compiler/rustc_borrowck/src/diagnostics/region_errors.rs

   1 //! Error reporting machinery for lifetime errors.
   2
   3 use rustc_errors::{Applicability, DiagnosticBuilder};
   4 use rustc_infer::infer::{
   5     error_reporting::nice_region_error::NiceRegionError,
   6     error_reporting::unexpected_hidden_region_diagnostic, NllRegionVariableOrigin,
   7 };
   8 use rustc_middle::hir::place::PlaceBase;
   9 use rustc_middle::mir::{ConstraintCategory, ReturnConstraint};
  10 use rustc_middle::ty::subst::{InternalSubsts, Subst};
  11 use rustc_middle::ty::{self, RegionVid, Ty};
  12 use rustc_span::symbol::{kw, sym};
  13 use rustc_span::{BytePos, Span};
  14
  15 use crate::borrowck_errors;
  16
  17 use super::{OutlivesSuggestionBuilder, RegionName};
  18 use crate::region_infer::BlameConstraint;
  19 use crate::{
  20     nll::ConstraintDescription,
  21     region_infer::{values::RegionElement, TypeTest},
  22     universal_regions::DefiningTy,
  23     MirBorrowckCtxt,
  24 };
  25
  26 impl ConstraintDescription for ConstraintCategory {
  27     fn description(&self) -> &'static str {
  28         // Must end with a space. Allows for empty names to be provided.
  29         match self {
  30             ConstraintCategory::Assignment => "assignment ",
  31             ConstraintCategory::Return(_) => "returning this value ",
  32             ConstraintCategory::Yield => "yielding this value ",
  33             ConstraintCategory::UseAsConst => "using this value as a constant ",
  34             ConstraintCategory::UseAsStatic => "using this value as a static ",
  35             ConstraintCategory::Cast => "cast ",
  36             ConstraintCategory::CallArgument => "argument ",
  37             ConstraintCategory::TypeAnnotation => "type annotation ",
  38             ConstraintCategory::ClosureBounds => "closure body ",
  39             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
  40             ConstraintCategory::CopyBound => "copying this value ",
  41             ConstraintCategory::OpaqueType => "opaque type ",
  42             ConstraintCategory::ClosureUpvar(_) => "closure capture ",
  43             ConstraintCategory::Usage => "this usage ",
  44             ConstraintCategory::Predicate(_)
  45             | ConstraintCategory::Boring
  46             | ConstraintCategory::BoringNoLocation
  47             | ConstraintCategory::Internal => "",
  48         }
  49     }
  50 }
  51
  52 /// A collection of errors encountered during region inference. This is needed to efficiently
  53 /// report errors after borrow checking.
  54 ///
  55 /// Usually we expect this to either be empty or contain a small number of items, so we can avoid
  56 /// allocation most of the time.
  57 crate type RegionErrors<'tcx> = Vec<RegionErrorKind<'tcx>>;
  58
  59 #[derive(Clone, Debug)]
  60 crate enum RegionErrorKind<'tcx> {
  61     /// A generic bound failure for a type test (`T: 'a`).
  62     TypeTestError { type_test: TypeTest<'tcx> },
  63
  64     /// An unexpected hidden region for an opaque type.
  65     UnexpectedHiddenRegion {
  66         /// The span for the member constraint.
  67         span: Span,
  68         /// The hidden type.
  69         hidden_ty: Ty<'tcx>,
  70         /// The unexpected region.
  71         member_region: ty::Region<'tcx>,
  72     },
  73
  74     /// Higher-ranked subtyping error.
  75     BoundUniversalRegionError {
  76         /// The placeholder free region.
  77         longer_fr: RegionVid,
  78         /// The region element that erroneously must be outlived by `longer_fr`.
  79         error_element: RegionElement,
  80         /// The placeholder region.
  81         placeholder: ty::PlaceholderRegion,
  82     },
  83
  84     /// Any other lifetime error.
  85     RegionError {
  86         /// The origin of the region.
  87         fr_origin: NllRegionVariableOrigin,
  88         /// The region that should outlive `shorter_fr`.
  89         longer_fr: RegionVid,
  90         /// The region that should be shorter, but we can't prove it.
