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::MatchExpressionArm { arm_span, source } => match source {
 491                 hir::MatchSource::IfLetDesugar { .. } => {
 492                     let msg = "`if let` arm with an incompatible type";
 493                     if self.tcx.sess.source_map().is_multiline(arm_span) {
 494                         err.span_note(arm_span, msg);
 495                     } else {
 496                         err.span_label(arm_span, msg);
 497                     }
 498                 }
 499                 hir::MatchSource::TryDesugar => {}
 500                 _ => {
 501                     let msg = "match arm with an incompatible type";
 502                     if self.tcx.sess.source_map().is_multiline(arm_span) {
 503                         err.span_note(arm_span, msg);
 504                     } else {
 505                         err.span_label(arm_span, msg);
 506                     }
 507                 }
 508             },
 509             _ => (),
 510         }
 511     }
 512
 513     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
 514     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
 515     /// populate `other_value` with `other_ty`.
 516     ///
 517     /// ```text
 518     /// Foo<Bar<Qux>>
 519     /// ^^^^--------^ this is highlighted
 520     /// |   |
 521     /// |   this type argument is exactly the same as the other type, not highlighted
 522     /// this is highlighted
 523     /// Bar<Qux>
 524     /// -------- this type is the same as a type argument in the other type, not highlighted
 525     /// ```
 526     fn highlight_outer(
 527         &self,
 528         value: &mut DiagnosticStyledString,
 529         other_value: &mut DiagnosticStyledString,
 530         name: String,
 531         sub: &ty::subst::Substs<'tcx>,
 532         pos: usize,
 533         other_ty: &Ty<'tcx>,
 534     ) {
 535         // `value` and `other_value` hold two incomplete type representation for display.
 536         // `name` is the path of both types being compared. `sub`
 537         value.push_highlighted(name);
 538         let len = sub.len();
 539         if len > 0 {
 540             value.push_highlighted("<");
 541         }
 542
 543         // Output the lifetimes for the first type
 544         let lifetimes = sub.regions()
 545             .map(|lifetime| {
 546                 let s = lifetime.to_string();
 547                 if s.is_empty() {
 548                     "'_".to_string()
 549                 } else {
 550                     s
 551                 }
 552             })
 553             .collect::<Vec<_>>()
 554             .join(", ");
 555         if !lifetimes.is_empty() {
 556             if sub.regions().count() < len {
 557                 value.push_normal(lifetimes + &", ");
 558             } else {
 559                 value.push_normal(lifetimes);
 560             }
 561         }
 562
 563         // Highlight all the type arguments that aren't at `pos` and compare the type argument at
 564         // `pos` and `other_ty`.
 565         for (i, type_arg) in sub.types().enumerate() {
 566             if i == pos {
 567                 let values = self.cmp(type_arg, other_ty);
 568                 value.0.extend((values.0).0);
 569                 other_value.0.extend((values.1).0);
 570             } else {
 571                 value.push_highlighted(type_arg.to_string());
 572             }
 573
 574             if len > 0 && i != len - 1 {
 575                 value.push_normal(", ");
 576             }
 577             //self.push_comma(&mut value, &mut other_value, len, i);
 578         }
 579         if len > 0 {
 580             value.push_highlighted(">");
 581         }
 582     }
 583
 584     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 585     /// as that is the difference to the other type.
 586     ///
 587     /// For the following code:
 588     ///
 589     /// ```norun
 590     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 591     /// ```
 592     ///
 593     /// The type error output will behave in the following way:
 594     ///
 595     /// ```text
 596     /// Foo<Bar<Qux>>
 597     /// ^^^^--------^ this is highlighted
 598     /// |   |
 599     /// |   this type argument is exactly the same as the other type, not highlighted
 600     /// this is highlighted
 601     /// Bar<Qux>
 602     /// -------- this type is the same as a type argument in the other type, not highlighted
 603     /// ```
 604     fn cmp_type_arg(
 605         &self,
 606         mut t1_out: &mut DiagnosticStyledString,
 607         mut t2_out: &mut DiagnosticStyledString,
 608         path: String,
 609         sub: &ty::subst::Substs<'tcx>,
 610         other_path: String,
 611         other_ty: &Ty<'tcx>,
 612     ) -> Option<()> {
 613         for (i, ta) in sub.types().enumerate() {
 614             if &ta == other_ty {
 615                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 616                 return Some(());
 617             }
 618             if let &ty::Adt(def, _) = &ta.sty {
 619                 let path_ = self.tcx.item_path_str(def.did.clone());
 620                 if path_ == other_path {
 621                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 622                     return Some(());
 623                 }
 624             }
 625         }
 626         None
 627     }
 628
 629     /// Add a `,` to the type representation only if it is appropriate.
