src/librustc/infer/error_reporting/mod.rs

   1 //! Error Reporting Code for the inference engine
   2 //!
   3 //! Because of the way inference, and in particular region inference,
   4 //! works, it often happens that errors are not detected until far after
   5 //! the relevant line of code has been type-checked. Therefore, there is
   6 //! an elaborate system to track why a particular constraint in the
   7 //! inference graph arose so that we can explain to the user what gave
   8 //! rise to a particular error.
   9 //!
  10 //! The basis of the system are the "origin" types. An "origin" is the
  11 //! reason that a constraint or inference variable arose. There are
  12 //! different "origin" enums for different kinds of constraints/variables
  13 //! (e.g., `TypeOrigin`, `RegionVariableOrigin`). An origin always has
  14 //! a span, but also more information so that we can generate a meaningful
  15 //! error message.
  16 //!
  17 //! Having a catalog of all the different reasons an error can arise is
  18 //! also useful for other reasons, like cross-referencing FAQs etc, though
  19 //! we are not really taking advantage of this yet.
  20 //!
  21 //! # Region Inference
  22 //!
  23 //! Region inference is particularly tricky because it always succeeds "in
  24 //! the moment" and simply registers a constraint. Then, at the end, we
  25 //! can compute the full graph and report errors, so we need to be able to
  26 //! store and later report what gave rise to the conflicting constraints.
  27 //!
  28 //! # Subtype Trace
  29 //!
  30 //! Determining whether `T1 <: T2` often involves a number of subtypes and
  31 //! subconstraints along the way. A "TypeTrace" is an extended version
  32 //! of an origin that traces the types and other values that were being
  33 //! compared. It is not necessarily comprehensive (in fact, at the time of
  34 //! this writing it only tracks the root values being compared) but I'd
  35 //! like to extend it to include significant "waypoints". For example, if
  36 //! you are comparing `(T1, T2) <: (T3, T4)`, and the problem is that `T2
  37 //! <: T4` fails, I'd like the trace to include enough information to say
  38 //! "in the 2nd element of the tuple". Similarly, failures when comparing
  39 //! arguments or return types in fn types should be able to cite the
  40 //! specific position, etc.
  41 //!
  42 //! # Reality vs plan
  43 //!
  44 //! Of course, there is still a LOT of code in typeck that has yet to be
  45 //! ported to this system, and which relies on string concatenation at the
  46 //! time of error detection.
  47
  48 use super::lexical_region_resolve::RegionResolutionError;
  49 use super::region_constraints::GenericKind;
  50 use super::{InferCtxt, RegionVariableOrigin, SubregionOrigin, TypeTrace, ValuePairs};
  51 use crate::infer::{self, SuppressRegionErrors};
  52
  53 use crate::hir;
  54 use crate::hir::def_id::DefId;
  55 use crate::hir::Node;
  56 use crate::infer::opaque_types;
  57 use crate::middle::region;
  58 use crate::traits::{IfExpressionCause, MatchExpressionArmCause, ObligationCause};
  59 use crate::traits::{ObligationCauseCode};
  60 use crate::ty::error::TypeError;
  61 use crate::ty::{self, subst::{Subst, SubstsRef}, Region, Ty, TyCtxt, TypeFoldable};
  62 use errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString};
  63 use std::{cmp, fmt};
  64 use syntax_pos::{Pos, Span};
  65
  66 mod note;
  67
  68 mod need_type_info;
  69
  70 pub mod nice_region_error;
  71
  72 impl<'tcx> TyCtxt<'tcx> {
  73     pub fn note_and_explain_region(
  74         self,
  75         region_scope_tree: &region::ScopeTree,
  76         err: &mut DiagnosticBuilder<'_>,
  77         prefix: &str,
  78         region: ty::Region<'tcx>,
  79         suffix: &str,
  80     ) {
  81         let (description, span) = match *region {
  82             ty::ReScope(scope) => {
  83                 let new_string;
  84                 let unknown_scope = || {
  85                     format!(
  86                         "{}unknown scope: {:?}{}.  Please report a bug.",
  87                         prefix, scope, suffix
  88                     )
  89                 };
  90                 let span = scope.span(self, region_scope_tree);
  91                 let tag = match self.hir().find(scope.hir_id(region_scope_tree)) {
  92                     Some(Node::Block(_)) => "block",
  93                     Some(Node::Expr(expr)) => match expr.kind {
  94                         hir::ExprKind::Call(..) => "call",
  95                         hir::ExprKind::MethodCall(..) => "method call",
  96                         hir::ExprKind::Match(.., hir::MatchSource::IfLetDesugar { .. }) => "if let",
  97                         hir::ExprKind::Match(.., hir::MatchSource::WhileLetDesugar) => "while let",
  98                         hir::ExprKind::Match(.., hir::MatchSource::ForLoopDesugar) => "for",
  99                         hir::ExprKind::Match(..) => "match",
 100                         _ => "expression",
 101                     },
 102                     Some(Node::Stmt(_)) => "statement",
 103                     Some(Node::Item(it)) => Self::item_scope_tag(&it),
 104                     Some(Node::TraitItem(it)) => Self::trait_item_scope_tag(&it),
 105                     Some(Node::ImplItem(it)) => Self::impl_item_scope_tag(&it),
 106                     Some(_) | None => {
 107                         err.span_note(span, &unknown_scope());
 108                         return;
 109                     }
 110                 };
 111                 let scope_decorated_tag = match scope.data {
 112                     region::ScopeData::Node => tag,
 113                     region::ScopeData::CallSite => "scope of call-site for function",
 114                     region::ScopeData::Arguments => "scope of function body",
 115                     region::ScopeData::Destruction => {
 116                         new_string = format!("destruction scope surrounding {}", tag);
 117                         &new_string[..]
 118                     }
 119                     region::ScopeData::Remainder(first_statement_index) => {
 120                         new_string = format!(
 121                             "block suffix following statement {}",
 122                             first_statement_index.index()
 123                         );
 124                         &new_string[..]
 125                     }
 126                 };
 127                 self.explain_span(scope_decorated_tag, span)
 128             }
 129
 130             ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic => {
 131                 self.msg_span_from_free_region(region)
 132             }
 133
 134             ty::ReEmpty => ("the empty lifetime".to_owned(), None),
 135
 136             ty::RePlaceholder(_) => (format!("any other region"), None),
 137
 138             // FIXME(#13998) RePlaceholder should probably print like
 139             // ReFree rather than dumping Debug output on the user.
 140             //
 141             // We shouldn't really be having unification failures with ReVar
 142             // and ReLateBound though.
 143             ty::ReVar(_) | ty::ReLateBound(..) | ty::ReErased => {
 144                 (format!("lifetime {:?}", region), None)
 145             }
 146
 147             // We shouldn't encounter an error message with ReClosureBound.
