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