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_inference::{RegionResolutionError, ConcreteFailure, SubSupConflict,
  61                               GenericBoundFailure, GenericKind};
  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};
  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 mod note;
  76
  77 mod need_type_info;
  78
  79 mod named_anon_conflict;
  80 #[macro_use]
  81 mod util;
  82 mod different_lifetimes;
  83
  84 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
  85     pub fn note_and_explain_region(self,
  86                                    region_scope_tree: &region::ScopeTree,
  87                                    err: &mut DiagnosticBuilder,
  88                                    prefix: &str,
  89                                    region: ty::Region<'tcx>,
  90                                    suffix: &str) {
  91         fn item_scope_tag(item: &hir::Item) -> &'static str {
  92             match item.node {
  93                 hir::ItemImpl(..) => "impl",
  94                 hir::ItemStruct(..) => "struct",
  95                 hir::ItemUnion(..) => "union",
  96                 hir::ItemEnum(..) => "enum",
  97                 hir::ItemTrait(..) => "trait",
  98                 hir::ItemFn(..) => "function body",
  99                 _ => "item"
 100             }
 101         }
 102
 103         fn trait_item_scope_tag(item: &hir::TraitItem) -> &'static str {
 104             match item.node {
 105                 hir::TraitItemKind::Method(..) => "method body",
 106                 hir::TraitItemKind::Const(..) |
 107                 hir::TraitItemKind::Type(..) => "associated item"
 108             }
 109         }
 110
 111         fn impl_item_scope_tag(item: &hir::ImplItem) -> &'static str {
 112             match item.node {
 113                 hir::ImplItemKind::Method(..) => "method body",
 114                 hir::ImplItemKind::Const(..) |
 115                 hir::ImplItemKind::Type(_) => "associated item"
 116             }
 117         }
 118
 119         fn explain_span<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 120                                         heading: &str, span: Span)
 121                                         -> (String, Option<Span>) {
 122             let lo = tcx.sess.codemap().lookup_char_pos_adj(span.lo());
 123             (format!("the {} at {}:{}", heading, lo.line, lo.col.to_usize() + 1),
 124              Some(span))
 125         }
 126
 127         let (description, span) = match *region {
 128             ty::ReScope(scope) => {
 129                 let new_string;
 130                 let unknown_scope = || {
 131                     format!("{}unknown scope: {:?}{}.  Please report a bug.",
 132                             prefix, scope, suffix)
 133                 };
 134                 let span = scope.span(self, region_scope_tree);
 135                 let tag = match self.hir.find(scope.node_id(self, region_scope_tree)) {
 136                     Some(hir_map::NodeBlock(_)) => "block",
 137                     Some(hir_map::NodeExpr(expr)) => match expr.node {
 138                         hir::ExprCall(..) => "call",
 139                         hir::ExprMethodCall(..) => "method call",
 140                         hir::ExprMatch(.., hir::MatchSource::IfLetDesugar { .. }) => "if let",
 141                         hir::ExprMatch(.., hir::MatchSource::WhileLetDesugar) =>  "while let",
 142                         hir::ExprMatch(.., hir::MatchSource::ForLoopDesugar) =>  "for",
 143                         hir::ExprMatch(..) => "match",
 144                         _ => "expression",
 145                     },
 146                     Some(hir_map::NodeStmt(_)) => "statement",
 147                     Some(hir_map::NodeItem(it)) => item_scope_tag(&it),
 148                     Some(hir_map::NodeTraitItem(it)) => trait_item_scope_tag(&it),
 149                     Some(hir_map::NodeImplItem(it)) => impl_item_scope_tag(&it),
 150                     Some(_) | None => {
 151                         err.span_note(span, &unknown_scope());
 152                         return;
 153                     }
 154                 };
 155                 let scope_decorated_tag = match scope {
 156                     region::Scope::Node(_) => tag,
 157                     region::Scope::CallSite(_) => {
 158                         "scope of call-site for function"
 159                     }
 160                     region::Scope::Arguments(_) => {
 161                         "scope of function body"
 162                     }
 163                     region::Scope::Destruction(_) => {
 164                         new_string = format!("destruction scope surrounding {}", tag);
 165                         &new_string[..]
 166                     }
 167                     region::Scope::Remainder(r) => {
 168                         new_string = format!("block suffix following statement {}",
 169                                              r.first_statement_index);
 170                         &new_string[..]