 630     fn push_comma(
 631         &self,
 632         value: &mut DiagnosticStyledString,
 633         other_value: &mut DiagnosticStyledString,
 634         len: usize,
 635         pos: usize,
 636     ) {
 637         if len > 0 && pos != len - 1 {
 638             value.push_normal(", ");
 639             other_value.push_normal(", ");
 640         }
 641     }
 642
 643     /// For generic types with parameters with defaults, remove the parameters corresponding to
 644     /// the defaults. This repeats a lot of the logic found in `PrintContext::parameterized`.
 645     fn strip_generic_default_params(
 646         &self,
 647         def_id: DefId,
 648         substs: &ty::subst::Substs<'tcx>,
 649     ) -> &'tcx ty::subst::Substs<'tcx> {
 650         let generics = self.tcx.generics_of(def_id);
 651         let mut num_supplied_defaults = 0;
 652         let mut type_params = generics
 653             .params
 654             .iter()
 655             .rev()
 656             .filter_map(|param| match param.kind {
 657                 ty::GenericParamDefKind::Lifetime => None,
 658                 ty::GenericParamDefKind::Type { has_default, .. } => {
 659                     Some((param.def_id, has_default))
 660                 }
 661             })
 662             .peekable();
 663         let has_default = {
 664             let has_default = type_params.peek().map(|(_, has_default)| has_default);
 665             *has_default.unwrap_or(&false)
 666         };
 667         if has_default {
 668             let types = substs.types().rev();
 669             for ((def_id, has_default), actual) in type_params.zip(types) {
 670                 if !has_default {
 671                     break;
 672                 }
 673                 if self.tcx.type_of(def_id).subst(self.tcx, substs) != actual {
 674                     break;
 675                 }
 676                 num_supplied_defaults += 1;
 677             }
 678         }
 679         let len = generics.params.len();
 680         let mut generics = generics.clone();
 681         generics.params.truncate(len - num_supplied_defaults);
 682         substs.truncate_to(self.tcx, &generics)
 683     }
 684
 685     /// Compare two given types, eliding parts that are the same between them and highlighting
 686     /// relevant differences, and return two representation of those types for highlighted printing.
 687     fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> (DiagnosticStyledString, DiagnosticStyledString) {
 688         fn equals<'tcx>(a: &Ty<'tcx>, b: &Ty<'tcx>) -> bool {
 689             match (&a.sty, &b.sty) {
 690                 (a, b) if *a == *b => true,
 691                 (&ty::Int(_), &ty::Infer(ty::InferTy::IntVar(_)))
 692                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Int(_))
 693                 | (&ty::Infer(ty::InferTy::IntVar(_)), &ty::Infer(ty::InferTy::IntVar(_)))
 694                 | (&ty::Float(_), &ty::Infer(ty::InferTy::FloatVar(_)))
 695                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Float(_))
 696                 | (&ty::Infer(ty::InferTy::FloatVar(_)), &ty::Infer(ty::InferTy::FloatVar(_))) => {
 697                     true
 698                 }
 699                 _ => false,
 700             }
 701         }
 702
 703         fn push_ty_ref<'tcx>(
 704             r: &ty::Region<'tcx>,
 705             ty: Ty<'tcx>,
 706             mutbl: hir::Mutability,
 707             s: &mut DiagnosticStyledString,
 708         ) {
 709             let r = &r.to_string();
 710             s.push_highlighted(format!(
 711                 "&{}{}{}",
 712                 r,
 713                 if r == "" { "" } else { " " },
 714                 if mutbl == hir::MutMutable { "mut " } else { "" }
 715             ));
 716             s.push_normal(ty.to_string());
 717         }
 718
 719         match (&t1.sty, &t2.sty) {
 720             (&ty::Adt(def1, sub1), &ty::Adt(def2, sub2)) => {
 721                 let sub_no_defaults_1 = self.strip_generic_default_params(def1.did, sub1);
 722                 let sub_no_defaults_2 = self.strip_generic_default_params(def2.did, sub2);
 723                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 724                 let path1 = self.tcx.item_path_str(def1.did.clone());
 725                 let path2 = self.tcx.item_path_str(def2.did.clone());
 726                 if def1.did == def2.did {
 727                     // Easy case. Replace same types with `_` to shorten the output and highlight
 728                     // the differing ones.
 729                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
 730                     //     Foo<Bar, _>
 731                     //     Foo<Quz, _>
 732                     //         ---  ^ type argument elided
 733                     //         |
 734                     //         highlighted in output
 735                     values.0.push_normal(path1);
 736                     values.1.push_normal(path2);
 737
 738                     // Avoid printing out default generic parameters that are common to both
 739                     // types.
