compiler/rustc_borrowck/src/diagnostics/region_errors.rs

   1 //! Error reporting machinery for lifetime errors.
   2
   3 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
   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             && let ty::BoundRegionKind::BrEnv = free_region.bound_region
 145             && let DefiningTy::Closure(_, substs) = self.regioncx.universal_regions().defining_ty
 146         {
 147             return substs.as_closure().kind() == ty::ClosureKind::FnMut;
 148         }
 149
 150         false
 151     }
 152
 153     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
 154     pub(crate) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
 155         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
 156         // buffered in the `MirBorrowckCtxt`.
 157
 158         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();
 159
 160         for nll_error in nll_errors.into_iter() {
 161             match nll_error {
 162                 RegionErrorKind::TypeTestError { type_test } => {
 163                     // Try to convert the lower-bound region into something named we can print for the user.
 164                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 165
 166                     let type_test_span = type_test.locations.span(&self.body);
 167
 168                     if let Some(lower_bound_region) = lower_bound_region {
 169                         self.buffer_error(self.infcx.construct_generic_bound_failure(
 170                             type_test_span,
 171                             None,
 172                             type_test.generic_kind,
 173                             lower_bound_region,
 174                         ));
 175                     } else {
 176                         // FIXME. We should handle this case better. It
 177                         // indicates that we have e.g., some region variable
 178                         // whose value is like `'a+'b` where `'a` and `'b` are
 179                         // distinct unrelated univesal regions that are not
 180                         // known to outlive one another. It'd be nice to have
 181                         // some examples where this arises to decide how best
 182                         // to report it; we could probably handle it by
 183                         // iterating over the universal regions and reporting
 184                         // an error that multiple bounds are required.
 185                         self.buffer_error(self.infcx.tcx.sess.struct_span_err(
 186                             type_test_span,
 187                             &format!("`{}` does not live long enough", type_test.generic_kind),
 188                         ));
 189                     }
 190                 }
 191
 192                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
 193                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 194                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 195                     self.buffer_error(unexpected_hidden_region_diagnostic(
 196                         self.infcx.tcx,
 197                         span,
 198                         named_ty,
 199                         named_region,
 200                     ));
 201                 }
 202
 203                 RegionErrorKind::BoundUniversalRegionError {
 204                     longer_fr,
 205                     placeholder,
 206                     error_element,
 207                 } => {
 208                     let error_vid = self.regioncx.region_from_element(longer_fr, &error_element);
 209
 210                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 211                     let (_, cause) = self.regioncx.find_outlives_blame_span(
 212                         &self.body,
 213                         longer_fr,
 214                         NllRegionVariableOrigin::Placeholder(placeholder),
 215                         error_vid,
 216                     );
 217
 218                     let universe = placeholder.universe;
 219                     let universe_info = self.regioncx.universe_info(universe);
 220
 221                     universe_info.report_error(self, placeholder, error_element, cause);
 222                 }
 223
 224                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 225                     if is_reported {
 226                         self.report_region_error(
 227                             longer_fr,
 228                             fr_origin,
 229                             shorter_fr,
 230                             &mut outlives_suggestion,
 231                         );
 232                     } else {
 233                         // We only report the first error, so as not to overwhelm the user. See
 234                         // `RegRegionErrorKind` docs.
 235                         //
 236                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 237                         // FIXME: try collecting these constraints on the outlives suggestion
 238                         // builder. Does it make the suggestions any better?
 239                         debug!(
 240                             "Unreported region error: can't prove that {:?}: {:?}",
 241                             longer_fr, shorter_fr
 242                         );
 243                     }
 244                 }
 245             }
 246         }
 247
 248         // Emit one outlives suggestions for each MIR def we borrowck
 249         outlives_suggestion.add_suggestion(self);
 250     }
 251
 252     /// Report an error because the universal region `fr` was required to outlive
 253     /// `outlived_fr` but it is not known to do so. For example:
 254     ///
 255     /// ```
 256     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 257     /// ```
 258     ///
 259     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 260     pub(crate) fn report_region_error(
 261         &mut self,
 262         fr: RegionVid,
 263         fr_origin: NllRegionVariableOrigin,
 264         outlived_fr: RegionVid,
 265         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 266     ) {
 267         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 268
 269         let BlameConstraint { category, cause, variance_info, from_closure: _ } =
 270             self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
 271                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 272             });
 273
 274         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);
 275         // Check if we can use one of the "nice region errors".
