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