 148             ty::ReClosureBound(..) => {
 149                 bug!("encountered unexpected ReClosureBound: {:?}", region,);
 150             }
 151         };
 152
 153         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 154     }
 155
 156     pub fn note_and_explain_free_region(
 157         self,
 158         err: &mut DiagnosticBuilder<'_>,
 159         prefix: &str,
 160         region: ty::Region<'tcx>,
 161         suffix: &str,
 162     ) {
 163         let (description, span) = self.msg_span_from_free_region(region);
 164
 165         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 166     }
 167
 168     fn msg_span_from_free_region(self, region: ty::Region<'tcx>) -> (String, Option<Span>) {
 169         match *region {
 170             ty::ReEarlyBound(_) | ty::ReFree(_) => {
 171                 self.msg_span_from_early_bound_and_free_regions(region)
 172             }
 173             ty::ReStatic => ("the static lifetime".to_owned(), None),
 174             ty::ReEmpty => ("an empty lifetime".to_owned(), None),
 175             _ => bug!("{:?}", region),
 176         }
 177     }
 178
 179     fn msg_span_from_early_bound_and_free_regions(
 180         self,
 181         region: ty::Region<'tcx>,
 182     ) -> (String, Option<Span>) {
 183         let cm = self.sess.source_map();
 184
 185         let scope = region.free_region_binding_scope(self);
 186         let node = self.hir().as_local_hir_id(scope).unwrap_or(hir::DUMMY_HIR_ID);
 187         let tag = match self.hir().find(node) {
 188             Some(Node::Block(_)) | Some(Node::Expr(_)) => "body",
 189             Some(Node::Item(it)) => Self::item_scope_tag(&it),
 190             Some(Node::TraitItem(it)) => Self::trait_item_scope_tag(&it),
 191             Some(Node::ImplItem(it)) => Self::impl_item_scope_tag(&it),
 192             _ => unreachable!(),
 193         };
 194         let (prefix, span) = match *region {
 195             ty::ReEarlyBound(ref br) => {
 196                 let mut sp = cm.def_span(self.hir().span(node));
 197                 if let Some(param) = self.hir()
 198                     .get_generics(scope)
 199                     .and_then(|generics| generics.get_named(br.name))
 200                 {
 201                     sp = param.span;
 202                 }
 203                 (format!("the lifetime {} as defined on", br.name), sp)
 204             }
 205             ty::ReFree(ty::FreeRegion {
 206                 bound_region: ty::BoundRegion::BrNamed(_, name),
 207                 ..
 208             }) => {
 209                 let mut sp = cm.def_span(self.hir().span(node));
 210                 if let Some(param) = self.hir()
 211                     .get_generics(scope)
 212                     .and_then(|generics| generics.get_named(name))
 213                 {
 214                     sp = param.span;
 215                 }
 216                 (format!("the lifetime {} as defined on", name), sp)
 217             }
 218             ty::ReFree(ref fr) => match fr.bound_region {
 219                 ty::BrAnon(idx) => (
 220                     format!("the anonymous lifetime #{} defined on", idx + 1),
 221                     self.hir().span(node),
 222                 ),
 223                 _ => (
 224                     format!("the lifetime {} as defined on", region),
 225                     cm.def_span(self.hir().span(node)),
 226                 ),
 227             },
 228             _ => bug!(),
 229         };
 230         let (msg, opt_span) = self.explain_span(tag, span);
 231         (format!("{} {}", prefix, msg), opt_span)
 232     }
 233
 234     fn emit_msg_span(
 235         err: &mut DiagnosticBuilder<'_>,
 236         prefix: &str,
 237         description: String,
 238         span: Option<Span>,
 239         suffix: &str,
 240     ) {
 241         let message = format!("{}{}{}", prefix, description, suffix);
 242
 243         if let Some(span) = span {
 244             err.span_note(span, &message);
 245         } else {
 246             err.note(&message);
 247         }
 248     }
 249
 250     fn item_scope_tag(item: &hir::Item) -> &'static str {
 251         match item.node {
 252             hir::ItemKind::Impl(..) => "impl",
 253             hir::ItemKind::Struct(..) => "struct",
 254             hir::ItemKind::Union(..) => "union",
 255             hir::ItemKind::Enum(..) => "enum",
 256             hir::ItemKind::Trait(..) => "trait",
 257             hir::ItemKind::Fn(..) => "function body",
 258             _ => "item",
 259         }
 260     }
 261
 262     fn trait_item_scope_tag(item: &hir::TraitItem) -> &'static str {
 263         match item.kind {
 264             hir::TraitItemKind::Method(..) => "method body",
 265             hir::TraitItemKind::Const(..) | hir::TraitItemKind::Type(..) => "associated item",
 266         }
 267     }
 268
 269     fn impl_item_scope_tag(item: &hir::ImplItem) -> &'static str {
 270         match item.kind {
 271             hir::ImplItemKind::Method(..) => "method body",
 272             hir::ImplItemKind::Const(..)
 273             | hir::ImplItemKind::OpaqueTy(..)
 274             | hir::ImplItemKind::TyAlias(..) => "associated item",
 275         }
 276     }
 277
 278     fn explain_span(self, heading: &str, span: Span) -> (String, Option<Span>) {
 279         let lo = self.sess.source_map().lookup_char_pos(span.lo());
 280         (
 281             format!("the {} at {}:{}", heading, lo.line, lo.col.to_usize() + 1),
 282             Some(span),
 283         )
 284     }
 285 }
 286
 287 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 288     pub fn report_region_errors(
 289         &self,
 290         region_scope_tree: &region::ScopeTree,
 291         errors: &Vec<RegionResolutionError<'tcx>>,
 292         suppress: SuppressRegionErrors,
 293     ) {
 294         debug!(
 295             "report_region_errors(): {} errors to start, suppress = {:?}",
 296             errors.len(),
 297             suppress
 298         );
 299
 300         if suppress.suppressed() {
 301             return;
 302         }
 303
 304         // try to pre-process the errors, which will group some of them
 305         // together into a `ProcessedErrors` group:
 306         let errors = self.process_errors(errors);
 307
 308         debug!(
 309             "report_region_errors: {} errors after preprocessing",
 310             errors.len()
 311         );
 312
 313         for error in errors {
 314             debug!("report_region_errors: error = {:?}", error);
 315
 316             if !self.try_report_nice_region_error(&error) {
 317                 match error.clone() {
 318                     // These errors could indicate all manner of different
 319                     // problems with many different solutions. Rather
 320                     // than generate a "one size fits all" error, what we
 321                     // attempt to do is go through a number of specific
 322                     // scenarios and try to find the best way to present
 323                     // the error. If all of these fails, we fall back to a rather
 324                     // general bit of code that displays the error information
 325                     RegionResolutionError::ConcreteFailure(origin, sub, sup) => {
 326                         if sub.is_placeholder() || sup.is_placeholder() {
 327                             self.report_placeholder_failure(region_scope_tree, origin, sub, sup)
 328                                 .emit();
 329                         } else {
 330                             self.report_concrete_failure(region_scope_tree, origin, sub, sup)
 331                                 .emit();
 332                         }
 333                     }
 334
 335                     RegionResolutionError::GenericBoundFailure(origin, param_ty, sub) => {
 336                         self.report_generic_bound_failure(
 337                             region_scope_tree,
 338                             origin.span(),
 339                             Some(origin),
 340                             param_ty,
 341                             sub,
 342                         );
 343                     }
 344
 345                     RegionResolutionError::SubSupConflict(
 346                         _,
 347                         var_origin,
 348                         sub_origin,
 349                         sub_r,
 350                         sup_origin,
 351                         sup_r,
 352                     ) => {
 353                         if sub_r.is_placeholder() {
 354                             self.report_placeholder_failure(
 355                                 region_scope_tree,
 356                                 sub_origin,
 357                                 sub_r,
 358                                 sup_r,
 359                             )
 360                                 .emit();
 361                         } else if sup_r.is_placeholder() {
 362                             self.report_placeholder_failure(
 363                                 region_scope_tree,
 364                                 sup_origin,
 365                                 sub_r,
 366                                 sup_r,
 367                             )
 368                                 .emit();
 369                         } else {
 370                             self.report_sub_sup_conflict(
 371                                 region_scope_tree,
 372                                 var_origin,
 373                                 sub_origin,
 374                                 sub_r,
 375                                 sup_origin,
 376                                 sup_r,
 377                             );
 378                         }
 379                     }
 380
 381                     RegionResolutionError::MemberConstraintFailure {
 382                         opaque_type_def_id,
 383                         hidden_ty,
 384                         member_region,
 385                         span: _,
 386                         choice_regions: _,
 387                     } => {
 388                         let hidden_ty = self.resolve_vars_if_possible(&hidden_ty);
 389                         opaque_types::unexpected_hidden_region_diagnostic(
 390                             self.tcx,
 391                             Some(region_scope_tree),
 392                             opaque_type_def_id,
 393                             hidden_ty,
 394                             member_region,
 395                         ).emit();
 396                     }
 397                 }
 398             }
 399         }
 400     }
 401
 402     // This method goes through all the errors and try to group certain types
 403     // of error together, for the purpose of suggesting explicit lifetime
 404     // parameters to the user. This is done so that we can have a more
 405     // complete view of what lifetimes should be the same.