  91         shorter_fr: RegionVid,
  92         /// Indicates whether this is a reported error. We currently only report the first error
  93         /// encountered and leave the rest unreported so as not to overwhelm the user.
  94         is_reported: bool,
  95     },
  96 }
  97
  98 /// Information about the various region constraints involved in a borrow checker error.
  99 #[derive(Clone, Debug)]
 100 pub struct ErrorConstraintInfo {
 101     // fr: outlived_fr
 102     pub(super) fr: RegionVid,
 103     pub(super) fr_is_local: bool,
 104     pub(super) outlived_fr: RegionVid,
 105     pub(super) outlived_fr_is_local: bool,
 106
 107     // Category and span for best blame constraint
 108     pub(super) category: ConstraintCategory,
 109     pub(super) span: Span,
 110 }
 111
 112 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 113     /// Converts a region inference variable into a `ty::Region` that
 114     /// we can use for error reporting. If `r` is universally bound,
 115     /// then we use the name that we have on record for it. If `r` is
 116     /// existentially bound, then we check its inferred value and try
 117     /// to find a good name from that. Returns `None` if we can't find
 118     /// one (e.g., this is just some random part of the CFG).
 119     pub(super) fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
 120         self.to_error_region_vid(r).and_then(|r| self.regioncx.region_definition(r).external_name)
 121     }
 122
 123     /// Returns the `RegionVid` corresponding to the region returned by
 124     /// `to_error_region`.
 125     pub(super) fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
 126         if self.regioncx.universal_regions().is_universal_region(r) {
 127             Some(r)
 128         } else {
 129             // We just want something nameable, even if it's not
 130             // actually an upper bound.
 131             let upper_bound = self.regioncx.approx_universal_upper_bound(r);
 132
 133             if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
 134                 self.to_error_region_vid(upper_bound)
 135             } else {
 136                 None
 137             }
 138         }
 139     }
 140
 141     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 142     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
 143         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr).as_deref() {
 144             if let ty::BoundRegionKind::BrEnv = free_region.bound_region {
 145                 if let DefiningTy::Closure(_, substs) =
 146                     self.regioncx.universal_regions().defining_ty
 147                 {
 148                     return substs.as_closure().kind() == ty::ClosureKind::FnMut;
 149                 }
 150             }
 151         }
 152
 153         false
 154     }
 155
 156     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
 157     pub(crate) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
 158         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
 159         // buffered in the `MirBorrowckCtxt`.
 160
 161         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();
 162
 163         for nll_error in nll_errors.into_iter() {
 164             match nll_error {
 165                 RegionErrorKind::TypeTestError { type_test } => {
 166                     // Try to convert the lower-bound region into something named we can print for the user.
 167                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 168
 169                     let type_test_span = type_test.locations.span(&self.body);
 170
 171                     if let Some(lower_bound_region) = lower_bound_region {
 172                         self.buffer_error(self.infcx.construct_generic_bound_failure(
 173                             type_test_span,
 174                             None,
 175                             type_test.generic_kind,
 176                             lower_bound_region,
 177                         ));
 178                     } else {
 179                         // FIXME. We should handle this case better. It
 180                         // indicates that we have e.g., some region variable
 181                         // whose value is like `'a+'b` where `'a` and `'b` are
 182                         // distinct unrelated univesal regions that are not
 183                         // known to outlive one another. It'd be nice to have
 184                         // some examples where this arises to decide how best
 185                         // to report it; we could probably handle it by
 186                         // iterating over the universal regions and reporting
 187                         // an error that multiple bounds are required.
