src/librustc/infer/error_reporting/mod.rs

   1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Error Reporting Code for the inference engine
  12 //!
  13 //! Because of the way inference, and in particular region inference,
  14 //! works, it often happens that errors are not detected until far after
  15 //! the relevant line of code has been type-checked. Therefore, there is
  16 //! an elaborate system to track why a particular constraint in the
  17 //! inference graph arose so that we can explain to the user what gave
  18 //! rise to a particular error.
  19 //!
  20 //! The basis of the system are the "origin" types. An "origin" is the
  21 //! reason that a constraint or inference variable arose. There are
  22 //! different "origin" enums for different kinds of constraints/variables
  23 //! (e.g., `TypeOrigin`, `RegionVariableOrigin`). An origin always has
  24 //! a span, but also more information so that we can generate a meaningful
  25 //! error message.
  26 //!
  27 //! Having a catalog of all the different reasons an error can arise is
  28 //! also useful for other reasons, like cross-referencing FAQs etc, though
  29 //! we are not really taking advantage of this yet.
  30 //!
  31 //! # Region Inference
  32 //!
  33 //! Region inference is particularly tricky because it always succeeds "in
  34 //! the moment" and simply registers a constraint. Then, at the end, we
  35 //! can compute the full graph and report errors, so we need to be able to
  36 //! store and later report what gave rise to the conflicting constraints.
  37 //!
  38 //! # Subtype Trace
  39 //!
  40 //! Determining whether `T1 <: T2` often involves a number of subtypes and
  41 //! subconstraints along the way. A "TypeTrace" is an extended version
  42 //! of an origin that traces the types and other values that were being
  43 //! compared. It is not necessarily comprehensive (in fact, at the time of
  44 //! this writing it only tracks the root values being compared) but I'd
  45 //! like to extend it to include significant "waypoints". For example, if
  46 //! you are comparing `(T1, T2) <: (T3, T4)`, and the problem is that `T2
  47 //! <: T4` fails, I'd like the trace to include enough information to say
  48 //! "in the 2nd element of the tuple". Similarly, failures when comparing
  49 //! arguments or return types in fn types should be able to cite the
  50 //! specific position, etc.
  51 //!
  52 //! # Reality vs plan
  53 //!
  54 //! Of course, there is still a LOT of code in typeck that has yet to be
  55 //! ported to this system, and which relies on string concatenation at the
  56 //! time of error detection.
  57
  58 use infer;
  59 use super::{InferCtxt, RegionVariableOrigin, SubregionOrigin, TypeTrace, ValuePairs};
  60 use super::region_constraints::GenericKind;
  61 use super::lexical_region_resolve::RegionResolutionError;
  62
  63 use std::fmt;
  64 use hir;
  65 use hir::map as hir_map;
  66 use hir::def_id::DefId;
  67 use middle::region;
  68 use traits::{ObligationCause, ObligationCauseCode};
  69 use ty::{self, Region, Ty, TyCtxt, TypeFoldable, TypeVariants};
  70 use ty::error::TypeError;
  71 use syntax::ast::DUMMY_NODE_ID;
  72 use syntax_pos::{Pos, Span};
  73 use errors::{DiagnosticBuilder, DiagnosticStyledString};
  74
  75 use rustc_data_structures::indexed_vec::Idx;
  76
  77 mod note;
  78
  79 mod need_type_info;
  80
  81 pub mod nice_region_error;
  82
  83 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
  84     pub fn note_and_explain_region(
  85         self,
  86         region_scope_tree: &region::ScopeTree,
  87         err: &mut DiagnosticBuilder,
  88         prefix: &str,
  89         region: ty::Region<'tcx>,
  90         suffix: &str,
  91     ) {
  92         let (description, span) = match *region {
  93             ty::ReScope(scope) => {
  94                 let new_string;
  95                 let unknown_scope = || {
  96                     format!(
  97                         "{}unknown scope: {:?}{}.  Please report a bug.",
  98                         prefix, scope, suffix
  99                     )
 100                 };
 101                 let span = scope.span(self, region_scope_tree);
 102                 let tag = match self.hir.find(scope.node_id(self, region_scope_tree)) {
 103                     Some(hir_map::NodeBlock(_)) => "block",
 104                     Some(hir_map::NodeExpr(expr)) => match expr.node {
 105                         hir::ExprCall(..) => "call",
 106                         hir::ExprMethodCall(..) => "method call",
 107                         hir::ExprMatch(.., hir::MatchSource::IfLetDesugar { .. }) => "if let",
 108                         hir::ExprMatch(.., hir::MatchSource::WhileLetDesugar) => "while let",
 109                         hir::ExprMatch(.., hir::MatchSource::ForLoopDesugar) => "for",
 110                         hir::ExprMatch(..) => "match",
 111                         _ => "expression",
 112                     },
 113                     Some(hir_map::NodeStmt(_)) => "statement",
 114                     Some(hir_map::NodeItem(it)) => Self::item_scope_tag(&it),
 115                     Some(hir_map::NodeTraitItem(it)) => Self::trait_item_scope_tag(&it),
 116                     Some(hir_map::NodeImplItem(it)) => Self::impl_item_scope_tag(&it),
 117                     Some(_) | None => {
 118                         err.span_note(span, &unknown_scope());
 119                         return;
 120                     }
 121                 };
 122                 let scope_decorated_tag = match scope.data() {
 123                     region::ScopeData::Node(_) => tag,
 124                     region::ScopeData::CallSite(_) => "scope of call-site for function",
 125                     region::ScopeData::Arguments(_) => "scope of function body",
 126                     region::ScopeData::Destruction(_) => {
 127                         new_string = format!("destruction scope surrounding {}", tag);
 128                         &new_string[..]
 129                     }
 130                     region::ScopeData::Remainder(r) => {
 131                         new_string = format!(
 132                             "block suffix following statement {}",
 133                             r.first_statement_index.index()
 134                         );
 135                         &new_string[..]
 136                     }
 137                 };
 138                 self.explain_span(scope_decorated_tag, span)
 139             }
 140
 141             ty::ReEarlyBound(_) | ty::ReFree(_) => self.msg_span_from_free_region(region),
 142
 143             ty::ReStatic => ("the static lifetime".to_owned(), None),
 144
 145             ty::ReEmpty => ("the empty lifetime".to_owned(), None),
 146
 147             // FIXME(#13998) ReSkolemized should probably print like
 148             // ReFree rather than dumping Debug output on the user.
