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::ast::DUMMY_NODE_ID;
  73 use syntax_pos::{Pos, Span};
  74 use errors::{DiagnosticBuilder, DiagnosticStyledString};
  75
  76 mod note;
  77
  78 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
  79     pub fn note_and_explain_region(self,
  80                                    err: &mut DiagnosticBuilder,
  81                                    prefix: &str,
  82                                    region: &'tcx ty::Region,
  83                                    suffix: &str) {
  84         fn item_scope_tag(item: &hir::Item) -> &'static str {
  85             match item.node {
  86                 hir::ItemImpl(..) => "impl",
  87                 hir::ItemStruct(..) => "struct",
  88                 hir::ItemUnion(..) => "union",
  89                 hir::ItemEnum(..) => "enum",
  90                 hir::ItemTrait(..) => "trait",
  91                 hir::ItemFn(..) => "function body",
  92                 _ => "item"
  93             }
  94         }
  95
  96         fn trait_item_scope_tag(item: &hir::TraitItem) -> &'static str {
  97             match item.node {
  98                 hir::TraitItemKind::Method(..) => "method body",
  99                 hir::TraitItemKind::Const(..) |
 100                 hir::TraitItemKind::Type(..) => "associated item"
 101             }
 102         }
 103
 104         fn impl_item_scope_tag(item: &hir::ImplItem) -> &'static str {
 105             match item.node {
 106                 hir::ImplItemKind::Method(..) => "method body",
 107                 hir::ImplItemKind::Const(..) |
 108                 hir::ImplItemKind::Type(_) => "associated item"
 109             }
 110         }
 111
 112         fn explain_span<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 113                                         heading: &str, span: Span)
 114                                         -> (String, Option<Span>) {
 115             let lo = tcx.sess.codemap().lookup_char_pos_adj(span.lo);
 116             (format!("the {} at {}:{}", heading, lo.line, lo.col.to_usize()),
 117              Some(span))
 118         }
 119
 120         let (description, span) = match *region {
 121             ty::ReScope(scope) => {
 122                 let new_string;
 123                 let unknown_scope = || {
 124                     format!("{}unknown scope: {:?}{}.  Please report a bug.",
 125                             prefix, scope, suffix)
 126                 };
 127                 let span = match scope.span(&self.region_maps(), &self.hir) {
 128                     Some(s) => s,
 129                     None => {
 130                         err.note(&unknown_scope());
 131                         return;
 132                     }
 133                 };
 134                 let tag = match self.hir.find(scope.node_id(&self.region_maps())) {
 135                     Some(hir_map::NodeBlock(_)) => "block",
 136                     Some(hir_map::NodeExpr(expr)) => match expr.node {
 137                         hir::ExprCall(..) => "call",
 138                         hir::ExprMethodCall(..) => "method call",
 139                         hir::ExprMatch(.., hir::MatchSource::IfLetDesugar { .. }) => "if let",
 140                         hir::ExprMatch(.., hir::MatchSource::WhileLetDesugar) =>  "while let",
 141                         hir::ExprMatch(.., hir::MatchSource::ForLoopDesugar) =>  "for",
 142                         hir::ExprMatch(..) => "match",
 143                         _ => "expression",
 144                     },
 145                     Some(hir_map::NodeStmt(_)) => "statement",
 146                     Some(hir_map::NodeItem(it)) => item_scope_tag(&it),
 147                     Some(hir_map::NodeTraitItem(it)) => trait_item_scope_tag(&it),
 148                     Some(hir_map::NodeImplItem(it)) => impl_item_scope_tag(&it),
 149                     Some(_) | None => {
 150                         err.span_note(span, &unknown_scope());
 151                         return;
 152                     }
 153                 };
 154                 let scope_decorated_tag = match self.region_maps().code_extent_data(scope) {
 155                     region::CodeExtentData::Misc(_) => tag,
 156                     region::CodeExtentData::CallSiteScope { .. } => {
 157                         "scope of call-site for function"
 158                     }
 159                     region::CodeExtentData::ParameterScope { .. } => {
 160                         "scope of function body"
 161                     }
 162                     region::CodeExtentData::DestructionScope(_) => {
 163                         new_string = format!("destruction scope surrounding {}", tag);
 164                         &new_string[..]
 165                     }
 166                     region::CodeExtentData::Remainder(r) => {
 167                         new_string = format!("block suffix following statement {}",
 168                                              r.first_statement_index);
 169                         &new_string[..]
