src/librustc/infer/error_reporting/mod.rs

   1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Error Reporting Code for the inference engine
  12 //!
  13 //! Because of the way inference, and in particular region inference,
  14 //! works, it often happens that errors are not detected until far after
  15 //! the relevant line of code has been type-checked. Therefore, there is
  16 //! an elaborate system to track why a particular constraint in the
  17 //! inference graph arose so that we can explain to the user what gave
  18 //! rise to a particular error.
  19 //!
  20 //! The basis of the system are the "origin" types. An "origin" is the
  21 //! reason that a constraint or inference variable arose. There are
  22 //! different "origin" enums for different kinds of constraints/variables
  23 //! (e.g., `TypeOrigin`, `RegionVariableOrigin`). An origin always has
  24 //! a span, but also more information so that we can generate a meaningful
  25 //! error message.
  26 //!
  27 //! Having a catalogue of all the different reasons an error can arise is
  28 //! also useful for other reasons, like cross-referencing FAQs etc, though
  29 //! we are not really taking advantage of this yet.
  30 //!
  31 //! # Region Inference
  32 //!
  33 //! Region inference is particularly tricky because it always succeeds "in
  34 //! the moment" and simply registers a constraint. Then, at the end, we
  35 //! can compute the full graph and report errors, so we need to be able to
  36 //! store and later report what gave rise to the conflicting constraints.
  37 //!
  38 //! # Subtype Trace
  39 //!
  40 //! Determining whether `T1 <: T2` often involves a number of subtypes and
  41 //! subconstraints along the way. A "TypeTrace" is an extended version
  42 //! of an origin that traces the types and other values that were being
  43 //! compared. It is not necessarily comprehensive (in fact, at the time of
  44 //! this writing it only tracks the root values being compared) but I'd
  45 //! like to extend it to include significant "waypoints". For example, if
  46 //! you are comparing `(T1, T2) <: (T3, T4)`, and the problem is that `T2
  47 //! <: T4` fails, I'd like the trace to include enough information to say
  48 //! "in the 2nd element of the tuple". Similarly, failures when comparing
  49 //! arguments or return types in fn types should be able to cite the
  50 //! specific position, etc.
  51 //!
  52 //! # Reality vs plan
  53 //!
  54 //! Of course, there is still a LOT of code in typeck that has yet to be
  55 //! ported to this system, and which relies on string concatenation at the
  56 //! time of error detection.
  57
  58 use infer;
  59 use super::{InferCtxt, TypeTrace, SubregionOrigin, RegionVariableOrigin, ValuePairs};
  60 use super::region_inference::{RegionResolutionError, ConcreteFailure, SubSupConflict,
  61                               GenericBoundFailure, GenericKind};
  62
  63 use std::fmt;
  64 use hir;
  65 use hir::map as hir_map;
  66 use hir::def_id::DefId;
  67 use middle::region;
  68 use traits::{ObligationCause, ObligationCauseCode};
  69 use ty::{self, TyCtxt, TypeFoldable};
  70 use ty::{Region, Issue32330};
  71 use ty::error::TypeError;
  72 use syntax_pos::{Pos, Span};
  73 use errors::DiagnosticBuilder;
  74
  75 mod note;
  76
  77 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
  78     pub fn note_and_explain_region(self,
  79                                    err: &mut DiagnosticBuilder,
  80                                    prefix: &str,
  81                                    region: &'tcx ty::Region,
  82                                    suffix: &str) {
  83         fn item_scope_tag(item: &hir::Item) -> &'static str {
  84             match item.node {
  85                 hir::ItemImpl(..) => "impl",
  86                 hir::ItemStruct(..) => "struct",
  87                 hir::ItemUnion(..) => "union",
  88                 hir::ItemEnum(..) => "enum",
  89                 hir::ItemTrait(..) => "trait",
  90                 hir::ItemFn(..) => "function body",
  91                 _ => "item"
