ReVar(_) | ty::ReLateBound(..) | ty::ReErased => {
 149                 (format!("lifetime {:?}", region), None)
 150             }
 151
 152             // We shouldn't encounter an error message with ReClosureBound.
 153             ty::ReClosureBound(..) => {
 154                 bug!("encountered unexpected ReClosureBound: {:?}", region,);
 155             }
 156         };
 157
 158         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 159     }
 160
 161     pub fn note_and_explain_free_region(
 162         self,
 163         err: &mut DiagnosticBuilder<'_>,
 164         prefix: &str,
 165         region: ty::Region<'tcx>,
 166         suffix: &str,
 167     ) {
 168         let (description, span) = self.msg_span_from_free_region(region);
 169
 170         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 171     }
 172
 173     fn msg_span_from_free_region(self, region: ty::Region<'tcx>) -> (String, Option<Span>) {
 174         match *region {
 175             ty::ReEarlyBound(_) | ty::ReFree(_) => {
 176                 self.msg_span_from_early_bound_and_free_regions(region)
 177             }
 178             ty::ReStatic => ("the static lifetime".to_owned(), None),
 179             ty::ReEmpty => ("an empty lifetime".to_owned(), None),
 180             _ => bug!("{:?}", region),
 181         }
 182     }
 183
 184     fn msg_span_from_early_bound_and_free_regions(
 185         self,
 186         region: ty::Region<'tcx>,
 187     ) -> (String, Option<Span>) {
 188         let cm = self.sess.source_map();
 189
 190         let scope = region.free_region_binding_scope(self);
 191         let node = self.hir().as_local_hir_id(scope).unwrap_or(hir::DUMMY_HIR_ID);
 192         let tag = match self.hir().find(node) {
 193             Some(Node::Block(_)) | Some(Node::Expr(_)) => "body",
 194             Some(Node::Item(it)) => Self::item_scope_tag(&it),
 195             Some(Node::TraitItem(it)) => Self::trait_item_scope_tag(&it),
 196             Some(Node::ImplItem(it)) => Self::impl_item_scope_tag(&it),
 197             _ => unreachable!(),
 198         };
 199         let (prefix, span) = match *region {
 200             ty::ReEarlyBound(ref br) => {
 201                 let mut sp = cm.def_span(self.hir().span(node));
 202                 if let Some(param) = self.hir()
 203                     .get_generics(scope)
 204                     .and_then(|generics| generics.get_named(br.name))
 205                 {
 206                     sp = param.span;
 207                 }
 208                 (format!("the lifetime `{}` as defined on", br.name), sp)
 209             }
 210             ty::ReFree(ty::FreeRegion {
 211                 bound_region: ty::BoundRegion::BrNamed(_, name),
 212                 ..
 213             }) => {
 214                 let mut sp = cm.def_span(self.hir().span(node));
 215                 if let Some(param) = self.hir()
 216                     .get_generics(scope)
 217                     .and_then(|generics| generics.get_named(name))
 218                 {
 219                     sp = param.span;
 220                 }
 221                 (format!("the lifetime `{}` as defined on", name), sp)
 222             }
 223             ty::ReFree(ref fr) => match fr.bound_region {
 224                 ty::BrAnon(idx) => (
 225                     format!("the anonymous lifetime #{} defined on", idx + 1),
 226                     self.hir().span(node),
 227                 ),
 228                 _ => (
 229                     format!("the lifetime `{}` as defined on", region),
 230                     cm.def_span(self.hir().span(node)),
 231                 ),
 232             },
 233             _ => bug!(),
 234         };
 235         let (msg, opt_span) = self.explain_span(tag, span);
 236         (format!("{} {}", prefix, msg), opt_span)
 237     }
 238
 239     fn emit_msg_span(
 240         err: &mut DiagnosticBuilder<'_>,
 241         prefix: &str,
 242         description: String,
 243         span: Option<Span>,
 244         suffix: &str,
 245     ) {
 246         let message = format!("{}{}{}", prefix, description, suffix);
 247
 248         if let Some(span) = span {
 249             err.span_note(span, &message);
 250         } else {
 251             err.note(&message);
 252         }
 253     }
 254
 255     fn item_scope_tag(item: &hir::Item<'_>) -> &'static str {
 256         match item.kind {
 257             hir::ItemKind::Impl(..) => "impl",
 258             hir::ItemKind::Struct(..) => "struct",
 259             hir::ItemKind::Union(..) => "union",
 260             hir::ItemKind::Enum(..) => "enum",
 261             hir::ItemKind::Trait(..) => "trait",
 262             hir::ItemKind::Fn(..) => "function body",
 263             _ => "item",
 264         }
 265     }
 266
 267     fn trait_item_scope_tag(item: &hir::TraitItem<'_>) -> &'static str {
 268         match item.kind {
 269             hir::TraitItemKind::Method(..) => "method body",
 270             hir::TraitItemKind::Const(..) | hir::TraitItemKind::Type(..) => "associated item",
 271         }
 272     }
 273
 274     fn impl_item_scope_tag(item: &hir::ImplItem<'_>) -> &'static str {
 275         match item.kind {
 276             hir::ImplItemKind::Method(..) => "method body",
 277             hir::ImplItemKind::Const(..)
 278             | hir::ImplItemKind::OpaqueTy(..)
 279             | hir::ImplItemKind::TyAlias(..) => "associated item",
 280         }
 281     }
 282
 283     fn explain_span(self, heading: &str, span: Span) -> (String, Option<Span>) {
 284         let lo = self.sess.source_map().lookup_char_pos(span.lo());
 285         (
 286             format!("the {} at {}:{}", heading, lo.line, lo.col.to_usize() + 1),
 287             Some(span),
 288         )
 289     }
 290 }
 291
 292 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 293     pub fn report_region_errors(
 294         &self,
 295         region_scope_tree: &region::ScopeTree,
 296         errors: &Vec<RegionResolutionError<'tcx>>,
 297         suppress: SuppressRegionErrors,
 298     ) {
 299         debug!(
 300             "report_region_errors(): {} errors to start, suppress = {:?}",
 301             errors.len(),
 302             suppress
 303         );
 304
 305         if suppress.suppressed() {
 306             return;
 307         }
 308
 309         // try to pre-process the errors, which will group some of them
 310         // together into a `ProcessedErrors` group:
 311         let errors = self.process_errors(errors);
 312
 313         debug!(
 314             "report_region_errors: {} errors after preprocessing",
 315             errors.len()
 316         );
 317
 318         for error in errors {
 319             debug!("report_region_errors: error = {:?}", error);
 320
 321             if !self.try_report_nice_region_error(&error) {
 322                 match error.clone() {
 323                     // These errors could indicate all manner of different
 324                     // problems with many different solutions. Rather
 325                     // than generate a "one size fits all" error, what we
 326                     // attempt to do is go through a number of specific
 327                     // scenarios and try to find the best way to present
 328                     // the error. If all of these fails, we fall back to a rather
 329                     // general bit of code that displays the error information
 330                     RegionResolutionError::ConcreteFailure(origin, sub, sup) => {
 331                         if sub.is_placeholder() || sup.is_placeholder() {
 332                             self.report_placeholder_failure(region_scope_tree, origin, sub, sup)
 333                                 .emit();
 334                         } else {
 335                             self.report_concrete_failure(region_scope_tree, origin, sub, sup)
 336                                 .emit();
 337                         }
 338                     }
 339
 340                     RegionResolutionError::GenericBoundFailure(origin, param_ty, sub) => {
 341                         self.report_generic_bound_failure(
 342                             region_scope_tree,
 343                             origin.span(),
 344                             Some(origin),
 345                             param_ty,
 346                             sub,
 347                         );
 348                     }
 349
 350                     RegionResolutionError::SubSupConflict(
 351                         _,
 352                         var_origin,
 353                         sub_origin,
 354                         sub_r,
 355                         sup_origin,
 356                         sup_r,
 357                     ) => {
 358                         if sub_r.is_placeholder() {
 359                             self.report_placeholder_failure(
 360                                 region_scope_tree,
 361                                 sub_origin,
 362                                 sub_r,
 363                                 sup_r,
 364                             )
 365                                 .emit();
 366                         } else if sup_r.is_placeholder() {
 367                             self.report_placeholder_failure(
 368                                 region_scope_tree,
 369                                 sup_origin,
 370                                 sub_r,
 371                                 sup_r,
 372                             )
 373                                 .emit();
 374                         } else {
 375                             self.report_sub_sup_conflict(
 376                                 region_scope_tree,
 377                                 var_origin,
 378                                 sub_origin,
 379                                 sub_r,
 380                                 sup_origin,
 381                                 sup_r,
 382                             );
 383                         }
 384                     }
 385
 386                     RegionResolutionError::MemberConstraintFailure {
 387                         opaque_type_def_id,
 388                         hidden_ty,
 389                         member_region,
 390                         span: _,
 391                         choice_regions: _,
 392                     } => {
 393                         let hidden_ty = self.resolve_vars_if_possible(&hidden_ty);
 394                         opaque_types::unexpected_hidden_region_diagnostic(
 395                             self.tcx,
 396                             Some(region_scope_tree),
 397                             opaque_type_def_id,
 398                             hidden_ty,
 399                             member_region,
 400                         ).emit();
 401                     }
 402                 }
 403             }
 404         }
 405     }
 406
 407     // This method goes through all the errors and try to group certain types
 408     // of error together, for the purpose of suggesting explicit lifetime
 409     // parameters to the user. This is done so that we can have a more
 410     // complete view of what lifetimes should be the same.
