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, DiagnosticStyledString};
  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     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
 369     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
 370     /// populate `other_value` with `other_ty`.
 371     ///
 372     /// ```text
 373     /// Foo<Bar<Qux>>
 374     /// ^^^^--------^ this is highlighted
 375     /// |   |
 376     /// |   this type argument is exactly the same as the other type, not highlighted
 377     /// this is highlighted
 378     /// Bar<Qux>
 379     /// -------- this type is the same as a type argument in the other type, not highlighted
 380     /// ```
 381     fn highlight_outer(&self,
 382                        mut value: &mut DiagnosticStyledString,
 383                        mut other_value: &mut DiagnosticStyledString,
 384                        name: String,
 385                        sub: &ty::subst::Substs<'tcx>,
 386                        pos: usize,
 387                        other_ty: &ty::Ty<'tcx>) {
 388         // `value` and `other_value` hold two incomplete type representation for display.
 389         // `name` is the path of both types being compared. `sub`
 390         value.push_highlighted(name);
 391         let len = sub.len();
 392         if len > 0 {
 393             value.push_highlighted("<");
 394         }
 395
 396         // Output the lifetimes fot the first type
 397         let lifetimes = sub.regions().map(|lifetime| {
 398             let s = format!("{}", lifetime);
 399             if s.is_empty() {
 400                 "'_".to_string()
 401             } else {
 402                 s
 403             }
 404         }).collect::<Vec<_>>().join(", ");
 405         if !lifetimes.is_empty() {
 406             if sub.regions().count() < len {
 407                 value.push_normal(lifetimes + &", ");
 408             } else {
 409                 value.push_normal(lifetimes);
 410             }
 411         }
 412
 413         // Highlight all the type arguments that aren't at `pos` and compare the type argument at
 414         // `pos` and `other_ty`.
 415         for (i, type_arg) in sub.types().enumerate() {
 416             if i == pos {
 417                 let values = self.cmp(type_arg, other_ty);
 418                 value.0.extend((values.0).0);
 419                 other_value.0.extend((values.1).0);
 420             } else {
 421                 value.push_highlighted(format!("{}", type_arg));
 422             }
 423
 424             if len > 0 && i != len - 1 {
 425                 value.push_normal(", ");
 426             }
 427             //self.push_comma(&mut value, &mut other_value, len, i);
 428         }
 429         if len > 0 {
 430             value.push_highlighted(">");
 431         }
 432     }
 433
 434     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 435     /// as that is the difference to the other type.
 436     ///
 437     /// For the following code:
 438     ///
 439     /// ```norun
 440     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 441     /// ```
 442     ///
 443     /// The type error output will behave in the following way:
 444     ///
 445     /// ```text
 446     /// Foo<Bar<Qux>>
 447     /// ^^^^--------^ this is highlighted
 448     /// |   |
 449     /// |   this type argument is exactly the same as the other type, not highlighted
 450     /// this is highlighted
 451     /// Bar<Qux>
 452     /// -------- this type is the same as a type argument in the other type, not highlighted
 453     /// ```
 454     fn cmp_type_arg(&self,
 455                     mut t1_out: &mut DiagnosticStyledString,
 456                     mut t2_out: &mut DiagnosticStyledString,
 457                     path: String,
 458                     sub: &ty::subst::Substs<'tcx>,
 459                     other_path: String,
 460                     other_ty: &ty::Ty<'tcx>) -> Option<()> {
 461         for (i, ta) in sub.types().enumerate() {
 462             if &ta == other_ty {
 463                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 464                 return Some(());
 465             }
 466             if let &ty::TyAdt(def, _) = &ta.sty {
 467                 let path_ = self.tcx.item_path_str(def.did.clone());
 468                 if path_ == other_path {
 469                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 470                     return Some(());
 471                 }
 472             }
 473         }
 474         None
 475     }
 476
 477     /// Add a `,` to the type representation only if it is appropriate.
 478     fn push_comma(&self,
 479                   value: &mut DiagnosticStyledString,
 480                   other_value: &mut DiagnosticStyledString,
 481                   len: usize,
 482                   pos: usize) {
 483         if len > 0 && pos != len - 1 {
 484             value.push_normal(", ");
 485             other_value.push_normal(", ");
 486         }
 487     }
 488
 489     /// Compare two given types, eliding parts that are the same between them and highlighting
 490     /// relevant differences, and return two representation of those types for highlighted printing.
