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