 188                         self.buffer_error(self.infcx.tcx.sess.struct_span_err(
 189                             type_test_span,
 190                             &format!("`{}` does not live long enough", type_test.generic_kind),
 191                         ));
 192                     }
 193                 }
 194
 195                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
 196                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 197                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 198                     self.buffer_error(unexpected_hidden_region_diagnostic(
 199                         self.infcx.tcx,
 200                         span,
 201                         named_ty,
 202                         named_region,
 203                     ));
 204                 }
 205
 206                 RegionErrorKind::BoundUniversalRegionError {
 207                     longer_fr,
 208                     placeholder,
 209                     error_element,
 210                 } => {
 211                     let error_vid = self.regioncx.region_from_element(longer_fr, &error_element);
 212
 213                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 214                     let (_, cause) = self.regioncx.find_outlives_blame_span(
 215                         &self.body,
 216                         longer_fr,
 217                         NllRegionVariableOrigin::Placeholder(placeholder),
 218                         error_vid,
 219                     );
 220
 221                     let universe = placeholder.universe;
 222                     let universe_info = self.regioncx.universe_info(universe);
 223
 224                     universe_info.report_error(self, placeholder, error_element, cause);
 225                 }
 226
 227                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 228                     if is_reported {
 229                         self.report_region_error(
 230                             longer_fr,
 231                             fr_origin,
 232                             shorter_fr,
 233                             &mut outlives_suggestion,
 234                         );
 235                     } else {
 236                         // We only report the first error, so as not to overwhelm the user. See
 237                         // `RegRegionErrorKind` docs.
 238                         //
 239                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 240                         // FIXME: try collecting these constraints on the outlives suggestion
 241                         // builder. Does it make the suggestions any better?
 242                         debug!(
 243                             "Unreported region error: can't prove that {:?}: {:?}",
 244                             longer_fr, shorter_fr
 245                         );
 246                     }
 247                 }
 248             }
 249         }
 250
 251         // Emit one outlives suggestions for each MIR def we borrowck
 252         outlives_suggestion.add_suggestion(self);
 253     }
 254
 255     /// Report an error because the universal region `fr` was required to outlive
 256     /// `outlived_fr` but it is not known to do so. For example:
 257     ///
 258     /// ```
 259     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 260     /// ```
 261     ///
 262     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 263     pub(crate) fn report_region_error(
 264         &mut self,
 265         fr: RegionVid,
 266         fr_origin: NllRegionVariableOrigin,
 267         outlived_fr: RegionVid,
 268         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 269     ) {
 270         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 271
 272         let BlameConstraint { category, cause, variance_info, from_closure: _ } =
 273             self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
 274                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 275             });
 276
 277         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);
 278         // Check if we can use one of the "nice region errors".
 279         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 280             let nice = NiceRegionError::new_from_span(self.infcx, cause.span, o, f);
 281             if let Some(diag) = nice.try_report_from_nll() {
 282                 self.buffer_error(diag);
 283                 return;
 284             }
 285         }
 286
 287         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 288             self.regioncx.universal_regions().is_local_free_region(fr),
 289             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 290         );
 291
 292         debug!(
 293             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 294             fr_is_local, outlived_fr_is_local, category
 295         );
 296
 297         let errci = ErrorConstraintInfo {
 298             fr,
 299             outlived_fr,
 300             fr_is_local,
 301             outlived_fr_is_local,
 302             category,
 303             span: cause.span,
 304         };
 305
 306         let mut diag = match (category, fr_is_local, outlived_fr_is_local) {
 307             (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
 308                 self.report_fnmut_error(&errci, kind)
 309             }
 310             (ConstraintCategory::Assignment, true, false)
 311             | (ConstraintCategory::CallArgument, true, false) => {
 312                 let mut db = self.report_escaping_data_error(&errci);
 313
 314                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 315                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 316
 317                 db
 318             }
 319             _ => {
 320                 let mut db = self.report_general_error(&errci);
 321
 322                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 323                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 324
 325                 db
 326             }
 327         };
 328
 329         match variance_info {
 330             ty::VarianceDiagInfo::None => {}