 406     // If the return value is an empty vector, it means that processing
 407     // failed (so the return value of this method should not be used).
 408     //
 409     // The method also attempts to weed out messages that seem like
 410     // duplicates that will be unhelpful to the end-user. But
 411     // obviously it never weeds out ALL errors.
 412     fn process_errors(
 413         &self,
 414         errors: &Vec<RegionResolutionError<'tcx>>,
 415     ) -> Vec<RegionResolutionError<'tcx>> {
 416         debug!("process_errors()");
 417
 418         // We want to avoid reporting generic-bound failures if we can
 419         // avoid it: these have a very high rate of being unhelpful in
 420         // practice. This is because they are basically secondary
 421         // checks that test the state of the region graph after the
 422         // rest of inference is done, and the other kinds of errors
 423         // indicate that the region constraint graph is internally
 424         // inconsistent, so these test results are likely to be
 425         // meaningless.
 426         //
 427         // Therefore, we filter them out of the list unless they are
 428         // the only thing in the list.
 429
 430         let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
 431             RegionResolutionError::GenericBoundFailure(..) => true,
 432             RegionResolutionError::ConcreteFailure(..)
 433                 | RegionResolutionError::SubSupConflict(..)
 434                 | RegionResolutionError::MemberConstraintFailure { .. } => false,
 435         };
 436
 437         let mut errors = if errors.iter().all(|e| is_bound_failure(e)) {
 438             errors.clone()
 439         } else {
 440             errors
 441             .iter()
 442             .filter(|&e| !is_bound_failure(e))
 443             .cloned()
 444             .collect()
 445         };
 446
 447         // sort the errors by span, for better error message stability.
 448         errors.sort_by_key(|u| match *u {
 449             RegionResolutionError::ConcreteFailure(ref sro, _, _) => sro.span(),
 450             RegionResolutionError::GenericBoundFailure(ref sro, _, _) => sro.span(),
 451             RegionResolutionError::SubSupConflict(_, ref rvo, _, _, _, _) => rvo.span(),
 452             RegionResolutionError::MemberConstraintFailure { span, .. } => span,
 453         });
 454         errors
 455     }
 456
 457     /// Adds a note if the types come from similarly named crates
 458     fn check_and_note_conflicting_crates(
 459         &self,
 460         err: &mut DiagnosticBuilder<'_>,
 461         terr: &TypeError<'tcx>,
 462         sp: Span,
 463     ) {
 464         use hir::def_id::CrateNum;
 465         use hir::map::DisambiguatedDefPathData;
 466         use ty::print::Printer;
 467         use ty::subst::GenericArg;
 468
 469         struct AbsolutePathPrinter<'tcx> {
 470             tcx: TyCtxt<'tcx>,
 471         }
 472
 473         struct NonTrivialPath;
 474
 475         impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
 476             type Error = NonTrivialPath;
 477
 478             type Path = Vec<String>;
 479             type Region = !;
 480             type Type = !;
 481             type DynExistential = !;
 482             type Const = !;
 483
 484             fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
 485                 self.tcx
 486             }
 487
 488             fn print_region(
 489                 self,
 490                 _region: ty::Region<'_>,
 491             ) -> Result<Self::Region, Self::Error> {
 492                 Err(NonTrivialPath)
 493             }
 494
 495             fn print_type(
 496                 self,
 497                 _ty: Ty<'tcx>,
 498             ) -> Result<Self::Type, Self::Error> {
 499                 Err(NonTrivialPath)
 500             }
 501
 502             fn print_dyn_existential(
 503                 self,
 504                 _predicates: &'tcx ty::List<ty::ExistentialPredicate<'tcx>>,
 505             ) -> Result<Self::DynExistential, Self::Error> {
 506                 Err(NonTrivialPath)
 507             }
 508
 509             fn print_const(
 510                 self,
 511                 _ct: &'tcx ty::Const<'tcx>,
 512             ) -> Result<Self::Const, Self::Error> {
 513                 Err(NonTrivialPath)
 514             }
 515
 516             fn path_crate(
 517                 self,
 518                 cnum: CrateNum,
 519             ) -> Result<Self::Path, Self::Error> {
 520                 Ok(vec![self.tcx.original_crate_name(cnum).to_string()])
 521             }
 522             fn path_qualified(
 523                 self,
 524                 _self_ty: Ty<'tcx>,
 525                 _trait_ref: Option<ty::TraitRef<'tcx>>,
 526             ) -> Result<Self::Path, Self::Error> {
 527                 Err(NonTrivialPath)
 528             }
 529
 530             fn path_append_impl(
 531                 self,
 532                 _print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 533                 _disambiguated_data: &DisambiguatedDefPathData,
 534                 _self_ty: Ty<'tcx>,
 535                 _trait_ref: Option<ty::TraitRef<'tcx>>,
 536             ) -> Result<Self::Path, Self::Error> {
 537                 Err(NonTrivialPath)
 538             }
 539             fn path_append(
 540                 self,
 541                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 542                 disambiguated_data: &DisambiguatedDefPathData,
 543             ) -> Result<Self::Path, Self::Error> {
 544                 let mut path = print_prefix(self)?;
 545                 path.push(disambiguated_data.data.as_interned_str().to_string());
 546                 Ok(path)
 547             }
 548             fn path_generic_args(
 549                 self,
 550                 print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
 551                 _args: &[GenericArg<'tcx>],
 552             ) -> Result<Self::Path, Self::Error> {
 553                 print_prefix(self)
 554             }
 555         }
 556
 557         let report_path_match = |err: &mut DiagnosticBuilder<'_>, did1: DefId, did2: DefId| {
 558             // Only external crates, if either is from a local
 559             // module we could have false positives
 560             if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
 561                 let abs_path = |def_id| {
 562                     AbsolutePathPrinter { tcx: self.tcx }
 563                         .print_def_path(def_id, &[])
 564                 };
 565
 566                 // We compare strings because DefPath can be different
 567                 // for imported and non-imported crates
 568                 let same_path = || -> Result<_, NonTrivialPath> {
 569                     Ok(
 570                         self.tcx.def_path_str(did1) == self.tcx.def_path_str(did2) ||
 571                         abs_path(did1)? == abs_path(did2)?