 149             //
 150             // We shouldn't really be having unification failures with ReVar
 151             // and ReLateBound though.
 152             ty::ReSkolemized(..) | ty::ReVar(_) | ty::ReLateBound(..) | ty::ReErased => {
 153                 (format!("lifetime {:?}", region), None)
 154             }
 155
 156             // We shouldn't encounter an error message with ReClosureBound.
 157             ty::ReClosureBound(..) => {
 158                 bug!("encountered unexpected ReClosureBound: {:?}", region,);
 159             }
 160         };
 161
 162         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 163     }
 164
 165     pub fn note_and_explain_free_region(
 166         self,
 167         err: &mut DiagnosticBuilder,
 168         prefix: &str,
 169         region: ty::Region<'tcx>,
 170         suffix: &str,
 171     ) {
 172         let (description, span) = self.msg_span_from_free_region(region);
 173
 174         TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
 175     }
 176
 177     fn msg_span_from_free_region(self, region: ty::Region<'tcx>) -> (String, Option<Span>) {
 178         let scope = region.free_region_binding_scope(self);
 179         let node = self.hir.as_local_node_id(scope).unwrap_or(DUMMY_NODE_ID);
 180         let unknown;
 181         let tag = match self.hir.find(node) {
 182             Some(hir_map::NodeBlock(_)) | Some(hir_map::NodeExpr(_)) => "body",
 183             Some(hir_map::NodeItem(it)) => Self::item_scope_tag(&it),
 184             Some(hir_map::NodeTraitItem(it)) => Self::trait_item_scope_tag(&it),
 185             Some(hir_map::NodeImplItem(it)) => Self::impl_item_scope_tag(&it),
 186
 187             // this really should not happen, but it does:
 188             // FIXME(#27942)
 189             Some(_) => {
 190                 unknown = format!(
 191                     "unexpected node ({}) for scope {:?}.  \
 192                      Please report a bug.",
 193                     self.hir.node_to_string(node),
 194                     scope
 195                 );
 196                 &unknown
 197             }
 198             None => {
 199                 unknown = format!(
 200                     "unknown node for scope {:?}.  \
 201                      Please report a bug.",
 202                     scope
 203                 );
 204                 &unknown
 205             }
 206         };
 207         let (prefix, span) = match *region {
 208             ty::ReEarlyBound(ref br) => (
 209                 format!("the lifetime {} as defined on", br.name),
 210                 self.sess.codemap().def_span(self.hir.span(node)),
 211             ),
 212             ty::ReFree(ref fr) => match fr.bound_region {
 213                 ty::BrAnon(idx) => (
 214                     format!("the anonymous lifetime #{} defined on", idx + 1),
 215                     self.hir.span(node),
 216                 ),
 217                 ty::BrFresh(_) => (
 218                     "an anonymous lifetime defined on".to_owned(),
 219                     self.hir.span(node),
 220                 ),
 221                 _ => (
 222                     format!("the lifetime {} as defined on", fr.bound_region),
 223                     self.sess.codemap().def_span(self.hir.span(node)),
 224                 ),
 225             },
 226             _ => bug!(),
 227         };
 228         let (msg, opt_span) = self.explain_span(tag, span);
 229         (format!("{} {}", prefix, msg), opt_span)
 230     }
 231
 232     fn emit_msg_span(
 233         err: &mut DiagnosticBuilder,
 234         prefix: &str,
 235         description: String,
 236         span: Option<Span>,
 237         suffix: &str,
 238     ) {
 239         let message = format!("{}{}{}", prefix, description, suffix);
 240
 241         if let Some(span) = span {
 242             err.span_note(span, &message);
 243         } else {
 244             err.note(&message);
 245         }
 246     }
 247
 248     fn item_scope_tag(item: &hir::Item) -> &'static str {
 249         match item.node {
 250             hir::ItemImpl(..) => "impl",
 251             hir::ItemStruct(..) => "struct",
 252             hir::ItemUnion(..) => "union",
 253             hir::ItemEnum(..) => "enum",
 254             hir::ItemTrait(..) => "trait",
 255             hir::ItemFn(..) => "function body",
 256             _ => "item",
 257         }
 258     }
 259
 260     fn trait_item_scope_tag(item: &hir::TraitItem) -> &'static str {
 261         match item.node {
 262             hir::TraitItemKind::Method(..) => "method body",
 263             hir::TraitItemKind::Const(..) | hir::TraitItemKind::Type(..) => "associated item",
 264         }
 265     }
 266
 267     fn impl_item_scope_tag(item: &hir::ImplItem) -> &'static str {
 268         match item.node {
 269             hir::ImplItemKind::Method(..) => "method body",
 270             hir::ImplItemKind::Const(..) | hir::ImplItemKind::Type(_) => "associated item",
 271         }
 272     }
 273
 274     fn explain_span(self, heading: &str, span: Span) -> (String, Option<Span>) {
 275         let lo = self.sess.codemap().lookup_char_pos_adj(span.lo());
 276         (
 277             format!("the {} at {}:{}", heading, lo.line, lo.col.to_usize() + 1),
 278             Some(span),
 279         )
 280     }
 281 }
 282
 283 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 284     pub fn report_region_errors(
 285         &self,
 286         region_scope_tree: &region::ScopeTree,
 287         errors: &Vec<RegionResolutionError<'tcx>>,
 288         will_later_be_reported_by_nll: bool,
 289     ) {
 290         debug!("report_region_errors(): {} errors to start", errors.len());
 291
 292         if will_later_be_reported_by_nll && self.tcx.nll() {
 293             // With `#![feature(nll)]`, we want to present a nice user
 294             // experience, so don't even mention the errors from the
 295             // AST checker.
 296             if self.tcx.features().nll {
 297                 return;
 298             }
 299
 300             // But with -Znll, it's nice to have some note for later.