 171                     }
 172                 };
 173                 explain_span(self, scope_decorated_tag, span)
 174             }
 175
 176             ty::ReEarlyBound(_) |
 177             ty::ReFree(_) => {
 178                 let scope = match *region {
 179                     ty::ReEarlyBound(ref br) => {
 180                         self.parent_def_id(br.def_id).unwrap()
 181                     }
 182                     ty::ReFree(ref fr) => fr.scope,
 183                     _ => bug!()
 184                 };
 185                 let prefix = match *region {
 186                     ty::ReEarlyBound(ref br) => {
 187                         format!("the lifetime {} as defined on", br.name)
 188                     }
 189                     ty::ReFree(ref fr) => {
 190                         match fr.bound_region {
 191                             ty::BrAnon(idx) => {
 192                                 format!("the anonymous lifetime #{} defined on", idx + 1)
 193                             }
 194                             ty::BrFresh(_) => "an anonymous lifetime defined on".to_owned(),
 195                             _ => {
 196                                 format!("the lifetime {} as defined on",
 197                                         fr.bound_region)
 198                             }
 199                         }
 200                     }
 201                     _ => bug!()
 202                 };
 203
 204                 let node = self.hir.as_local_node_id(scope)
 205                                    .unwrap_or(DUMMY_NODE_ID);
 206                 let unknown;
 207                 let tag = match self.hir.find(node) {
 208                     Some(hir_map::NodeBlock(_)) |
 209                     Some(hir_map::NodeExpr(_)) => "body",
 210                     Some(hir_map::NodeItem(it)) => item_scope_tag(&it),
 211                     Some(hir_map::NodeTraitItem(it)) => trait_item_scope_tag(&it),
 212                     Some(hir_map::NodeImplItem(it)) => impl_item_scope_tag(&it),
 213
 214                     // this really should not happen, but it does:
 215                     // FIXME(#27942)
 216                     Some(_) => {
 217                         unknown = format!("unexpected node ({}) for scope {:?}.  \
 218                                            Please report a bug.",
 219                                           self.hir.node_to_string(node), scope);
 220                         &unknown
 221                     }
 222                     None => {
 223                         unknown = format!("unknown node for scope {:?}.  \
 224                                            Please report a bug.", scope);
 225                         &unknown
 226                     }
 227                 };
 228                 let (msg, opt_span) = explain_span(self, tag, self.hir.span(node));
 229                 (format!("{} {}", prefix, msg), opt_span)
 230             }
 231
 232             ty::ReStatic => ("the static lifetime".to_owned(), None),
 233
 234             ty::ReEmpty => ("the empty lifetime".to_owned(), None),
 235
 236             // FIXME(#13998) ReSkolemized should probably print like
 237             // ReFree rather than dumping Debug output on the user.
 238             //
 239             // We shouldn't really be having unification failures with ReVar
 240             // and ReLateBound though.
 241             ty::ReSkolemized(..) |
 242             ty::ReVar(_) |
 243             ty::ReLateBound(..) |
 244             ty::ReErased => {
 245                 (format!("lifetime {:?}", region), None)
 246             }
 247         };
 248         let message = format!("{}{}{}", prefix, description, suffix);
 249         if let Some(span) = span {
 250             err.span_note(span, &message);
 251         } else {
 252             err.note(&message);
 253         }
 254     }
 255 }
 256
 257 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 258     pub fn report_region_errors(&self,
 259                                 region_scope_tree: &region::ScopeTree,
 260                                 errors: &Vec<RegionResolutionError<'tcx>>) {
 261         debug!("report_region_errors(): {} errors to start", errors.len());
 262
 263         // try to pre-process the errors, which will group some of them
 264         // together into a `ProcessedErrors` group:
 265         let errors = self.process_errors(errors);
 266
 267         debug!("report_region_errors: {} errors after preprocessing", errors.len());
 268
 269         for error in errors {
 270             debug!("report_region_errors: error = {:?}", error);
 271
 272             if !self.try_report_named_anon_conflict(&error) &&
 273                !self.try_report_anon_anon_conflict(&error) {
 274
 275                match error.clone() {
 276                   // These errors could indicate all manner of different
 277                   // problems with many different solutions. Rather
 278                   // than generate a "one size fits all" error, what we
 279                   // attempt to do is go through a number of specific
 280                   // scenarios and try to find the best way to present
 281                   // the error. If all of these fails, we fall back to a rather
 282                   // general bit of code that displays the error information
 283                   ConcreteFailure(origin, sub, sup) => {
 284                       self.report_concrete_failure(region_scope_tree, origin, sub, sup).emit();
 285                   }
 286
 287                   GenericBoundFailure(kind, param_ty, sub) => {
 288                       self.report_generic_bound_failure(region_scope_tree, kind, param_ty, sub);
 289                   }
 290
 291                   SubSupConflict(var_origin, sub_origin, sub_r, sup_origin, sup_r) => {
 292                         self.report_sub_sup_conflict(region_scope_tree,
 293                                                      var_origin,
 294                                                      sub_origin,
 295                                                      sub_r,
 296                                                      sup_origin,
 297                                                      sup_r);
 298                   }
 299                }
 300             }
 301         }
 302     }
 303
 304     // This method goes through all the errors and try to group certain types
 305     // of error together, for the purpose of suggesting explicit lifetime
 306     // parameters to the user. This is done so that we can have a more
 307     // complete view of what lifetimes should be the same.