 170                     }
 171                 };
 172                 explain_span(self, scope_decorated_tag, span)
 173             }
 174
 175             ty::ReFree(ref fr) => {
 176                 let prefix = match fr.bound_region {
 177                     ty::BrAnon(idx) => {
 178                         format!("the anonymous lifetime #{} defined on", idx + 1)
 179                     }
 180                     ty::BrFresh(_) => "an anonymous lifetime defined on".to_owned(),
 181                     _ => {
 182                         format!("the lifetime {} as defined on",
 183                                 fr.bound_region)
 184                     }
 185                 };
 186
 187                 let node = fr.scope.map(|s| s.node_id(&self.region_maps()))
 188                                    .unwrap_or(DUMMY_NODE_ID);
 189                 let unknown;
 190                 let tag = match self.hir.find(node) {
 191                     Some(hir_map::NodeBlock(_)) |
 192                     Some(hir_map::NodeExpr(_)) => "body",
 193                     Some(hir_map::NodeItem(it)) => item_scope_tag(&it),
 194                     Some(hir_map::NodeTraitItem(it)) => trait_item_scope_tag(&it),
 195                     Some(hir_map::NodeImplItem(it)) => impl_item_scope_tag(&it),
 196
 197                     // this really should not happen, but it does:
 198                     // FIXME(#27942)
 199                     Some(_) => {
 200                         unknown = format!("unexpected node ({}) for scope {:?}.  \
 201                                            Please report a bug.",
 202                                           self.hir.node_to_string(node), fr.scope);
 203                         &unknown
 204                     }
 205                     None => {
 206                         unknown = format!("unknown node for scope {:?}.  \
 207                                            Please report a bug.", fr.scope);
 208                         &unknown
 209                     }
 210                 };
 211                 let (msg, opt_span) = explain_span(self, tag, self.hir.span(node));
 212                 (format!("{} {}", prefix, msg), opt_span)
 213             }
 214
 215             ty::ReStatic => ("the static lifetime".to_owned(), None),
 216
 217             ty::ReEmpty => ("the empty lifetime".to_owned(), None),
 218
 219             ty::ReEarlyBound(ref data) => (data.name.to_string(), None),
 220
 221             // FIXME(#13998) ReSkolemized should probably print like
 222             // ReFree rather than dumping Debug output on the user.
 223             //
 224             // We shouldn't really be having unification failures with ReVar
 225             // and ReLateBound though.
 226             ty::ReSkolemized(..) |
 227             ty::ReVar(_) |
 228             ty::ReLateBound(..) |
 229             ty::ReErased => {
 230                 (format!("lifetime {:?}", region), None)
 231             }
 232         };
 233         let message = format!("{}{}{}", prefix, description, suffix);
 234         if let Some(span) = span {
 235             err.span_note(span, &message);
 236         } else {
 237             err.note(&message);
 238         }
 239     }
 240 }
 241
 242 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 243     pub fn report_region_errors(&self,
 244                                 errors: &Vec<RegionResolutionError<'tcx>>) {
 245         debug!("report_region_errors(): {} errors to start", errors.len());
 246
 247         // try to pre-process the errors, which will group some of them
 248         // together into a `ProcessedErrors` group:
 249         let errors = self.process_errors(errors);
 250
 251         debug!("report_region_errors: {} errors after preprocessing", errors.len());
 252
 253         for error in errors {
 254             debug!("report_region_errors: error = {:?}", error);
 255             match error.clone() {
 256                 ConcreteFailure(origin, sub, sup) => {
 257                     self.report_concrete_failure(origin, sub, sup).emit();
 258                 }
 259
 260                 GenericBoundFailure(kind, param_ty, sub) => {
 261                     self.report_generic_bound_failure(kind, param_ty, sub);
 262                 }
 263
 264                 SubSupConflict(var_origin,
 265                                sub_origin, sub_r,
 266                                sup_origin, sup_r) => {
 267                     self.report_sub_sup_conflict(var_origin,
 268                                                  sub_origin, sub_r,
 269                                                  sup_origin, sup_r);
 270                 }
 271             }
 272         }
 273     }
 274
 275     // This method goes through all the errors and try to group certain types
 276     // of error together, for the purpose of suggesting explicit lifetime
 277     // parameters to the user. This is done so that we can have a more
 278     // complete view of what lifetimes should be the same.
 279     // If the return value is an empty vector, it means that processing
 280     // failed (so the return value of this method should not be used).
 281     //
 282     // The method also attempts to weed out messages that seem like
 283     // duplicates that will be unhelpful to the end-user. But
 284     // obviously it never weeds out ALL errors.