 301             for error in errors {
 302                 match *error {
 303                     RegionResolutionError::ConcreteFailure(ref origin, ..)
 304                     | RegionResolutionError::GenericBoundFailure(ref origin, ..) => {
 305                         self.tcx
 306                             .sess
 307                             .span_warn(origin.span(), "not reporting region error due to -Znll");
 308                     }
 309
 310                     RegionResolutionError::SubSupConflict(ref rvo, ..) => {
 311                         self.tcx
 312                             .sess
 313                             .span_warn(rvo.span(), "not reporting region error due to -Znll");
 314                     }
 315                 }
 316             }
 317
 318             return;
 319         }
 320
 321         // try to pre-process the errors, which will group some of them
 322         // together into a `ProcessedErrors` group:
 323         let errors = self.process_errors(errors);
 324
 325         debug!(
 326             "report_region_errors: {} errors after preprocessing",
 327             errors.len()
 328         );
 329
 330         for error in errors {
 331             debug!("report_region_errors: error = {:?}", error);
 332
 333             if !self.try_report_nice_region_error(&error) {
 334                 match error.clone() {
 335                     // These errors could indicate all manner of different
 336                     // problems with many different solutions. Rather
 337                     // than generate a "one size fits all" error, what we
 338                     // attempt to do is go through a number of specific
 339                     // scenarios and try to find the best way to present
 340                     // the error. If all of these fails, we fall back to a rather
 341                     // general bit of code that displays the error information
 342                     RegionResolutionError::ConcreteFailure(origin, sub, sup) => {
 343                         self.report_concrete_failure(region_scope_tree, origin, sub, sup)
 344                             .emit();
 345                     }
 346
 347                     RegionResolutionError::GenericBoundFailure(origin, param_ty, sub) => {
 348                         self.report_generic_bound_failure(
 349                             region_scope_tree,
 350                             origin.span(),
 351                             Some(origin),
 352                             param_ty,
 353                             sub,
 354                         );
 355                     }
 356
 357                     RegionResolutionError::SubSupConflict(
 358                         var_origin,
 359                         sub_origin,
 360                         sub_r,
 361                         sup_origin,
 362                         sup_r,
 363                     ) => {
 364                         self.report_sub_sup_conflict(
 365                             region_scope_tree,
 366                             var_origin,
 367                             sub_origin,
 368                             sub_r,
 369                             sup_origin,
 370                             sup_r,
 371                         );
 372                     }
 373                 }
 374             }
 375         }
 376     }
 377
 378     // This method goes through all the errors and try to group certain types
 379     // of error together, for the purpose of suggesting explicit lifetime
 380     // parameters to the user. This is done so that we can have a more
 381     // complete view of what lifetimes should be the same.
 382     // If the return value is an empty vector, it means that processing
 383     // failed (so the return value of this method should not be used).
 384     //
 385     // The method also attempts to weed out messages that seem like
 386     // duplicates that will be unhelpful to the end-user. But
 387     // obviously it never weeds out ALL errors.
 388     fn process_errors(
 389         &self,
 390         errors: &Vec<RegionResolutionError<'tcx>>,
 391     ) -> Vec<RegionResolutionError<'tcx>> {
 392         debug!("process_errors()");
 393
 394         // We want to avoid reporting generic-bound failures if we can
 395         // avoid it: these have a very high rate of being unhelpful in
 396         // practice. This is because they are basically secondary
 397         // checks that test the state of the region graph after the
 398         // rest of inference is done, and the other kinds of errors
 399         // indicate that the region constraint graph is internally
 400         // inconsistent, so these test results are likely to be
 401         // meaningless.
 402         //
 403         // Therefore, we filter them out of the list unless they are
 404         // the only thing in the list.
 405
 406         let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
 407             RegionResolutionError::GenericBoundFailure(..) => true,
 408             RegionResolutionError::ConcreteFailure(..)
 409             | RegionResolutionError::SubSupConflict(..) => false,
 410         };
 411
 412         let mut errors = if errors.iter().all(|e| is_bound_failure(e)) {
 413             errors.clone()
 414         } else {
 415             errors
 416                 .iter()
 417                 .filter(|&e| !is_bound_failure(e))
 418                 .cloned()
 419                 .collect()
 420         };
 421
 422         // sort the errors by span, for better error message stability.
 423         errors.sort_by_key(|u| match *u {
 424             RegionResolutionError::ConcreteFailure(ref sro, _, _) => sro.span(),
 425             RegionResolutionError::GenericBoundFailure(ref sro, _, _) => sro.span(),
 426             RegionResolutionError::SubSupConflict(ref rvo, _, _, _, _) => rvo.span(),
 427         });
 428         errors
 429     }
 430
 431     /// Adds a note if the types come from similarly named crates
 432     fn check_and_note_conflicting_crates(
 433         &self,
 434         err: &mut DiagnosticBuilder,
 435         terr: &TypeError<'tcx>,
 436         sp: Span,
 437     ) {
 438         let report_path_match = |err: &mut DiagnosticBuilder, did1: DefId, did2: DefId| {
 439             // Only external crates, if either is from a local
 440             // module we could have false positives
 441             if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
 442                 let exp_path = self.tcx.item_path_str(did1);
 443                 let found_path = self.tcx.item_path_str(did2);
 444                 let exp_abs_path = self.tcx.absolute_item_path_str(did1);
 445                 let found_abs_path = self.tcx.absolute_item_path_str(did2);
 446                 // We compare strings because DefPath can be different
 447                 // for imported and non-imported crates
 448                 if exp_path == found_path || exp_abs_path == found_abs_path {
 449                     let crate_name = self.tcx.crate_name(did1.krate);
 450                     err.span_note(
 451                         sp,
 452                         &format!(
 453                             "Perhaps two different versions \
 454                              of crate `{}` are being used?",
 455                             crate_name
 456                         ),
 457                     );
 458                 }
 459             }
 460         };
 461         match *terr {
 462             TypeError::Sorts(ref exp_found) => {
 463                 // if they are both "path types", there's a chance of ambiguity
 464                 // due to different versions of the same crate
 465                 match (&exp_found.expected.sty, &exp_found.found.sty) {
 466                     (&ty::TyAdt(exp_adt, _), &ty::TyAdt(found_adt, _)) => {
 467                         report_path_match(err, exp_adt.did, found_adt.did);
 468                     }
 469                     _ => (),
 470                 }
 471             }
 472             TypeError::Traits(ref exp_found) => {
 473                 report_path_match(err, exp_found.expected, exp_found.found);
 474             }
 475             _ => (), // FIXME(#22750) handle traits and stuff
 476         }
 477     }
 478
 479     fn note_error_origin(&self, err: &mut DiagnosticBuilder<'tcx>, cause: &ObligationCause<'tcx>) {
 480         match cause.code {
 481             ObligationCauseCode::MatchExpressionArm { arm_span, source } => match source {
 482                 hir::MatchSource::IfLetDesugar { .. } => {
 483                     let msg = "`if let` arm with an incompatible type";
 484                     if self.tcx.sess.codemap().is_multiline(arm_span) {
 485                         err.span_note(arm_span, msg);
 486                     } else {
 487                         err.span_label(arm_span, msg);
 488                     }
 489                 }
 490                 _ => {
 491                     let msg = "match arm with an incompatible type";
 492                     if self.tcx.sess.codemap().is_multiline(arm_span) {
 493                         err.span_note(arm_span, msg);
 494                     } else {
 495                         err.span_label(arm_span, msg);
 496                     }
 497                 }
 498             },
 499             _ => (),
 500         }
 501     }
 502
 503     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
 504     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
 505     /// populate `other_value` with `other_ty`.