 411     // If the return value is an empty vector, it means that processing
 412     // failed (so the return value of this method should not be used).
 413     //
 414     // The method also attempts to weed out messages that seem like
 415     // duplicates that will be unhelpful to the end-user. But
 416     // obviously it never weeds out ALL errors.
 417     fn process_errors(
 418         &self,
 419         errors: &Vec<RegionResolutionError<'tcx>>,
 420     ) -> Vec<RegionResolutionError<'tcx>> {
 421         debug!("process_errors()");
 422
 423         // We want to avoid reporting generic-bound failures if we can
 424         // avoid it: these have a very high rate of being unhelpful in
 425         // practice. This is because they are basically secondary
 426         // checks that test the state of the region graph after the
 427         // rest of inference is done, and the other kinds of errors
 428         // indicate that the region constraint graph is internally
 429         // inconsistent, so these test results are likely to be
 430         // meaningless.
 431         //
 432         // Therefore, we filter them out of the list unless they are
 433         // the only thing in the list.
 434
 435         let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
 436             RegionResolutionError::GenericBoundFailure(..) => true,
 437             RegionResolutionError::ConcreteFailure(..)
 438                 | RegionResolutionError::SubSupConflict(..)
 439                 | RegionResolutionError::MemberConstraintFailure { .. } => false,
 440         };
 441
 442         let mut errors = if errors.iter().all(|e| is_bound_failure(e)) {
 443             errors.clone()
 444         } else {
 445             errors
 446             .iter()
 447             .filter(|&e| !is_bound_failure(e))
 448             .cloned()
 449             .collect()
 450         };
 451
 452         // sort the errors by span, for better error message stability.
 453         errors.sort_by_key(|u| match *u {
 454             RegionResolutionError::ConcreteFailure(ref sro, _, _) => sro.span(),
 455             RegionResolutionError::GenericBoundFailure(ref sro, _, _) => sro.span(),
 456             RegionResolutionError::SubSupConflict(_, ref rvo, _, _, _, _) => rvo.span(),
 457             RegionResolutionError::MemberConstraintFailure { span, .. } => span,
 458         });
 459         errors
 460     }
 461
 462     /// Adds a note if the types come from similarly named crates
 463     fn check_and_note_conflicting_crates(
 464         &self,
 465         err: &mut DiagnosticBuilder<'_>,
 466         terr: &TypeError<'tcx>,
 467     ) {
 468         use hir::def_id::CrateNum;
 469         use hir::map::DisambiguatedDefPathData;
 470         use ty::print::Printer;
 471         use ty::subst::GenericArg;
 472
 473         struct AbsolutePathPrinter<'tcx> {
 474             tcx: TyCtxt<'tcx>,
 475         }
 476
 477         struct NonTrivialPath;
 478
 479         impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
 480             type Error = NonTrivialPath;
 481
 482             type Path = Vec<String>;
 483             type Region = !;
 484             type Type = !;
 485             type DynExistential = !;
 486             type Const = !;
 487
 488             fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
 489                 self.tcx
 490             }
 491
 492             fn print_region(
 493                 self,
 494                 _region: ty::Region<'_>,
 495             ) -> Result<Self::Region, Self::Error> {
 496                 Err(NonTrivialPath)
 497             }
 498
 499             fn print_type(
 500                 self,
 501                 _ty: Ty<'tcx>,
 502             ) -> Result<Self::Type, Self::Error> {
 503                 Err(NonTrivialPath)
 504             }
 505
 506             fn print_dyn_existential(
 507                 self,
 508                 _predicates: &'tcx ty::List<ty::ExistentialPredicate<'tcx>>,
 509             ) -> Result<Self::DynExistential, Self::Error> {
 510                 Err(NonTrivialPath)
 511             }
 512
 513             fn print_const(
 514                 self,
 515                 _ct: &'tcx ty::Const<'tcx>,
 516             ) -> Result<Self::Const, Self::Error> {
 517                 Err(NonTrivialPath)
 518             }
 519
 520             fn path_crate(
 521                 self,
 522                 cnum: CrateNum,
 523             ) -> Result<Self::Path, Self::Error> {
 524                 Ok(vec![self.tcx.original_crate_name(cnum).to_string()])
 525             }
 526             fn path_qualified(
 527                 self,
 528                 _self_ty: Ty<'tcx>,
 529                 _trait_ref: Option<ty::TraitRef<'tcx>>,
 530             ) -> Result<Self::Path, Self::Error> {
 531                 Err(NonTrivialPath)
 532             }
 533
 534             fn path_append_impl(
 535                 self,
 536                 _print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 537                 _disambiguated_data: &DisambiguatedDefPathData,
 538                 _self_ty: Ty<'tcx>,
 539                 _trait_ref: Option<ty::TraitRef<'tcx>>,
 540             ) -> Result<Self::Path, Self::Error> {
 541                 Err(NonTrivialPath)
 542             }
 543             fn path_append(
 544                 self,
 545                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 546                 disambiguated_data: &DisambiguatedDefPathData,
 547             ) -> Result<Self::Path, Self::Error> {
 548                 let mut path = print_prefix(self)?;
 549                 path.push(disambiguated_data.data.as_symbol().to_string());
 550                 Ok(path)
 551             }
 552             fn path_generic_args(
 553                 self,
 554                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 555                 _args: &[GenericArg<'tcx>],
 556             ) -> Result<Self::Path, Self::Error> {
 557                 print_prefix(self)
 558             }
 559         }
 560
 561         let report_path_match = |err: &mut DiagnosticBuilder<'_>, did1: DefId, did2: DefId| {
 562             // Only external crates, if either is from a local
 563             // module we could have false positives
 564             if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
 565                 let abs_path = |def_id| {
 566                     AbsolutePathPrinter { tcx: self.tcx }
 567                         .print_def_path(def_id, &[])
 568                 };
 569
 570                 // We compare strings because DefPath can be different
 571                 // for imported and non-imported crates
 572                 let same_path = || -> Result<_, NonTrivialPath> {
 573                     Ok(
 574                         self.tcx.def_path_str(did1) == self.tcx.def_path_str(did2) ||
 575                         abs_path(did1)? == abs_path(did2)?
 576                     )
 577                 };
 578                 if same_path().unwrap_or(false) {
 579                     let crate_name = self.tcx.crate_name(did1.krate);
 580                     err.note(&format!(
 581                         "perhaps two different versions of crate `{}` are being used?",
 582                         crate_name
 583                     ));
 584                 }
 585             }
 586         };
 587         match *terr {
 588             TypeError::Sorts(ref exp_found) => {
 589                 // if they are both "path types", there's a chance of ambiguity
 590                 // due to different versions of the same crate
 591                 if let (&ty::Adt(exp_adt, _), &ty::Adt(found_adt, _))
 592                      = (&exp_found.expected.kind, &exp_found.found.kind)
 593                 {
 594                     report_path_match(err, exp_adt.did, found_adt.did);
 595                 }
 596             }
 597             TypeError::Traits(ref exp_found) => {
 598                 report_path_match(err, exp_found.expected, exp_found.found);
 599             }
 600             _ => (), // FIXME(#22750) handle traits and stuff
 601         }
 602     }
 603
 604     fn note_error_origin(
 605         &self,
 606         err: &mut DiagnosticBuilder<'tcx>,
 607         cause: &ObligationCause<'tcx>,
 608         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
 609     ) {
 610         match cause.code {
 611             ObligationCauseCode::MatchExpressionArmPattern { span, ty } => {
 612                 if ty.is_suggestable() {  // don't show type `_`
 613                     err.span_label(span, format!("this match expression has type `{}`", ty));
 614                 }
 615                 if let Some(ty::error::ExpectedFound { found, .. }) = exp_found {
 616                     if ty.is_box() && ty.boxed_ty() == found {
 617                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 618                             err.span_suggestion(
 619                                 span,
 620                                 "consider dereferencing the boxed value",
 621                                 format!("*{}", snippet),
 622                                 Applicability::MachineApplicable,
 623                             );
 624                         }
 625                     }
 626                 }
 627             }
 628             ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
 629                 source,
 630                 ref prior_arms,
 631                 last_ty,
 632                 discrim_hir_id,
 633                 ..