 491     fn cmp(&self, t1: ty::Ty<'tcx>, t2: ty::Ty<'tcx>)
 492         -> (DiagnosticStyledString, DiagnosticStyledString)
 493     {
 494         match (&t1.sty, &t2.sty) {
 495             (&ty::TyAdt(def1, sub1), &ty::TyAdt(def2, sub2)) => {
 496                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 497                 let path1 = self.tcx.item_path_str(def1.did.clone());
 498                 let path2 = self.tcx.item_path_str(def2.did.clone());
 499                 if def1.did == def2.did {
 500                     // Easy case. Replace same types with `_` to shorten the output and highlight
 501                     // the differing ones.
 502                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
 503                     //     Foo<Bar, _>
 504                     //     Foo<Quz, _>
 505                     //         ---  ^ type argument elided
 506                     //         |
 507                     //         highlighted in output
 508                     values.0.push_normal(path1);
 509                     values.1.push_normal(path2);
 510
 511                     // Only draw `<...>` if there're lifetime/type arguments.
 512                     let len = sub1.len();
 513                     if len > 0 {
 514                         values.0.push_normal("<");
 515                         values.1.push_normal("<");
 516                     }
 517
 518                     fn lifetime_display(lifetime: &Region) -> String {
 519                         let s = format!("{}", lifetime);
 520                         if s.is_empty() {
 521                             "'_".to_string()
 522                         } else {
 523                             s
 524                         }
 525                     }
 526                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
 527                     // all diagnostics that use this output
 528                     //
 529                     //     Foo<'x, '_, Bar>
 530                     //     Foo<'y, '_, Qux>
 531                     //         ^^  ^^  --- type arguments are not elided
 532                     //         |   |
 533                     //         |   elided as they were the same
 534                     //         not elided, they were different, but irrelevant
 535                     let lifetimes = sub1.regions().zip(sub2.regions());
 536                     for (i, lifetimes) in lifetimes.enumerate() {
 537                         let l1 = lifetime_display(lifetimes.0);
 538                         let l2 = lifetime_display(lifetimes.1);
 539                         if l1 == l2 {
 540                             values.0.push_normal("'_");
 541                             values.1.push_normal("'_");
 542                         } else {
 543                             values.0.push_highlighted(l1);
 544                             values.1.push_highlighted(l2);
 545                         }
 546                         self.push_comma(&mut values.0, &mut values.1, len, i);
 547                     }
 548
 549                     // We're comparing two types with the same path, so we compare the type
 550                     // arguments for both. If they are the same, do not highlight and elide from the
 551                     // output.
 552                     //     Foo<_, Bar>
 553                     //     Foo<_, Qux>
 554                     //         ^ elided type as this type argument was the same in both sides
 555                     let type_arguments = sub1.types().zip(sub2.types());
 556                     let regions_len = sub1.regions().collect::<Vec<_>>().len();
 557                     for (i, (ta1, ta2)) in type_arguments.enumerate() {
 558                         let i = i + regions_len;
 559                         if ta1 == ta2 {
 560                             values.0.push_normal("_");
 561                             values.1.push_normal("_");
 562                         } else {
 563                             let (x1, x2) = self.cmp(ta1, ta2);
 564                             (values.0).0.extend(x1.0);
 565                             (values.1).0.extend(x2.0);
 566                         }
 567                         self.push_comma(&mut values.0, &mut values.1, len, i);
 568                     }
 569
 570                     // Close the type argument bracket.
 571                     // Only draw `<...>` if there're lifetime/type arguments.
 572                     if len > 0 {
 573                         values.0.push_normal(">");
 574                         values.1.push_normal(">");
 575                     }
 576                     values
 577                 } else {
 578                     // Check for case:
 579                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
 580                     //     Foo<Bar<Qux>
 581                     //         ------- this type argument is exactly the same as the other type
 582                     //     Bar<Qux>
 583                     if self.cmp_type_arg(&mut values.0,
 584                                          &mut values.1,
 585                                          path1.clone(),
 586                                          sub1,
 587                                          path2.clone(),
 588                                          &t2).is_some() {
 589                         return values;
 590                     }
 591                     // Check for case:
 592                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
 593                     //     Bar<Qux>
 594                     //     Foo<Bar<Qux>>
 595                     //         ------- this type argument is exactly the same as the other type
 596                     if self.cmp_type_arg(&mut values.1,
 597                                          &mut values.0,
 598                                          path2,
 599                                          sub2,
 600                                          path1,
 601                                          &t1).is_some() {
 602                         return values;
 603                     }
 604
 605                     // We couldn't find anything in common, highlight everything.