 506     ///
 507     /// ```text
 508     /// Foo<Bar<Qux>>
 509     /// ^^^^--------^ this is highlighted
 510     /// |   |
 511     /// |   this type argument is exactly the same as the other type, not highlighted
 512     /// this is highlighted
 513     /// Bar<Qux>
 514     /// -------- this type is the same as a type argument in the other type, not highlighted
 515     /// ```
 516     fn highlight_outer(
 517         &self,
 518         value: &mut DiagnosticStyledString,
 519         other_value: &mut DiagnosticStyledString,
 520         name: String,
 521         sub: &ty::subst::Substs<'tcx>,
 522         pos: usize,
 523         other_ty: &Ty<'tcx>,
 524     ) {
 525         // `value` and `other_value` hold two incomplete type representation for display.
 526         // `name` is the path of both types being compared. `sub`
 527         value.push_highlighted(name);
 528         let len = sub.len();
 529         if len > 0 {
 530             value.push_highlighted("<");
 531         }
 532
 533         // Output the lifetimes fot the first type
 534         let lifetimes = sub.regions()
 535             .map(|lifetime| {
 536                 let s = format!("{}", lifetime);
 537                 if s.is_empty() {
 538                     "'_".to_string()
 539                 } else {
 540                     s
 541                 }
 542             })
 543             .collect::<Vec<_>>()
 544             .join(", ");
 545         if !lifetimes.is_empty() {
 546             if sub.regions().count() < len {
 547                 value.push_normal(lifetimes + &", ");
 548             } else {
 549                 value.push_normal(lifetimes);
 550             }
 551         }
 552
 553         // Highlight all the type arguments that aren't at `pos` and compare the type argument at
 554         // `pos` and `other_ty`.
 555         for (i, type_arg) in sub.types().enumerate() {
 556             if i == pos {
 557                 let values = self.cmp(type_arg, other_ty);
 558                 value.0.extend((values.0).0);
 559                 other_value.0.extend((values.1).0);
 560             } else {
 561                 value.push_highlighted(format!("{}", type_arg));
 562             }
 563
 564             if len > 0 && i != len - 1 {
 565                 value.push_normal(", ");
 566             }
 567             //self.push_comma(&mut value, &mut other_value, len, i);
 568         }
 569         if len > 0 {
 570             value.push_highlighted(">");
 571         }
 572     }
 573
 574     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 575     /// as that is the difference to the other type.
 576     ///
 577     /// For the following code:
 578     ///
 579     /// ```norun
 580     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 581     /// ```
 582     ///
 583     /// The type error output will behave in the following way:
 584     ///
 585     /// ```text
 586     /// Foo<Bar<Qux>>
 587     /// ^^^^--------^ this is highlighted
 588     /// |   |
 589     /// |   this type argument is exactly the same as the other type, not highlighted
 590     /// this is highlighted
 591     /// Bar<Qux>
 592     /// -------- this type is the same as a type argument in the other type, not highlighted
 593     /// ```
 594     fn cmp_type_arg(
 595         &self,
 596         mut t1_out: &mut DiagnosticStyledString,
 597         mut t2_out: &mut DiagnosticStyledString,
 598         path: String,
 599         sub: &ty::subst::Substs<'tcx>,
 600         other_path: String,
 601         other_ty: &Ty<'tcx>,
 602     ) -> Option<()> {
 603         for (i, ta) in sub.types().enumerate() {
 604             if &ta == other_ty {
 605                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 606                 return Some(());
 607             }
 608             if let &ty::TyAdt(def, _) = &ta.sty {
 609                 let path_ = self.tcx.item_path_str(def.did.clone());
 610                 if path_ == other_path {
 611                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 612                     return Some(());
 613                 }
 614             }
 615         }
 616         None
 617     }
 618
 619     /// Add a `,` to the type representation only if it is appropriate.
 620     fn push_comma(
 621         &self,
 622         value: &mut DiagnosticStyledString,
 623         other_value: &mut DiagnosticStyledString,
 624         len: usize,
 625         pos: usize,
 626     ) {
 627         if len > 0 && pos != len - 1 {
 628             value.push_normal(", ");
 629             other_value.push_normal(", ");
 630         }
 631     }
 632
 633     /// Compare two given types, eliding parts that are the same between them and highlighting
 634     /// relevant differences, and return two representation of those types for highlighted printing.