 285     fn process_errors(&self, errors: &Vec<RegionResolutionError<'tcx>>)
 286                       -> Vec<RegionResolutionError<'tcx>> {
 287         debug!("process_errors()");
 288
 289         // We want to avoid reporting generic-bound failures if we can
 290         // avoid it: these have a very high rate of being unhelpful in
 291         // practice. This is because they are basically secondary
 292         // checks that test the state of the region graph after the
 293         // rest of inference is done, and the other kinds of errors
 294         // indicate that the region constraint graph is internally
 295         // inconsistent, so these test results are likely to be
 296         // meaningless.
 297         //
 298         // Therefore, we filter them out of the list unless they are
 299         // the only thing in the list.
 300
 301         let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
 302             ConcreteFailure(..) => false,
 303             SubSupConflict(..) => false,
 304             GenericBoundFailure(..) => true,
 305         };
 306
 307         if errors.iter().all(|e| is_bound_failure(e)) {
 308             errors.clone()
 309         } else {
 310             errors.iter().filter(|&e| !is_bound_failure(e)).cloned().collect()
 311         }
 312     }
 313
 314     /// Adds a note if the types come from similarly named crates
 315     fn check_and_note_conflicting_crates(&self,
 316                                          err: &mut DiagnosticBuilder,
 317                                          terr: &TypeError<'tcx>,
 318                                          sp: Span) {
 319         let report_path_match = |err: &mut DiagnosticBuilder, did1: DefId, did2: DefId| {
 320             // Only external crates, if either is from a local
 321             // module we could have false positives
 322             if !(did1.is_local() || did2.is_local()) && did1.krate != did2.krate {
 323                 let exp_path = self.tcx.item_path_str(did1);
 324                 let found_path = self.tcx.item_path_str(did2);
 325                 // We compare strings because DefPath can be different
 326                 // for imported and non-imported crates
 327                 if exp_path == found_path {
 328                     let crate_name = self.tcx.sess.cstore.crate_name(did1.krate);
 329                     err.span_note(sp, &format!("Perhaps two different versions \
 330                                                 of crate `{}` are being used?",
 331                                                crate_name));
 332                 }
 333             }
 334         };
 335         match *terr {
 336             TypeError::Sorts(ref exp_found) => {
 337                 // if they are both "path types", there's a chance of ambiguity
 338                 // due to different versions of the same crate
 339                 match (&exp_found.expected.sty, &exp_found.found.sty) {
 340                     (&ty::TyAdt(exp_adt, _), &ty::TyAdt(found_adt, _)) => {
 341                         report_path_match(err, exp_adt.did, found_adt.did);
 342                     },
 343                     _ => ()
 344                 }
 345             },
 346             TypeError::Traits(ref exp_found) => {
 347                 report_path_match(err, exp_found.expected, exp_found.found);
 348             },
 349             _ => () // FIXME(#22750) handle traits and stuff
 350         }
 351     }
 352
 353     fn note_error_origin(&self,
 354                          err: &mut DiagnosticBuilder<'tcx>,
 355                          cause: &ObligationCause<'tcx>)
 356     {
 357         match cause.code {
 358             ObligationCauseCode::MatchExpressionArm { arm_span, source } => match source {
 359                 hir::MatchSource::IfLetDesugar {..} => {
 360                     err.span_note(arm_span, "`if let` arm with an incompatible type");
 361                 }
 362                 _ => {
 363                     err.span_note(arm_span, "match arm with an incompatible type");
 364                 }
 365             },
 366             _ => ()
 367         }
 368     }
 369
 370     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
 371     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
 372     /// populate `other_value` with `other_ty`.
 373     ///
 374     /// ```text
 375     /// Foo<Bar<Qux>>
 376     /// ^^^^--------^ this is highlighted
 377     /// |   |
 378     /// |   this type argument is exactly the same as the other type, not highlighted
 379     /// this is highlighted
 380     /// Bar<Qux>
 381     /// -------- this type is the same as a type argument in the other type, not highlighted
 382     /// ```
 383     fn highlight_outer(&self,
 384                        mut value: &mut DiagnosticStyledString,
 385                        mut other_value: &mut DiagnosticStyledString,
 386                        name: String,
 387                        sub: &ty::subst::Substs<'tcx>,
 388                        pos: usize,
 389                        other_ty: &ty::Ty<'tcx>) {
 390         // `value` and `other_value` hold two incomplete type representation for display.