 572                     )
 573                 };
 574                 if same_path().unwrap_or(false) {
 575                     let crate_name = self.tcx.crate_name(did1.krate);
 576                     err.span_note(
 577                         sp,
 578                         &format!(
 579                             "Perhaps two different versions \
 580                              of crate `{}` are being used?",
 581                             crate_name
 582                         ),
 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             //self.push_comma(&mut value, &mut other_value, len, i);
 766         }
 767         if len > 0 {
 768             value.push_highlighted(">");
 769         }
 770     }
 771
 772     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 773     /// as that is the difference to the other type.
 774     ///
 775     /// For the following code:
 776     ///
 777     /// ```norun
 778     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 779     /// ```
 780     ///
 781     /// The type error output will behave in the following way:
 782     ///
 783     /// ```text
 784     /// Foo<Bar<Qux>>
 785     /// ^^^^--------^ this is highlighted
 786     /// |   |
 787     /// |   this type argument is exactly the same as the other type, not highlighted
 788     /// this is highlighted
 789     /// Bar<Qux>
 790     /// -------- this type is the same as a type argument in the other type, not highlighted
 791     /// ```
 792     fn cmp_type_arg(
 793         &self,
 794         mut t1_out: &mut DiagnosticStyledString,
 795         mut t2_out: &mut DiagnosticStyledString,
 796         path: String,
 797         sub: ty::subst::SubstsRef<'tcx>,
 798         other_path: String,
 799         other_ty: Ty<'tcx>,
 800     ) -> Option<()> {
 801         for (i, ta) in sub.types().enumerate() {
 802             if ta == other_ty {
 803                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 804                 return Some(());
 805             }
 806             if let &ty::Adt(def, _) = &ta.kind {
 807                 let path_ = self.tcx.def_path_str(def.did.clone());
 808                 if path_ == other_path {
 809                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 810                     return Some(());
 811                 }
 812             }
 813         }
 814         None
 815     }
 816
 817     /// Adds a `,` to the type representation only if it is appropriate.
 818     fn push_comma(
 819         &self,
 820         value: &mut DiagnosticStyledString,
 821         other_value: &mut DiagnosticStyledString,
 822         len: usize,
 823         pos: usize,
 824     ) {
 825         if len > 0 && pos != len - 1 {
 826             value.push_normal(", ");
 827             other_value.push_normal(", ");
 828         }
 829     }
 830
 831     /// For generic types with parameters with defaults, remove the parameters corresponding to
 832     /// the defaults. This repeats a lot of the logic found in `ty::print::pretty`.
 833     fn strip_generic_default_params(
 834         &self,
 835         def_id: DefId,
 836         substs: ty::subst::SubstsRef<'tcx>,
 837     ) -> SubstsRef<'tcx> {
 838         let generics = self.tcx.generics_of(def_id);
 839         let mut num_supplied_defaults = 0;
 840         let mut type_params = generics.params.iter().rev().filter_map(|param| match param.kind {
 841             ty::GenericParamDefKind::Lifetime => None,
 842             ty::GenericParamDefKind::Type { has_default, .. } => Some((param.def_id, has_default)),
 843             ty::GenericParamDefKind::Const => None, // FIXME(const_generics:defaults)
 844         }).peekable();
 845         let has_default = {
 846             let has_default = type_params.peek().map(|(_, has_default)| has_default);
 847             *has_default.unwrap_or(&false)
 848         };
 849         if has_default {
 850             let types = substs.types().rev();
 851             for ((def_id, has_default), actual) in type_params.zip(types) {
 852                 if !has_default {
 853                     break;
 854                 }
 855                 if self.tcx.type_of(def_id).subst(self.tcx, substs) != actual {
 856                     break;
 857                 }
 858                 num_supplied_defaults += 1;
 859             }
 860         }
 861         let len = generics.params.len();
 862         let mut generics = generics.clone();
 863         generics.params.truncate(len - num_supplied_defaults);
 864         substs.truncate_to(self.tcx, &generics)
 865     }
 866
 867     /// Compares two given types, eliding parts that are the same between them and highlighting
 868     /// relevant differences, and return two representation of those types for highlighted printing.
 869     fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> (DiagnosticStyledString, DiagnosticStyledString) {
 870         fn equals<'tcx>(a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 871             match (&a.kind, &b.kind) {
 872                 (a, b) if *a == *b => true,
 873                 (&ty::Int(_), &ty::Infer(ty::InferTy::IntVar(_)))
 874                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Int(_))
 875                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Infer(ty::InferTy::IntVar(_)))
 876                 | (&ty::Float(_), &ty::Infer(ty::InferTy::FloatVar(_)))
 877                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Float(_))
 878                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Infer(ty::InferTy::FloatVar(_))) => {
 879                     true
 880                 }
 881                 _ => false,
 882             }
 883         }
 884
 885         fn push_ty_ref<'tcx>(
 886             r: &ty::Region<'tcx>,
 887             ty: Ty<'tcx>,
 888             mutbl: hir::Mutability,
 889             s: &mut DiagnosticStyledString,
 890         ) {
 891             let mut r = r.to_string();
 892             if r == "'_" {
 893                 r.clear();
 894             } else {
 895                 r.push(' ');
 896             }
 897             s.push_highlighted(format!(
 898                 "&{}{}",
 899                 r,
 900                 if mutbl == hir::MutMutable { "mut " } else { "" }
 901             ));
 902             s.push_normal(ty.to_string());
 903         }
 904
 905         match (&t1.kind, &t2.kind) {
 906             (&ty::Adt(def1, sub1), &ty::Adt(def2, sub2)) => {
 907                 let sub_no_defaults_1 = self.strip_generic_default_params(def1.did, sub1);
 908                 let sub_no_defaults_2 = self.strip_generic_default_params(def2.did, sub2);
 909                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 910                 let path1 = self.tcx.def_path_str(def1.did.clone());
 911                 let path2 = self.tcx.def_path_str(def2.did.clone());
 912                 if def1.did == def2.did {
 913                     // Easy case. Replace same types with `_` to shorten the output and highlight
 914                     // the differing ones.