 635     fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> (DiagnosticStyledString, DiagnosticStyledString) {
 636         fn equals<'tcx>(a: &Ty<'tcx>, b: &Ty<'tcx>) -> bool {
 637             match (&a.sty, &b.sty) {
 638                 (a, b) if *a == *b => true,
 639                 (&ty::TyInt(_), &ty::TyInfer(ty::InferTy::IntVar(_)))
 640                 | (&ty::TyInfer(ty::InferTy::IntVar(_)), &ty::TyInt(_))
 641                 | (&ty::TyInfer(ty::InferTy::IntVar(_)), &ty::TyInfer(ty::InferTy::IntVar(_)))
 642                 | (&ty::TyFloat(_), &ty::TyInfer(ty::InferTy::FloatVar(_)))
 643                 | (&ty::TyInfer(ty::InferTy::FloatVar(_)), &ty::TyFloat(_))
 644                 | (
 645                     &ty::TyInfer(ty::InferTy::FloatVar(_)),
 646                     &ty::TyInfer(ty::InferTy::FloatVar(_)),
 647                 ) => true,
 648                 _ => false,
 649             }
 650         }
 651
 652         fn push_ty_ref<'tcx>(
 653             r: &ty::Region<'tcx>,
 654             tnm: &ty::TypeAndMut<'tcx>,
 655             s: &mut DiagnosticStyledString,
 656         ) {
 657             let r = &format!("{}", r);
 658             s.push_highlighted(format!(
 659                 "&{}{}{}",
 660                 r,
 661                 if r == "" { "" } else { " " },
 662                 if tnm.mutbl == hir::MutMutable {
 663                     "mut "
 664                 } else {
 665                     ""
 666                 }
 667             ));
 668             s.push_normal(format!("{}", tnm.ty));
 669         }
 670
 671         match (&t1.sty, &t2.sty) {
 672             (&ty::TyAdt(def1, sub1), &ty::TyAdt(def2, sub2)) => {
 673                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 674                 let path1 = self.tcx.item_path_str(def1.did.clone());
 675                 let path2 = self.tcx.item_path_str(def2.did.clone());
 676                 if def1.did == def2.did {
 677                     // Easy case. Replace same types with `_` to shorten the output and highlight
 678                     // the differing ones.
 679                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
 680                     //     Foo<Bar, _>
 681                     //     Foo<Quz, _>
 682                     //         ---  ^ type argument elided
 683                     //         |
 684                     //         highlighted in output
 685                     values.0.push_normal(path1);
 686                     values.1.push_normal(path2);
 687
 688                     // Only draw `<...>` if there're lifetime/type arguments.
 689                     let len = sub1.len();
 690                     if len > 0 {
 691                         values.0.push_normal("<");
 692                         values.1.push_normal("<");
 693                     }
 694
 695                     fn lifetime_display(lifetime: Region) -> String {
 696                         let s = format!("{}", lifetime);
 697                         if s.is_empty() {
 698                             "'_".to_string()
 699                         } else {
 700                             s
 701                         }
 702                     }
 703                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
 704                     // all diagnostics that use this output
 705                     //
 706                     //     Foo<'x, '_, Bar>
 707                     //     Foo<'y, '_, Qux>
 708                     //         ^^  ^^  --- type arguments are not elided
 709                     //         |   |
 710                     //         |   elided as they were the same
 711                     //         not elided, they were different, but irrelevant
 712                     let lifetimes = sub1.regions().zip(sub2.regions());
 713                     for (i, lifetimes) in lifetimes.enumerate() {
 714                         let l1 = lifetime_display(lifetimes.0);
 715                         let l2 = lifetime_display(lifetimes.1);
 716                         if l1 == l2 {
 717                             values.0.push_normal("'_");
 718                             values.1.push_normal("'_");
 719                         } else {
 720                             values.0.push_highlighted(l1);
 721                             values.1.push_highlighted(l2);
 722                         }
 723                         self.push_comma(&mut values.0, &mut values.1, len, i);
 724                     }
 725
 726                     // We're comparing two types with the same path, so we compare the type
 727                     // arguments for both. If they are the same, do not highlight and elide from the
 728                     // output.
 729                     //     Foo<_, Bar>
 730                     //     Foo<_, Qux>
 731                     //         ^ elided type as this type argument was the same in both sides
 732                     let type_arguments = sub1.types().zip(sub2.types());
 733                     let regions_len = sub1.regions().collect::<Vec<_>>().len();
 734                     for (i, (ta1, ta2)) in type_arguments.enumerate() {
 735                         let i = i + regions_len;
 736                         if ta1 == ta2 {
 737                             values.0.push_normal("_");
 738                             values.1.push_normal("_");
 739                         } else {
 740                             let (x1, x2) = self.cmp(ta1, ta2);
 741                             (values.0).0.extend(x1.0);
 742                             (values.1).0.extend(x2.0);
 743                         }
 744                         self.push_comma(&mut values.0, &mut values.1, len, i);
 745                     }
 746
 747                     // Close the type argument bracket.
 748                     // Only draw `<...>` if there're lifetime/type arguments.
 749                     if len > 0 {
 750                         values.0.push_normal(">");
 751                         values.1.push_normal(">");
 752                     }
 753                     values
 754                 } else {
 755                     // Check for case:
 756                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
 757                     //     Foo<Bar<Qux>
 758                     //         ------- this type argument is exactly the same as the other type
 759                     //     Bar<Qux>
 760                     if self.cmp_type_arg(
 761                         &mut values.0,
 762                         &mut values.1,
 763                         path1.clone(),
 764                         sub1,
 765                         path2.clone(),
 766                         &t2,
 767                     ).is_some()
 768                     {
 769                         return values;
 770                     }
 771                     // Check for case:
 772                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
 773                     //     Bar<Qux>
 774                     //     Foo<Bar<Qux>>
 775                     //         ------- this type argument is exactly the same as the other type
 776                     if self.cmp_type_arg(&mut values.1, &mut values.0, path2, sub2, path1, &t1)
 777                         .is_some()
 778                     {
 779                         return values;
 780                     }
 781
 782                     // We couldn't find anything in common, highlight everything.