 331             ty::VarianceDiagInfo::Invariant { ty, param_index } => {
 332                 let (desc, note) = match ty.kind() {
 333                     ty::RawPtr(ty_mut) => {
 334                         assert_eq!(ty_mut.mutbl, rustc_hir::Mutability::Mut);
 335                         (
 336                             format!("a mutable pointer to {}", ty_mut.ty),
 337                             "mutable pointers are invariant over their type parameter".to_string(),
 338                         )
 339                     }
 340                     ty::Ref(_, inner_ty, mutbl) => {
 341                         assert_eq!(*mutbl, rustc_hir::Mutability::Mut);
 342                         (
 343                             format!("a mutable reference to {}", inner_ty),
 344                             "mutable references are invariant over their type parameter"
 345                                 .to_string(),
 346                         )
 347                     }
 348                     ty::Adt(adt, substs) => {
 349                         let generic_arg = substs[param_index as usize];
 350                         let identity_substs =
 351                             InternalSubsts::identity_for_item(self.infcx.tcx, adt.did);
 352                         let base_ty = self.infcx.tcx.mk_adt(adt, identity_substs);
 353                         let base_generic_arg = identity_substs[param_index as usize];
 354                         let adt_desc = adt.descr();
 355
 356                         let desc = format!(
 357                             "the type {ty}, which makes the generic argument {generic_arg} invariant"
 358                         );
 359                         let note = format!(
 360                             "the {adt_desc} {base_ty} is invariant over the parameter {base_generic_arg}"
 361                         );
 362                         (desc, note)
 363                     }
 364                     _ => panic!("Unexpected type {:?}", ty),
 365                 };
 366                 diag.note(&format!("requirement occurs because of {desc}",));
 367                 diag.note(&note);
 368                 diag.help("see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance");
 369             }
 370         }
 371
 372         self.buffer_error(diag);
 373     }
 374
 375     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 376     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 377     ///
 378     /// ```text
 379     /// error: captured variable cannot escape `FnMut` closure body
 380     ///   --> $DIR/issue-53040.rs:15:8
 381     ///    |
 382     /// LL |     || &mut v;
 383     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 384     ///    |     |
 385     ///    |     inferred to be a `FnMut` closure
 386     ///    |
 387     ///    = note: `FnMut` closures only have access to their captured variables while they are
 388     ///            executing...
 389     ///    = note: ...therefore, returned references to captured variables will escape the closure
 390     /// ```
 391     fn report_fnmut_error(
 392         &self,
 393         errci: &ErrorConstraintInfo,
 394         kind: ReturnConstraint,
 395     ) -> DiagnosticBuilder<'tcx> {
 396         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 397
 398         let mut diag = self
 399             .infcx
 400             .tcx
 401             .sess
 402             .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
 403
 404         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 405         if let ty::Opaque(def_id, _) = *output_ty.kind() {
 406             output_ty = self.infcx.tcx.type_of(def_id)
 407         };
 408
 409         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 410
 411         let message = match output_ty.kind() {
 412             ty::Closure(_, _) => {
 413                 "returns a closure that contains a reference to a captured variable, which then \
 414                  escapes the closure body"
 415             }
 416             ty::Adt(def, _) if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did) => {
 417                 "returns an `async` block that contains a reference to a captured variable, which then \
 418                  escapes the closure body"
 419             }
 420             _ => "returns a reference to a captured variable which escapes the closure body",
 421         };
 422
 423         diag.span_label(*span, message);
 424
 425         if let ReturnConstraint::ClosureUpvar(upvar_field) = kind {
 426             let def_id = match self.regioncx.universal_regions().defining_ty {
 427                 DefiningTy::Closure(def_id, _) => def_id,
 428                 ty => bug!("unexpected DefiningTy {:?}", ty),
 429             };
 430
 431             let captured_place = &self.upvars[upvar_field.index()].place;
 432             let defined_hir = match captured_place.place.base {
 433                 PlaceBase::Local(hirid) => Some(hirid),
 434                 PlaceBase::Upvar(upvar) => Some(upvar.var_path.hir_id),
 435                 _ => None,
 436             };
 437
 438             if defined_hir.is_some() {
 439                 let upvars_map = self.infcx.tcx.upvars_mentioned(def_id).unwrap();
 440                 let upvar_def_span = self.infcx.tcx.hir().span(defined_hir.unwrap());
 441                 let upvar_span = upvars_map.get(&defined_hir.unwrap()).unwrap().span;
 442                 diag.span_label(upvar_def_span, "variable defined here");
 443                 diag.span_label(upvar_span, "variable captured here");
 444             }
 445         }
 446
 447         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
 448             diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 449         }
 450
 451         diag.note(
 452             "`FnMut` closures only have access to their captured variables while they are \
 453              executing...",
 454         );
 455         diag.note("...therefore, they cannot allow references to captured variables to escape");
 456
 457         diag
 458     }
 459
 460     /// Reports an error specifically for when data is escaping a closure.
