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