  92             }
  93         }
  94
  95         fn trait_item_scope_tag(item: &hir::TraitItem) -> &'static str {
  96             match item.node {
  97                 hir::TraitItemKind::Method(..) => "method body",
  98                 hir::TraitItemKind::Const(..) |
  99                 hir::TraitItemKind::Type(..) => "associated item"
 100             }
 101         }
 102
 103         fn impl_item_scope_tag(item: &hir::ImplItem) -> &'static str {
 104             match item.node {
 105                 hir::ImplItemKind::Method(..) => "method body",
 106                 hir::ImplItemKind::Const(..) |
 107                 hir::ImplItemKind::Type(_) => "associated item"
 108             }
 109         }
 110
 111         fn explain_span<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 112                                         heading: &str, span: Span)
 113                                         -> (String, Option<Span>) {
 114             let lo = tcx.sess.codemap().lookup_char_pos_adj(span.lo);
 115             (format!("the {} at {}:{}", heading, lo.line, lo.col.to_usize()),
 116              Some(span))
 117         }
 118
 119         let (description, span) = match *region {
 120             ty::ReScope(scope) => {
 121                 let new_string;
 122                 let unknown_scope = || {
 123                     format!("{}unknown scope: {:?}{}.  Please report a bug.",
 124                             prefix, scope, suffix)
 125                 };
 126                 let span = match scope.span(&self.region_maps, &self.hir) {
 127                     Some(s) => s,
 128                     None => {
 129                         err.note(&unknown_scope());
 130                         return;
 131                     }
 132                 };
 133                 let tag = match self.hir.find(scope.node_id(&self.region_maps)) {
 134                     Some(hir_map::NodeBlock(_)) => "block",
 135                     Some(hir_map::NodeExpr(expr)) => match expr.node {
 136                         hir::ExprCall(..) => "call",
 137                         hir::ExprMethodCall(..) => "method call",
 138                         hir::ExprMatch(.., hir::MatchSource::IfLetDesugar { .. }) => "if let",
 139                         hir::ExprMatch(.., hir::MatchSource::WhileLetDesugar) =>  "while let",
 140                         hir::ExprMatch(.., hir::MatchSource::ForLoopDesugar) =>  "for",
 141                         hir::ExprMatch(..) => "match",
 142                         _ => "expression",
 143                     },
 144                     Some(hir_map::NodeStmt(_)) => "statement",
 145                     Some(hir_map::NodeItem(it)) => item_scope_tag(&it),
 146                     Some(hir_map::NodeTraitItem(it)) => trait_item_scope_tag(&it),
 147                     Some(hir_map::NodeImplItem(it)) => impl_item_scope_tag(&it),
 148                     Some(_) | None => {
 149                         err.span_note(span, &unknown_scope());
 150                         return;
 151                     }
 152                 };
 153                 let scope_decorated_tag = match self.region_maps.code_extent_data(scope) {
 154                     region::CodeExtentData::Misc(_) => tag,
 155                     region::CodeExtentData::CallSiteScope { .. } => {
 156                         "scope of call-site for function"
 157                     }
 158                     region::CodeExtentData::ParameterScope { .. } => {
 159                         "scope of function body"
 160                     }
 161                     region::CodeExtentData::DestructionScope(_) => {
 162                         new_string = format!("destruction scope surrounding {}", tag);
 163                         &new_string[..]
 164                     }
 165                     region::CodeExtentData::Remainder(r) => {
 166                         new_string = format!("block suffix following statement {}",
 167                                              r.first_statement_index);
 168                         &new_string[..]