 740                     let len1 = sub_no_defaults_1.len();
 741                     let len2 = sub_no_defaults_2.len();
 742                     let common_len = cmp::min(len1, len2);
 743                     let remainder1: Vec<_> = sub1.types().skip(common_len).collect();
 744                     let remainder2: Vec<_> = sub2.types().skip(common_len).collect();
 745                     let common_default_params = remainder1
 746                         .iter()
 747                         .rev()
 748                         .zip(remainder2.iter().rev())
 749                         .filter(|(a, b)| a == b)
 750                         .count();
 751                     let len = sub1.len() - common_default_params;
 752
 753                     // Only draw `<...>` if there're lifetime/type arguments.
 754                     if len > 0 {
 755                         values.0.push_normal("<");
 756                         values.1.push_normal("<");
 757                     }
 758
 759                     fn lifetime_display(lifetime: Region<'_>) -> String {
 760                         let s = lifetime.to_string();
 761                         if s.is_empty() {
 762                             "'_".to_string()
 763                         } else {
 764                             s
 765                         }
 766                     }
 767                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
 768                     // all diagnostics that use this output
 769                     //
 770                     //     Foo<'x, '_, Bar>
 771                     //     Foo<'y, '_, Qux>
 772                     //         ^^  ^^  --- type arguments are not elided
 773                     //         |   |
 774                     //         |   elided as they were the same
 775                     //         not elided, they were different, but irrelevant
 776                     let lifetimes = sub1.regions().zip(sub2.regions());
 777                     for (i, lifetimes) in lifetimes.enumerate() {
 778                         let l1 = lifetime_display(lifetimes.0);
 779                         let l2 = lifetime_display(lifetimes.1);
 780                         if l1 == l2 {
 781                             values.0.push_normal("'_");
 782                             values.1.push_normal("'_");
 783                         } else {
 784                             values.0.push_highlighted(l1);
 785                             values.1.push_highlighted(l2);
 786                         }
 787                         self.push_comma(&mut values.0, &mut values.1, len, i);
 788                     }
 789
 790                     // We're comparing two types with the same path, so we compare the type
 791                     // arguments for both. If they are the same, do not highlight and elide from the
 792                     // output.
 793                     //     Foo<_, Bar>
 794                     //     Foo<_, Qux>
 795                     //         ^ elided type as this type argument was the same in both sides
 796                     let type_arguments = sub1.types().zip(sub2.types());
 797                     let regions_len = sub1.regions().count();
 798                     for (i, (ta1, ta2)) in type_arguments.take(len).enumerate() {
 799                         let i = i + regions_len;
 800                         if ta1 == ta2 {
 801                             values.0.push_normal("_");
 802                             values.1.push_normal("_");
 803                         } else {
 804                             let (x1, x2) = self.cmp(ta1, ta2);
 805                             (values.0).0.extend(x1.0);
 806                             (values.1).0.extend(x2.0);
 807                         }
 808                         self.push_comma(&mut values.0, &mut values.1, len, i);
 809                     }
 810
 811                     // Close the type argument bracket.
 812                     // Only draw `<...>` if there're lifetime/type arguments.
 813                     if len > 0 {
 814                         values.0.push_normal(">");
 815                         values.1.push_normal(">");
 816                     }
 817                     values
 818                 } else {
 819                     // Check for case:
 820                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
 821                     //     Foo<Bar<Qux>
 822                     //         ------- this type argument is exactly the same as the other type
 823                     //     Bar<Qux>
 824                     if self.cmp_type_arg(
 825                         &mut values.0,
 826                         &mut values.1,
 827                         path1.clone(),
 828                         sub_no_defaults_1,
 829                         path2.clone(),
 830                         &t2,
 831                     ).is_some()
 832                     {
 833                         return values;
 834                     }
 835                     // Check for case:
 836                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
 837                     //     Bar<Qux>
 838                     //     Foo<Bar<Qux>>
 839                     //         ------- this type argument is exactly the same as the other type
 840                     if self.cmp_type_arg(
 841                         &mut values.1,
 842                         &mut values.0,
 843                         path2,
 844                         sub_no_defaults_2,
 845                         path1,
 846                         &t1,
 847                     ).is_some()
 848                     {
 849                         return values;
 850                     }
 851
 852                     // We couldn't find anything in common, highlight everything.