 308     // If the return value is an empty vector, it means that processing
 309     // failed (so the return value of this method should not be used).
 310     //
 311     // The method also attempts to weed out messages that seem like
 312     // duplicates that will be unhelpful to the end-user. But
 313     // obviously it never weeds out ALL errors.
 314     fn process_errors(&self, errors: &Vec<RegionResolutionError<'tcx>>)
 315                       -> Vec<RegionResolutionError<'tcx>> {
 316         debug!("process_errors()");
 317
 318         // We want to avoid reporting generic-bound failures if we can
 319         // avoid it: these have a very high rate of being unhelpful in
 320         // practice. This is because they are basically secondary
 321         // checks that test the state of the region graph after the
 322         // rest of inference is done, and the other kinds of errors
 323         // indicate that the region constraint graph is internally
 324         // inconsistent, so these test results are likely to be
 325         // meaningless.
 326         //
 327         // Therefore, we filter them out of the list unless they are
 328         // the only thing in the list.
 329
 330         let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
 331             ConcreteFailure(..) => false,
 332             SubSupConflict(..) => false,
 333             GenericBoundFailure(..) => true,
 334         };
 335
 336         if errors.iter().all(|e| is_bound_failure(e)) {
 337             errors.clone()
 338         } else {
 339             errors.iter().filter(|&e| !is_bound_failure(e)).cloned().collect()
 340         }
 341     }
 342
 343     /// Adds a note if the types come from similarly named crates
 344     fn check_and_note_conflicting_crates(&self,
 345                                          err: &mut DiagnosticBuilder,
 346                                          terr: &TypeError<'tcx>,
 347                                          sp: Span) {
 348         let report_path_match = |err: &mut DiagnosticBuilder, did1: DefId, did2: DefId| {
 349             // Only external crates, if either is from a local
 350             // module we could have false positives
 351             if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
 352                 let exp_path = self.tcx.item_path_str(did1);
 353                 let found_path = self.tcx.item_path_str(did2);
 354                 let exp_abs_path = self.tcx.absolute_item_path_str(did1);
 355                 let found_abs_path = self.tcx.absolute_item_path_str(did2);
 356                 // We compare strings because DefPath can be different
 357                 // for imported and non-imported crates
 358                 if exp_path == found_path
 359                 || exp_abs_path == found_abs_path {
 360                     let crate_name = self.tcx.crate_name(did1.krate);
 361                     err.span_note(sp, &format!("Perhaps two different versions \
 362                                                 of crate `{}` are being used?",
 363                                                crate_name));
 364                 }
 365             }
 366         };
 367         match *terr {
 368             TypeError::Sorts(ref exp_found) => {
 369                 // if they are both "path types", there's a chance of ambiguity
 370                 // due to different versions of the same crate
 371                 match (&exp_found.expected.sty, &exp_found.found.sty) {
 372                     (&ty::TyAdt(exp_adt, _), &ty::TyAdt(found_adt, _)) => {
 373                         report_path_match(err, exp_adt.did, found_adt.did);
 374                     },
 375                     _ => ()
 376                 }
 377             },
 378             TypeError::Traits(ref exp_found) => {
 379                 report_path_match(err, exp_found.expected, exp_found.found);
 380             },
 381             _ => () // FIXME(#22750) handle traits and stuff
 382         }
 383     }
 384
 385     fn note_error_origin(&self,
 386                          err: &mut DiagnosticBuilder<'tcx>,
 387                          cause: &ObligationCause<'tcx>)
 388     {
 389         match cause.code {
 390             ObligationCauseCode::MatchExpressionArm { arm_span, source } => match source {
 391                 hir::MatchSource::IfLetDesugar {..} => {
 392                     err.span_note(arm_span, "`if let` arm with an incompatible type");
 393                 }
 394                 _ => {
 395                     err.span_note(arm_span, "match arm with an incompatible type");
 396                 }
 397             },
 398             _ => ()
 399         }
 400     }
 401
 402     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
 403     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
 404     /// populate `other_value` with `other_ty`.