 169                     }
 170                 };
 171                 explain_span(self, scope_decorated_tag, span)
 172             }
 173
 174             ty::ReFree(ref fr) => {
 175                 let prefix = match fr.bound_region {
 176                     ty::BrAnon(idx) => {
 177                         format!("the anonymous lifetime #{} defined on", idx + 1)
 178                     }
 179                     ty::BrFresh(_) => "an anonymous lifetime defined on".to_owned(),
 180                     _ => {
 181                         format!("the lifetime {} as defined on",
 182                                 fr.bound_region)
 183                     }
 184                 };
 185
 186                 let node = fr.scope.node_id(&self.region_maps);
 187                 let unknown;
 188                 let tag = match self.hir.find(node) {
 189                     Some(hir_map::NodeBlock(_)) |
 190                     Some(hir_map::NodeExpr(_)) => "body",
 191                     Some(hir_map::NodeItem(it)) => item_scope_tag(&it),
 192                     Some(hir_map::NodeTraitItem(it)) => trait_item_scope_tag(&it),
 193                     Some(hir_map::NodeImplItem(it)) => impl_item_scope_tag(&it),
 194
 195                     // this really should not happen, but it does:
 196                     // FIXME(#27942)
 197                     Some(_) => {
 198                         unknown = format!("unexpected node ({}) for scope {:?}.  \
 199                                            Please report a bug.",
 200                                           self.hir.node_to_string(node), fr.scope);
 201                         &unknown
 202                     }
 203                     None => {
 204                         unknown = format!("unknown node for scope {:?}.  \
 205                                            Please report a bug.", fr.scope);
 206                         &unknown
 207                     }
 208                 };
 209                 let (msg, opt_span) = explain_span(self, tag, self.hir.span(node));
 210                 (format!("{} {}", prefix, msg), opt_span)
 211             }
 212
 213             ty::ReStatic => ("the static lifetime".to_owned(), None),
 214
 215             ty::ReEmpty => ("the empty lifetime".to_owned(), None),
 216
 217             ty::ReEarlyBound(ref data) => (data.name.to_string(), None),
 218
 219             // FIXME(#13998) ReSkolemized should probably print like
 220             // ReFree rather than dumping Debug output on the user.
 221             //
 222             // We shouldn't really be having unification failures with ReVar
 223             // and ReLateBound though.
 224             ty::ReSkolemized(..) |
 225             ty::ReVar(_) |
 226             ty::ReLateBound(..) |
 227             ty::ReErased => {
 228                 (format!("lifetime {:?}", region), None)
 229             }
 230         };
 231         let message = format!("{}{}{}", prefix, description, suffix);
 232         if let Some(span) = span {
 233             err.span_note(span, &message);
 234         } else {
 235             err.note(&message);
 236         }
 237     }
 238 }
 239
 240 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 241     pub fn report_region_errors(&self,
 242                                 errors: &Vec<RegionResolutionError<'tcx>>) {
 243         debug!("report_region_errors(): {} errors to start", errors.len());
 244
 245         // try to pre-process the errors, which will group some of them
 246         // together into a `ProcessedErrors` group:
 247         let errors = self.process_errors(errors);
 248
 249         debug!("report_region_errors: {} errors after preprocessing", errors.len());
 250
 251         for error in errors {
 252             debug!("report_region_errors: error = {:?}", error);
 253             match error.clone() {
 254                 ConcreteFailure(origin, sub, sup) => {
 255                     self.report_concrete_failure(origin, sub, sup).emit();
 256                 }
 257
 258                 GenericBoundFailure(kind, param_ty, sub) => {
 259                     self.report_generic_bound_failure(kind, param_ty, sub);
 260                 }
 261
 262                 SubSupConflict(var_origin,
 263                                sub_origin, sub_r,
 264                                sup_origin, sup_r) => {
 265                     self.report_sub_sup_conflict(var_origin,
 266                                                  sub_origin, sub_r,
 267                                                  sup_origin, sup_r);
 268                 }
 269             }
 270         }
 271     }
 272
 273     // This method goes through all the errors and try to group certain types
 274     // of error together, for the purpose of suggesting explicit lifetime
 275     // parameters to the user. This is done so that we can have a more
 276     // complete view of what lifetimes should be the same.
 277     // If the return value is an empty vector, it means that processing
 278     // failed (so the return value of this method should not be used).
 279     //
 280     // The method also attempts to weed out messages that seem like
 281     // duplicates that will be unhelpful to the end-user. But
 282     // obviously it never weeds out ALL errors.
