src/librustc/infer/error_reporting/mod.rs

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