 634             }) => match source {
 635                 hir::MatchSource::IfLetDesugar { .. } => {
 636                     let msg = "`if let` arms have incompatible types";
 637                     err.span_label(cause.span, msg);
 638                 }
 639                 hir::MatchSource::TryDesugar => {
 640                     if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
 641                         let discrim_expr = self.tcx.hir().expect_expr(discrim_hir_id);
 642                         let discrim_ty = if let hir::ExprKind::Call(_, args) = &discrim_expr.kind {
 643                             let arg_expr = args.first().expect("try desugaring call w/out arg");
 644                             self.in_progress_tables.and_then(|tables| {
 645                                 tables.borrow().expr_ty_opt(arg_expr)
 646                             })
 647                         } else {
 648                             bug!("try desugaring w/out call expr as discriminant");
 649                         };
 650
 651                         match discrim_ty {
 652                             Some(ty) if expected == ty => {
 653                                 let source_map = self.tcx.sess.source_map();
 654                                 err.span_suggestion(
 655                                     source_map.end_point(cause.span),
 656                                     "try removing this `?`",
 657                                     "".to_string(),
 658                                     Applicability::MachineApplicable,
 659                                 );
 660                             },
 661                             _ => {},
 662                         }
 663                     }
 664                 }
 665                 _ => {
 666                     // `last_ty` can be `!`, `expected` will have better info when present.
 667                     let t = self.resolve_vars_if_possible(&match exp_found {
 668                         Some(ty::error::ExpectedFound { expected, .. }) => expected,
 669                         _ => last_ty,
 670                     });
 671                     let msg = "`match` arms have incompatible types";
 672                     err.span_label(cause.span, msg);
 673                     if prior_arms.len() <= 4 {
 674                         for sp in prior_arms {
 675                             err.span_label( *sp, format!("this is found to be of type `{}`", t));
 676                         }
 677                     } else if let Some(sp) = prior_arms.last() {
 678                         err.span_label(
 679                             *sp,
 680                             format!("this and all prior arms are found to be of type `{}`", t),
 681                         );
 682                     }
 683                 }
 684             },
 685             ObligationCauseCode::IfExpression(box IfExpressionCause { then, outer, semicolon }) => {
 686                 err.span_label(then, "expected because of this");
 687                 outer.map(|sp| err.span_label(sp, "if and else have incompatible types"));
 688                 if let Some(sp) = semicolon {
 689                     err.span_suggestion_short(
 690                         sp,
 691                         "consider removing this semicolon",
 692                         String::new(),
 693                         Applicability::MachineApplicable,
 694                     );
 695                 }
 696             }
 697             _ => (),
 698         }
 699     }
 700
 701     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
 702     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
 703     /// populate `other_value` with `other_ty`.
 704     ///
 705     /// ```text
 706     /// Foo<Bar<Qux>>
 707     /// ^^^^--------^ this is highlighted
 708     /// |   |
 709     /// |   this type argument is exactly the same as the other type, not highlighted
 710     /// this is highlighted
 711     /// Bar<Qux>
 712     /// -------- this type is the same as a type argument in the other type, not highlighted
 713     /// ```
 714     fn highlight_outer(
 715         &self,
 716         value: &mut DiagnosticStyledString,
 717         other_value: &mut DiagnosticStyledString,
 718         name: String,
 719         sub: ty::subst::SubstsRef<'tcx>,
 720         pos: usize,
 721         other_ty: Ty<'tcx>,
 722     ) {
 723         // `value` and `other_value` hold two incomplete type representation for display.
 724         // `name` is the path of both types being compared. `sub`
 725         value.push_highlighted(name);
 726         let len = sub.len();
 727         if len > 0 {
 728             value.push_highlighted("<");
 729         }
 730
 731         // Output the lifetimes for the first type
 732         let lifetimes = sub.regions()
 733             .map(|lifetime| {
 734                 let s = lifetime.to_string();
 735                 if s.is_empty() {
 736                     "'_".to_string()
 737                 } else {
 738                     s
 739                 }
 740             })
 741             .collect::<Vec<_>>()
 742             .join(", ");
 743         if !lifetimes.is_empty() {
 744             if sub.regions().count() < len {
 745                 value.push_normal(lifetimes + &", ");
 746             } else {
 747                 value.push_normal(lifetimes);
 748             }
 749         }
 750
 751         // Highlight all the type arguments that aren't at `pos` and compare the type argument at
 752         // `pos` and `other_ty`.
 753         for (i, type_arg) in sub.types().enumerate() {
 754             if i == pos {
 755                 let values = self.cmp(type_arg, other_ty);
 756                 value.0.extend((values.0).0);
 757                 other_value.0.extend((values.1).0);
 758             } else {
 759                 value.push_highlighted(type_arg.to_string());
 760             }
 761
 762             if len > 0 && i != len - 1 {
 763                 value.push_normal(", ");
 764             }
 765         }
 766         if len > 0 {
 767             value.push_highlighted(">");
 768         }
 769     }
 770
 771     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 772     /// as that is the difference to the other type.
 773     ///
 774     /// For the following code:
 775     ///
 776     /// ```norun
 777     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 778     /// ```
 779     ///
 780     /// The type error output will behave in the following way:
 781     ///
 782     /// ```text
 783     /// Foo<Bar<Qux>>
 784     /// ^^^^--------^ this is highlighted
 785     /// |   |
 786     /// |   this type argument is exactly the same as the other type, not highlighted
 787     /// this is highlighted
 788     /// Bar<Qux>
 789     /// -------- this type is the same as a type argument in the other type, not highlighted
 790     /// ```
 791     fn cmp_type_arg(
 792         &self,
 793         mut t1_out: &mut DiagnosticStyledString,
 794         mut t2_out: &mut DiagnosticStyledString,
 795         path: String,
 796         sub: ty::subst::SubstsRef<'tcx>,
 797         other_path: String,
 798         other_ty: Ty<'tcx>,
 799     ) -> Option<()> {
 800         for (i, ta) in sub.types().enumerate() {
 801             if ta == other_ty {
 802                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 803                 return Some(());
 804             }
 805             if let &ty::Adt(def, _) = &ta.kind {
 806                 let path_ = self.tcx.def_path_str(def.did.clone());
 807                 if path_ == other_path {
 808                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 809                     return Some(());
 810                 }
 811             }
 812         }
 813         None
 814     }
 815
 816     /// Adds a `,` to the type representation only if it is appropriate.
 817     fn push_comma(
 818         &self,
 819         value: &mut DiagnosticStyledString,
 820         other_value: &mut DiagnosticStyledString,
 821         len: usize,
 822         pos: usize,
 823     ) {
 824         if len > 0 && pos != len - 1 {
 825             value.push_normal(", ");
 826             other_value.push_normal(", ");
 827         }
 828     }
 829
 830     /// For generic types with parameters with defaults, remove the parameters corresponding to
 831     /// the defaults. This repeats a lot of the logic found in `ty::print::pretty`.
 832     fn strip_generic_default_params(
 833         &self,
 834         def_id: DefId,
 835         substs: ty::subst::SubstsRef<'tcx>,
 836     ) -> SubstsRef<'tcx> {
 837         let generics = self.tcx.generics_of(def_id);
 838         let mut num_supplied_defaults = 0;
 839         let mut type_params = generics.params.iter().rev().filter_map(|param| match param.kind {
 840             ty::GenericParamDefKind::Lifetime => None,
 841             ty::GenericParamDefKind::Type { has_default, .. } => Some((param.def_id, has_default)),
 842             ty::GenericParamDefKind::Const => None, // FIXME(const_generics:defaults)
 843         }).peekable();
 844         let has_default = {
 845             let has_default = type_params.peek().map(|(_, has_default)| has_default);
 846             *has_default.unwrap_or(&false)
 847         };
 848         if has_default {
 849             let types = substs.types().rev();
 850             for ((def_id, has_default), actual) in type_params.zip(types) {
 851                 if !has_default {
 852                     break;
 853                 }
 854                 if self.tcx.type_of(def_id).subst(self.tcx, substs) != actual {
 855                     break;
 856                 }
 857                 num_supplied_defaults += 1;
 858             }
 859         }
 860         let len = generics.params.len();
 861         let mut generics = generics.clone();
 862         generics.params.truncate(len - num_supplied_defaults);
 863         substs.truncate_to(self.tcx, &generics)
 864     }
 865
 866     /// Given two `fn` signatures highlight only sub-parts that are different.