 276         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 277             let nice = NiceRegionError::new_from_span(self.infcx, cause.span, o, f);
 278             if let Some(diag) = nice.try_report_from_nll() {
 279                 self.buffer_error(diag);
 280                 return;
 281             }
 282         }
 283
 284         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 285             self.regioncx.universal_regions().is_local_free_region(fr),
 286             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 287         );
 288
 289         debug!(
 290             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 291             fr_is_local, outlived_fr_is_local, category
 292         );
 293
 294         let errci = ErrorConstraintInfo {
 295             fr,
 296             outlived_fr,
 297             fr_is_local,
 298             outlived_fr_is_local,
 299             category,
 300             span: cause.span,
 301         };
 302
 303         let mut diag = match (category, fr_is_local, outlived_fr_is_local) {
 304             (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
 305                 self.report_fnmut_error(&errci, kind)
 306             }
 307             (ConstraintCategory::Assignment, true, false)
 308             | (ConstraintCategory::CallArgument, true, false) => {
 309                 let mut db = self.report_escaping_data_error(&errci);
 310
 311                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 312                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 313
 314                 db
 315             }
 316             _ => {
 317                 let mut db = self.report_general_error(&errci);
 318
 319                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 320                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 321
 322                 db
 323             }
 324         };
 325
 326         match variance_info {
 327             ty::VarianceDiagInfo::None => {}
 328             ty::VarianceDiagInfo::Invariant { ty, param_index } => {
 329                 let (desc, note) = match ty.kind() {
 330                     ty::RawPtr(ty_mut) => {
 331                         assert_eq!(ty_mut.mutbl, rustc_hir::Mutability::Mut);
 332                         (
 333                             format!("a mutable pointer to {}", ty_mut.ty),
 334                             "mutable pointers are invariant over their type parameter".to_string(),
 335                         )
 336                     }
 337                     ty::Ref(_, inner_ty, mutbl) => {
 338                         assert_eq!(*mutbl, rustc_hir::Mutability::Mut);
 339                         (
 340                             format!("a mutable reference to {}", inner_ty),
 341                             "mutable references are invariant over their type parameter"
 342                                 .to_string(),
 343                         )
 344                     }
 345                     ty::Adt(adt, substs) => {
 346                         let generic_arg = substs[param_index as usize];
 347                         let identity_substs =
 348                             InternalSubsts::identity_for_item(self.infcx.tcx, adt.did());
 349                         let base_ty = self.infcx.tcx.mk_adt(*adt, identity_substs);
 350                         let base_generic_arg = identity_substs[param_index as usize];
 351                         let adt_desc = adt.descr();
 352
 353                         let desc = format!(
 354                             "the type {ty}, which makes the generic argument {generic_arg} invariant"
 355                         );
 356                         let note = format!(
 357                             "the {adt_desc} {base_ty} is invariant over the parameter {base_generic_arg}"
 358                         );
 359                         (desc, note)
 360                     }
 361                     _ => panic!("Unexpected type {:?}", ty),
 362                 };
 363                 diag.note(&format!("requirement occurs because of {desc}",));
 364                 diag.note(&note);
 365                 diag.help("see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance");
 366             }
 367         }
 368
 369         self.buffer_error(diag);
 370     }
 371
 372     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 373     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 374     ///
 375     /// ```text
 376     /// error: captured variable cannot escape `FnMut` closure body
 377     ///   --> $DIR/issue-53040.rs:15:8
 378     ///    |
 379     /// LL |     || &mut v;
 380     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 381     ///    |     |
 382     ///    |     inferred to be a `FnMut` closure
 383     ///    |
 384     ///    = note: `FnMut` closures only have access to their captured variables while they are
 385     ///            executing...