 606                     //     let x: Bar<Qux> = y::<Foo<Zar>>();
 607                     (DiagnosticStyledString::highlighted(format!("{}", t1)),
 608                      DiagnosticStyledString::highlighted(format!("{}", t2)))
 609                 }
 610             }
 611             _ => {
 612                 if t1 == t2 {
 613                     // The two types are the same, elide and don't highlight.
 614                     (DiagnosticStyledString::normal("_"), DiagnosticStyledString::normal("_"))
 615                 } else {
 616                     // We couldn't find anything in common, highlight everything.
 617                     (DiagnosticStyledString::highlighted(format!("{}", t1)),
 618                      DiagnosticStyledString::highlighted(format!("{}", t2)))
 619                 }
 620             }
 621         }
 622     }
 623
 624     pub fn note_type_err(&self,
 625                          diag: &mut DiagnosticBuilder<'tcx>,
 626                          cause: &ObligationCause<'tcx>,
 627                          secondary_span: Option<(Span, String)>,
 628                          values: Option<ValuePairs<'tcx>>,
 629                          terr: &TypeError<'tcx>)
 630     {
 631         let (expected_found, is_simple_error) = match values {
 632             None => (None, false),
 633             Some(values) => {
 634                 let is_simple_error = match values {
 635                     ValuePairs::Types(exp_found) => {
 636                         exp_found.expected.is_primitive() && exp_found.found.is_primitive()
 637                     }
 638                     _ => false,
 639                 };
 640                 let vals = match self.values_str(&values) {
 641                     Some((expected, found)) => Some((expected, found)),
 642                     None => {
 643                         // Derived error. Cancel the emitter.
 644                         self.tcx.sess.diagnostic().cancel(diag);
 645                         return
 646                     }
 647                 };
 648                 (vals, is_simple_error)
 649             }
 650         };
 651
 652         let span = cause.span;
 653
 654         if let Some((expected, found)) = expected_found {
 655             match (terr, is_simple_error, expected == found) {
 656                 (&TypeError::Sorts(ref values), false, true) => {
 657                     diag.note_expected_found_extra(
 658                         &"type", expected, found,
 659                         &format!(" ({})", values.expected.sort_string(self.tcx)),
 660                         &format!(" ({})", values.found.sort_string(self.tcx)));
 661                 }
 662                 (_, false,  _) => {
 663                     diag.note_expected_found(&"type", expected, found);
 664                 }
 665                 _ => (),
 666             }
 667         }
 668
 669         diag.span_label(span, &terr);
 670         if let Some((sp, msg)) = secondary_span {
 671             diag.span_label(sp, &msg);
 672         }
 673
 674         self.note_error_origin(diag, &cause);
 675         self.check_and_note_conflicting_crates(diag, terr, span);
 676         self.tcx.note_and_explain_type_err(diag, terr, span);
 677     }
 678
 679     pub fn note_issue_32330(&self,
 680                             diag: &mut DiagnosticBuilder<'tcx>,
 681                             terr: &TypeError<'tcx>)
 682     {
 683         debug!("note_issue_32330: terr={:?}", terr);
 684         match *terr {
 685             TypeError::RegionsInsufficientlyPolymorphic(_, _, Some(box Issue32330 {
 686                 fn_def_id, region_name
 687             })) |
 688             TypeError::RegionsOverlyPolymorphic(_, _, Some(box Issue32330 {
 689                 fn_def_id, region_name
 690             })) => {
 691                 diag.note(
 692                     &format!("lifetime parameter `{0}` declared on fn `{1}` \
 693                               appears only in the return type, \
 694                               but here is required to be higher-ranked, \
 695                               which means that `{0}` must appear in both \
 696                               argument and return types",
 697                              region_name,
 698                              self.tcx.item_path_str(fn_def_id)));
 699                 diag.note(
 700                     &format!("this error is the result of a recent bug fix; \
 701                               for more information, see issue #33685 \
 702                               <https://github.com/rust-lang/rust/issues/33685>"));
 703             }
 704             _ => {}
 705         }
 706     }
 707
 708     pub fn report_and_explain_type_error(&self,
 709                                          trace: TypeTrace<'tcx>,
 710                                          terr: &TypeError<'tcx>)
 711                                          -> DiagnosticBuilder<'tcx>
 712     {
 713         let span = trace.cause.span;
 714         let failure_str = trace.cause.as_failure_str();
 715         let mut diag = match trace.cause.code {
 716             ObligationCauseCode::IfExpressionWithNoElse => {
 717                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
 718             }
 719             ObligationCauseCode::MainFunctionType => {
 720                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
 721             }
 722             _ => {
 723                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
 724             }
 725         };
 726         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
 727         self.note_issue_32330(&mut diag, terr);
 728         diag
 729     }
 730
 731     fn values_str(&self, values: &ValuePairs<'tcx>)
 732         -> Option<(DiagnosticStyledString, DiagnosticStyledString)>
 733     {
 734         match *values {
 735             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
 736             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 737             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 738         }
 739     }
 740
 741     fn expected_found_str_ty(&self,
 742                              exp_found: &ty::error::ExpectedFound<ty::Ty<'tcx>>)
 743                              -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
 744         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 745         if exp_found.references_error() {
 746             return None;
 747         }
 748
 749         Some(self.cmp(exp_found.expected, exp_found.found))
 750     }
 751
 752     /// Returns a string of the form "expected `{}`, found `{}`".