 853                     //     let x: Bar<Qux> = y::<Foo<Zar>>();
 854                     (
 855                         DiagnosticStyledString::highlighted(t1.to_string()),
 856                         DiagnosticStyledString::highlighted(t2.to_string()),
 857                     )
 858                 }
 859             }
 860
 861             // When finding T != &T, highlight only the borrow
 862             (&ty::Ref(r1, ref_ty1, mutbl1), _) if equals(&ref_ty1, &t2) => {
 863                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 864                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
 865                 values.1.push_normal(t2.to_string());
 866                 values
 867             }
 868             (_, &ty::Ref(r2, ref_ty2, mutbl2)) if equals(&t1, &ref_ty2) => {
 869                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 870                 values.0.push_normal(t1.to_string());
 871                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
 872                 values
 873             }
 874
 875             // When encountering &T != &mut T, highlight only the borrow
 876             (&ty::Ref(r1, ref_ty1, mutbl1), &ty::Ref(r2, ref_ty2, mutbl2))
 877                 if equals(&ref_ty1, &ref_ty2) =>
 878             {
 879                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 880                 push_ty_ref(&r1, ref_ty1, mutbl1, &mut values.0);
 881                 push_ty_ref(&r2, ref_ty2, mutbl2, &mut values.1);
 882                 values
 883             }
 884
 885             _ => {
 886                 if t1 == t2 {
 887                     // The two types are the same, elide and don't highlight.
 888                     (
 889                         DiagnosticStyledString::normal("_"),
 890                         DiagnosticStyledString::normal("_"),
 891                     )
 892                 } else {
 893                     // We couldn't find anything in common, highlight everything.
 894                     (
 895                         DiagnosticStyledString::highlighted(t1.to_string()),
 896                         DiagnosticStyledString::highlighted(t2.to_string()),
 897                     )
 898                 }
 899             }
 900         }
 901     }
 902
 903     pub fn note_type_err(
 904         &self,
 905         diag: &mut DiagnosticBuilder<'tcx>,
 906         cause: &ObligationCause<'tcx>,
 907         secondary_span: Option<(Span, String)>,
 908         mut values: Option<ValuePairs<'tcx>>,
 909         terr: &TypeError<'tcx>,
 910     ) {
 911         // For some types of errors, expected-found does not make
 912         // sense, so just ignore the values we were given.
 913         match terr {
 914             TypeError::CyclicTy(_) => {
 915                 values = None;
 916             }
 917             _ => {}
 918         }
 919
 920         let (expected_found, exp_found, is_simple_error) = match values {
 921             None => (None, None, false),
 922             Some(values) => {
 923                 let (is_simple_error, exp_found) = match values {
 924                     ValuePairs::Types(exp_found) => {
 925                         let is_simple_err =
 926                             exp_found.expected.is_primitive() && exp_found.found.is_primitive();
 927
 928                         (is_simple_err, Some(exp_found))
 929                     }
 930                     _ => (false, None),
 931                 };
 932                 let vals = match self.values_str(&values) {
 933                     Some((expected, found)) => Some((expected, found)),
 934                     None => {
 935                         // Derived error. Cancel the emitter.
 936                         self.tcx.sess.diagnostic().cancel(diag);
 937                         return;
 938                     }
 939                 };
 940                 (vals, exp_found, is_simple_error)
 941             }
 942         };
 943
 944         let span = cause.span(&self.tcx);
 945
 946         diag.span_label(span, terr.to_string());
 947         if let Some((sp, msg)) = secondary_span {
 948             diag.span_label(sp, msg);
 949         }
 950
 951         if let Some((expected, found)) = expected_found {
 952             match (terr, is_simple_error, expected == found) {
 953                 (&TypeError::Sorts(ref values), false, true) => {
 954                     diag.note_expected_found_extra(
 955                         &"type",
 956                         expected,
 957                         found,
 958                         &format!(" ({})", values.expected.sort_string(self.tcx)),
 959                         &format!(" ({})", values.found.sort_string(self.tcx)),
 960                     );
 961                 }
 962                 (_, false, _) => {
 963                     if let Some(exp_found) = exp_found {
 964                         let (def_id, ret_ty) = match exp_found.found.sty {
 965                             TyKind::FnDef(def, _) => {
 966                                 (Some(def), Some(self.tcx.fn_sig(def).output()))
 967                             }
 968                             _ => (None, None),
 969                         };
 970
 971                         let exp_is_struct = match exp_found.expected.sty {
 972                             TyKind::Adt(def, _) => def.is_struct(),
 973                             _ => false,
 974                         };
 975
 976                         if let (Some(def_id), Some(ret_ty)) = (def_id, ret_ty) {
 977                             if exp_is_struct && &exp_found.expected == ret_ty.skip_binder() {
 978                                 let message = format!(
 979                                     "did you mean `{}(/* fields */)`?",
 980                                     self.tcx.item_path_str(def_id)
 981                                 );
 982                                 diag.span_label(span, message);
 983                             }
 984                         }
 985                         self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
 986                     }
 987
 988                     diag.note_expected_found(&"type", expected, found);
 989                 }
 990                 _ => (),
 991             }
 992         }
 993
 994         self.check_and_note_conflicting_crates(diag, terr, span);
 995         self.tcx.note_and_explain_type_err(diag, terr, span);
 996
 997         // It reads better to have the error origin as the final
 998         // thing.