 867     fn cmp_fn_sig(
 868         &self,
 869         sig1: &ty::PolyFnSig<'tcx>,
 870         sig2: &ty::PolyFnSig<'tcx>,
 871     ) -> (DiagnosticStyledString, DiagnosticStyledString) {
 872         let get_lifetimes = |sig| {
 873             use crate::hir::def::Namespace;
 874             let mut s = String::new();
 875             let (_, (sig, reg)) = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS)
 876                 .name_all_regions(sig)
 877                 .unwrap();
 878             let lts: Vec<String> = reg.into_iter().map(|(_, kind)| kind.to_string()).collect();
 879             (if lts.is_empty() {
 880                 String::new()
 881             } else {
 882                 format!("for<{}> ", lts.join(", "))
 883             }, sig)
 884         };
 885
 886         let (lt1, sig1) = get_lifetimes(sig1);
 887         let (lt2, sig2) = get_lifetimes(sig2);
 888
 889         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 890         let mut values = (
 891             DiagnosticStyledString::normal("".to_string()),
 892             DiagnosticStyledString::normal("".to_string()),
 893         );
 894
 895         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 896         // ^^^^^^
 897         values.0.push(sig1.unsafety.prefix_str(), sig1.unsafety != sig2.unsafety);
 898         values.1.push(sig2.unsafety.prefix_str(), sig1.unsafety != sig2.unsafety);
 899
 900         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 901         //        ^^^^^^^^^^
 902         if sig1.abi != abi::Abi::Rust {
 903             values.0.push(format!("extern {} ", sig1.abi), sig1.abi != sig2.abi);
 904         }
 905         if sig2.abi != abi::Abi::Rust {
 906             values.1.push(format!("extern {} ", sig2.abi), sig1.abi != sig2.abi);
 907         }
 908
 909         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 910         //                   ^^^^^^^^
 911         let lifetime_diff = lt1 != lt2;
 912         values.0.push(lt1, lifetime_diff);
 913         values.1.push(lt2, lifetime_diff);
 914
 915         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 916         //                           ^^^
 917         values.0.push_normal("fn(");
 918         values.1.push_normal("fn(");
 919
 920         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 921         //                              ^^^^^
 922         let len1 = sig1.inputs().len();
 923         let len2 = sig2.inputs().len();
 924         if len1 == len2 {
 925             for (i, (l, r)) in sig1.inputs().iter().zip(sig2.inputs().iter()).enumerate() {
 926                 let (x1, x2) = self.cmp(l, r);
 927                 (values.0).0.extend(x1.0);
 928                 (values.1).0.extend(x2.0);
 929                 self.push_comma(&mut values.0, &mut values.1, len1, i);
 930             }
 931         } else {
 932             for (i, l) in sig1.inputs().iter().enumerate() {
 933                 values.0.push_highlighted(l.to_string());
 934                 if i != len1 - 1 {
 935                     values.0.push_highlighted(", ");
 936                 }
 937             }
 938             for (i, r) in sig2.inputs().iter().enumerate() {
 939                 values.1.push_highlighted(r.to_string());
 940                 if i != len2 - 1 {
 941                     values.1.push_highlighted(", ");
 942                 }
 943             }
 944         }
 945
 946         if sig1.c_variadic {
 947             if len1 > 0 {
 948                 values.0.push_normal(", ");
 949             }
 950             values.0.push("...", !sig2.c_variadic);
 951         }
 952         if sig2.c_variadic {
 953             if len2 > 0 {
 954                 values.1.push_normal(", ");
 955             }
 956             values.1.push("...", !sig1.c_variadic);
 957         }
 958
 959         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 960         //                                   ^
 961         values.0.push_normal(")");
 962         values.1.push_normal(")");
 963
 964         // unsafe extern "C" for<'a> fn(&'a T) -> &'a T
 965         //                                     ^^^^^^^^
 966         let output1 = sig1.output();
 967         let output2 = sig2.output();
 968         let (x1, x2) = self.cmp(output1, output2);
 969         if !output1.is_unit() {
 970             values.0.push_normal(" -> ");
 971             (values.0).0.extend(x1.0);
 972         }
 973         if !output2.is_unit() {
 974             values.1.push_normal(" -> ");
 975             (values.1).0.extend(x2.0);
 976         }
 977         values
 978     }
 979
 980     /// Compares two given types, eliding parts that are the same between them and highlighting
 981     /// relevant differences, and return two representation of those types for highlighted printing.
 982     fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> (DiagnosticStyledString, DiagnosticStyledString) {
 983         debug!("cmp(t1={}, t1.kind={:?}, t2={}, t2.kind={:?})", t1, t1.kind, t2, t2.kind);
 984
 985         // helper functions
 986         fn equals<'tcx>(a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 987             match (&a.kind, &b.kind) {
 988                 (a, b) if *a == *b => true,
 989                 (&ty::Int(_), &ty::Infer(ty::InferTy::IntVar(_)))
 990                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Int(_))
 991                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Infer(ty::InferTy::IntVar(_)))
 992                 | (&ty::Float(_), &ty::Infer(ty::InferTy::FloatVar(_)))
 993                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Float(_))
 994                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Infer(ty::InferTy::FloatVar(_))) => {
 995                     true
 996                 }
 997                 _ => false,
 998             }
 999         }
1000
1001         fn push_ty_ref<'tcx>(
1002             r: &ty::Region<'tcx>,
1003             ty: Ty<'tcx>,
1004             mutbl: hir::Mutability,
1005             s: &mut DiagnosticStyledString,
1006         ) {
1007             let mut r = r.to_string();
1008             if r == "'_" {
1009                 r.clear();
1010             } else {
1011                 r.push(' ');
1012             }
1013             s.push_highlighted(format!("&{}{}", r, mutbl.prefix_str()));
1014             s.push_normal(ty.to_string());
1015         }
1016
1017         // process starts here
1018         match (&t1.kind, &t2.kind) {
1019             (&ty::Adt(def1, sub1), &ty::Adt(def2, sub2)) => {
1020                 let sub_no_defaults_1 = self.strip_generic_default_params(def1.did, sub1);
1021                 let sub_no_defaults_2 = self.strip_generic_default_params(def2.did, sub2);
1022                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1023                 let path1 = self.tcx.def_path_str(def1.did.clone());
1024                 let path2 = self.tcx.def_path_str(def2.did.clone());
1025                 if def1.did == def2.did {
1026                     // Easy case. Replace same types with `_` to shorten the output and highlight
1027                     // the differing ones.
1028                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
1029                     //     Foo<Bar, _>
1030                     //     Foo<Quz, _>
1031                     //         ---  ^ type argument elided
1032                     //         |
1033                     //         highlighted in output
1034                     values.0.push_normal(path1);
1035                     values.1.push_normal(path2);
1036
1037                     // Avoid printing out default generic parameters that are common to both
1038                     // types.
1039                     let len1 = sub_no_defaults_1.len();
1040                     let len2 = sub_no_defaults_2.len();
1041                     let common_len = cmp::min(len1, len2);
1042                     let remainder1: Vec<_> = sub1.types().skip(common_len).collect();
1043                     let remainder2: Vec<_> = sub2.types().skip(common_len).collect();
1044                     let common_default_params = remainder1
1045                         .iter()
1046                         .rev()
1047                         .zip(remainder2.iter().rev())
1048                         .filter(|(a, b)| a == b)
1049                         .count();
1050                     let len = sub1.len() - common_default_params;
1051                     let consts_offset = len - sub1.consts().count();
1052
1053                     // Only draw `<...>` if there're lifetime/type arguments.
1054                     if len > 0 {
1055                         values.0.push_normal("<");
1056                         values.1.push_normal("<");
1057                     }
1058
1059                     fn lifetime_display(lifetime: Region<'_>) -> String {
1060                         let s = lifetime.to_string();
1061                         if s.is_empty() {
1062                             "'_".to_string()
1063                         } else {
1064                             s
1065                         }
1066                     }
1067                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
1068                     // all diagnostics that use this output
1069                     //
1070                     //     Foo<'x, '_, Bar>
1071                     //     Foo<'y, '_, Qux>
1072                     //         ^^  ^^  --- type arguments are not elided
1073                     //         |   |
1074                     //         |   elided as they were the same
1075                     //         not elided, they were different, but irrelevant
1076                     let lifetimes = sub1.regions().zip(sub2.regions());
1077                     for (i, lifetimes) in lifetimes.enumerate() {
1078                         let l1 = lifetime_display(lifetimes.0);
1079                         let l2 = lifetime_display(lifetimes.1);
1080                         if lifetimes.0 == lifetimes.1 {
1081                             values.0.push_normal("'_");
1082                             values.1.push_normal("'_");
1083                         } else {
1084                             values.0.push_highlighted(l1);
1085                             values.1.push_highlighted(l2);
1086                         }
1087                         self.push_comma(&mut values.0, &mut values.1, len, i);
1088                     }
1089
1090                     // We're comparing two types with the same path, so we compare the type
1091                     // arguments for both. If they are the same, do not highlight and elide from the
1092                     // output.