 391         // `name` is the path of both types being compared. `sub`
 392         value.push_highlighted(name);
 393         let len = sub.len();
 394         if len > 0 {
 395             value.push_highlighted("<");
 396         }
 397
 398         // Output the lifetimes fot the first type
 399         let lifetimes = sub.regions().map(|lifetime| {
 400             let s = format!("{}", lifetime);
 401             if s.is_empty() {
 402                 "'_".to_string()
 403             } else {
 404                 s
 405             }
 406         }).collect::<Vec<_>>().join(", ");
 407         if !lifetimes.is_empty() {
 408             if sub.regions().count() < len {
 409                 value.push_normal(lifetimes + &", ");
 410             } else {
 411                 value.push_normal(lifetimes);
 412             }
 413         }
 414
 415         // Highlight all the type arguments that aren't at `pos` and compare the type argument at
 416         // `pos` and `other_ty`.
 417         for (i, type_arg) in sub.types().enumerate() {
 418             if i == pos {
 419                 let values = self.cmp(type_arg, other_ty);
 420                 value.0.extend((values.0).0);
 421                 other_value.0.extend((values.1).0);
 422             } else {
 423                 value.push_highlighted(format!("{}", type_arg));
 424             }
 425
 426             if len > 0 && i != len - 1 {
 427                 value.push_normal(", ");
 428             }
 429             //self.push_comma(&mut value, &mut other_value, len, i);
 430         }
 431         if len > 0 {
 432             value.push_highlighted(">");
 433         }
 434     }
 435
 436     /// If `other_ty` is the same as a type argument present in `sub`, highlight `path` in `t1_out`,
 437     /// as that is the difference to the other type.
 438     ///
 439     /// For the following code:
 440     ///
 441     /// ```norun
 442     /// let x: Foo<Bar<Qux>> = foo::<Bar<Qux>>();
 443     /// ```
 444     ///
 445     /// The type error output will behave in the following way:
 446     ///
 447     /// ```text
 448     /// Foo<Bar<Qux>>
 449     /// ^^^^--------^ this is highlighted
 450     /// |   |
 451     /// |   this type argument is exactly the same as the other type, not highlighted
 452     /// this is highlighted
 453     /// Bar<Qux>
 454     /// -------- this type is the same as a type argument in the other type, not highlighted
 455     /// ```
 456     fn cmp_type_arg(&self,
 457                     mut t1_out: &mut DiagnosticStyledString,
 458                     mut t2_out: &mut DiagnosticStyledString,
 459                     path: String,
 460                     sub: &ty::subst::Substs<'tcx>,
 461                     other_path: String,
 462                     other_ty: &ty::Ty<'tcx>) -> Option<()> {
 463         for (i, ta) in sub.types().enumerate() {
 464             if &ta == other_ty {
 465                 self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 466                 return Some(());
 467             }
 468             if let &ty::TyAdt(def, _) = &ta.sty {
 469                 let path_ = self.tcx.item_path_str(def.did.clone());
 470                 if path_ == other_path {
 471                     self.highlight_outer(&mut t1_out, &mut t2_out, path, sub, i, &other_ty);
 472                     return Some(());
 473                 }
 474             }
 475         }
 476         None
 477     }
 478
 479     /// Add a `,` to the type representation only if it is appropriate.
 480     fn push_comma(&self,
 481                   value: &mut DiagnosticStyledString,
 482                   other_value: &mut DiagnosticStyledString,
 483                   len: usize,
 484                   pos: usize) {
 485         if len > 0 && pos != len - 1 {
 486             value.push_normal(", ");
 487             other_value.push_normal(", ");
 488         }
 489     }
 490
 491     /// Compare two given types, eliding parts that are the same between them and highlighting
 492     /// relevant differences, and return two representation of those types for highlighted printing.
 493     fn cmp(&self, t1: ty::Ty<'tcx>, t2: ty::Ty<'tcx>)
 494         -> (DiagnosticStyledString, DiagnosticStyledString)
 495     {
 496         match (&t1.sty, &t2.sty) {
 497             (&ty::TyAdt(def1, sub1), &ty::TyAdt(def2, sub2)) => {
 498                 let mut values = (DiagnosticStyledString::new(), DiagnosticStyledString::new());
 499                 let path1 = self.tcx.item_path_str(def1.did.clone());
 500                 let path2 = self.tcx.item_path_str(def2.did.clone());
 501                 if def1.did == def2.did {
 502                     // Easy case. Replace same types with `_` to shorten the output and highlight
 503                     // the differing ones.
 504                     //     let x: Foo<Bar, Qux> = y::<Foo<Quz, Qux>>();
 505                     //     Foo<Bar, _>
 506                     //     Foo<Quz, _>
 507                     //         ---  ^ type argument elided
 508                     //         |
 509                     //         highlighted in output
 510                     values.0.push_normal(path1);
 511                     values.1.push_normal(path2);
 512
 513                     // Only draw `<...>` if there're lifetime/type arguments.