 783                     //     let x: Bar<Qux> = y::<Foo<Zar>>();
 784                     (
 785                         DiagnosticStyledString::highlighted(format!("{}", t1)),
 786                         DiagnosticStyledString::highlighted(format!("{}", t2)),
 787                     )
 788                 }
 789             }
 790
 791             // When finding T != &T, highlight only the borrow
 792             (&ty::TyRef(r1, ref tnm1), _) if equals(&tnm1.ty, &t2) => {
 793                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 794                 push_ty_ref(&r1, tnm1, &mut values.0);
 795                 values.1.push_normal(format!("{}", t2));
 796                 values
 797             }
 798             (_, &ty::TyRef(r2, ref tnm2)) if equals(&t1, &tnm2.ty) => {
 799                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 800                 values.0.push_normal(format!("{}", t1));
 801                 push_ty_ref(&r2, tnm2, &mut values.1);
 802                 values
 803             }
 804
 805             // When encountering &T != &mut T, highlight only the borrow
 806             (&ty::TyRef(r1, ref tnm1), &ty::TyRef(r2, ref tnm2)) if equals(&tnm1.ty, &tnm2.ty) => {
 807                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 808                 push_ty_ref(&r1, tnm1, &mut values.0);
 809                 push_ty_ref(&r2, tnm2, &mut values.1);
 810                 values
 811             }
 812
 813             _ => {
 814                 if t1 == t2 {
 815                     // The two types are the same, elide and don't highlight.
 816                     (
 817                         DiagnosticStyledString::normal("_"),
 818                         DiagnosticStyledString::normal("_"),
 819                     )
 820                 } else {
 821                     // We couldn't find anything in common, highlight everything.
 822                     (
 823                         DiagnosticStyledString::highlighted(format!("{}", t1)),
 824                         DiagnosticStyledString::highlighted(format!("{}", t2)),
 825                     )
 826                 }
 827             }
 828         }
 829     }
 830
 831     pub fn note_type_err(
 832         &self,
 833         diag: &mut DiagnosticBuilder<'tcx>,
 834         cause: &ObligationCause<'tcx>,
 835         secondary_span: Option<(Span, String)>,
 836         mut values: Option<ValuePairs<'tcx>>,
 837         terr: &TypeError<'tcx>,
 838     ) {
 839         // For some types of errors, expected-found does not make
 840         // sense, so just ignore the values we were given.
 841         match terr {
 842             TypeError::CyclicTy(_) => {
 843                 values = None;
 844             }
 845             _ => {}
 846         }
 847
 848         let (expected_found, exp_found, is_simple_error) = match values {
 849             None => (None, None, false),
 850             Some(values) => {
 851                 let (is_simple_error, exp_found) = match values {
 852                     ValuePairs::Types(exp_found) => {
 853                         let is_simple_err =
 854                             exp_found.expected.is_primitive() && exp_found.found.is_primitive();
 855
 856                         (is_simple_err, Some(exp_found))
 857                     }
 858                     _ => (false, None),
 859                 };
 860                 let vals = match self.values_str(&values) {
 861                     Some((expected, found)) => Some((expected, found)),
 862                     None => {
 863                         // Derived error. Cancel the emitter.
 864                         self.tcx.sess.diagnostic().cancel(diag);
 865                         return;
 866                     }
 867                 };
 868                 (vals, exp_found, is_simple_error)
 869             }
 870         };
 871
 872         let span = cause.span(&self.tcx);
 873
 874         diag.span_label(span, terr.to_string());
 875         if let Some((sp, msg)) = secondary_span {
 876             diag.span_label(sp, msg);
 877         }
 878
 879         if let Some((expected, found)) = expected_found {
 880             match (terr, is_simple_error, expected == found) {
 881                 (&TypeError::Sorts(ref values), false, true) => {
 882                     diag.note_expected_found_extra(
 883                         &"type",
 884                         expected,
 885                         found,
 886                         &format!(" ({})", values.expected.sort_string(self.tcx)),
 887                         &format!(" ({})", values.found.sort_string(self.tcx)),
 888                     );
 889                 }
 890                 (_, false, _) => {
 891                     if let Some(exp_found) = exp_found {
 892                         let (def_id, ret_ty) = match exp_found.found.sty {
 893                             TypeVariants::TyFnDef(def, _) => {
 894                                 (Some(def), Some(self.tcx.fn_sig(def).output()))
 895                             }
 896                             _ => (None, None),
 897                         };
 898
 899                         let exp_is_struct = match exp_found.expected.sty {
 900                             TypeVariants::TyAdt(def, _) => def.is_struct(),
 901                             _ => false,
 902                         };
 903
 904                         if let (Some(def_id), Some(ret_ty)) = (def_id, ret_ty) {
 905                             if exp_is_struct && exp_found.expected == ret_ty.0 {
 906                                 let message = format!(
 907                                     "did you mean `{}(/* fields */)`?",
 908                                     self.tcx.item_path_str(def_id)
 909                                 );
 910                                 diag.span_label(span, message);
 911                             }
 912                         }
 913                     }
 914
 915                     diag.note_expected_found(&"type", expected, found);
 916                 }
 917                 _ => (),
 918             }
 919         }
 920
 921         self.check_and_note_conflicting_crates(diag, terr, span);
 922         self.tcx.note_and_explain_type_err(diag, terr, span);
 923
 924         // It reads better to have the error origin as the final
 925         // thing.
 926         self.note_error_origin(diag, &cause);
 927     }
 928
 929     pub fn report_and_explain_type_error(
 930         &self,
 931         trace: TypeTrace<'tcx>,
 932         terr: &TypeError<'tcx>,
 933     ) -> DiagnosticBuilder<'tcx> {
 934         debug!(
 935             "report_and_explain_type_error(trace={:?}, terr={:?})",
 936             trace, terr
 937         );
 938
 939         let span = trace.cause.span(&self.tcx);
 940         let failure_code = trace.cause.as_failure_code(terr);
 941         let mut diag = match failure_code {
 942             FailureCode::Error0317(failure_str) => {
 943                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
 944             }
 945             FailureCode::Error0580(failure_str) => {
 946                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
 947             }
 948             FailureCode::Error0308(failure_str) => {
 949                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
 950             }
 951             FailureCode::Error0644(failure_str) => {
 952                 struct_span_err!(self.tcx.sess, span, E0644, "{}", failure_str)
 953             }
 954         };
 955         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
 956         diag
 957     }
 958
 959     fn values_str(
 960         &self,
 961         values: &ValuePairs<'tcx>,
 962     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
 963         match *values {
 964             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
 965             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 966             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 967         }
 968     }
 969
 970     fn expected_found_str_ty(
 971         &self,
 972         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
 973     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
 974         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 975         if exp_found.references_error() {
 976             return None;
 977         }
 978
 979         Some(self.cmp(exp_found.expected, exp_found.found))
 980     }
 981
 982     /// Returns a string of the form "expected `{}`, found `{}`".