 283     fn process_errors(&self, errors: &Vec<RegionResolutionError<'tcx>>)
 284                       -> Vec<RegionResolutionError<'tcx>> {
 285         debug!("process_errors()");
 286
 287         // We want to avoid reporting generic-bound failures if we can
 288         // avoid it: these have a very high rate of being unhelpful in
 289         // practice. This is because they are basically secondary
 290         // checks that test the state of the region graph after the
 291         // rest of inference is done, and the other kinds of errors
 292         // indicate that the region constraint graph is internally
 293         // inconsistent, so these test results are likely to be
 294         // meaningless.
 295         //
 296         // Therefore, we filter them out of the list unless they are
 297         // the only thing in the list.
 298
 299         let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
 300             ConcreteFailure(..) => false,
 301             SubSupConflict(..) => false,
 302             GenericBoundFailure(..) => true,
 303         };
 304
 305         if errors.iter().all(|e| is_bound_failure(e)) {
 306             errors.clone()
 307         } else {
 308             errors.iter().filter(|&e| !is_bound_failure(e)).cloned().collect()
 309         }
 310     }
 311
 312     /// Adds a note if the types come from similarly named crates
 313     fn check_and_note_conflicting_crates(&self,
 314                                          err: &mut DiagnosticBuilder,
 315                                          terr: &TypeError<'tcx>,
 316                                          sp: Span) {
 317         let report_path_match = |err: &mut DiagnosticBuilder, did1: DefId, did2: DefId| {
 318             // Only external crates, if either is from a local
 319             // module we could have false positives
 320             if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
 321                 let exp_path = self.tcx.item_path_str(did1);
 322                 let found_path = self.tcx.item_path_str(did2);
 323                 // We compare strings because DefPath can be different
 324                 // for imported and non-imported crates
 325                 if exp_path == found_path {
 326                     let crate_name = self.tcx.sess.cstore.crate_name(did1.krate);
 327                     err.span_note(sp, &format!("Perhaps two different versions \
 328                                                 of crate `{}` are being used?",
 329                                                crate_name));
 330                 }
 331             }
 332         };
 333         match *terr {
 334             TypeError::Sorts(ref exp_found) => {
 335                 // if they are both "path types", there's a chance of ambiguity
 336                 // due to different versions of the same crate
 337                 match (&exp_found.expected.sty, &exp_found.found.sty) {
 338                     (&ty::TyAdt(exp_adt, _), &ty::TyAdt(found_adt, _)) => {
 339                         report_path_match(err, exp_adt.did, found_adt.did);
 340                     },
 341                     _ => ()
 342                 }
 343             },
 344             TypeError::Traits(ref exp_found) => {
 345                 report_path_match(err, exp_found.expected, exp_found.found);
 346             },
 347             _ => () // FIXME(#22750) handle traits and stuff
 348         }
 349     }
 350
 351     fn note_error_origin(&self,
 352                          err: &mut DiagnosticBuilder<'tcx>,
 353                          cause: &ObligationCause<'tcx>)
 354     {
 355         match cause.code {
 356             ObligationCauseCode::MatchExpressionArm { arm_span, source } => match source {
 357                 hir::MatchSource::IfLetDesugar {..} => {
 358                     err.span_note(arm_span, "`if let` arm with an incompatible type");
 359                 }
 360                 _ => {
 361                     err.span_note(arm_span, "match arm with an incompatible type");
 362                 }
 363             },
 364             _ => ()
 365         }
 366     }
 367
 368     pub fn note_type_err(&self,
 369                          diag: &mut DiagnosticBuilder<'tcx>,
 370                          cause: &ObligationCause<'tcx>,
 371                          secondary_span: Option<(Span, String)>,
 372                          values: Option<ValuePairs<'tcx>>,
 373                          terr: &TypeError<'tcx>)
 374     {
 375         let (expected_found, is_simple_error) = match values {
 376             None => (None, false),
 377             Some(values) => {
 378                 let is_simple_error = match values {
 379                     ValuePairs::Types(exp_found) => {
 380                         exp_found.expected.is_primitive() && exp_found.found.is_primitive()
 381                     }
 382                     _ => false,
 383                 };
 384                 let vals = match self.values_str(&values) {
 385                     Some((expected, found)) => Some((expected, found)),
 386                     None => {
 387                         // Derived error. Cancel the emitter.