 461     ///
 462     /// ```text
 463     /// error: borrowed data escapes outside of function
 464     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 465     ///    |
 466     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 467     ///    |              - `x` is a reference that is only valid in the function body
 468     /// LL |     // but ref_obj will not, so warn.
 469     /// LL |     ref_obj(x)
 470     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 471     /// ```
 472     fn report_escaping_data_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 473         let ErrorConstraintInfo { span, category, .. } = errci;
 474
 475         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 476             self.infcx.tcx,
 477             &self.body,
 478             &self.local_names,
 479             &self.upvars,
 480             errci.fr,
 481         );
 482         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 483             self.infcx.tcx,
 484             &self.body,
 485             &self.local_names,
 486             &self.upvars,
 487             errci.outlived_fr,
 488         );
 489
 490         let (_, escapes_from) = self
 491             .infcx
 492             .tcx
 493             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());
 494
 495         // Revert to the normal error in these cases.
 496         // Assignments aren't "escapes" in function items.
 497         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 498             || (*category == ConstraintCategory::Assignment
 499                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 500             || self.regioncx.universal_regions().defining_ty.is_const()
 501         {
 502             return self.report_general_error(&ErrorConstraintInfo {
 503                 fr_is_local: true,
 504                 outlived_fr_is_local: false,
 505                 ..*errci
 506             });
 507         }
 508
 509         let mut diag =
 510             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 511
 512         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 513             diag.span_label(
 514                 outlived_fr_span,
 515                 format!(
 516                     "`{}` declared here, outside of the {} body",
 517                     outlived_fr_name, escapes_from
 518                 ),
 519             );
 520         }
 521
 522         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 523             diag.span_label(
 524                 fr_span,
 525                 format!(
 526                     "`{}` is a reference that is only valid in the {} body",
 527                     fr_name, escapes_from
 528                 ),
 529             );
 530
 531             diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
 532         }
 533
 534         // Only show an extra note if we can find an 'error region' for both of the region
 535         // variables. This avoids showing a noisy note that just mentions 'synthetic' regions
 536         // that don't help the user understand the error.
 537         if self.to_error_region(errci.fr).is_some()
 538             && self.to_error_region(errci.outlived_fr).is_some()
 539         {
 540             let fr_region_name = self.give_region_a_name(errci.fr).unwrap();
 541             fr_region_name.highlight_region_name(&mut diag);
 542             let outlived_fr_region_name = self.give_region_a_name(errci.outlived_fr).unwrap();
 543             outlived_fr_region_name.highlight_region_name(&mut diag);
 544
 545             diag.span_label(
 546                 *span,
 547                 format!(
 548                     "{}requires that `{}` must outlive `{}`",
 549                     category.description(),
 550                     fr_region_name,
 551                     outlived_fr_region_name,
 552                 ),
 553             );
 554         }
 555         diag
 556     }
 557
 558     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 559     /// explain what is happening.
 560     ///
 561     /// ```text
 562     /// error: unsatisfied lifetime constraints
 563     ///   --> $DIR/regions-creating-enums3.rs:17:5
 564     ///    |
 565     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 566     ///    |                -- -- lifetime `'b` defined here
 567     ///    |                |
 568     ///    |                lifetime `'a` defined here
 569     /// LL |     ast::add(x, y)
 570     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 571     ///    |                    is returning data with lifetime `'b`
 572     /// ```
 573     fn report_general_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 574         let ErrorConstraintInfo {
 575             fr,
 576             fr_is_local,
 577             outlived_fr,
 578             outlived_fr_is_local,
 579             span,
 580             category,
 581             ..