 405     ///
 406     /// ```text
 407     /// Foo<Bar<Qux>>
 408     /// ^^^^--------^ this is highlighted
 409     /// |   |
 410     /// |   this type argument is exactly the same as the other type, not highlighted
 411     /// this is highlighted
 412     /// Bar<Qux>
 413     /// -------- this type is the same as a type argument in the other type, not highlighted
 414     /// ```
 415     fn highlight_outer(&self,
 416                        value: &mut DiagnosticStyledString,
 417                        other_value: &mut DiagnosticStyledString,
 418                        name: String,
 419                        sub: &ty::subst::Substs<'tcx>,
 420                        pos: usize,
 421                        other_ty: &Ty<'tcx>) {
 422         // `value` and `other_value` hold two incomplete type representation for display.
 423         // `name` is the path of both types being compared. `sub`
 424         value.push_highlighted(name);
 425         let len = sub.len();
 426         if len > 0 {
 427             value.push_highlighted("<");
 428         }
 429
 430         // Output the lifetimes fot the first type
 431         let lifetimes = sub.regions().map(|lifetime| {
 432             let s = format!("{}", lifetime);
 433             if s.is_empty() {
 434                 "'_".to_string()
 435             } else {
 436                 s
 437             }
 438         }).collect::<Vec<_>>().join(", ");
 439         if !lifetimes.is_empty() {
 440             if sub.regions().count() < len {
 441                 value.push_normal(lifetimes + &", ");
 442             } else {
 443                 value.push_normal(lifetimes);
 444             }
 445         }
 446
 447         // Highlight all the type arguments that aren't at `pos` and compare the type argument at
 448         // `pos` and `other_ty`.
 449         for (i, type_arg) in sub.types().enumerate() {
 450             if i == pos {
 451                 let values = self.cmp(type_arg, other_ty);
 452                 value.0.extend((values.0).0);
 453                 other_value.0.extend((values.1).0);
 454             } else {
 455                 value.push_highlighted(format!("{}", type_arg));
 456             }
 457
 458             if len > 0 && i != len - 1 {
 459                 value.push_normal(", ");
 460             }
 461             //self.push_comma(&mut value, &mut other_value, len, i);
 462         }
 463         if len > 0 {
 464             value.push_highlighted(">");
 465         }
 466     }
 467
 468     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 469     /// as that is the difference to the other type.
 470     ///
 471     /// For the following code:
 472     ///
 473     /// ```norun
 474     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 475     /// ```
 476     ///
 477     /// The type error output will behave in the following way:
 478     ///
 479     /// ```text
 480     /// Foo<Bar<Qux>>
 481     /// ^^^^--------^ this is highlighted
 482     /// |   |
 483     /// |   this type argument is exactly the same as the other type, not highlighted
 484     /// this is highlighted
 485     /// Bar<Qux>
 486     /// -------- this type is the same as a type argument in the other type, not highlighted
 487     /// ```
 488     fn cmp_type_arg(&self,
 489                     mut t1_out: &mut DiagnosticStyledString,
 490                     mut t2_out: &mut DiagnosticStyledString,
 491                     path: String,
 492                     sub: &ty::subst::Substs<'tcx>,
 493                     other_path: String,
 494                     other_ty: &Ty<'tcx>) -> Option<()> {
 495         for (i, ta) in sub.types().enumerate() {
 496             if &ta == other_ty {
 497                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 498                 return Some(());
 499             }
 500             if let &ty::TyAdt(def, _) = &ta.sty {
 501                 let path_ = self.tcx.item_path_str(def.did.clone());
 502                 if path_ == other_path {
 503                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 504                     return Some(());
 505                 }
 506             }
 507         }
 508         None
 509     }
 510
 511     /// Add a `,` to the type representation only if it is appropriate.