 388                         self.tcx.sess.diagnostic().cancel(diag);
 389                         return
 390                     }
 391                 };
 392                 (vals, is_simple_error)
 393             }
 394         };
 395
 396         let span = cause.span;
 397
 398         if let Some((expected, found)) = expected_found {
 399             match (terr, is_simple_error, expected == found) {
 400                 (&TypeError::Sorts(ref values), false,  true) => {
 401                     diag.note_expected_found_extra(
 402                         &"type", &expected, &found,
 403                         &format!(" ({})", values.expected.sort_string(self.tcx)),
 404                         &format!(" ({})", values.found.sort_string(self.tcx)));
 405                 }
 406                 (_, false,  _) => {
 407                     diag.note_expected_found(&"type", &expected, &found);
 408                 }
 409                 _ => (),
 410             }
 411         }
 412
 413         diag.span_label(span, &terr);
 414         if let Some((sp, msg)) = secondary_span {
 415             diag.span_label(sp, &msg);
 416         }
 417
 418         self.note_error_origin(diag, &cause);
 419         self.check_and_note_conflicting_crates(diag, terr, span);
 420         self.tcx.note_and_explain_type_err(diag, terr, span);
 421     }
 422
 423     pub fn note_issue_32330(&self,
 424                             diag: &mut DiagnosticBuilder<'tcx>,
 425                             terr: &TypeError<'tcx>)
 426     {
 427         debug!("note_issue_32330: terr={:?}", terr);
 428         match *terr {
 429             TypeError::RegionsInsufficientlyPolymorphic(_, _, Some(box Issue32330 {
 430                 fn_def_id, region_name
 431             })) |
 432             TypeError::RegionsOverlyPolymorphic(_, _, Some(box Issue32330 {
 433                 fn_def_id, region_name
 434             })) => {
 435                 diag.note(
 436                     &format!("lifetime parameter `{0}` declared on fn `{1}` \
 437                               appears only in the return type, \
 438                               but here is required to be higher-ranked, \
 439                               which means that `{0}` must appear in both \
 440                               argument and return types",
 441                              region_name,
 442                              self.tcx.item_path_str(fn_def_id)));
 443                 diag.note(
 444                     &format!("this error is the result of a recent bug fix; \
 445                               for more information, see issue #33685 \
 446                               <https://github.com/rust-lang/rust/issues/33685>"));
 447             }
 448             _ => {}
 449         }
 450     }
 451
 452     pub fn report_and_explain_type_error(&self,
 453                                          trace: TypeTrace<'tcx>,
 454                                          terr: &TypeError<'tcx>)
 455                                          -> DiagnosticBuilder<'tcx>
 456     {
 457         let span = trace.cause.span;
 458         let failure_str = trace.cause.as_failure_str();
 459         let mut diag = match trace.cause.code {
 460             ObligationCauseCode::IfExpressionWithNoElse => {
 461                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
 462             }
 463             ObligationCauseCode::MainFunctionType => {
 464                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
 465             }
 466             _ => {
 467                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
 468             }
 469         };
 470         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
 471         self.note_issue_32330(&mut diag, terr);
 472         diag
 473     }
 474
 475     /// Returns a string of the form "expected `{}`, found `{}`".