 983     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
 984         &self,
 985         exp_found: &ty::error::ExpectedFound<T>,
 986     ) -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
 987         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 988         if exp_found.references_error() {
 989             return None;
 990         }
 991
 992         Some((
 993             DiagnosticStyledString::highlighted(format!("{}", exp_found.expected)),
 994             DiagnosticStyledString::highlighted(format!("{}", exp_found.found)),
 995         ))
 996     }
 997
 998     pub fn report_generic_bound_failure(
 999         &self,
1000         region_scope_tree: &region::ScopeTree,
1001         span: Span,
1002         origin: Option<SubregionOrigin<'tcx>>,
1003         bound_kind: GenericKind<'tcx>,
1004         sub: Region<'tcx>,
1005     ) {
1006         // Attempt to obtain the span of the parameter so we can
1007         // suggest adding an explicit lifetime bound to it.
1008         let type_param_span = match (self.in_progress_tables, bound_kind) {
1009             (Some(ref table), GenericKind::Param(ref param)) => {
1010                 let table = table.borrow();
1011                 table.local_id_root.and_then(|did| {
1012                     let generics = self.tcx.generics_of(did);
1013                     // Account for the case where `did` corresponds to `Self`, which doesn't have
1014                     // the expected type argument.
1015                     if !param.is_self() {
1016                         let type_param = generics.type_param(param, self.tcx);
1017                         let hir = &self.tcx.hir;
1018                         hir.as_local_node_id(type_param.def_id).map(|id| {
1019                             // Get the `hir::TyParam` to verify whether it already has any bounds.
1020                             // We do this to avoid suggesting code that ends up as `T: 'a'b`,
1021                             // instead we suggest `T: 'a + 'b` in that case.
1022                             let has_lifetimes = if let hir_map::NodeTyParam(ref p) = hir.get(id) {
1023                                 p.bounds.len() > 0
1024                             } else {
1025                                 false
1026                             };
1027                             let sp = hir.span(id);
1028                             // `sp` only covers `T`, change it so that it covers
1029                             // `T:` when appropriate
1030                             let sp = if has_lifetimes {
1031                                 sp.to(self.tcx
1032                                     .sess
1033                                     .codemap()
1034                                     .next_point(self.tcx.sess.codemap().next_point(sp)))
1035                             } else {
1036                                 sp
1037                             };
1038                             (sp, has_lifetimes)
1039                         })
1040                     } else {
1041                         None
1042                     }
1043                 })
1044             }
1045             _ => None,
1046         };
1047
1048         let labeled_user_string = match bound_kind {
1049             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
1050             GenericKind::Projection(ref p) => format!("the associated type `{}`", p),
1051         };
1052
1053         if let Some(SubregionOrigin::CompareImplMethodObligation {
1054             span,
1055             item_name,
1056             impl_item_def_id,
1057             trait_item_def_id,
1058         }) = origin
1059         {
1060             self.report_extra_impl_obligation(
1061                 span,
1062                 item_name,
1063                 impl_item_def_id,
1064                 trait_item_def_id,
1065                 &format!("`{}: {}`", bound_kind, sub),
1066             ).emit();
1067             return;
1068         }
1069
1070         fn binding_suggestion<'tcx, S: fmt::Display>(
1071             err: &mut DiagnosticBuilder<'tcx>,
1072             type_param_span: Option<(Span, bool)>,
1073             bound_kind: GenericKind<'tcx>,
1074             sub: S,
1075         ) {
1076             let consider = &format!(
1077                 "consider adding an explicit lifetime bound `{}: {}`...",
1078                 bound_kind, sub
1079             );
1080             if let Some((sp, has_lifetimes)) = type_param_span {
1081                 let tail = if has_lifetimes { " + " } else { "" };
1082                 let suggestion = format!("{}: {}{}", bound_kind, sub, tail);
1083                 err.span_suggestion_short(sp, consider, suggestion);
1084             } else {
1085                 err.help(consider);
1086             }
1087         }
1088
1089         let mut err = match *sub {
1090             ty::ReEarlyBound(_)
1091             | ty::ReFree(ty::FreeRegion {
1092                 bound_region: ty::BrNamed(..),
1093                 ..
1094             }) => {
1095                 // Does the required lifetime have a nice name we can print?
1096                 let mut err = struct_span_err!(
1097                     self.tcx.sess,
1098                     span,
1099                     E0309,
1100                     "{} may not live long enough",
1101                     labeled_user_string
1102                 );
1103                 binding_suggestion(&mut err, type_param_span, bound_kind, sub);
1104                 err
1105             }
1106
1107             ty::ReStatic => {
1108                 // Does the required lifetime have a nice name we can print?
1109                 let mut err = struct_span_err!(
1110                     self.tcx.sess,
1111                     span,
1112                     E0310,
1113                     "{} may not live long enough",
1114                     labeled_user_string
1115                 );
1116                 binding_suggestion(&mut err, type_param_span, bound_kind, "'static");
1117                 err
1118             }
1119
1120             _ => {
1121                 // If not, be less specific.