 514                     let len = sub1.len();
 515                     if len > 0 {
 516                         values.0.push_normal("<");
 517                         values.1.push_normal("<");
 518                     }
 519
 520                     fn lifetime_display(lifetime: &Region) -> String {
 521                         let s = format!("{}", lifetime);
 522                         if s.is_empty() {
 523                             "'_".to_string()
 524                         } else {
 525                             s
 526                         }
 527                     }
 528                     // At one point we'd like to elide all lifetimes here, they are irrelevant for
 529                     // all diagnostics that use this output
 530                     //
 531                     //     Foo<'x, '_, Bar>
 532                     //     Foo<'y, '_, Qux>
 533                     //         ^^  ^^  --- type arguments are not elided
 534                     //         |   |
 535                     //         |   elided as they were the same
 536                     //         not elided, they were different, but irrelevant
 537                     let lifetimes = sub1.regions().zip(sub2.regions());
 538                     for (i, lifetimes) in lifetimes.enumerate() {
 539                         let l1 = lifetime_display(lifetimes.0);
 540                         let l2 = lifetime_display(lifetimes.1);
 541                         if l1 == l2 {
 542                             values.0.push_normal("'_");
 543                             values.1.push_normal("'_");
 544                         } else {
 545                             values.0.push_highlighted(l1);
 546                             values.1.push_highlighted(l2);
 547                         }
 548                         self.push_comma(&mut values.0, &mut values.1, len, i);
 549                     }
 550
 551                     // We're comparing two types with the same path, so we compare the type
 552                     // arguments for both. If they are the same, do not highlight and elide from the
 553                     // output.
 554                     //     Foo<_, Bar>
 555                     //     Foo<_, Qux>
 556                     //         ^ elided type as this type argument was the same in both sides
 557                     let type_arguments = sub1.types().zip(sub2.types());
 558                     let regions_len = sub1.regions().collect::<Vec<_>>().len();
 559                     for (i, (ta1, ta2)) in type_arguments.enumerate() {
 560                         let i = i + regions_len;
 561                         if ta1 == ta2 {
 562                             values.0.push_normal("_");
 563                             values.1.push_normal("_");
 564                         } else {
 565                             let (x1, x2) = self.cmp(ta1, ta2);
 566                             (values.0).0.extend(x1.0);
 567                             (values.1).0.extend(x2.0);
 568                         }
 569                         self.push_comma(&mut values.0, &mut values.1, len, i);
 570                     }
 571
 572                     // Close the type argument bracket.
 573                     // Only draw `<...>` if there're lifetime/type arguments.
 574                     if len > 0 {
 575                         values.0.push_normal(">");
 576                         values.1.push_normal(">");
 577                     }
 578                     values
 579                 } else {
 580                     // Check for case:
 581                     //     let x: Foo<Bar<Qux> = foo::<Bar<Qux>>();
 582                     //     Foo<Bar<Qux>
 583                     //         ------- this type argument is exactly the same as the other type
 584                     //     Bar<Qux>
 585                     if self.cmp_type_arg(&mut values.0,
 586                                          &mut values.1,
 587                                          path1.clone(),
 588                                          sub1,
 589                                          path2.clone(),
 590                                          &t2).is_some() {
 591                         return values;
 592                     }
 593                     // Check for case:
 594                     //     let x: Bar<Qux> = y:<Foo<Bar<Qux>>>();
 595                     //     Bar<Qux>
 596                     //     Foo<Bar<Qux>>
 597                     //         ------- this type argument is exactly the same as the other type
 598                     if self.cmp_type_arg(&mut values.1,
 599                                          &mut values.0,
 600                                          path2,
 601                                          sub2,
 602                                          path1,
 603                                          &t1).is_some() {
 604                         return values;
 605                     }
 606
 607                     // We couldn't find anything in common, highlight everything.
 608                     //     let x: Bar<Qux> = y::<Foo<Zar>>();
 609                     (DiagnosticStyledString::highlighted(format!("{}", t1)),
 610                      DiagnosticStyledString::highlighted(format!("{}", t2)))
 611                 }
 612             }
 613             _ => {
 614                 if t1 == t2 {
 615                     // The two types are the same, elide and don't highlight.
 616                     (DiagnosticStyledString::normal("_"), DiagnosticStyledString::normal("_"))
 617                 } else {
 618                     // We couldn't find anything in common, highlight everything.