 476     fn values_str(&self, values: &ValuePairs<'tcx>) -> Option<(String, String)> {
 477         match *values {
 478             infer::Types(ref exp_found) => self.expected_found_str(exp_found),
 479             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 480             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 481         }
 482     }
 483
 484     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
 485         &self,
 486         exp_found: &ty::error::ExpectedFound<T>)
 487         -> Option<(String, String)>
 488     {
 489         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 490         if exp_found.references_error() {
 491             return None;
 492         }
 493
 494         Some((format!("{}", exp_found.expected), format!("{}", exp_found.found)))
 495     }
 496
 497     fn report_generic_bound_failure(&self,
 498                                     origin: SubregionOrigin<'tcx>,
 499                                     bound_kind: GenericKind<'tcx>,
 500                                     sub: &'tcx Region)
 501     {
 502         // FIXME: it would be better to report the first error message
 503         // with the span of the parameter itself, rather than the span
 504         // where the error was detected. But that span is not readily
 505         // accessible.
 506
 507         let labeled_user_string = match bound_kind {
 508             GenericKind::Param(ref p) =>
 509                 format!("the parameter type `{}`", p),
 510             GenericKind::Projection(ref p) =>
 511                 format!("the associated type `{}`", p),
 512         };
 513
 514         if let SubregionOrigin::CompareImplMethodObligation {
 515             span, item_name, impl_item_def_id, trait_item_def_id, lint_id
 516         } = origin {
 517             self.report_extra_impl_obligation(span,
 518                                               item_name,
 519                                               impl_item_def_id,
 520                                               trait_item_def_id,
 521                                               &format!("`{}: {}`", bound_kind, sub),
 522                                               lint_id)
 523                 .emit();
 524             return;
 525         }
 526
 527         let mut err = match *sub {
 528             ty::ReFree(ty::FreeRegion {bound_region: ty::BrNamed(..), ..}) => {
 529                 // Does the required lifetime have a nice name we can print?
 530                 let mut err = struct_span_err!(self.tcx.sess,
 531                                                origin.span(),
 532                                                E0309,
 533                                                "{} may not live long enough",
 534                                                labeled_user_string);
 535                 err.help(&format!("consider adding an explicit lifetime bound `{}: {}`...",
 536                          bound_kind,
 537                          sub));
 538                 err
 539             }
 540
 541             ty::ReStatic => {
 542                 // Does the required lifetime have a nice name we can print?
 543                 let mut err = struct_span_err!(self.tcx.sess,
 544                                                origin.span(),
 545                                                E0310,
 546                                                "{} may not live long enough",
 547                                                labeled_user_string);
 548                 err.help(&format!("consider adding an explicit lifetime \
 549                                    bound `{}: 'static`...",
 550                                   bound_kind));
 551                 err
 552             }
 553
 554             _ => {
 555                 // If not, be less specific.
 556                 let mut err = struct_span_err!(self.tcx.sess,
 557                                                origin.span(),
 558                                                E0311,
 559                                                "{} may not live long enough",
 560                                                labeled_user_string);
 561                 err.help(&format!("consider adding an explicit lifetime bound for `{}`",
 562                                   bound_kind));
 563                 self.tcx.note_and_explain_region(
 564                     &mut err,
 565                     &format!("{} must be valid for ", labeled_user_string),
 566                     sub,
 567                     "...");
 568                 err
 569             }
 570         };
 571
 572         self.note_region_origin(&mut err, &origin);
 573         err.emit();
 574     }
 575
 576     fn report_sub_sup_conflict(&self,
 577                                var_origin: RegionVariableOrigin,
 578                                sub_origin: SubregionOrigin<'tcx>,
 579                                sub_region: &'tcx Region,
 580                                sup_origin: SubregionOrigin<'tcx>,
 581                                sup_region: &'tcx Region) {