 512     fn push_comma(&self,
 513                   value: &mut DiagnosticStyledString,
 514                   other_value: &mut DiagnosticStyledString,
 515                   len: usize,
 516                   pos: usize) {
 517         if len > 0 && pos != len - 1 {
 518             value.push_normal(", ");
 519             other_value.push_normal(", ");
 520         }
 521     }
 522
 523     /// Compare two given types, eliding parts that are the same between them and highlighting
 524     /// relevant differences, and return two representation of those types for highlighted printing.
 525     fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>)
 526         -> (DiagnosticStyledString, DiagnosticStyledString)
 527     {
 528         match (&t1.sty, &t2.sty) {
 529             (&ty::TyAdt(def1, sub1), &ty::TyAdt(def2, sub2)) => {
 530                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 531                 let path1 = self.tcx.item_path_str(def1.did.clone());
 532                 let path2 = self.tcx.item_path_str(def2.did.clone());
 533                 if def1.did == def2.did {
 534                     // Easy case. Replace same types with `_` to shorten the output and highlight
 535                     // the differing ones.
 536                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
 537                     //     Foo<Bar, _>
 538                     //     Foo<Quz, _>
 539                     //         ---  ^ type argument elided
 540                     //         |
 541                     //         highlighted in output
 542                     values.0.push_normal(path1);
 543                     values.1.push_normal(path2);
 544
 545                     // Only draw `<...>` if there're lifetime/type arguments.
 546                     let len = sub1.len();
 547                     if len > 0 {
 548                         values.0.push_normal("<");
 549                         values.1.push_normal("<");
 550                     }
 551
 552                     fn lifetime_display(lifetime: Region) -> String {
 553                         let s = format!("{}", lifetime);
 554                         if s.is_empty() {
 555                             "'_".to_string()
 556                         } else {
 557                             s
 558                         }
 559                     }
 560                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
 561                     // all diagnostics that use this output
 562                     //
 563                     //     Foo<'x, '_, Bar>
 564                     //     Foo<'y, '_, Qux>
 565                     //         ^^  ^^  --- type arguments are not elided
 566                     //         |   |
 567                     //         |   elided as they were the same
 568                     //         not elided, they were different, but irrelevant
 569                     let lifetimes = sub1.regions().zip(sub2.regions());
 570                     for (i, lifetimes) in lifetimes.enumerate() {
 571                         let l1 = lifetime_display(lifetimes.0);
 572                         let l2 = lifetime_display(lifetimes.1);
 573                         if l1 == l2 {
 574                             values.0.push_normal("'_");
 575                             values.1.push_normal("'_");
 576                         } else {
 577                             values.0.push_highlighted(l1);
 578                             values.1.push_highlighted(l2);
 579                         }
 580                         self.push_comma(&mut values.0, &mut values.1, len, i);
 581                     }
 582
 583                     // We're comparing two types with the same path, so we compare the type
 584                     // arguments for both. If they are the same, do not highlight and elide from the
 585                     // output.
 586                     //     Foo<_, Bar>
 587                     //     Foo<_, Qux>
 588                     //         ^ elided type as this type argument was the same in both sides
 589                     let type_arguments = sub1.types().zip(sub2.types());
 590                     let regions_len = sub1.regions().collect::<Vec<_>>().len();
 591                     for (i, (ta1, ta2)) in type_arguments.enumerate() {
 592                         let i = i + regions_len;
 593                         if ta1 == ta2 {
 594                             values.0.push_normal("_");
 595                             values.1.push_normal("_");
 596                         } else {
 597                             let (x1, x2) = self.cmp(ta1, ta2);
 598                             (values.0).0.extend(x1.0);
 599                             (values.1).0.extend(x2.0);
 600                         }
 601                         self.push_comma(&mut values.0, &mut values.1, len, i);
 602                     }
 603
 604                     // Close the type argument bracket.
 605                     // Only draw `<...>` if there're lifetime/type arguments.