 619                     (DiagnosticStyledString::highlighted(format!("{}", t1)),
 620                      DiagnosticStyledString::highlighted(format!("{}", t2)))
 621                 }
 622             }
 623         }
 624     }
 625
 626     pub fn note_type_err(&self,
 627                          diag: &mut DiagnosticBuilder<'tcx>,
 628                          cause: &ObligationCause<'tcx>,
 629                          secondary_span: Option<(Span, String)>,
 630                          values: Option<ValuePairs<'tcx>>,
 631                          terr: &TypeError<'tcx>)
 632     {
 633         let (expected_found, is_simple_error) = match values {
 634             None => (None, false),
 635             Some(values) => {
 636                 let is_simple_error = match values {
 637                     ValuePairs::Types(exp_found) => {
 638                         exp_found.expected.is_primitive() && exp_found.found.is_primitive()
 639                     }
 640                     _ => false,
 641                 };
 642                 let vals = match self.values_str(&values) {
 643                     Some((expected, found)) => Some((expected, found)),
 644                     None => {
 645                         // Derived error. Cancel the emitter.
 646                         self.tcx.sess.diagnostic().cancel(diag);
 647                         return
 648                     }
 649                 };
 650                 (vals, is_simple_error)
 651             }
 652         };
 653
 654         let span = cause.span;
 655
 656         if let Some((expected, found)) = expected_found {
 657             match (terr, is_simple_error, expected == found) {
 658                 (&TypeError::Sorts(ref values), false, true) => {
 659                     diag.note_expected_found_extra(
 660                         &"type", expected, found,
 661                         &format!(" ({})", values.expected.sort_string(self.tcx)),
 662                         &format!(" ({})", values.found.sort_string(self.tcx)));
 663                 }
 664                 (_, false,  _) => {
 665                     diag.note_expected_found(&"type", expected, found);
 666                 }
 667                 _ => (),
 668             }
 669         }
 670
 671         diag.span_label(span, &terr);
 672         if let Some((sp, msg)) = secondary_span {
 673             diag.span_label(sp, &msg);
 674         }
 675
 676         self.note_error_origin(diag, &cause);
 677         self.check_and_note_conflicting_crates(diag, terr, span);
 678         self.tcx.note_and_explain_type_err(diag, terr, span);
 679     }
 680
 681     pub fn note_issue_32330(&self,
 682                             diag: &mut DiagnosticBuilder<'tcx>,
 683                             terr: &TypeError<'tcx>)
 684     {
 685         debug!("note_issue_32330: terr={:?}", terr);
 686         match *terr {
 687             TypeError::RegionsInsufficientlyPolymorphic(_, _, Some(box Issue32330 {
 688                 fn_def_id, region_name
 689             })) |
 690             TypeError::RegionsOverlyPolymorphic(_, _, Some(box Issue32330 {
 691                 fn_def_id, region_name
 692             })) => {
 693                 diag.note(
 694                     &format!("lifetime parameter `{0}` declared on fn `{1}` \
 695                               appears only in the return type, \
 696                               but here is required to be higher-ranked, \
 697                               which means that `{0}` must appear in both \
 698                               argument and return types",
 699                              region_name,
 700                              self.tcx.item_path_str(fn_def_id)));
 701                 diag.note(
 702                     &format!("this error is the result of a recent bug fix; \
 703                               for more information, see issue #33685 \
 704                               <https://github.com/rust-lang/rust/issues/33685>"));
 705             }
 706             _ => {}
 707         }
 708     }
 709
 710     pub fn report_and_explain_type_error(&self,
 711                                          trace: TypeTrace<'tcx>,
 712                                          terr: &TypeError<'tcx>)
 713                                          -> DiagnosticBuilder<'tcx>
 714     {
 715         let span = trace.cause.span;
 716         let failure_str = trace.cause.as_failure_str();
 717         let mut diag = match trace.cause.code {
 718             ObligationCauseCode::IfExpressionWithNoElse => {
 719                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
 720             }
 721             ObligationCauseCode::MainFunctionType => {
 722                 struct_span_err!(self.tcx.sess, span, E0580, "{}", failure_str)
 723             }
 724             _ => {
 725                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
 726             }
 727         };
 728         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
 729         self.note_issue_32330(&mut diag, terr);
 730         diag
 731     }
 732
 733     fn values_str(&self, values: &ValuePairs<'tcx>)
 734         -> Option<(DiagnosticStyledString, DiagnosticStyledString)>
 735     {
 736         match *values {
 737             infer::Types(ref exp_found) => self.expected_found_str_ty(exp_found),
 738             infer::TraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 739             infer::PolyTraitRefs(ref exp_found) => self.expected_found_str(exp_found),
 740         }
 741     }
 742
 743     fn expected_found_str_ty(&self,
 744                              exp_found: &ty::error::ExpectedFound<ty::Ty<'tcx>>)
 745                              -> Option<(DiagnosticStyledString, DiagnosticStyledString)> {
 746         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 747         if exp_found.references_error() {
 748             return None;
 749         }
 750
 751         Some(self.cmp(exp_found.expected, exp_found.found))
 752     }
 753
 754     /// Returns a string of the form "expected `{}`, found `{}`".