 915                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
 916                     //     Foo<Bar, _>
 917                     //     Foo<Quz, _>
 918                     //         ---  ^ type argument elided
 919                     //         |
 920                     //         highlighted in output
 921                     values.0.push_normal(path1);
 922                     values.1.push_normal(path2);
 923
 924                     // Avoid printing out default generic parameters that are common to both
 925                     // types.
 926                     let len1 = sub_no_defaults_1.len();
 927                     let len2 = sub_no_defaults_2.len();
 928                     let common_len = cmp::min(len1, len2);
 929                     let remainder1: Vec<_> = sub1.types().skip(common_len).collect();
 930                     let remainder2: Vec<_> = sub2.types().skip(common_len).collect();
 931                     let common_default_params = remainder1
 932                         .iter()
 933                         .rev()
 934                         .zip(remainder2.iter().rev())
 935                         .filter(|(a, b)| a == b)
 936                         .count();
 937                     let len = sub1.len() - common_default_params;
 938
 939                     // Only draw `<...>` if there're lifetime/type arguments.
 940                     if len > 0 {
 941                         values.0.push_normal("<");
 942                         values.1.push_normal("<");
 943                     }
 944
 945                     fn lifetime_display(lifetime: Region<'_>) -> String {
 946                         let s = lifetime.to_string();
 947                         if s.is_empty() {
 948                             "'_".to_string()
 949                         } else {
 950                             s
 951                         }
 952                     }
 953                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
 954                     // all diagnostics that use this output
 955                     //
 956                     //     Foo<'x, '_, Bar>
 957                     //     Foo<'y, '_, Qux>
 958                     //         ^^  ^^  --- type arguments are not elided
 959                     //         |   |
 960                     //         |   elided as they were the same
 961                     //         not elided, they were different, but irrelevant
 962                     let lifetimes = sub1.regions().zip(sub2.regions());
 963                     for (i, lifetimes) in lifetimes.enumerate() {
 964                         let l1 = lifetime_display(lifetimes.0);
 965                         let l2 = lifetime_display(lifetimes.1);
 966                         if l1 == l2 {
 967                             values.0.push_normal("'_");
 968                             values.1.push_normal("'_");
 969                         } else {
 970                             values.0.push_highlighted(l1);
 971                             values.1.push_highlighted(l2);
 972                         }
 973                         self.push_comma(&mut values.0, &mut values.1, len, i);
 974                     }
 975
 976                     // We're comparing two types with the same path, so we compare the type
 977                     // arguments for both. If they are the same, do not highlight and elide from the
 978                     // output.
 979                     //     Foo<_, Bar>
 980                     //     Foo<_, Qux>
 981                     //         ^ elided type as this type argument was the same in both sides
 982                     let type_arguments = sub1.types().zip(sub2.types());
 983                     let regions_len = sub1.regions().count();
 984                     for (i, (ta1, ta2)) in type_arguments.take(len).enumerate() {
 985                         let i = i + regions_len;
 986                         if ta1 == ta2 {
 987                             values.0.push_normal("_");
 988                             values.1.push_normal("_");
 989                         } else {
 990                             let (x1, x2) = self.cmp(ta1, ta2);
 991                             (values.0).0.extend(x1.0);
 992                             (values.1).0.extend(x2.0);
 993                         }
 994                         self.push_comma(&mut values.0, &mut values.1, len, i);
 995                     }
 996
 997                     // Close the type argument bracket.
 998                     // Only draw `<...>` if there're lifetime/type arguments.
 999                     if len > 0 {
1000                         values.0.push_normal(">");
1001                         values.1.push_normal(">");
1002                     }
1003                     values
1004                 } else {
1005                     // Check for case:
1006                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
1007                     //     Foo<Bar<Qux>
1008                     //         ------- this type argument is exactly the same as the other type
1009                     //     Bar<Qux>
1010                     if self.cmp_type_arg(
1011                         &mut values.0,
1012                         &mut values.1,
1013                         path1.clone(),
1014                         sub_no_defaults_1,
1015                         path2.clone(),
1016                         &t2,
1017                     ).is_some()
1018                     {
1019                         return values;
1020                     }
1021                     // Check for case:
1022                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
1023                     //     Bar<Qux>
1024                     //     Foo<Bar<Qux>>
1025                     //         ------- this type argument is exactly the same as the other type
1026                     if self.cmp_type_arg(
1027                         &mut values.1,
1028                         &mut values.0,
1029                         path2,
1030                         sub_no_defaults_2,
1031                         path1,
1032                         &t1,
1033                     ).is_some()
1034                     {
1035                         return values;
1036                     }
1037
1038                     // We couldn't find anything in common, highlight everything.
1039                     //     let x: Bar<Qux> = y::<Foo<Zar>>();
1040                     (
1041                         DiagnosticStyledString::highlighted(t1.to_string()),
1042                         DiagnosticStyledString::highlighted(t2.to_string()),
1043                     )
1044                 }
1045             }
1046
1047             // When finding T != &T, highlight only the borrow
1048             (&ty::Ref(r1, ref_ty1, mutbl1), _) if equals(&ref_ty1, &t2) => {
1049                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1050                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
1051                 values.1.push_normal(t2.to_string());
1052                 values
1053             }
1054             (_, &ty::Ref(r2, ref_ty2, mutbl2)) if equals(&t1, &ref_ty2) => {
1055                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1056                 values.0.push_normal(t1.to_string());
1057                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
1058                 values
1059             }
1060
1061             // When encountering &T != &mut T, highlight only the borrow
1062             (&ty::Ref(r1, ref_ty1, mutbl1), &ty::Ref(r2, ref_ty2, mutbl2))
1063                 if equals(&ref_ty1, &ref_ty2) =>
1064             {
1065                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
1066                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
1067                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
1068                 values
1069             }
1070
1071             _ => {
1072                 if t1 == t2 {
1073                     // The two types are the same, elide and don't highlight.
1074                     (
1075                         DiagnosticStyledString::normal("_"),
1076                         DiagnosticStyledString::normal("_"),
1077                     )
1078                 } else {
1079                     // We couldn't find anything in common, highlight everything.
1080                     (
1081                         DiagnosticStyledString::highlighted(t1.to_string()),
1082                         DiagnosticStyledString::highlighted(t2.to_string()),
1083                     )
1084                 }
1085             }
1086         }
1087     }
1088
1089     pub fn note_type_err(
1090         &self,
1091         diag: &mut DiagnosticBuilder<'tcx>,
1092         cause: &ObligationCause<'tcx>,
1093         secondary_span: Option<(Span, String)>,
1094         mut values: Option<ValuePairs<'tcx>>,
1095         terr: &TypeError<'tcx>,
1096     ) {
1097         // For some types of errors, expected-found does not make
1098         // sense, so just ignore the values we were given.
