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