1093                     //     Foo<_, Bar>
1094                     //     Foo<_, Qux>
1095                     //         ^ elided type as this type argument was the same in both sides
1096                     let type_arguments = sub1.types().zip(sub2.types());
1097                     let regions_len = sub1.regions().count();
1098                     let num_display_types = consts_offset - regions_len;
1099                     for (i, (ta1, ta2)) in type_arguments.take(num_display_types).enumerate() {
1100                         let i = i + regions_len;
1101                         if ta1 == ta2 {
1102                             values.0.push_normal("_");
1103                             values.1.push_normal("_");
1104                         } else {
1105                             let (x1, x2) = self.cmp(ta1, ta2);
1106                             (values.0).0.extend(x1.0);
1107                             (values.1).0.extend(x2.0);
1108                         }
1109                         self.push_comma(&mut values.0, &mut values.1, len, i);
1110                     }
1111
1112                     // Do the same for const arguments, if they are equal, do not highlight and
1113                     // elide them from the output.
1114                     let const_arguments = sub1.consts().zip(sub2.consts());
1115                     for (i, (ca1, ca2)) in const_arguments.enumerate() {
1116                         let i = i + consts_offset;
1117                         if ca1 == ca2 {
1118                             values.0.push_normal("_");
1119                             values.1.push_normal("_");
1120                         } else {
1121                             values.0.push_highlighted(ca1.to_string());
1122                             values.1.push_highlighted(ca2.to_string());
1123                         }
1124                         self.push_comma(&mut values.0, &mut values.1, len, i);
1125                     }
1126
1127                     // Close the type argument bracket.
1128                     // Only draw `<...>` if there're lifetime/type arguments.
1129                     if len > 0 {
1130                         values.0.push_normal(">");
1131                         values.1.push_normal(">");
1132                     }
1133                     values
1134                 } else {
1135                     // Check for case:
1136                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
1137                     //     Foo<Bar<Qux>
1138                     //         ------- this type argument is exactly the same as the other type
1139                     //     Bar<Qux>
1140                     if self.cmp_type_arg(
1141                         &mut values.0,
1142                         &mut values.1,
1143                         path1.clone(),
1144                         sub_no_defaults_1,
1145                         path2.clone(),
1146                         &t2,
1147                     ).is_some()
1148                     {
1149                         return values;
1150                     }
1151                     // Check for case:
1152                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
1153                     //     Bar<Qux>
1154                     //     Foo<Bar<Qux>>
1155                     //         ------- this type argument is exactly the same as the other type
1156                     if self.cmp_type_arg(
1157                         &mut values.1,
1158                         &mut values.0,
1159                         path2,
1160                         sub_no_defaults_2,
1161                         path1,
1162                         &t1,
1163                     ).is_some()
1164                     {
1165                         return values;
1166                     }
1167
1168                     // We can't find anything in common, highlight relevant part of type path.
1169                     //     let x: foo::bar::Baz<Qux> = y:<foo::bar::Bar<Zar>>();
1170                     //     foo::bar::Baz<Qux>
1171                     //     foo::bar::Bar<Zar>
1172                     //               -------- this part of the path is different
1173
1174                     let t1_str = t1.to_string();
1175                     let t2_str = t2.to_string();
1176                     let min_len = t1_str.len().min(t2_str.len());
1177
1178                     const SEPARATOR: &str = "::";
1179                     let separator_len = SEPARATOR.len();
1180                     let split_idx: usize =
1181                         t1_str.split(SEPARATOR)
1182                             .zip(t2_str.split(SEPARATOR))
1183                             .take_while(|(mod1_str, mod2_str)| mod1_str == mod2_str)
1184                             .map(|(mod_str, _)| mod_str.len() + separator_len)
1185                             .sum();
1186
1187                     debug!("cmp: separator_len={}, split_idx={}, min_len={}",
1188                         separator_len, split_idx, min_len
1189                     );
1190
1191                     if split_idx >= min_len {
1192                         // paths are identical, highlight everything
1193                         (
1194                             DiagnosticStyledString::highlighted(t1_str),
1195                             DiagnosticStyledString::highlighted(t2_str)
1196                         )
1197                     } else {
1198                         let (common, uniq1) = t1_str.split_at(split_idx);
1199                         let (_, uniq2) = t2_str.split_at(split_idx);
1200                         debug!("cmp: common={}, uniq1={}, uniq2={}", common, uniq1, uniq2);
1201
1202                         values.0.push_normal(common);
1203                         values.0.push_highlighted(uniq1);
1204                         values.1.push_normal(common);
1205                         values.1.push_highlighted(uniq2);
1206
1207                         values
1208                     }
1209                 }
1210             }
1211
1212             // When finding T != &T, highlight only the borrow
1213             (&ty::Ref(r1, ref_ty1, mutbl1), _) if equals(&ref_ty1, &t2) => {
1214                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1215                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
1216                 values.1.push_normal(t2.to_string());
1217                 values
1218             }
1219             (_, &ty::Ref(r2, ref_ty2, mutbl2)) if equals(&t1, &ref_ty2) => {
1220                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1221                 values.0.push_normal(t1.to_string());
1222                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
1223                 values
1224             }
1225
1226             // When encountering &T != &mut T, highlight only the borrow
1227             (&ty::Ref(r1, ref_ty1, mutbl1), &ty::Ref(r2, ref_ty2, mutbl2))
1228                 if equals(&ref_ty1, &ref_ty2) =>
1229             {
1230                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1231                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
1232                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
1233                 values
1234             }
1235
1236             // When encountering tuples of the same size, highlight only the differing types
1237             (&ty::Tuple(substs1), &ty::Tuple(substs2)) if substs1.len() == substs2.len() => {
1238                 let mut values = (
1239                     DiagnosticStyledString::normal("("),
1240                     DiagnosticStyledString::normal("("),
1241                 );
1242                 let len = substs1.len();
1243                 for (i, (left, right)) in substs1.types().zip(substs2.types()).enumerate() {
1244                     let (x1, x2) = self.cmp(left, right);
1245                     (values.0).0.extend(x1.0);
1246                     (values.1).0.extend(x2.0);
1247                     self.push_comma(&mut values.0, &mut values.1, len, i);
1248                 }
1249                 if len == 1 { // Keep the output for single element tuples as `(ty,)`.
1250                     values.0.push_normal(",");
1251                     values.1.push_normal(",");
1252                 }
1253                 values.0.push_normal(")");
1254                 values.1.push_normal(")");
1255                 values
1256             }
1257
1258             (ty::FnDef(did1, substs1), ty::FnDef(did2, substs2)) => {
1259                 let sig1 = self.tcx.fn_sig(*did1).subst(self.tcx, substs1);
1260                 let sig2 = self.tcx.fn_sig(*did2).subst(self.tcx, substs2);
1261                 let mut values = self.cmp_fn_sig(&sig1, &sig2);
1262                 let path1 = format!(" {{{}}}", self.tcx.def_path_str_with_substs(*did1, substs1));
1263                 let path2 = format!(" {{{}}}", self.tcx.def_path_str_with_substs(*did2, substs2));
1264                 let same_path = path1 == path2;
1265                 values.0.push(path1, !same_path);
1266                 values.1.push(path2, !same_path);
1267                 values
1268             }
1269
1270             (ty::FnDef(did1, substs1), ty::FnPtr(sig2)) => {
1271                 let sig1 = self.tcx.fn_sig(*did1).subst(self.tcx, substs1);
1272                 let mut values = self.cmp_fn_sig(&sig1, sig2);
1273                 values.0.push_normal(format!(
1274                     " {{{}}}",
1275                     self.tcx.def_path_str_with_substs(*did1, substs1)),
1276                 );
1277                 values
1278             }
1279
1280             (ty::FnPtr(sig1), ty::FnDef(did2, substs2)) => {
1281                 let sig2 = self.tcx.fn_sig(*did2).subst(self.tcx, substs2);
1282                 let mut values = self.cmp_fn_sig(sig1, &sig2);
1283                 values.1.push_normal(format!(
1284                     " {{{}}}",
1285                     self.tcx.def_path_str_with_substs(*did2, substs2)),
1286                 );
1287                 values
1288             }
1289
1290             (ty::FnPtr(sig1), ty::FnPtr(sig2)) => {
1291                 self.cmp_fn_sig(sig1, sig2)
1292             }
1293
1294             _ => {
1295                 if t1 == t2 {
1296                     // The two types are the same, elide and don't highlight.