 606                     if len > 0 {
 607                         values.0.push_normal(">");
 608                         values.1.push_normal(">");
 609                     }
 610                     values
 611                 } else {
 612                     // Check for case:
 613                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
 614                     //     Foo<Bar<Qux>
 615                     //         ------- this type argument is exactly the same as the other type
 616                     //     Bar<Qux>
 617                     if self.cmp_type_arg(&mut values.0,
 618                                          &mut values.1,
 619                                          path1.clone(),
 620                                          sub1,
 621                                          path2.clone(),
 622                                          &t2).is_some() {
 623                         return values;
 624                     }
 625                     // Check for case:
 626                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
 627                     //     Bar<Qux>
 628                     //     Foo<Bar<Qux>>
 629                     //         ------- this type argument is exactly the same as the other type
 630                     if self.cmp_type_arg(&mut values.1,
 631                                          &mut values.0,
 632                                          path2,
 633                                          sub2,
 634                                          path1,
 635                                          &t1).is_some() {
 636                         return values;
 637                     }
 638
 639                     // We couldn't find anything in common, highlight everything.
 640                     //     let x: Bar<Qux> = y::<Foo<Zar>>();
 641                     (DiagnosticStyledString::highlighted(format!("{}", t1)),
 642                      DiagnosticStyledString::highlighted(format!("{}", t2)))
 643                 }
 644             }
 645             _ => {
 646                 if t1 == t2 {
 647                     // The two types are the same, elide and don't highlight.
 648                     (DiagnosticStyledString::normal("_"), DiagnosticStyledString::normal("_"))
 649                 } else {
 650                     // We couldn't find anything in common, highlight everything.
 651                     (DiagnosticStyledString::highlighted(format!("{}", t1)),
 652                      DiagnosticStyledString::highlighted(format!("{}", t2)))
 653                 }
 654             }
 655         }
 656     }
 657
 658     pub fn note_type_err(&self,
 659                          diag: &mut DiagnosticBuilder<'tcx>,
 660                          cause: &ObligationCause<'tcx>,
 661                          secondary_span: Option<(Span, String)>,
 662                          values: Option<ValuePairs<'tcx>>,
 663                          terr: &TypeError<'tcx>)
 664     {
 665         let (expected_found, is_simple_error) = match values {
 666             None => (None, false),
 667             Some(values) => {
 668                 let is_simple_error = match values {
 669                     ValuePairs::Types(exp_found) => {
 670                         exp_found.expected.is_primitive() && exp_found.found.is_primitive()
 671                     }
 672                     _ => false,
 673                 };
 674                 let vals = match self.values_str(&values) {
 675                     Some((expected, found)) => Some((expected, found)),
 676                     None => {
 677                         // Derived error. Cancel the emitter.
 678                         self.tcx.sess.diagnostic().cancel(diag);
 679                         return
 680                     }
 681                 };
 682                 (vals, is_simple_error)
 683             }
 684         };
 685
 686         let span = cause.span;
 687
 688         if let Some((expected, found)) = expected_found {
 689             match (terr, is_simple_error, expected == found) {
 690                 (&TypeError::Sorts(ref values), false, true) => {
 691                     diag.note_expected_found_extra(
 692                         &"type", expected, found,
 693                         &format!(" ({})", values.expected.sort_string(self.tcx)),
 694                         &format!(" ({})", values.found.sort_string(self.tcx)));
 695                 }
 696                 (_, false,  _) => {
 697                     diag.note_expected_found(&"type", expected, found);
 698                 }
 699                 _ => (),
 700             }
 701         }
 702
 703         diag.span_label(span, terr.to_string());
 704         if let Some((sp, msg)) = secondary_span {
 705             diag.span_label(sp, msg);
 706         }
 707
 708         self.note_error_origin(diag, &cause);
 709         self.check_and_note_conflicting_crates(diag, terr, span);
 710         self.tcx.note_and_explain_type_err(diag, terr, span);
 711     }
 712
 713     pub fn report_and_explain_type_error(&self,
 714                                          trace: TypeTrace<'tcx>,
 715                                          terr: &TypeError<'tcx>)
 716                                          -> DiagnosticBuilder<'tcx>
 717     {
 718         let span = trace.cause.span;
 719         let failure_str = trace.cause.as_failure_str();
 720         let mut diag = match trace.cause.code {
 721             ObligationCauseCode::IfExpressionWithNoElse => {
 722                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
 723             }
 724             ObligationCauseCode::MainFunctionType => {
 725                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
 726             }
 727             _ => {
 728                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
 729             }
 730         };
 731         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
 732         diag
 733     }
 734
 735     fn values_str(&self, values: &ValuePairs<'tcx>)
 736         -> Option<(DiagnosticStyledString, DiagnosticStyledString)>
 737     {
 738         match *values {
 739             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
 740             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 741             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 742         }
 743     }
 744
 745     fn expected_found_str_ty(&self,
 746                              exp_found: &ty::error::ExpectedFound<Ty<'tcx>>)
 747                              -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
 748         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 749         if exp_found.references_error() {
 750             return None;
 751         }
 752
 753         Some(self.cmp(exp_found.expected, exp_found.found))
 754     }
 755
 756     /// Returns a string of the form "expected `{}`, found `{}`".