 753     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
 754         &self,
 755         exp_found: &ty::error::ExpectedFound<T>)
 756         -> Option<(DiagnosticStyledString, DiagnosticStyledString)>
 757     {
 758         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 759         if exp_found.references_error() {
 760             return None;
 761         }
 762
 763         Some((DiagnosticStyledString::highlighted(format!("{}", exp_found.expected)),
 764               DiagnosticStyledString::highlighted(format!("{}", exp_found.found))))
 765     }
 766
 767     fn report_generic_bound_failure(&self,
 768                                     origin: SubregionOrigin<'tcx>,
 769                                     bound_kind: GenericKind<'tcx>,
 770                                     sub: &'tcx Region)
 771     {
 772         // FIXME: it would be better to report the first error message
 773         // with the span of the parameter itself, rather than the span
 774         // where the error was detected. But that span is not readily
 775         // accessible.
 776
 777         let labeled_user_string = match bound_kind {
 778             GenericKind::Param(ref p) =>
 779                 format!("the parameter type `{}`", p),
 780             GenericKind::Projection(ref p) =>
 781                 format!("the associated type `{}`", p),
 782         };
 783
 784         if let SubregionOrigin::CompareImplMethodObligation {
 785             span, item_name, impl_item_def_id, trait_item_def_id, lint_id
 786         } = origin {
 787             self.report_extra_impl_obligation(span,
 788                                               item_name,
 789                                               impl_item_def_id,
 790                                               trait_item_def_id,
 791                                               &format!("`{}: {}`", bound_kind, sub),
 792                                               lint_id)
 793                 .emit();
 794             return;
 795         }
 796
 797         let mut err = match *sub {
 798             ty::ReFree(ty::FreeRegion {bound_region: ty::BrNamed(..), ..}) => {
 799                 // Does the required lifetime have a nice name we can print?
 800                 let mut err = struct_span_err!(self.tcx.sess,
 801                                                origin.span(),
 802                                                E0309,
 803                                                "{} may not live long enough",
 804                                                labeled_user_string);
 805                 err.help(&format!("consider adding an explicit lifetime bound `{}: {}`...",
 806                          bound_kind,
 807                          sub));
 808                 err
 809             }
 810
 811             ty::ReStatic => {
 812                 // Does the required lifetime have a nice name we can print?
 813                 let mut err = struct_span_err!(self.tcx.sess,
 814                                                origin.span(),
 815                                                E0310,
 816                                                "{} may not live long enough",
 817                                                labeled_user_string);
 818                 err.help(&format!("consider adding an explicit lifetime \
 819                                    bound `{}: 'static`...",
 820                                   bound_kind));
 821                 err
 822             }
 823
 824             _ => {
 825                 // If not, be less specific.