1122                 let mut err = struct_span_err!(
1123                     self.tcx.sess,
1124                     span,
1125                     E0311,
1126                     "{} may not live long enough",
1127                     labeled_user_string
1128                 );
1129                 err.help(&format!(
1130                     "consider adding an explicit lifetime bound for `{}`",
1131                     bound_kind
1132                 ));
1133                 self.tcx.note_and_explain_region(
1134                     region_scope_tree,
1135                     &mut err,
1136                     &format!("{} must be valid for ", labeled_user_string),
1137                     sub,
1138                     "...",
1139                 );
1140                 err
1141             }
1142         };
1143
1144         if let Some(origin) = origin {
1145             self.note_region_origin(&mut err, &origin);
1146         }
1147         err.emit();
1148     }
1149
1150     fn report_sub_sup_conflict(
1151         &self,
1152         region_scope_tree: &region::ScopeTree,
1153         var_origin: RegionVariableOrigin,
1154         sub_origin: SubregionOrigin<'tcx>,
1155         sub_region: Region<'tcx>,
1156         sup_origin: SubregionOrigin<'tcx>,
1157         sup_region: Region<'tcx>,
1158     ) {
1159         let mut err = self.report_inference_failure(var_origin);
1160
1161         self.tcx.note_and_explain_region(
1162             region_scope_tree,
1163             &mut err,
1164             "first, the lifetime cannot outlive ",
1165             sup_region,
1166             "...",
1167         );
1168
1169         match (&sup_origin, &sub_origin) {
1170             (&infer::Subtype(ref sup_trace), &infer::Subtype(ref sub_trace)) => {
1171                 if let (Some((sup_expected, sup_found)), Some((sub_expected, sub_found))) = (
1172                     self.values_str(&sup_trace.values),
1173                     self.values_str(&sub_trace.values),
1174                 ) {
1175                     if sub_expected == sup_expected && sub_found == sup_found {
1176                         self.tcx.note_and_explain_region(
1177                             region_scope_tree,
1178                             &mut err,
1179                             "...but the lifetime must also be valid for ",
1180                             sub_region,
1181                             "...",
1182                         );
1183                         err.note(&format!(
1184                             "...so that the {}:\nexpected {}\n   found {}",
1185                             sup_trace.cause.as_requirement_str(),
1186                             sup_expected.content(),
1187                             sup_found.content()
1188                         ));
1189                         err.emit();
1190                         return;
1191                     }
1192                 }
1193             }
1194             _ => {}
1195         }
1196
1197         self.note_region_origin(&mut err, &sup_origin);
1198
1199         self.tcx.note_and_explain_region(
1200             region_scope_tree,
1201             &mut err,
1202             "but, the lifetime must be valid for ",
1203             sub_region,
1204             "...",
1205         );
1206
1207         self.note_region_origin(&mut err, &sub_origin);
1208         err.emit();
1209     }
1210 }
1211
1212 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1213     fn report_inference_failure(
1214         &self,
1215         var_origin: RegionVariableOrigin,
1216     ) -> DiagnosticBuilder<'tcx> {
1217         let br_string = |br: ty::BoundRegion| {
1218             let mut s = br.to_string();
1219             if !s.is_empty() {
1220                 s.push_str(" ");
1221             }
1222             s
1223         };
1224         let var_description = match var_origin {
1225             infer::MiscVariable(_) => "".to_string(),
1226             infer::PatternRegion(_) => " for pattern".to_string(),
1227             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
1228             infer::Autoref(_) => " for autoref".to_string(),
1229             infer::Coercion(_) => " for automatic coercion".to_string(),
1230             infer::LateBoundRegion(_, br, infer::FnCall) => {
1231                 format!(" for lifetime parameter {}in function call", br_string(br))
1232             }
1233             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
1234                 format!(" for lifetime parameter {}in generic type", br_string(br))
1235             }
1236             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(def_id)) => format!(
1237                 " for lifetime parameter {}in trait containing associated type `{}`",
1238                 br_string(br),
1239                 self.tcx.associated_item(def_id).name
1240             ),
1241             infer::EarlyBoundRegion(_, name) => format!(" for lifetime parameter `{}`", name),
1242             infer::BoundRegionInCoherence(name) => {
1243                 format!(" for lifetime parameter `{}` in coherence check", name)
1244             }
1245             infer::UpvarRegion(ref upvar_id, _) => {
1246                 let var_node_id = self.tcx.hir.hir_to_node_id(upvar_id.var_id);
1247                 let var_name = self.tcx.hir.name(var_node_id);
1248                 format!(" for capture of `{}` by closure", var_name)
1249             }
1250             infer::NLL(..) => bug!("NLL variable found in lexical phase"),
1251         };
1252
1253         struct_span_err!(
1254             self.tcx.sess,
1255             var_origin.span(),
1256             E0495,
1257             "cannot infer an appropriate lifetime{} \
1258              due to conflicting requirements",
1259             var_description
1260         )
1261     }
1262 }
1263
1264 enum FailureCode {
1265     Error0317(&'static str),
1266     Error0580(&'static str),
1267     Error0308(&'static str),
1268     Error0644(&'static str),
1269 }
1270
1271 impl<'tcx> ObligationCause<'tcx> {
1272     fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode {
1273         use self::FailureCode::*;
1274         use traits::ObligationCauseCode::*;
1275         match self.code {
1276             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
1277             MatchExpressionArm { source, .. } => Error0308(match source {
1278                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have incompatible types",
1279                 _ => "match arms have incompatible types",
1280             }),
1281             IfExpression => Error0308("if and else have incompatible types"),
1282             IfExpressionWithNoElse => Error0317("if may be missing an else clause"),
1283             EquatePredicate => Error0308("equality predicate not satisfied"),
1284             MainFunctionType => Error0580("main function has wrong type"),
1285             StartFunctionType => Error0308("start function has wrong type"),
1286             IntrinsicType => Error0308("intrinsic has wrong type"),
1287             MethodReceiver => Error0308("mismatched method receiver"),
1288
1289             // In the case where we have no more specific thing to
1290             // say, also take a look at the error code, maybe we can
1291             // tailor to that.
1292             _ => match terr {
1293                 TypeError::CyclicTy(ty) if ty.is_closure() || ty.is_generator() => {
1294                     Error0644("closure/generator type that references itself")
1295                 }
1296                 _ => Error0308("mismatched types"),
1297             },
1298         }
1299     }
1300
1301     fn as_requirement_str(&self) -> &'static str {
1302         use traits::ObligationCauseCode::*;
1303         match self.code {
1304             CompareImplMethodObligation { .. } => "method type is compatible with trait",
1305             ExprAssignable => "expression is assignable",
1306             MatchExpressionArm { source, .. } => match source {
1307                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have compatible types",
1308                 _ => "match arms have compatible types",
1309             },
1310             IfExpression => "if and else have compatible types",
1311             IfExpressionWithNoElse => "if missing an else returns ()",
1312             EquatePredicate => "equality where clause is satisfied",
1313             MainFunctionType => "`main` function has the correct type",
1314             StartFunctionType => "`start` function has the correct type",
1315             IntrinsicType => "intrinsic has the correct type",
1316             MethodReceiver => "method receiver has the correct type",
1317             _ => "types are compatible",
1318         }
1319     }
1320 }