 757     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
 758         &self,
 759         exp_found: &ty::error::ExpectedFound<T>)
 760         -> Option<(DiagnosticStyledString, DiagnosticStyledString)>
 761     {
 762         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 763         if exp_found.references_error() {
 764             return None;
 765         }
 766
 767         Some((DiagnosticStyledString::highlighted(format!("{}", exp_found.expected)),
 768               DiagnosticStyledString::highlighted(format!("{}", exp_found.found))))
 769     }
 770
 771     fn report_generic_bound_failure(&self,
 772                                     region_scope_tree: &region::ScopeTree,
 773                                     origin: SubregionOrigin<'tcx>,
 774                                     bound_kind: GenericKind<'tcx>,
 775                                     sub: Region<'tcx>)
 776     {
 777         // FIXME: it would be better to report the first error message
 778         // with the span of the parameter itself, rather than the span
 779         // where the error was detected. But that span is not readily
 780         // accessible.
 781
 782         let labeled_user_string = match bound_kind {
 783             GenericKind::Param(ref p) =>
 784                 format!("the parameter type `{}`", p),
 785             GenericKind::Projection(ref p) =>
 786                 format!("the associated type `{}`", p),
 787         };
 788
 789         if let SubregionOrigin::CompareImplMethodObligation {
 790             span, item_name, impl_item_def_id, trait_item_def_id, lint_id
 791         } = origin {
 792             self.report_extra_impl_obligation(span,
 793                                               item_name,
 794                                               impl_item_def_id,
 795                                               trait_item_def_id,
 796                                               &format!("`{}: {}`", bound_kind, sub),
 797                                               lint_id)
 798                 .emit();
 799             return;
 800         }
 801
 802         let mut err = match *sub {
 803             ty::ReEarlyBound(_) |
 804             ty::ReFree(ty::FreeRegion {bound_region: ty::BrNamed(..), ..}) => {
 805                 // Does the required lifetime have a nice name we can print?
 806                 let mut err = struct_span_err!(self.tcx.sess,
 807                                                origin.span(),
 808                                                E0309,
 809                                                "{} may not live long enough",
 810                                                labeled_user_string);
 811                 err.help(&format!("consider adding an explicit lifetime bound `{}: {}`...",
 812                          bound_kind,
 813                          sub));
 814                 err
 815             }
 816
 817             ty::ReStatic => {
 818                 // Does the required lifetime have a nice name we can print?
 819                 let mut err = struct_span_err!(self.tcx.sess,
 820                                                origin.span(),
 821                                                E0310,
 822                                                "{} may not live long enough",
 823                                                labeled_user_string);
 824                 err.help(&format!("consider adding an explicit lifetime \
 825                                    bound `{}: 'static`...",
 826                                   bound_kind));
 827                 err
 828             }
 829
 830             _ => {
 831                 // If not, be less specific.