 755     fn expected_found_str<T: fmt::Display + TypeFoldable<'tcx>>(
 756         &self,
 757         exp_found: &ty::error::ExpectedFound<T>)
 758         -> Option<(DiagnosticStyledString, DiagnosticStyledString)>
 759     {
 760         let exp_found = self.resolve_type_vars_if_possible(exp_found);
 761         if exp_found.references_error() {
 762             return None;
 763         }
 764
 765         Some((DiagnosticStyledString::highlighted(format!("{}", exp_found.expected)),
 766               DiagnosticStyledString::highlighted(format!("{}", exp_found.found))))
 767     }
 768
 769     fn report_generic_bound_failure(&self,
 770                                     origin: SubregionOrigin<'tcx>,
 771                                     bound_kind: GenericKind<'tcx>,
 772                                     sub: &'tcx Region)
 773     {
 774         // FIXME: it would be better to report the first error message
 775         // with the span of the parameter itself, rather than the span
 776         // where the error was detected. But that span is not readily
 777         // accessible.
 778
 779         let labeled_user_string = match bound_kind {
 780             GenericKind::Param(ref p) =>
 781                 format!("the parameter type `{}`", p),
 782             GenericKind::Projection(ref p) =>
 783                 format!("the associated type `{}`", p),
 784         };
 785
 786         if let SubregionOrigin::CompareImplMethodObligation {
 787             span, item_name, impl_item_def_id, trait_item_def_id, lint_id
 788         } = origin {
 789             self.report_extra_impl_obligation(span,
 790                                               item_name,
 791                                               impl_item_def_id,
 792                                               trait_item_def_id,
 793                                               &format!("`{}: {}`", bound_kind, sub),
 794                                               lint_id)
 795                 .emit();
 796             return;
 797         }
 798
 799         let mut err = match *sub {
 800             ty::ReFree(ty::FreeRegion {bound_region: ty::BrNamed(..), ..}) => {
 801                 // Does the required lifetime have a nice name we can print?
 802                 let mut err = struct_span_err!(self.tcx.sess,
 803                                                origin.span(),
 804                                                E0309,
 805                                                "{} may not live long enough",
 806                                                labeled_user_string);
 807                 err.help(&format!("consider adding an explicit lifetime bound `{}: {}`...",
 808                          bound_kind,
 809                          sub));
 810                 err
 811             }
 812
 813             ty::ReStatic => {
 814                 // Does the required lifetime have a nice name we can print?
 815                 let mut err = struct_span_err!(self.tcx.sess,
 816                                                origin.span(),
 817                                                E0310,
 818                                                "{} may not live long enough",
 819                                                labeled_user_string);
 820                 err.help(&format!("consider adding an explicit lifetime \
 821                                    bound `{}: 'static`...",
 822                                   bound_kind));
 823                 err
 824             }
 825
 826             _ => {
 827                 // If not, be less specific.