1099         match terr {
1100             TypeError::CyclicTy(_) => {
1101                 values = None;
1102             }
1103             _ => {}
1104         }
1105
1106         let (expected_found, exp_found, is_simple_error) = match values {
1107             None => (None, None, false),
1108             Some(values) => {
1109                 let (is_simple_error, exp_found) = match values {
1110                     ValuePairs::Types(exp_found) => {
1111                         let is_simple_err =
1112                             exp_found.expected.is_primitive() && exp_found.found.is_primitive();
1113
1114                         (is_simple_err, Some(exp_found))
1115                     }
1116                     _ => (false, None),
1117                 };
1118                 let vals = match self.values_str(&values) {
1119                     Some((expected, found)) => Some((expected, found)),
1120                     None => {
1121                         // Derived error. Cancel the emitter.
1122                         diag.cancel();
1123                         return;
1124                     }
1125                 };
1126                 (vals, exp_found, is_simple_error)
1127             }
1128         };
1129
1130         let span = cause.span(self.tcx);
1131
1132         diag.span_label(span, terr.to_string());
1133         if let Some((sp, msg)) = secondary_span {
1134             diag.span_label(sp, msg);
1135         }
1136
1137         if let Some((expected, found)) = expected_found {
1138             match (terr, is_simple_error, expected == found) {
1139                 (&TypeError::Sorts(ref values), false, true) => {
1140                     let sort_string = | a_type: Ty<'tcx> |
1141                         if let ty::Opaque(def_id, _) = a_type.kind {
1142                             format!(" (opaque type at {})", self.tcx.sess.source_map()
1143                                 .mk_substr_filename(self.tcx.def_span(def_id)))
1144                         } else {
1145                             format!(" ({})", a_type.sort_string(self.tcx))
1146                         };
1147                     diag.note_expected_found_extra(
1148                         &"type",
1149                         expected,
1150                         found,
1151                         &sort_string(values.expected),
1152                         &sort_string(values.found),
1153                     );
1154                 }
1155                 (_, false, _) => {
1156                     if let Some(exp_found) = exp_found {
1157                         self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
1158                     }
1159
1160                     diag.note_expected_found(&"type", expected, found);
1161                 }
1162                 _ => (),
1163             }
1164         }
1165
1166         self.check_and_note_conflicting_crates(diag, terr, span);
1167         self.tcx.note_and_explain_type_err(diag, terr, span);
1168
1169         // It reads better to have the error origin as the final
1170         // thing.
1171         self.note_error_origin(diag, &cause, exp_found);
1172     }
1173
1174     /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
1175     /// suggest it.
1176     fn suggest_as_ref_where_appropriate(
1177         &self,
1178         span: Span,
1179         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1180         diag: &mut DiagnosticBuilder<'tcx>,
1181     ) {
1182         match (&exp_found.expected.kind, &exp_found.found.kind) {
1183             (ty::Adt(exp_def, exp_substs), ty::Ref(_, found_ty, _)) => {
1184                 if let ty::Adt(found_def, found_substs) = found_ty.kind {
1185                     let path_str = format!("{:?}", exp_def);
1186                     if exp_def == &found_def {
1187                         let opt_msg = "you can convert from `&Option<T>` to `Option<&T>` using \
1188                                        `.as_ref()`";
1189                         let result_msg = "you can convert from `&Result<T, E>` to \
1190                                           `Result<&T, &E>` using `.as_ref()`";
1191                         let have_as_ref = &[
1192                             ("std::option::Option", opt_msg),
1193                             ("core::option::Option", opt_msg),
1194                             ("std::result::Result", result_msg),
1195                             ("core::result::Result", result_msg),
1196                         ];
1197                         if let Some(msg) = have_as_ref.iter()
1198                             .filter_map(|(path, msg)| if &path_str == path {
1199                                 Some(msg)
1200                             } else {
1201                                 None
1202                             }).next()
1203                         {
1204                             let mut show_suggestion = true;
1205                             for (exp_ty, found_ty) in exp_substs.types().zip(found_substs.types()) {
1206                                 match exp_ty.kind {
1207                                     ty::Ref(_, exp_ty, _) => {
1208                                         match (&exp_ty.kind, &found_ty.kind) {
1209                                             (_, ty::Param(_)) |
1210                                             (_, ty::Infer(_)) |
1211                                             (ty::Param(_), _) |
1212                                             (ty::Infer(_), _) => {}
1213                                             _ if ty::TyS::same_type(exp_ty, found_ty) => {}
1214                                             _ => show_suggestion = false,
1215                                         };
1216                                     }
1217                                     ty::Param(_) | ty::Infer(_) => {}
1218                                     _ => show_suggestion = false,
1219                                 }
1220                             }
1221                             if let (Ok(snippet), true) = (
1222                                 self.tcx.sess.source_map().span_to_snippet(span),
1223                                 show_suggestion,
1224                             ) {
1225                                 diag.span_suggestion(
1226                                     span,
1227                                     msg,
1228                                     format!("{}.as_ref()", snippet),
1229                                     Applicability::MachineApplicable,
1230                                 );
1231                             }
1232                         }
1233                     }
1234                 }
1235             }
1236             _ => {}
1237         }
1238     }
1239
1240     pub fn report_and_explain_type_error(
1241         &self,
1242         trace: TypeTrace<'tcx>,
1243         terr: &TypeError<'tcx>,
1244     ) -> DiagnosticBuilder<'tcx> {
1245         debug!(
1246             "report_and_explain_type_error(trace={:?}, terr={:?})",
1247             trace, terr
1248         );
1249
1250         let span = trace.cause.span(self.tcx);
1251         let failure_code = trace.cause.as_failure_code(terr);
1252         let mut diag = match failure_code {
1253             FailureCode::Error0317(failure_str) => {
1254                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
1255             }
1256             FailureCode::Error0580(failure_str) => {
1257                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
1258             }
1259             FailureCode::Error0308(failure_str) => {
1260                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
1261             }
1262             FailureCode::Error0644(failure_str) => {
1263                 struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
1264             }
1265         };
1266         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
1267         diag
1268     }
1269
1270     fn values_str(
1271         &self,
1272         values: &ValuePairs<'tcx>,
1273     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1274         match *values {
1275             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
1276             infer::Regions(ref exp_found) => self.expected_found_str(exp_found),
1277             infer::Consts(ref exp_found) => self.expected_found_str(exp_found),
1278             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
1279             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
1280         }
1281     }
1282
1283     fn expected_found_str_ty(
1284         &self,
1285         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1286     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1287         let exp_found = self.resolve_vars_if_possible(exp_found);
1288         if exp_found.references_error() {
1289             return None;
1290         }
1291
1292         Some(self.cmp(exp_found.expected, exp_found.found))
1293     }
1294
1295     /// Returns a string of the form "expected `{}`, found `{}`".