 832                 let mut err = struct_span_err!(self.tcx.sess,
 833                                                origin.span(),
 834                                                E0311,
 835                                                "{} may not live long enough",
 836                                                labeled_user_string);
 837                 err.help(&format!("consider adding an explicit lifetime bound for `{}`",
 838                                   bound_kind));
 839                 self.tcx.note_and_explain_region(
 840                     region_scope_tree,
 841                     &mut err,
 842                     &format!("{} must be valid for ", labeled_user_string),
 843                     sub,
 844                     "...");
 845                 err
 846             }
 847         };
 848
 849         self.note_region_origin(&mut err, &origin);
 850         err.emit();
 851     }
 852
 853     fn report_sub_sup_conflict(&self,
 854                                region_scope_tree: &region::ScopeTree,
 855                                var_origin: RegionVariableOrigin,
 856                                sub_origin: SubregionOrigin<'tcx>,
 857                                sub_region: Region<'tcx>,
 858                                sup_origin: SubregionOrigin<'tcx>,
 859                                sup_region: Region<'tcx>) {
 860         let mut err = self.report_inference_failure(var_origin);
 861
 862         self.tcx.note_and_explain_region(region_scope_tree, &mut err,
 863             "first, the lifetime cannot outlive ",
 864             sup_region,
 865             "...");
 866
 867         self.note_region_origin(&mut err, &sup_origin);
 868
 869         self.tcx.note_and_explain_region(region_scope_tree, &mut err,
 870             "but, the lifetime must be valid for ",
 871             sub_region,
 872             "...");
 873
 874         self.note_region_origin(&mut err, &sub_origin);
 875         err.emit();
 876     }
 877 }
 878
 879 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 880     fn report_inference_failure(&self,
 881                                 var_origin: RegionVariableOrigin)
 882                                 -> DiagnosticBuilder<'tcx> {
 883         let br_string = |br: ty::BoundRegion| {
 884             let mut s = br.to_string();
 885             if !s.is_empty() {
 886                 s.push_str(" ");
 887             }
 888             s
 889         };
 890         let var_description = match var_origin {
 891             infer::MiscVariable(_) => "".to_string(),
 892             infer::PatternRegion(_) => " for pattern".to_string(),
 893             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
 894             infer::Autoref(_) => " for autoref".to_string(),
 895             infer::Coercion(_) => " for automatic coercion".to_string(),
 896             infer::LateBoundRegion(_, br, infer::FnCall) => {
 897                 format!(" for lifetime parameter {}in function call",
 898                         br_string(br))
 899             }
 900             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
 901                 format!(" for lifetime parameter {}in generic type", br_string(br))
 902             }
 903             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(def_id)) => {
 904                 format!(" for lifetime parameter {}in trait containing associated type `{}`",
 905                         br_string(br), self.tcx.associated_item(def_id).name)
 906             }
 907             infer::EarlyBoundRegion(_, name) => {
 908                 format!(" for lifetime parameter `{}`",
 909                         name)
 910             }
 911             infer::BoundRegionInCoherence(name) => {
 912                 format!(" for lifetime parameter `{}` in coherence check",
 913                         name)
 914             }
 915             infer::UpvarRegion(ref upvar_id, _) => {
 916                 let var_node_id = self.tcx.hir.hir_to_node_id(upvar_id.var_id);
 917                 let var_name = self.tcx.hir.name(var_node_id);
 918                 format!(" for capture of `{}` by closure", var_name)
 919             }
 920         };
 921
 922         struct_span_err!(self.tcx.sess, var_origin.span(), E0495,
 923                   "cannot infer an appropriate lifetime{} \
 924                    due to conflicting requirements",
 925                   var_description)
 926     }
 927 }
 928
 929 impl<'tcx> ObligationCause<'tcx> {
 930     fn as_failure_str(&self) -> &'static str {
 931         use traits::ObligationCauseCode::*;
 932         match self.code {
 933             CompareImplMethodObligation { .. } => "method not compatible with trait",
 934             MatchExpressionArm { source, .. } => match source {
 935                 hir::MatchSource::IfLetDesugar{..} => "`if let` arms have incompatible types",
 936                 _ => "match arms have incompatible types",
 937             },
 938             IfExpression => "if and else have incompatible types",
 939             IfExpressionWithNoElse => "if may be missing an else clause",
 940             EquatePredicate => "equality predicate not satisfied",
 941             MainFunctionType => "main function has wrong type",
 942             StartFunctionType => "start function has wrong type",
 943             IntrinsicType => "intrinsic has wrong type",
 944             MethodReceiver => "mismatched method receiver",
 945             _ => "mismatched types",
 946         }
 947     }
 948
 949     fn as_requirement_str(&self) -> &'static str {
 950         use traits::ObligationCauseCode::*;
 951         match self.code {
 952             CompareImplMethodObligation { .. } => "method type is compatible with trait",
 953             ExprAssignable => "expression is assignable",
 954             MatchExpressionArm { source, .. } => match source {
 955                 hir::MatchSource::IfLetDesugar{..} => "`if let` arms have compatible types",
 956                 _ => "match arms have compatible types",
 957             },
 958             IfExpression => "if and else have compatible types",
 959             IfExpressionWithNoElse => "if missing an else returns ()",
 960             EquatePredicate => "equality where clause is satisfied",
 961             MainFunctionType => "`main` function has the correct type",
 962             StartFunctionType => "`start` function has the correct type",
 963             IntrinsicType => "intrinsic has the correct type",
 964             MethodReceiver => "method receiver has the correct type",
 965             _ => "types are compatible",
 966         }
 967     }
 968 }