 826                 let mut err = struct_span_err!(self.tcx.sess,
 827                                                origin.span(),
 828                                                E0311,
 829                                                "{} may not live long enough",
 830                                                labeled_user_string);
 831                 err.help(&format!("consider adding an explicit lifetime bound for `{}`",
 832                                   bound_kind));
 833                 self.tcx.note_and_explain_region(
 834                     &mut err,
 835                     &format!("{} must be valid for ", labeled_user_string),
 836                     sub,
 837                     "...");
 838                 err
 839             }
 840         };
 841
 842         self.note_region_origin(&mut err, &origin);
 843         err.emit();
 844     }
 845
 846     fn report_sub_sup_conflict(&self,
 847                                var_origin: RegionVariableOrigin,
 848                                sub_origin: SubregionOrigin<'tcx>,
 849                                sub_region: &'tcx Region,
 850                                sup_origin: SubregionOrigin<'tcx>,
 851                                sup_region: &'tcx Region) {
 852         let mut err = self.report_inference_failure(var_origin);
 853
 854         self.tcx.note_and_explain_region(&mut err,
 855             "first, the lifetime cannot outlive ",
 856             sup_region,
 857             "...");
 858
 859         self.note_region_origin(&mut err, &sup_origin);
 860
 861         self.tcx.note_and_explain_region(&mut err,
 862             "but, the lifetime must be valid for ",
 863             sub_region,
 864             "...");
 865
 866         self.note_region_origin(&mut err, &sub_origin);
 867         err.emit();
 868     }
 869 }
 870
 871 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 872     fn report_inference_failure(&self,
 873                                 var_origin: RegionVariableOrigin)
 874                                 -> DiagnosticBuilder<'tcx> {
 875         let br_string = |br: ty::BoundRegion| {
 876             let mut s = br.to_string();
 877             if !s.is_empty() {
 878                 s.push_str(" ");
 879             }
 880             s
 881         };
 882         let var_description = match var_origin {
 883             infer::MiscVariable(_) => "".to_string(),
 884             infer::PatternRegion(_) => " for pattern".to_string(),
 885             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
 886             infer::Autoref(_) => " for autoref".to_string(),
 887             infer::Coercion(_) => " for automatic coercion".to_string(),
 888             infer::LateBoundRegion(_, br, infer::FnCall) => {
 889                 format!(" for lifetime parameter {}in function call",
 890                         br_string(br))
 891             }
 892             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
 893                 format!(" for lifetime parameter {}in generic type", br_string(br))
 894             }
 895             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(type_name)) => {
 896                 format!(" for lifetime parameter {}in trait containing associated type `{}`",
 897                         br_string(br), type_name)
 898             }
 899             infer::EarlyBoundRegion(_, name, _) => {
 900                 format!(" for lifetime parameter `{}`",
 901                         name)
 902             }
 903             infer::BoundRegionInCoherence(name) => {
 904                 format!(" for lifetime parameter `{}` in coherence check",
 905                         name)
 906             }
 907             infer::UpvarRegion(ref upvar_id, _) => {
 908                 format!(" for capture of `{}` by closure",
 909                         self.tcx.local_var_name_str(upvar_id.var_id).to_string())
 910             }
 911         };
 912
 913         struct_span_err!(self.tcx.sess, var_origin.span(), E0495,
 914                   "cannot infer an appropriate lifetime{} \
 915                    due to conflicting requirements",
 916                   var_description)
 917     }
 918 }
 919
 920 impl<'tcx> ObligationCause<'tcx> {
 921     fn as_failure_str(&self) -> &'static str {
 922         use traits::ObligationCauseCode::*;
 923         match self.code {
 924             CompareImplMethodObligation { .. } => "method not compatible with trait",
 925             MatchExpressionArm { source, .. } => match source {
 926                 hir::MatchSource::IfLetDesugar{..} => "`if let` arms have incompatible types",
 927                 _ => "match arms have incompatible types",
 928             },
 929             IfExpression => "if and else have incompatible types",
 930             IfExpressionWithNoElse => "if may be missing an else clause",
 931             EquatePredicate => "equality predicate not satisfied",
 932             MainFunctionType => "main function has wrong type",
 933             StartFunctionType => "start function has wrong type",
 934             IntrinsicType => "intrinsic has wrong type",
 935             MethodReceiver => "mismatched method receiver",
 936             _ => "mismatched types",
 937         }
 938     }
 939
 940     fn as_requirement_str(&self) -> &'static str {
 941         use traits::ObligationCauseCode::*;
 942         match self.code {
 943             CompareImplMethodObligation { .. } => "method type is compatible with trait",
 944             ExprAssignable => "expression is assignable",
 945             MatchExpressionArm { source, .. } => match source {
 946                 hir::MatchSource::IfLetDesugar{..} => "`if let` arms have compatible types",
 947                 _ => "match arms have compatible types",
 948             },
 949             IfExpression => "if and else have compatible types",
 950             IfExpressionWithNoElse => "if missing an else returns ()",
 951             EquatePredicate => "equality where clause is satisfied",
 952             MainFunctionType => "`main` function has the correct type",
 953             StartFunctionType => "`start` function has the correct type",
 954             IntrinsicType => "intrinsic has the correct type",
 955             MethodReceiver => "method receiver has the correct type",
 956             _ => "types are compatible",
 957         }
 958     }
 959 }