 999         self.note_error_origin(diag, &cause);
1000     }
1001
1002     /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
1003     /// suggest it.
1004     fn suggest_as_ref_where_appropriate(
1005         &self,
1006         span: Span,
1007         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1008         diag: &mut DiagnosticBuilder<'tcx>,
1009     ) {
1010         match (&exp_found.expected.sty, &exp_found.found.sty) {
1011             (TyKind::Adt(exp_def, exp_substs), TyKind::Ref(_, found_ty, _)) => {
1012                 if let TyKind::Adt(found_def, found_substs) = found_ty.sty {
1013                     let path_str = format!("{:?}", exp_def);
1014                     if exp_def == &found_def {
1015                         let opt_msg = "you can convert from `&Option<T>` to `Option<&T>` using \
1016                                        `.as_ref()`";
1017                         let result_msg = "you can convert from `&Result<T, E>` to \
1018                                           `Result<&T, &E>` using `.as_ref()`";
1019                         let have_as_ref = &[
1020                             ("std::option::Option", opt_msg),
1021                             ("core::option::Option", opt_msg),
1022                             ("std::result::Result", result_msg),
1023                             ("core::result::Result", result_msg),
1024                         ];
1025                         if let Some(msg) = have_as_ref.iter()
1026                             .filter_map(|(path, msg)| if &path_str == path {
1027                                 Some(msg)
1028                             } else {
1029                                 None
1030                             }).next()
1031                         {
1032                             let mut show_suggestion = true;
1033                             for (exp_ty, found_ty) in exp_substs.types().zip(found_substs.types()) {
1034                                 match exp_ty.sty {
1035                                     TyKind::Ref(_, exp_ty, _) => {
1036                                         match (&exp_ty.sty, &found_ty.sty) {
1037                                             (_, TyKind::Param(_)) |
1038                                             (_, TyKind::Infer(_)) |
1039                                             (TyKind::Param(_), _) |
1040                                             (TyKind::Infer(_), _) => {}
1041                                             _ if ty::TyS::same_type(exp_ty, found_ty) => {}
1042                                             _ => show_suggestion = false,
1043                                         };
1044                                     }
1045                                     TyKind::Param(_) | TyKind::Infer(_) => {}
1046                                     _ => show_suggestion = false,
1047                                 }
1048                             }
1049                             if let (Ok(snippet), true) = (
1050                                 self.tcx.sess.source_map().span_to_snippet(span),
1051                                 show_suggestion,
1052                             ) {
1053                                 diag.span_suggestion_with_applicability(
1054                                     span,
1055                                     msg,
1056                                     format!("{}.as_ref()", snippet),
1057                                     Applicability::MachineApplicable,
1058                                 );
1059                             }
1060                         }
1061                     }
1062                 }
1063             }
1064             _ => {}
1065         }
1066     }
1067
1068     pub fn report_and_explain_type_error(
1069         &self,
1070         trace: TypeTrace<'tcx>,
1071         terr: &TypeError<'tcx>,
1072     ) -> DiagnosticBuilder<'tcx> {
1073         debug!(
1074             "report_and_explain_type_error(trace={:?}, terr={:?})",
1075             trace, terr
1076         );
1077
1078         let span = trace.cause.span(&self.tcx);
1079         let failure_code = trace.cause.as_failure_code(terr);
1080         let mut diag = match failure_code {
1081             FailureCode::Error0317(failure_str) => {
1082                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
1083             }
1084             FailureCode::Error0580(failure_str) => {
1085                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
1086             }
1087             FailureCode::Error0308(failure_str) => {
1088                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
1089             }
1090             FailureCode::Error0644(failure_str) => {
1091                 struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
1092             }
1093         };
1094         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
1095         diag
1096     }
1097
1098     fn values_str(
1099         &self,
1100         values: &ValuePairs<'tcx>,
1101     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1102         match *values {
1103             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
1104             infer::Regions(ref exp_found) => self.expected_found_str(exp_found),
1105             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
1106             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
1107         }
1108     }
1109
1110     fn expected_found_str_ty(
1111         &self,
1112         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
1113     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1114         let exp_found = self.resolve_type_vars_if_possible(exp_found);
1115         if exp_found.references_error() {
1116             return None;
1117         }
1118
1119         Some(self.cmp(exp_found.expected, exp_found.found))
1120     }
1121
1122     /// Returns a string of the form "expected `{}`, found `{}`".