 582         } = errci;
 583
 584         let mut diag =
 585             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
 586
 587         let (_, mir_def_name) =
 588             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());
 589
 590         let fr_name = self.give_region_a_name(*fr).unwrap();
 591         fr_name.highlight_region_name(&mut diag);
 592         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 593         outlived_fr_name.highlight_region_name(&mut diag);
 594
 595         match (category, outlived_fr_is_local, fr_is_local) {
 596             (ConstraintCategory::Return(_), true, _) => {
 597                 diag.span_label(
 598                     *span,
 599                     format!(
 600                         "{} was supposed to return data with lifetime `{}` but it is returning \
 601                          data with lifetime `{}`",
 602                         mir_def_name, outlived_fr_name, fr_name
 603                     ),
 604                 );
 605             }
 606             _ => {
 607                 diag.span_label(
 608                     *span,
 609                     format!(
 610                         "{}requires that `{}` must outlive `{}`",
 611                         category.description(),
 612                         fr_name,
 613                         outlived_fr_name,
 614                     ),
 615                 );
 616             }
 617         }
 618
 619         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 620
 621         diag
 622     }
 623
 624     /// Adds a suggestion to errors where an `impl Trait` is returned.
 625     ///
 626     /// ```text
 627     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 628     ///       a constraint
 629     ///    |
 630     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 631     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 632     /// ```
 633     fn add_static_impl_trait_suggestion(
 634         &self,
 635         diag: &mut DiagnosticBuilder<'tcx>,
 636         fr: RegionVid,
 637         // We need to pass `fr_name` - computing it again will label it twice.
 638         fr_name: RegionName,
 639         outlived_fr: RegionVid,
 640     ) {
 641         if let (Some(f), Some(ty::ReStatic)) =
 642             (self.to_error_region(fr), self.to_error_region(outlived_fr).as_deref())
 643         {
 644             if let Some(&ty::Opaque(did, substs)) = self
 645                 .infcx
 646                 .tcx
 647                 .is_suitable_region(f)
 648                 .map(|r| r.def_id)
 649                 .and_then(|id| self.infcx.tcx.return_type_impl_trait(id))
 650                 .map(|(ty, _)| ty.kind())
 651             {
 652                 // Check whether or not the impl trait return type is intended to capture
 653                 // data with the static lifetime.
 654                 //
 655                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 656                 let has_static_predicate = {
 657                     let bounds = self.infcx.tcx.explicit_item_bounds(did);
 658
 659                     let mut found = false;
 660                     for (bound, _) in bounds {
 661                         if let ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(_, r)) =
 662                             bound.kind().skip_binder()
 663                         {
 664                             let r = r.subst(self.infcx.tcx, substs);
 665                             if r.is_static() {
 666                                 found = true;
 667                                 break;
 668                             } else {
 669                                 // If there's already a lifetime bound, don't
 670                                 // suggest anything.
 671                                 return;
 672                             }
 673                         }
 674                     }
 675
 676                     found
 677                 };
 678
 679                 debug!(
 680                     "add_static_impl_trait_suggestion: has_static_predicate={:?}",
 681                     has_static_predicate
 682                 );
 683                 let static_str = kw::StaticLifetime;
 684                 // If there is a static predicate, then the only sensible suggestion is to replace
 685                 // fr with `'static`.
 686                 if has_static_predicate {
 687                     diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
 688                 } else {
 689                     // Otherwise, we should suggest adding a constraint on the return type.
 690                     let span = self.infcx.tcx.def_span(did);
 691                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 692                         let suggestable_fr_name = if fr_name.was_named() {
 693                             fr_name.to_string()
 694                         } else {
 695                             "'_".to_string()
 696                         };
 697                         let span = if snippet.ends_with(';') {
 698                             // `type X = impl Trait;`
 699                             span.with_hi(span.hi() - BytePos(1))
 700                         } else {
 701                             span
 702                         };
 703                         let suggestion = format!(" + {}", suggestable_fr_name);
 704                         let span = span.shrink_to_hi();
 705                         diag.span_suggestion(
 706                             span,
 707                             &format!(
 708                                 "to allow this `impl Trait` to capture borrowed data with lifetime \
 709                                  `{}`, add `{}` as a bound",
 710                                 fr_name, suggestable_fr_name,
 711                             ),
 712                             suggestion,
 713                             Applicability::MachineApplicable,
 714                         );
 715                     }
 716                 }
 717             }
 718         }
 719     }
 720 }