1297                     (
1298                         DiagnosticStyledString::normal("_"),
1299                         DiagnosticStyledString::normal("_"),
1300                     )
1301                 } else {
1302                     // We couldn't find anything in common, highlight everything.
1303                     (
1304                         DiagnosticStyledString::highlighted(t1.to_string()),
1305                         DiagnosticStyledString::highlighted(t2.to_string()),
1306                     )
1307                 }
1308             }
1309         }
1310     }
1311
1312     pub fn note_type_err(
1313         &self,
1314         diag: &mut DiagnosticBuilder<'tcx>,
1315         cause: &ObligationCause<'tcx>,
1316         secondary_span: Option<(Span, String)>,
1317         mut values: Option<ValuePairs<'tcx>>,
1318         terr: &TypeError<'tcx>,
1319     ) {
1320         // For some types of errors, expected-found does not make
1321         // sense, so just ignore the values we were given.
1322         match terr {
1323             TypeError::CyclicTy(_) => {
1324                 values = None;
1325             }
1326             _ => {}
1327         }
1328
1329         debug!("note_type_err(diag={:?})", diag);
1330         let (expected_found, exp_found, is_simple_error) = match values {
1331             None => (None, None, false),
1332             Some(values) => {
1333                 let (is_simple_error, exp_found) = match values {
1334                     ValuePairs::Types(exp_found) => {
1335                         let is_simple_err = exp_found.expected.is_simple_text()
1336                             && exp_found.found.is_simple_text();
1337
1338                         (is_simple_err, Some(exp_found))
1339                     }
1340                     _ => (false, None),
1341                 };
1342                 let vals = match self.values_str(&values) {
1343                     Some((expected, found)) => Some((expected, found)),
1344                     None => {
1345                         // Derived error. Cancel the emitter.
1346                         diag.cancel();
1347                         return;
1348                     }
1349                 };
1350                 (vals, exp_found, is_simple_error)
1351             }
1352         };
1353
1354         let span = cause.span(self.tcx);
1355
1356         // Ignore msg for object safe coercion
1357         // since E0038 message will be printed
1358         match terr {
1359             TypeError::ObjectUnsafeCoercion(_) => {}
1360             _ => {
1361                 diag.span_label(span, terr.to_string());
1362                 if let Some((sp, msg)) = secondary_span {
1363                     diag.span_label(sp, msg);
1364                 }
1365             }
1366         };
1367
1368         if let Some((expected, found)) = expected_found {
1369             let expected_label = exp_found.map_or("type".into(), |ef| ef.expected.prefix_string());
1370             let found_label = exp_found.map_or("type".into(), |ef| ef.found.prefix_string());
1371             match (&terr, expected == found) {
1372                 (TypeError::Sorts(values), extra) => {
1373                     let sort_string = |ty: Ty<'tcx>| match (extra, &ty.kind) {
1374                         (true, ty::Opaque(def_id, _)) => format!(
1375                             " (opaque type at {})",
1376                             self.tcx.sess.source_map()
1377                                 .mk_substr_filename(self.tcx.def_span(*def_id)),
1378                         ),
1379                         (true, _) => format!(" ({})", ty.sort_string(self.tcx)),
1380                         (false, _) => "".to_string(),
1381                     };
1382                     if !(values.expected.is_simple_text() && values.found.is_simple_text()) || (
1383                         exp_found.map_or(false, |ef| {
1384                             // This happens when the type error is a subset of the expectation,
1385                             // like when you have two references but one is `usize` and the other
1386                             // is `f32`. In those cases we still want to show the `note`. If the
1387                             // value from `ef` is `Infer(_)`, then we ignore it.
1388                             if !ef.expected.is_ty_infer() {
1389                                 ef.expected != values.expected
1390                             } else if !ef.found.is_ty_infer() {
1391                                 ef.found != values.found
1392                             } else {
1393                                 false
1394                             }
1395                         })
1396                     ) {
1397                         diag.note_expected_found_extra(
1398                             &expected_label,
1399                             expected,
1400                             &found_label,
1401                             found,
1402                             &sort_string(values.expected),
1403                             &sort_string(values.found),
1404                         );
1405                     }
1406                 }
1407                 (TypeError::ObjectUnsafeCoercion(_), _) => {
1408                     diag.note_unsuccessfull_coercion(found, expected);
1409                 }
1410                 (_, _) => {
1411                     debug!(
1412                         "note_type_err: exp_found={:?}, expected={:?} found={:?}",
1413                         exp_found, expected, found
1414                     );
1415                     if !is_simple_error || terr.must_include_note() {
1416                         diag.note_expected_found(&expected_label, expected, &found_label, found);
1417                     }
1418                 }
1419             }
1420         }
1421         if let Some(exp_found) = exp_found {
1422             self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
1423         }
1424
1425         // In some (most?) cases cause.body_id points to actual body, but in some cases
1426         // it's a actual definition. According to the comments (e.g. in
1427         // librustc_typeck/check/compare_method.rs:compare_predicate_entailment) the latter
1428         // is relied upon by some other code. This might (or might not) need cleanup.
1429         let body_owner_def_id = self.tcx.hir().opt_local_def_id(cause.body_id)
1430             .unwrap_or_else(|| {
1431                 self.tcx.hir().body_owner_def_id(hir::BodyId { hir_id: cause.body_id })
1432             });
1433         self.check_and_note_conflicting_crates(diag, terr);
1434         self.tcx.note_and_explain_type_err(diag, terr, span, body_owner_def_id);
1435
1436         // It reads better to have the error origin as the final
1437         // thing.
1438         self.note_error_origin(diag, &cause, exp_found);
1439     }
1440
1441     /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
1442     /// suggests it.