1123     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
1124         &self,
1125         exp_found: &ty::error::ExpectedFound<T>,
1126     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
1127         let exp_found = self.resolve_type_vars_if_possible(exp_found);
1128         if exp_found.references_error() {
1129             return None;
1130         }
1131
1132         Some((
1133             DiagnosticStyledString::highlighted(exp_found.expected.to_string()),
1134             DiagnosticStyledString::highlighted(exp_found.found.to_string()),
1135         ))
1136     }
1137
1138     pub fn report_generic_bound_failure(
1139         &self,
1140         region_scope_tree: &region::ScopeTree,
1141         span: Span,
1142         origin: Option<SubregionOrigin<'tcx>>,
1143         bound_kind: GenericKind<'tcx>,
1144         sub: Region<'tcx>,
1145     ) {
1146         self.construct_generic_bound_failure(region_scope_tree, span, origin, bound_kind, sub)
1147             .emit()
1148     }
1149
1150     pub fn construct_generic_bound_failure(
1151         &self,
1152         region_scope_tree: &region::ScopeTree,
1153         span: Span,
1154         origin: Option<SubregionOrigin<'tcx>>,
1155         bound_kind: GenericKind<'tcx>,
1156         sub: Region<'tcx>,
1157     ) -> DiagnosticBuilder<'a> {
1158         // Attempt to obtain the span of the parameter so we can
1159         // suggest adding an explicit lifetime bound to it.
1160         let type_param_span = match (self.in_progress_tables, bound_kind) {
1161             (Some(ref table), GenericKind::Param(ref param)) => {
1162                 let table = table.borrow();
1163                 table.local_id_root.and_then(|did| {
1164                     let generics = self.tcx.generics_of(did);
1165                     // Account for the case where `did` corresponds to `Self`, which doesn't have
1166                     // the expected type argument.
1167                     if !param.is_self() {
1168                         let type_param = generics.type_param(param, self.tcx);
1169                         let hir = &self.tcx.hir();
1170                         hir.as_local_node_id(type_param.def_id).map(|id| {
1171                             // Get the `hir::Param` to verify whether it already has any bounds.
1172                             // We do this to avoid suggesting code that ends up as `T: 'a'b`,
1173                             // instead we suggest `T: 'a + 'b` in that case.
1174                             let mut has_bounds = false;
1175                             if let Node::GenericParam(ref param) = hir.get(id) {
1176                                 has_bounds = !param.bounds.is_empty();
1177                             }
1178                             let sp = hir.span(id);
1179                             // `sp` only covers `T`, change it so that it covers
1180                             // `T:` when appropriate
1181                             let is_impl_trait = bound_kind.to_string().starts_with("impl ");
1182                             let sp = if has_bounds && !is_impl_trait {
1183                                 sp.to(self.tcx
1184                                     .sess
1185                                     .source_map()
1186                                     .next_point(self.tcx.sess.source_map().next_point(sp)))
1187                             } else {
1188                                 sp
1189                             };
1190                             (sp, has_bounds, is_impl_trait)
1191                         })
1192                     } else {
1193                         None
1194                     }
1195                 })
1196             }
1197             _ => None,
1198         };
1199
1200         let labeled_user_string = match bound_kind {
1201             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
1202             GenericKind::Projection(ref p) => format!("the associated type `{}`", p),
1203         };
1204
1205         if let Some(SubregionOrigin::CompareImplMethodObligation {
1206             span,
1207             item_name,
1208             impl_item_def_id,
1209             trait_item_def_id,
1210         }) = origin
1211         {
1212             return self.report_extra_impl_obligation(
1213                 span,
1214                 item_name,
1215                 impl_item_def_id,
1216                 trait_item_def_id,
1217                 &format!("`{}: {}`", bound_kind, sub),
1218             );
1219         }
1220
1221         fn binding_suggestion<'tcx, S: fmt::Display>(
1222             err: &mut DiagnosticBuilder<'tcx>,
1223             type_param_span: Option<(Span, bool, bool)>,
1224             bound_kind: GenericKind<'tcx>,
1225             sub: S,
1226         ) {
1227             let consider = format!(
1228                 "consider adding an explicit lifetime bound {}",
1229                 if type_param_span.map(|(_, _, is_impl_trait)| is_impl_trait).unwrap_or(false) {
1230                     format!(" `{}` to `{}`...", sub, bound_kind)
1231                 } else {
1232                     format!("`{}: {}`...", bound_kind, sub)
1233                 },
1234             );
1235             if let Some((sp, has_lifetimes, is_impl_trait)) = type_param_span {
1236                 let suggestion = if is_impl_trait {
1237                     format!("{} + {}", bound_kind, sub)
1238                 } else {
1239                     let tail = if has_lifetimes { " + " } else { "" };
1240                     format!("{}: {}{}", bound_kind, sub, tail)
1241                 };
1242                 err.span_suggestion_short_with_applicability(
1243                     sp,
1244                     &consider,
1245                     suggestion,
1246                     Applicability::MaybeIncorrect, // Issue #41966
1247                 );
1248             } else {
1249                 err.help(&consider);
1250             }
1251         }
1252
1253         let mut err = match *sub {
1254             ty::ReEarlyBound(_)
1255             | ty::ReFree(ty::FreeRegion {
1256                 bound_region: ty::BrNamed(..),
1257                 ..