 582         let mut err = self.report_inference_failure(var_origin);
 583
 584         self.tcx.note_and_explain_region(&mut err,
 585             "first, the lifetime cannot outlive ",
 586             sup_region,
 587             "...");
 588
 589         self.note_region_origin(&mut err, &sup_origin);
 590
 591         self.tcx.note_and_explain_region(&mut err,
 592             "but, the lifetime must be valid for ",
 593             sub_region,
 594             "...");
 595
 596         self.note_region_origin(&mut err, &sub_origin);
 597         err.emit();
 598     }
 599 }
 600
 601 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 602     fn report_inference_failure(&self,
 603                                 var_origin: RegionVariableOrigin)
 604                                 -> DiagnosticBuilder<'tcx> {
 605         let br_string = |br: ty::BoundRegion| {
 606             let mut s = br.to_string();
 607             if !s.is_empty() {
 608                 s.push_str(" ");
 609             }
 610             s
 611         };
 612         let var_description = match var_origin {
 613             infer::MiscVariable(_) => "".to_string(),
 614             infer::PatternRegion(_) => " for pattern".to_string(),
 615             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
 616             infer::Autoref(_) => " for autoref".to_string(),
 617             infer::Coercion(_) => " for automatic coercion".to_string(),
 618             infer::LateBoundRegion(_, br, infer::FnCall) => {
 619                 format!(" for lifetime parameter {}in function call",
 620                         br_string(br))
 621             }
 622             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
 623                 format!(" for lifetime parameter {}in generic type", br_string(br))
 624             }
 625             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(type_name)) => {
 626                 format!(" for lifetime parameter {}in trait containing associated type `{}`",
 627                         br_string(br), type_name)
 628             }
 629             infer::EarlyBoundRegion(_, name, _) => {
 630                 format!(" for lifetime parameter `{}`",
 631                         name)
 632             }
 633             infer::BoundRegionInCoherence(name) => {
 634                 format!(" for lifetime parameter `{}` in coherence check",
 635                         name)
 636             }
 637             infer::UpvarRegion(ref upvar_id, _) => {
 638                 format!(" for capture of `{}` by closure",
 639                         self.tcx.local_var_name_str(upvar_id.var_id).to_string())
 640             }
 641         };
 642
 643         struct_span_err!(self.tcx.sess, var_origin.span(), E0495,
 644                   "cannot infer an appropriate lifetime{} \
 645                    due to conflicting requirements",
 646                   var_description)
 647     }
 648 }
 649
 650 impl<'tcx> ObligationCause<'tcx> {
 651     fn as_failure_str(&self) -> &'static str {
 652         use traits::ObligationCauseCode::*;
 653         match self.code {
 654             CompareImplMethodObligation { .. } => "method not compatible with trait",
 655             MatchExpressionArm { source, .. } => match source {
 656                 hir::MatchSource::IfLetDesugar{..} => "`if let` arms have incompatible types",
 657                 _ => "match arms have incompatible types",
 658             },
 659             IfExpression => "if and else have incompatible types",
 660             IfExpressionWithNoElse => "if may be missing an else clause",
 661             EquatePredicate => "equality predicate not satisfied",
 662             MainFunctionType => "main function has wrong type",
 663             StartFunctionType => "start function has wrong type",
 664             IntrinsicType => "intrinsic has wrong type",
 665             MethodReceiver => "mismatched method receiver",
 666             _ => "mismatched types",
 667         }
 668     }
 669
 670     fn as_requirement_str(&self) -> &'static str {
 671         use traits::ObligationCauseCode::*;
 672         match self.code {
 673             CompareImplMethodObligation { .. } => "method type is compatible with trait",
 674             ExprAssignable => "expression is assignable",
 675             MatchExpressionArm { source, .. } => match source {
 676                 hir::MatchSource::IfLetDesugar{..} => "`if let` arms have compatible types",
 677                 _ => "match arms have compatible types",
 678             },
 679             IfExpression => "if and else have compatible types",
 680             IfExpressionWithNoElse => "if missing an else returns ()",
 681             EquatePredicate => "equality where clause is satisfied",
 682             MainFunctionType => "`main` function has the correct type",
 683             StartFunctionType => "`start` function has the correct type",
 684             IntrinsicType => "intrinsic has the correct type",
 685             MethodReceiver => "method receiver has the correct type",
 686             _ => "types are compatible",
 687         }
 688     }
 689 }