1443     fn suggest_as_ref_where_appropriate(
1444         &self,
1445         span: Span,
1446         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1447         diag: &mut DiagnosticBuilder<'tcx>,
1448     ) {
1449         match (&exp_found.expected.kind, &exp_found.found.kind) {
1450             (ty::Adt(exp_def, exp_substs), ty::Ref(_, found_ty, _)) => {
1451                 if let ty::Adt(found_def, found_substs) = found_ty.kind {
1452                     let path_str = format!("{:?}", exp_def);
1453                     if exp_def == &found_def {
1454                         let opt_msg = "you can convert from `&Option<T>` to `Option<&T>` using \
1455                                        `.as_ref()`";
1456                         let result_msg = "you can convert from `&Result<T, E>` to \
1457                                           `Result<&T, &E>` using `.as_ref()`";
1458                         let have_as_ref = &[
1459                             ("std::option::Option", opt_msg),
1460                             ("core::option::Option", opt_msg),
1461                             ("std::result::Result", result_msg),
1462                             ("core::result::Result", result_msg),
1463                         ];
1464                         if let Some(msg) = have_as_ref.iter()
1465                             .filter_map(|(path, msg)| if &path_str == path {
1466                                 Some(msg)
1467                             } else {
1468                                 None
1469                             }).next()
1470                         {
1471                             let mut show_suggestion = true;
1472                             for (exp_ty, found_ty) in exp_substs.types().zip(found_substs.types()) {
1473                                 match exp_ty.kind {
1474                                     ty::Ref(_, exp_ty, _) => {
1475                                         match (&exp_ty.kind, &found_ty.kind) {
1476                                             (_, ty::Param(_)) |
1477                                             (_, ty::Infer(_)) |
1478                                             (ty::Param(_), _) |
1479                                             (ty::Infer(_), _) => {}
1480                                             _ if ty::TyS::same_type(exp_ty, found_ty) => {}
1481                                             _ => show_suggestion = false,
1482                                         };
1483                                     }
1484                                     ty::Param(_) | ty::Infer(_) => {}
1485                                     _ => show_suggestion = false,
1486                                 }
1487                             }
1488                             if let (Ok(snippet), true) = (
1489                                 self.tcx.sess.source_map().span_to_snippet(span),
1490                                 show_suggestion,
1491                             ) {
1492                                 diag.span_suggestion(
1493                                     span,
1494                                     msg,
1495                                     format!("{}.as_ref()", snippet),
1496                                     Applicability::MachineApplicable,
1497                                 );
1498                             }
1499                         }
1500                     }
1501                 }
1502             }
1503             _ => {}
1504         }
1505     }
1506
1507     pub fn report_and_explain_type_error(
1508         &self,
1509         trace: TypeTrace<'tcx>,
1510         terr: &TypeError<'tcx>,
1511     ) -> DiagnosticBuilder<'tcx> {
1512         debug!(
1513             "report_and_explain_type_error(trace={:?}, terr={:?})",
1514             trace, terr
1515         );
1516
1517         let span = trace.cause.span(self.tcx);
1518         let failure_code = trace.cause.as_failure_code(terr);
1519         let mut diag = match failure_code {
1520             FailureCode::Error0038(did) => {
1521                 let violations = self.tcx.object_safety_violations(did);
1522                 self.tcx.report_object_safety_error(span, did, violations)
1523             }
1524             FailureCode::Error0317(failure_str) => {
1525                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
1526             }
1527             FailureCode::Error0580(failure_str) => {
1528                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
1529             }
1530             FailureCode::Error0308(failure_str) => {
1531                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
1532             }
1533             FailureCode::Error0644(failure_str) => {
1534                 struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
1535             }
1536         };
1537         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
1538         diag
1539     }
1540
1541     fn values_str(
1542         &self,
1543         values: &ValuePairs<'tcx>,
1544     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1545         match *values {
1546             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
1547             infer::Regions(ref exp_found) => self.expected_found_str(exp_found),
1548             infer::Consts(ref exp_found) => self.expected_found_str(exp_found),
1549             infer::TraitRefs(ref exp_found) => {
1550                 let pretty_exp_found = ty::error::ExpectedFound {
1551                     expected: exp_found.expected.print_only_trait_path(),
1552                     found: exp_found.found.print_only_trait_path()
1553                 };
1554                 self.expected_found_str(&pretty_exp_found)
1555             },
1556             infer::PolyTraitRefs(ref exp_found) => {
1557                 let pretty_exp_found = ty::error::ExpectedFound {
1558                     expected: exp_found.expected.print_only_trait_path(),
1559                     found: exp_found.found.print_only_trait_path()
1560                 };
1561                 self.expected_found_str(&pretty_exp_found)
1562             },
1563         }
1564     }
1565
1566     fn expected_found_str_ty(
1567         &self,
1568         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1569     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1570         let exp_found = self.resolve_vars_if_possible(exp_found);
1571         if exp_found.references_error() {
1572             return None;
1573         }
1574
1575         Some(self.cmp(exp_found.expected, exp_found.found))
1576     }
1577
1578     /// Returns a string of the form "expected `{}`, found `{}`".
1579     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
1580         &self,
1581         exp_found: &ty::error::ExpectedFound<T>,
1582     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1583         let exp_found = self.resolve_vars_if_possible(exp_found);
1584         if exp_found.references_error() {
1585             return None;
1586         }
1587
1588         Some((
1589             DiagnosticStyledString::highlighted(exp_found.expected.to_string()),
1590             DiagnosticStyledString::highlighted(exp_found.found.to_string()),
1591         ))
1592     }
1593
1594     pub fn report_generic_bound_failure(
1595         &self,
1596         region_scope_tree: &region::ScopeTree,
1597         span: Span,
1598         origin: Option<SubregionOrigin<'tcx>>,
1599         bound_kind: GenericKind<'tcx>,
1600         sub: Region<'tcx>,
1601     ) {
1602         self.construct_generic_bound_failure(region_scope_tree, span, origin, bound_kind, sub)
1603             .emit()
1604     }
1605
1606     pub fn construct_generic_bound_failure(
1607         &self,
1608         region_scope_tree: &region::ScopeTree,
1609         span: Span,
1610         origin: Option<SubregionOrigin<'tcx>>,
1611         bound_kind: GenericKind<'tcx>,
1612         sub: Region<'tcx>,
1613     ) -> DiagnosticBuilder<'a> {
1614         // Attempt to obtain the span of the parameter so we can
1615         // suggest adding an explicit lifetime bound to it.
1616         let type_param_span = match (self.in_progress_tables, bound_kind) {
1617             (Some(ref table), GenericKind::Param(ref param)) => {
1618                 let table = table.borrow();
1619                 table.local_id_root.and_then(|did| {
1620                     let generics = self.tcx.generics_of(did);
1621                     // Account for the case where `did` corresponds to `Self`, which doesn't have
1622                     // the expected type argument.
1623                     if !(generics.has_self && param.index == 0) {
1624                         let type_param = generics.type_param(param, self.tcx);
1625                         let hir = &self.tcx.hir();
1626                         hir.as_local_hir_id(type_param.def_id).map(|id| {
1627                             // Get the `hir::Param` to verify whether it already has any bounds.
1628                             // We do this to avoid suggesting code that ends up as `T: 'a'b`,
1629                             // instead we suggest `T: 'a + 'b` in that case.
1630                             let mut has_bounds = false;
1631                             if let Node::GenericParam(param) = hir.get(id) {
1632                                 has_bounds = !param.bounds.is_empty();
1633                             }
1634                             let sp = hir.span(id);
1635                             // `sp` only covers `T`, change it so that it covers
1636                             // `T:` when appropriate
1637                             let is_impl_trait = bound_kind.to_string().starts_with("impl ");
1638                             let sp = if has_bounds && !is_impl_trait {
1639                                 sp.to(self.tcx
1640                                     .sess
1641                                     .source_map()
1642                                     .next_point(self.tcx.sess.source_map().next_point(sp)))
1643                             } else {
1644                                 sp
1645                             };
1646                             (sp, has_bounds, is_impl_trait)
1647                         })
1648                     } else {
1649                         None
1650                     }
1651                 })
1652             }
1653             _ => None,
1654         };
1655
1656         let labeled_user_string = match bound_kind {
1657             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
1658             GenericKind::Projection(ref p) => format!("the associated type `{}`", p),
1659         };
1660
1661         if let Some(SubregionOrigin::CompareImplMethodObligation {
1662             span,
1663             item_name,
1664             impl_item_def_id,
1665             trait_item_def_id,
1666         }) = origin
1667         {
1668             return self.report_extra_impl_obligation(
1669                 span,
1670                 item_name,
1671                 impl_item_def_id,
1672                 trait_item_def_id,
1673                 &format!("`{}: {}`", bound_kind, sub),
1674             );
1675         }
1676
1677         fn binding_suggestion<'tcx, S: fmt::Display>(
1678             err: &mut DiagnosticBuilder<'tcx>,
1679             type_param_span: Option<(Span, bool, bool)>,
1680             bound_kind: GenericKind<'tcx>,
1681             sub: S,
1682         ) {
1683             let consider = format!(
1684                 "consider adding an explicit lifetime bound {}",
1685                 if type_param_span.map(|(_, _, is_impl_trait)| is_impl_trait).unwrap_or(false) {
1686                     format!(" `{}` to `{}`...", sub, bound_kind)
1687                 } else {
1688                     format!("`{}: {}`...", bound_kind, sub)
1689                 },
1690             );
1691             if let Some((sp, has_lifetimes, is_impl_trait)) = type_param_span {
1692                 let suggestion = if is_impl_trait {
1693                     format!("{} + {}", bound_kind, sub)
1694                 } else {
1695                     let tail = if has_lifetimes { " + " } else { "" };
1696                     format!("{}: {}{}", bound_kind, sub, tail)
1697                 };
1698                 err.span_suggestion_short(
1699                     sp,
1700                     &consider,
1701                     suggestion,
1702                     Applicability::MaybeIncorrect, // Issue #41966
1703                 );
1704             } else {
1705                 err.help(&consider);
1706             }
1707         }
1708
1709         let mut err = match *sub {
1710             ty::ReEarlyBound(_)
1711             | ty::ReFree(ty::FreeRegion {
1712                 bound_region: ty::BrNamed(..),
1713                 ..
1714             }) => {
1715                 // Does the required lifetime have a nice name we can print?
1716                 let mut err = struct_span_err!(
1717                     self.tcx.sess,
1718                     span,
1719                     E0309,
1720                     "{} may not live long enough",
1721                     labeled_user_string
1722                 );
1723                 binding_suggestion(&mut err, type_param_span, bound_kind, sub);
1724                 err
1725             }
1726
1727             ty::ReStatic => {
1728                 // Does the required lifetime have a nice name we can print?
1729                 let mut err = struct_span_err!(
1730                     self.tcx.sess,
1731                     span,
1732                     E0310,
1733                     "{} may not live long enough",
1734                     labeled_user_string
1735                 );
1736                 binding_suggestion(&mut err, type_param_span, bound_kind, "'static");
1737                 err
1738             }
1739
1740             _ => {
1741                 // If not, be less specific.