1258             }) => {
1259                 // Does the required lifetime have a nice name we can print?
1260                 let mut err = struct_span_err!(
1261                     self.tcx.sess,
1262                     span,
1263                     E0309,
1264                     "{} may not live long enough",
1265                     labeled_user_string
1266                 );
1267                 binding_suggestion(&mut err, type_param_span, bound_kind, sub);
1268                 err
1269             }
1270
1271             ty::ReStatic => {
1272                 // Does the required lifetime have a nice name we can print?
1273                 let mut err = struct_span_err!(
1274                     self.tcx.sess,
1275                     span,
1276                     E0310,
1277                     "{} may not live long enough",
1278                     labeled_user_string
1279                 );
1280                 binding_suggestion(&mut err, type_param_span, bound_kind, "'static");
1281                 err
1282             }
1283
1284             _ => {
1285                 // If not, be less specific.
1286                 let mut err = struct_span_err!(
1287                     self.tcx.sess,
1288                     span,
1289                     E0311,
1290                     "{} may not live long enough",
1291                     labeled_user_string
1292                 );
1293                 err.help(&format!(
1294                     "consider adding an explicit lifetime bound for `{}`",
1295                     bound_kind
1296                 ));
1297                 self.tcx.note_and_explain_region(
1298                     region_scope_tree,
1299                     &mut err,
1300                     &format!("{} must be valid for ", labeled_user_string),
1301                     sub,
1302                     "...",
1303                 );
1304                 err
1305             }
1306         };
1307
1308         if let Some(origin) = origin {
1309             self.note_region_origin(&mut err, &origin);
1310         }
1311         err
1312     }
1313
1314     fn report_sub_sup_conflict(
1315         &self,
1316         region_scope_tree: &region::ScopeTree,
1317         var_origin: RegionVariableOrigin,
1318         sub_origin: SubregionOrigin<'tcx>,
1319         sub_region: Region<'tcx>,
1320         sup_origin: SubregionOrigin<'tcx>,
1321         sup_region: Region<'tcx>,
1322     ) {
1323         let mut err = self.report_inference_failure(var_origin);
1324
1325         self.tcx.note_and_explain_region(
1326             region_scope_tree,
1327             &mut err,
1328             "first, the lifetime cannot outlive ",
1329             sup_region,
1330             "...",
1331         );
1332
1333         match (&sup_origin, &sub_origin) {
1334             (&infer::Subtype(ref sup_trace), &infer::Subtype(ref sub_trace)) => {
1335                 debug!("report_sub_sup_conflict: var_origin={:?}", var_origin);
1336                 debug!("report_sub_sup_conflict: sub_region={:?}", sub_region);
1337                 debug!("report_sub_sup_conflict: sub_origin={:?}", sub_origin);
1338                 debug!("report_sub_sup_conflict: sup_region={:?}", sup_region);
1339                 debug!("report_sub_sup_conflict: sup_origin={:?}", sup_origin);
1340                 debug!("report_sub_sup_conflict: sup_trace={:?}", sup_trace);
1341                 debug!("report_sub_sup_conflict: sub_trace={:?}", sub_trace);
1342                 debug!("report_sub_sup_conflict: sup_trace.values={:?}", sup_trace.values);
1343                 debug!("report_sub_sup_conflict: sub_trace.values={:?}", sub_trace.values);
1344
1345                 if let (Some((sup_expected, sup_found)), Some((sub_expected, sub_found))) = (
1346                     self.values_str(&sup_trace.values),
1347                     self.values_str(&sub_trace.values),
1348                 ) {
1349                     if sub_expected == sup_expected && sub_found == sup_found {
1350                         self.tcx.note_and_explain_region(
1351                             region_scope_tree,
1352                             &mut err,
1353                             "...but the lifetime must also be valid for ",
1354                             sub_region,
1355                             "...",
1356                         );
1357                         err.note(&format!(
1358                             "...so that the {}:\nexpected {}\n   found {}",
1359                             sup_trace.cause.as_requirement_str(),
1360                             sup_expected.content(),
1361                             sup_found.content()
1362                         ));
1363                         err.emit();
1364                         return;
1365                     }
1366                 }
1367             }
1368             _ => {}
1369         }
1370
1371         self.note_region_origin(&mut err, &sup_origin);
1372
1373         self.tcx.note_and_explain_region(
1374             region_scope_tree,
1375             &mut err,
1376             "but, the lifetime must be valid for ",
1377             sub_region,
1378             "...",
1379         );
1380
1381         self.note_region_origin(&mut err, &sub_origin);
1382         err.emit();
1383     }
1384 }
1385