 828                 let mut err = struct_span_err!(self.tcx.sess,
 829                                                origin.span(),
 830                                                E0311,
 831                                                "{} may not live long enough",
 832                                                labeled_user_string);
 833                 err.help(&format!("consider adding an explicit lifetime bound for `{}`",
 834                                   bound_kind));
 835                 self.tcx.note_and_explain_region(
 836                     &mut err,
 837                     &format!("{} must be valid for ", labeled_user_string),
 838                     sub,
 839                     "...");
 840                 err
 841             }
 842         };
 843
 844         self.note_region_origin(&mut err, &origin);
 845         err.emit();
 846     }
 847
 848     fn report_sub_sup_conflict(&self,
 849                                var_origin: RegionVariableOrigin,
 850                                sub_origin: SubregionOrigin<'tcx>,
 851                                sub_region: &'tcx Region,
 852                                sup_origin: SubregionOrigin<'tcx>,
 853                                sup_region: &'tcx Region) {
 854         let mut err = self.report_inference_failure(var_origin);
 855
 856         self.tcx.note_and_explain_region(&mut err,
 857             "first, the lifetime cannot outlive ",
 858             sup_region,
 859             "...");
 860
 861         self.note_region_origin(&mut err, &sup_origin);
 862
 863         self.tcx.note_and_explain_region(&mut err,
 864             "but, the lifetime must be valid for ",
 865             sub_region,
 866             "...");
 867
 868         self.note_region_origin(&mut err, &sub_origin);
 869         err.emit();
 870     }
 871 }
 872
 873 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 874     fn report_inference_failure(&self,
 875                                 var_origin: RegionVariableOrigin)
 876                                 -> DiagnosticBuilder<'tcx> {
 877         let br_string = |br: ty::BoundRegion| {
 878             let mut s = br.to_string();
 879             if !s.is_empty() {
 880                 s.push_str(" ");
 881             }
 882             s
 883         };
 884         let var_description = match var_origin {
 885             infer::MiscVariable(_) => "".to_string(),
 886             infer::PatternRegion(_) => " for pattern".to_string(),
 887             infer::AddrOfRegion(_) => " for borrow expression".to_string(),
 888             infer::Autoref(_) => " for autoref".to_string(),
 889             infer::Coercion(_) => " for automatic coercion".to_string(),
 890             infer::LateBoundRegion(_, br, infer::FnCall) => {
 891                 format!(" for lifetime parameter {}in function call",
 892                         br_string(br))
 893             }
 894             infer::LateBoundRegion(_, br, infer::HigherRankedType) => {
 895                 format!(" for lifetime parameter {}in generic type", br_string(br))
 896             }
 897             infer::LateBoundRegion(_, br, infer::AssocTypeProjection(type_name)) => {
 898                 format!(" for lifetime parameter {}in trait containing associated type `{}`",
 899                         br_string(br), type_name)
 900             }
 901             infer::EarlyBoundRegion(_, name, _) => {
 902                 format!(" for lifetime parameter `{}`",
 903                         name)
 904             }
 905             infer::BoundRegionInCoherence(name) => {
 906                 format!(" for lifetime parameter `{}` in coherence check",
 907                         name)
 908             }
 909             infer::UpvarRegion(ref upvar_id, _) => {
 910                 format!(" for capture of `{}` by closure",
 911                         self.tcx.local_var_name_str(upvar_id.var_id).to_string())
 912             }
 913         };
 914
 915         struct_span_err!(self.tcx.sess, var_origin.span(), E0495,
 916                   "cannot infer an appropriate lifetime{} \
 917                    due to conflicting requirements",
 918                   var_description)
 919     }
 920 }
 921
 922 impl<'tcx> ObligationCause<'tcx> {
 923     fn as_failure_str(&self) -> &'static str {
 924         use traits::ObligationCauseCode::*;
 925         match self.code {
 926             CompareImplMethodObligation { .. } => "method not compatible with trait",
 927             MatchExpressionArm { source, .. } => match source {
 928                 hir::MatchSource::IfLetDesugar{..} => "`if let` arms have incompatible types",
 929                 _ => "match arms have incompatible types",
 930             },
 931             IfExpression => "if and else have incompatible types",
 932             IfExpressionWithNoElse => "if may be missing an else clause",
 933             EquatePredicate => "equality predicate not satisfied",
 934             MainFunctionType => "main function has wrong type",
 935             StartFunctionType => "start function has wrong type",
 936             IntrinsicType => "intrinsic has wrong type",
 937             MethodReceiver => "mismatched method receiver",
 938             _ => "mismatched types",
 939         }
 940     }
 941
 942     fn as_requirement_str(&self) -> &'static str {
 943         use traits::ObligationCauseCode::*;
 944         match self.code {
 945             CompareImplMethodObligation { .. } => "method type is compatible with trait",
 946             ExprAssignable => "expression is assignable",
 947             MatchExpressionArm { source, .. } => match source {
 948                 hir::MatchSource::IfLetDesugar{..} => "`if let` arms have compatible types",
 949                 _ => "match arms have compatible types",
 950             },
 951             IfExpression => "if and else have compatible types",
 952             IfExpressionWithNoElse => "if missing an else returns ()",
 953             EquatePredicate => "equality where clause is satisfied",
 954             MainFunctionType => "`main` function has the correct type",
 955             StartFunctionType => "`start` function has the correct type",
 956             IntrinsicType => "intrinsic has the correct type",
 957             MethodReceiver => "method receiver has the correct type",
 958             _ => "types are compatible",
 959         }
 960     }
 961 }