1296     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
1297         &self,
1298         exp_found: &ty::error::ExpectedFound<T>,
1299     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1300         let exp_found = self.resolve_vars_if_possible(exp_found);
1301         if exp_found.references_error() {
1302             return None;
1303         }
1304
1305         Some((
1306             DiagnosticStyledString::highlighted(exp_found.expected.to_string()),
1307             DiagnosticStyledString::highlighted(exp_found.found.to_string()),
1308         ))
1309     }
1310
1311     pub fn report_generic_bound_failure(
1312         &self,
1313         region_scope_tree: &region::ScopeTree,
1314         span: Span,
1315         origin: Option<SubregionOrigin<'tcx>>,
1316         bound_kind: GenericKind<'tcx>,
1317         sub: Region<'tcx>,
1318     ) {
1319         self.construct_generic_bound_failure(region_scope_tree, span, origin, bound_kind, sub)
1320             .emit()
1321     }
1322
1323     pub fn construct_generic_bound_failure(
1324         &self,
1325         region_scope_tree: &region::ScopeTree,
1326         span: Span,
1327         origin: Option<SubregionOrigin<'tcx>>,
1328         bound_kind: GenericKind<'tcx>,
1329         sub: Region<'tcx>,
1330     ) -> DiagnosticBuilder<'a> {
1331         // Attempt to obtain the span of the parameter so we can
1332         // suggest adding an explicit lifetime bound to it.
1333         let type_param_span = match (self.in_progress_tables, bound_kind) {
1334             (Some(ref table), GenericKind::Param(ref param)) => {
1335                 let table = table.borrow();
1336                 table.local_id_root.and_then(|did| {
1337                     let generics = self.tcx.generics_of(did);
1338                     // Account for the case where `did` corresponds to `Self`, which doesn't have
1339                     // the expected type argument.
1340                     if !(generics.has_self && param.index == 0) {
1341                         let type_param = generics.type_param(param, self.tcx);
1342                         let hir = &self.tcx.hir();
1343                         hir.as_local_hir_id(type_param.def_id).map(|id| {
1344                             // Get the `hir::Param` to verify whether it already has any bounds.
1345                             // We do this to avoid suggesting code that ends up as `T: 'a'b`,
1346                             // instead we suggest `T: 'a + 'b` in that case.
1347                             let mut has_bounds = false;
1348                             if let Node::GenericParam(param) = hir.get(id) {
1349                                 has_bounds = !param.bounds.is_empty();
1350                             }
1351                             let sp = hir.span(id);
1352                             // `sp` only covers `T`, change it so that it covers
1353                             // `T:` when appropriate
1354                             let is_impl_trait = bound_kind.to_string().starts_with("impl ");
1355                             let sp = if has_bounds && !is_impl_trait {
1356                                 sp.to(self.tcx
1357                                     .sess
1358                                     .source_map()
1359                                     .next_point(self.tcx.sess.source_map().next_point(sp)))
1360                             } else {
1361                                 sp
1362                             };
1363                             (sp, has_bounds, is_impl_trait)
1364                         })
1365                     } else {
1366                         None
1367                     }
1368                 })
1369             }
1370             _ => None,
1371         };
1372
1373         let labeled_user_string = match bound_kind {
1374             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
1375             GenericKind::Projection(ref p) => format!("the associated type `{}`", p),
1376         };
1377
1378         if let Some(SubregionOrigin::CompareImplMethodObligation {
1379             span,
1380             item_name,
1381             impl_item_def_id,
1382             trait_item_def_id,
1383         }) = origin
1384         {
1385             return self.report_extra_impl_obligation(
1386                 span,
1387                 item_name,
1388                 impl_item_def_id,
1389                 trait_item_def_id,
1390                 &format!("`{}: {}`", bound_kind, sub),
1391             );
1392         }
1393
1394         fn binding_suggestion<'tcx, S: fmt::Display>(
1395             err: &mut DiagnosticBuilder<'tcx>,
1396             type_param_span: Option<(Span, bool, bool)>,
1397             bound_kind: GenericKind<'tcx>,
1398             sub: S,
1399         ) {
1400             let consider = format!(
1401                 "consider adding an explicit lifetime bound {}",
1402                 if type_param_span.map(|(_, _, is_impl_trait)| is_impl_trait).unwrap_or(false) {
1403                     format!(" `{}` to `{}`...", sub, bound_kind)
1404                 } else {
1405                     format!("`{}: {}`...", bound_kind, sub)
1406                 },
1407             );
1408             if let Some((sp, has_lifetimes, is_impl_trait)) = type_param_span {
1409                 let suggestion = if is_impl_trait {
1410                     format!("{} + {}", bound_kind, sub)
1411                 } else {
1412                     let tail = if has_lifetimes { " + " } else { "" };
1413                     format!("{}: {}{}", bound_kind, sub, tail)
1414                 };
1415                 err.span_suggestion_short(
1416                     sp,
1417                     &consider,
1418                     suggestion,
1419                     Applicability::MaybeIncorrect, // Issue #41966
1420                 );
1421             } else {
1422                 err.help(&consider);
1423             }
1424         }
1425
1426         let mut err = match *sub {
1427             ty::ReEarlyBound(_)
1428             | ty::ReFree(ty::FreeRegion {
1429                 bound_region: ty::BrNamed(..),
1430                 ..
1431             }) => {
1432                 // Does the required lifetime have a nice name we can print?
1433                 let mut err = struct_span_err!(
1434                     self.tcx.sess,
1435                     span,
1436                     E0309,
1437                     "{} may not live long enough",
1438                     labeled_user_string
1439                 );
1440                 binding_suggestion(&mut err, type_param_span, bound_kind, sub);
1441                 err
1442             }
1443
1444             ty::ReStatic => {
1445                 // Does the required lifetime have a nice name we can print?
1446                 let mut err = struct_span_err!(
1447                     self.tcx.sess,
1448                     span,
1449                     E0310,
1450                     "{} may not live long enough",
1451                     labeled_user_string
1452                 );
1453                 binding_suggestion(&mut err, type_param_span, bound_kind, "'static");
1454                 err
1455             }
1456
1457             _ => {
1458                 // If not, be less specific.