1386 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1387     fn report_inference_failure(
1388         &self,
1389         var_origin: RegionVariableOrigin,
1390     ) -> DiagnosticBuilder<'tcx> {
1391         let br_string = |br: ty::BoundRegion| {
1392             let mut s = br.to_string();
1393             if !s.is_empty() {
1394                 s.push_str(" ");
1395             }
1396             s
1397         };
1398         let var_description = match var_origin {
1399             infer::MiscVariable(_) => String::new(),
1400             infer::PatternRegion(_) => " for pattern".to_string(),
1401             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
1402             infer::Autoref(_) => " for autoref".to_string(),
1403             infer::Coercion(_) => " for automatic coercion".to_string(),
1404             infer::LateBoundRegion(_, br, infer::FnCall) => {
1405                 format!(" for lifetime parameter {}in function call", br_string(br))
1406             }
1407             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
1408                 format!(" for lifetime parameter {}in generic type", br_string(br))
1409             }
1410             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
1411                 " for lifetime parameter {}in trait containing associated type `{}`",
1412                 br_string(br),
1413                 self.tcx.associated_item(def_id).ident
1414             ),
1415             infer::EarlyBoundRegion(_, name) => format!(" for lifetime parameter `{}`", name),
1416             infer::BoundRegionInCoherence(name) => {
1417                 format!(" for lifetime parameter `{}` in coherence check", name)
1418             }
1419             infer::UpvarRegion(ref upvar_id, _) => {
1420                 let var_node_id = self.tcx.hir().hir_to_node_id(upvar_id.var_path.hir_id);
1421                 let var_name = self.tcx.hir().name(var_node_id);
1422                 format!(" for capture of `{}` by closure", var_name)
1423             }
1424             infer::NLL(..) => bug!("NLL variable found in lexical phase"),
1425         };
1426
1427         struct_span_err!(
1428             self.tcx.sess,
1429             var_origin.span(),
1430             E0495,
1431             "cannot infer an appropriate lifetime{} \
1432              due to conflicting requirements",
1433             var_description
1434         )
1435     }
1436 }
1437
1438 enum FailureCode {
1439     Error0317(&'static str),
1440     Error0580(&'static str),
1441     Error0308(&'static str),
1442     Error0644(&'static str),
1443 }
1444
1445 impl<'tcx> ObligationCause<'tcx> {
1446     fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode {
1447         use self::FailureCode::*;
1448         use traits::ObligationCauseCode::*;
1449         match self.code {
1450             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
1451             MatchExpressionArm { source, .. } => Error0308(match source {
1452                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have incompatible types",
1453                 hir::MatchSource::TryDesugar => {
1454                     "try expression alternatives have incompatible types"
1455                 }
1456                 _ => "match arms have incompatible types",
1457             }),
1458             IfExpression => Error0308("if and else have incompatible types"),
1459             IfExpressionWithNoElse => Error0317("if may be missing an else clause"),
1460             MainFunctionType => Error0580("main function has wrong type"),
1461             StartFunctionType => Error0308("start function has wrong type"),
1462             IntrinsicType => Error0308("intrinsic has wrong type"),
1463             MethodReceiver => Error0308("mismatched method receiver"),
1464
1465             // In the case where we have no more specific thing to
1466             // say, also take a look at the error code, maybe we can
1467             // tailor to that.
1468             _ => match terr {
1469                 TypeError::CyclicTy(ty) if ty.is_closure() || ty.is_generator() => {
1470                     Error0644("closure/generator type that references itself")
1471                 }
1472                 _ => Error0308("mismatched types"),
1473             },
1474         }
1475     }
1476
1477     fn as_requirement_str(&self) -> &'static str {
1478         use traits::ObligationCauseCode::*;
1479         match self.code {
1480             CompareImplMethodObligation { .. } => "method type is compatible with trait",
1481             ExprAssignable => "expression is assignable",
1482             MatchExpressionArm { source, .. } => match source {
1483                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have compatible types",
1484                 _ => "match arms have compatible types",
1485             },
1486             IfExpression => "if and else have compatible types",
1487             IfExpressionWithNoElse => "if missing an else returns ()",
1488             MainFunctionType => "`main` function has the correct type",
1489             StartFunctionType => "`start` function has the correct type",
1490             IntrinsicType => "intrinsic has the correct type",
1491             MethodReceiver => "method receiver has the correct type",
1492             _ => "types are compatible",
1493         }
1494     }
1495 }