1742                 let mut err = struct_span_err!(
1743                     self.tcx.sess,
1744                     span,
1745                     E0311,
1746                     "{} may not live long enough",
1747                     labeled_user_string
1748                 );
1749                 err.help(&format!(
1750                     "consider adding an explicit lifetime bound for `{}`",
1751                     bound_kind
1752                 ));
1753                 self.tcx.note_and_explain_region(
1754                     region_scope_tree,
1755                     &mut err,
1756                     &format!("{} must be valid for ", labeled_user_string),
1757                     sub,
1758                     "...",
1759                 );
1760                 err
1761             }
1762         };
1763
1764         if let Some(origin) = origin {
1765             self.note_region_origin(&mut err, &origin);
1766         }
1767         err
1768     }
1769
1770     fn report_sub_sup_conflict(
1771         &self,
1772         region_scope_tree: &region::ScopeTree,
1773         var_origin: RegionVariableOrigin,
1774         sub_origin: SubregionOrigin<'tcx>,
1775         sub_region: Region<'tcx>,
1776         sup_origin: SubregionOrigin<'tcx>,
1777         sup_region: Region<'tcx>,
1778     ) {
1779         let mut err = self.report_inference_failure(var_origin);
1780
1781         self.tcx.note_and_explain_region(
1782             region_scope_tree,
1783             &mut err,
1784             "first, the lifetime cannot outlive ",
1785             sup_region,
1786             "...",
1787         );
1788
1789         match (&sup_origin, &sub_origin) {
1790             (&infer::Subtype(ref sup_trace), &infer::Subtype(ref sub_trace)) => {
1791                 debug!("report_sub_sup_conflict: var_origin={:?}", var_origin);
1792                 debug!("report_sub_sup_conflict: sub_region={:?}", sub_region);
1793                 debug!("report_sub_sup_conflict: sub_origin={:?}", sub_origin);
1794                 debug!("report_sub_sup_conflict: sup_region={:?}", sup_region);
1795                 debug!("report_sub_sup_conflict: sup_origin={:?}", sup_origin);
1796                 debug!("report_sub_sup_conflict: sup_trace={:?}", sup_trace);
1797                 debug!("report_sub_sup_conflict: sub_trace={:?}", sub_trace);
1798                 debug!("report_sub_sup_conflict: sup_trace.values={:?}", sup_trace.values);
1799                 debug!("report_sub_sup_conflict: sub_trace.values={:?}", sub_trace.values);
1800
1801                 if let (Some((sup_expected, sup_found)), Some((sub_expected, sub_found))) = (
1802                     self.values_str(&sup_trace.values),
1803                     self.values_str(&sub_trace.values),
1804                 ) {
1805                     if sub_expected == sup_expected && sub_found == sup_found {
1806                         self.tcx.note_and_explain_region(
1807                             region_scope_tree,
1808                             &mut err,
1809                             "...but the lifetime must also be valid for ",
1810                             sub_region,
1811                             "...",
1812                         );
1813                         err.span_note(sup_trace.cause.span, &format!(
1814                             "...so that the {}",
1815                             sup_trace.cause.as_requirement_str()
1816                         ));
1817
1818                         err.note_expected_found(
1819                             &"",
1820                             sup_expected,
1821                             &"",
1822                             sup_found
1823                         );
1824                         err.emit();
1825                         return;
1826                     }
1827                 }
1828             }
1829             _ => {}
1830         }
1831
1832         self.note_region_origin(&mut err, &sup_origin);
1833
1834         self.tcx.note_and_explain_region(
1835             region_scope_tree,
1836             &mut err,
1837             "but, the lifetime must be valid for ",
1838             sub_region,
1839             "...",
1840         );
1841
1842         self.note_region_origin(&mut err, &sub_origin);
1843         err.emit();
1844     }
1845 }
1846
1847 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
1848     fn report_inference_failure(
1849         &self,
1850         var_origin: RegionVariableOrigin,
1851     ) -> DiagnosticBuilder<'tcx> {
1852         let br_string = |br: ty::BoundRegion| {
1853             let mut s = match br {
1854                 ty::BrNamed(_, name) => name.to_string(),
1855                 _ => String::new(),
1856             };
1857             if !s.is_empty() {
1858                 s.push_str(" ");
1859             }
1860             s
1861         };
1862         let var_description = match var_origin {
1863             infer::MiscVariable(_) => String::new(),
1864             infer::PatternRegion(_) => " for pattern".to_string(),
1865             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
1866             infer::Autoref(_) => " for autoref".to_string(),
1867             infer::Coercion(_) => " for automatic coercion".to_string(),
1868             infer::LateBoundRegion(_, br, infer::FnCall) => {
1869                 format!(" for lifetime parameter {}in function call", br_string(br))
1870             }
1871             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
1872                 format!(" for lifetime parameter {}in generic type", br_string(br))
1873             }
1874             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
1875                 " for lifetime parameter {}in trait containing associated type `{}`",
1876                 br_string(br),
1877                 self.tcx.associated_item(def_id).ident
1878             ),
1879             infer::EarlyBoundRegion(_, name) => format!(" for lifetime parameter `{}`", name),
1880             infer::BoundRegionInCoherence(name) => {
1881                 format!(" for lifetime parameter `{}` in coherence check", name)
1882             }
1883             infer::UpvarRegion(ref upvar_id, _) => {
1884                 let var_name = self.tcx.hir().name(upvar_id.var_path.hir_id);
1885                 format!(" for capture of `{}` by closure", var_name)
1886             }
1887             infer::NLL(..) => bug!("NLL variable found in lexical phase"),
1888         };
1889
1890         struct_span_err!(
1891             self.tcx.sess,
1892             var_origin.span(),
1893             E0495,
1894             "cannot infer an appropriate lifetime{} \
1895              due to conflicting requirements",
1896             var_description
1897         )
1898     }
1899 }
1900
1901 enum FailureCode {
1902     Error0038(DefId),
1903     Error0317(&'static str),
1904     Error0580(&'static str),
1905     Error0308(&'static str),
1906     Error0644(&'static str),
1907 }
1908
1909 impl<'tcx> ObligationCause<'tcx> {
1910     fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode {
1911         use self::FailureCode::*;
1912         use crate::traits::ObligationCauseCode::*;
1913         match self.code {
1914             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
1915             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) =>
1916                 Error0308(match source {
1917                     hir::MatchSource::IfLetDesugar { .. } =>
1918                         "`if let` arms have incompatible types",
1919                     hir::MatchSource::TryDesugar => {
1920                         "try expression alternatives have incompatible types"
1921                     }
1922                     _ => "match arms have incompatible types",
1923                 }),
1924             IfExpression { .. } => Error0308("if and else have incompatible types"),
1925             IfExpressionWithNoElse => Error0317("if may be missing an else clause"),
1926             MainFunctionType => Error0580("main function has wrong type"),
1927             StartFunctionType => Error0308("start function has wrong type"),
1928             IntrinsicType => Error0308("intrinsic has wrong type"),
1929             MethodReceiver => Error0308("mismatched `self` parameter type"),
1930
1931             // In the case where we have no more specific thing to
1932             // say, also take a look at the error code, maybe we can
1933             // tailor to that.
1934             _ => match terr {
1935                 TypeError::CyclicTy(ty) if ty.is_closure() || ty.is_generator() => {
1936                     Error0644("closure/generator type that references itself")
1937                 }
1938                 TypeError::IntrinsicCast => {
1939                     Error0308("cannot coerce intrinsics to function pointers")
1940                 }
1941                 TypeError::ObjectUnsafeCoercion(did) => Error0038(did.clone()),
1942                 _ => Error0308("mismatched types"),
1943             },
1944         }
1945     }
1946
1947     fn as_requirement_str(&self) -> &'static str {
1948         use crate::traits::ObligationCauseCode::*;
1949         match self.code {
1950             CompareImplMethodObligation { .. } => "method type is compatible with trait",
1951             ExprAssignable => "expression is assignable",
1952             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => match source {
1953                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have compatible types",
1954                 _ => "match arms have compatible types",
1955             },
1956             IfExpression { .. } => "if and else have incompatible types",
1957             IfExpressionWithNoElse => "if missing an else returns ()",
1958             MainFunctionType => "`main` function has the correct type",
1959             StartFunctionType => "`start` function has the correct type",
1960             IntrinsicType => "intrinsic has the correct type",
1961             MethodReceiver => "method receiver has the correct type",
1962             _ => "types are compatible",
1963         }
1964     }
1965 }