1459                 let mut err = struct_span_err!(
1460                     self.tcx.sess,
1461                     span,
1462                     E0311,
1463                     "{} may not live long enough",
1464                     labeled_user_string
1465                 );
1466                 err.help(&format!(
1467                     "consider adding an explicit lifetime bound for `{}`",
1468                     bound_kind
1469                 ));
1470                 self.tcx.note_and_explain_region(
1471                     region_scope_tree,
1472                     &mut err,
1473                     &format!("{} must be valid for ", labeled_user_string),
1474                     sub,
1475                     "...",
1476                 );
1477                 err
1478             }
1479         };
1480
1481         if let Some(origin) = origin {
1482             self.note_region_origin(&mut err, &origin);
1483         }
1484         err
1485     }
1486
1487     fn report_sub_sup_conflict(
1488         &self,
1489         region_scope_tree: &region::ScopeTree,
1490         var_origin: RegionVariableOrigin,
1491         sub_origin: SubregionOrigin<'tcx>,
1492         sub_region: Region<'tcx>,
1493         sup_origin: SubregionOrigin<'tcx>,
1494         sup_region: Region<'tcx>,
1495     ) {
1496         let mut err = self.report_inference_failure(var_origin);
1497
1498         self.tcx.note_and_explain_region(
1499             region_scope_tree,
1500             &mut err,
1501             "first, the lifetime cannot outlive ",
1502             sup_region,
1503             "...",
1504         );
1505
1506         match (&sup_origin, &sub_origin) {
1507             (&infer::Subtype(ref sup_trace), &infer::Subtype(ref sub_trace)) => {
1508                 debug!("report_sub_sup_conflict: var_origin={:?}", var_origin);
1509                 debug!("report_sub_sup_conflict: sub_region={:?}", sub_region);
1510                 debug!("report_sub_sup_conflict: sub_origin={:?}", sub_origin);
1511                 debug!("report_sub_sup_conflict: sup_region={:?}", sup_region);
1512                 debug!("report_sub_sup_conflict: sup_origin={:?}", sup_origin);
1513                 debug!("report_sub_sup_conflict: sup_trace={:?}", sup_trace);
1514                 debug!("report_sub_sup_conflict: sub_trace={:?}", sub_trace);
1515                 debug!("report_sub_sup_conflict: sup_trace.values={:?}", sup_trace.values);
1516                 debug!("report_sub_sup_conflict: sub_trace.values={:?}", sub_trace.values);
1517
1518                 if let (Some((sup_expected, sup_found)), Some((sub_expected, sub_found))) = (
1519                     self.values_str(&sup_trace.values),
1520                     self.values_str(&sub_trace.values),
1521                 ) {
1522                     if sub_expected == sup_expected && sub_found == sup_found {
1523                         self.tcx.note_and_explain_region(
1524                             region_scope_tree,
1525                             &mut err,
1526                             "...but the lifetime must also be valid for ",
1527                             sub_region,
1528                             "...",
1529                         );
1530                         err.note(&format!(
1531                             "...so that the {}:\nexpected {}\n   found {}",
1532                             sup_trace.cause.as_requirement_str(),
1533                             sup_expected.content(),
1534                             sup_found.content()
1535                         ));
1536                         err.emit();
1537                         return;
1538                     }
1539                 }
1540             }
1541             _ => {}
1542         }
1543
1544         self.note_region_origin(&mut err, &sup_origin);
1545
1546         self.tcx.note_and_explain_region(
1547             region_scope_tree,
1548             &mut err,
1549             "but, the lifetime must be valid for ",
1550             sub_region,
1551             "...",
1552         );
1553
1554         self.note_region_origin(&mut err, &sub_origin);
1555         err.emit();
1556     }
1557 }
1558
1559 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
1560     fn report_inference_failure(
1561         &self,
1562         var_origin: RegionVariableOrigin,
1563     ) -> DiagnosticBuilder<'tcx> {
1564         let br_string = |br: ty::BoundRegion| {
1565             let mut s = match br {
1566                 ty::BrNamed(_, name) => name.to_string(),
1567                 _ => String::new(),
1568             };
1569             if !s.is_empty() {
1570                 s.push_str(" ");
1571             }
1572             s
1573         };
1574         let var_description = match var_origin {
1575             infer::MiscVariable(_) => String::new(),
1576             infer::PatternRegion(_) => " for pattern".to_string(),
1577             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
1578             infer::Autoref(_) => " for autoref".to_string(),
1579             infer::Coercion(_) => " for automatic coercion".to_string(),
1580             infer::LateBoundRegion(_, br, infer::FnCall) => {
1581                 format!(" for lifetime parameter {}in function call", br_string(br))
1582             }
1583             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
1584                 format!(" for lifetime parameter {}in generic type", br_string(br))
1585             }
1586             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
1587                 " for lifetime parameter {}in trait containing associated type `{}`",
1588                 br_string(br),
1589                 self.tcx.associated_item(def_id).ident
1590             ),
1591             infer::EarlyBoundRegion(_, name) => format!(" for lifetime parameter `{}`", name),
1592             infer::BoundRegionInCoherence(name) => {
1593                 format!(" for lifetime parameter `{}` in coherence check", name)
1594             }
1595             infer::UpvarRegion(ref upvar_id, _) => {
1596                 let var_name = self.tcx.hir().name(upvar_id.var_path.hir_id);
1597                 format!(" for capture of `{}` by closure", var_name)
1598             }
1599             infer::NLL(..) => bug!("NLL variable found in lexical phase"),
1600         };
1601
1602         struct_span_err!(
1603             self.tcx.sess,
1604             var_origin.span(),
1605             E0495,
1606             "cannot infer an appropriate lifetime{} \
1607              due to conflicting requirements",
1608             var_description
1609         )
1610     }
1611 }
1612
1613 enum FailureCode {
1614     Error0317(&'static str),
1615     Error0580(&'static str),
1616     Error0308(&'static str),
1617     Error0644(&'static str),
1618 }
1619
1620 impl<'tcx> ObligationCause<'tcx> {
1621     fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode {
1622         use self::FailureCode::*;
1623         use crate::traits::ObligationCauseCode::*;
1624         match self.code {
1625             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
1626             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) =>
1627                 Error0308(match source {
1628                     hir::MatchSource::IfLetDesugar { .. } =>
1629                         "`if let` arms have incompatible types",
1630                     hir::MatchSource::TryDesugar => {
1631                         "try expression alternatives have incompatible types"
1632                     }
1633                     _ => "match arms have incompatible types",
1634                 }),
1635             IfExpression { .. } => Error0308("if and else have incompatible types"),
1636             IfExpressionWithNoElse => Error0317("if may be missing an else clause"),
1637             MainFunctionType => Error0580("main function has wrong type"),
1638             StartFunctionType => Error0308("start function has wrong type"),
1639             IntrinsicType => Error0308("intrinsic has wrong type"),
1640             MethodReceiver => Error0308("mismatched `self` parameter type"),
1641
1642             // In the case where we have no more specific thing to
1643             // say, also take a look at the error code, maybe we can
1644             // tailor to that.
1645             _ => match terr {
1646                 TypeError::CyclicTy(ty) if ty.is_closure() || ty.is_generator() => {
1647                     Error0644("closure/generator type that references itself")
1648                 }
1649                 TypeError::IntrinsicCast => {
1650                     Error0308("cannot coerce intrinsics to function pointers")
1651                 }
1652                 _ => Error0308("mismatched types"),
1653             },
1654         }
1655     }
1656
1657     fn as_requirement_str(&self) -> &'static str {
1658         use crate::traits::ObligationCauseCode::*;
1659         match self.code {
1660             CompareImplMethodObligation { .. } => "method type is compatible with trait",
1661             ExprAssignable => "expression is assignable",
1662             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => match source {
1663                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have compatible types",
1664                 _ => "match arms have compatible types",
1665             },
1666             IfExpression { .. } => "if and else have incompatible types",
1667             IfExpressionWithNoElse => "if missing an else returns ()",
1668             MainFunctionType => "`main` function has the correct type",
1669             StartFunctionType => "`start` function has the correct type",
1670             IntrinsicType => "intrinsic has the correct type",
1671             MethodReceiver => "method receiver has the correct type",
1672             _ => "types are compatible",
1673         }
1674     }
1675 }