 386     ///    = note: ...therefore, returned references to captured variables will escape the closure
 387     /// ```
 388     fn report_fnmut_error(
 389         &self,
 390         errci: &ErrorConstraintInfo,
 391         kind: ReturnConstraint,
 392     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 393         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 394
 395         let mut diag = self
 396             .infcx
 397             .tcx
 398             .sess
 399             .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
 400
 401         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 402         if let ty::Opaque(def_id, _) = *output_ty.kind() {
 403             output_ty = self.infcx.tcx.type_of(def_id)
 404         };
 405
 406         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 407
 408         let message = match output_ty.kind() {
 409             ty::Closure(_, _) => {
 410                 "returns a closure that contains a reference to a captured variable, which then \
 411                  escapes the closure body"
 412             }
 413             ty::Adt(def, _) if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did()) => {
 414                 "returns an `async` block that contains a reference to a captured variable, which then \
 415                  escapes the closure body"
 416             }
 417             _ => "returns a reference to a captured variable which escapes the closure body",
 418         };
 419
 420         diag.span_label(*span, message);
 421
 422         if let ReturnConstraint::ClosureUpvar(upvar_field) = kind {
 423             let def_id = match self.regioncx.universal_regions().defining_ty {
 424                 DefiningTy::Closure(def_id, _) => def_id,
 425                 ty => bug!("unexpected DefiningTy {:?}", ty),
 426             };
 427
 428             let captured_place = &self.upvars[upvar_field.index()].place;
 429             let defined_hir = match captured_place.place.base {
 430                 PlaceBase::Local(hirid) => Some(hirid),
 431                 PlaceBase::Upvar(upvar) => Some(upvar.var_path.hir_id),
 432                 _ => None,
 433             };
 434
 435             if defined_hir.is_some() {
 436                 let upvars_map = self.infcx.tcx.upvars_mentioned(def_id).unwrap();
 437                 let upvar_def_span = self.infcx.tcx.hir().span(defined_hir.unwrap());
 438                 let upvar_span = upvars_map.get(&defined_hir.unwrap()).unwrap().span;
 439                 diag.span_label(upvar_def_span, "variable defined here");
 440                 diag.span_label(upvar_span, "variable captured here");
 441             }
 442         }
 443
 444         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
 445             diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 446         }
 447
 448         diag.note(
 449             "`FnMut` closures only have access to their captured variables while they are \
 450              executing...",
 451         );
 452         diag.note("...therefore, they cannot allow references to captured variables to escape");
 453
 454         diag
 455     }
 456
 457     /// Reports an error specifically for when data is escaping a closure.
 458     ///
 459     /// ```text
 460     /// error: borrowed data escapes outside of function
 461     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 462     ///    |
 463     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 464     ///    |              - `x` is a reference that is only valid in the function body
 465     /// LL |     // but ref_obj will not, so warn.
 466     /// LL |     ref_obj(x)
 467     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 468     /// ```
 469     fn report_escaping_data_error(
 470         &self,
 471         errci: &ErrorConstraintInfo,
 472     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 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(
 574         &self,
 575         errci: &ErrorConstraintInfo,
 576     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 577         let ErrorConstraintInfo {
 578             fr,
 579             fr_is_local,
 580             outlived_fr,
 581             outlived_fr_is_local,
 582             span,
 583             category,
 584             ..
 585         } = errci;
 586
 587         let mut diag =
 588             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
 589
 590         let (_, mir_def_name) =
 591             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());
 592
 593         let fr_name = self.give_region_a_name(*fr).unwrap();
 594         fr_name.highlight_region_name(&mut diag);
 595         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 596         outlived_fr_name.highlight_region_name(&mut diag);
 597
 598         match (category, outlived_fr_is_local, fr_is_local) {
 599             (ConstraintCategory::Return(_), true, _) => {
 600                 diag.span_label(
 601                     *span,
 602                     format!(
 603                         "{} was supposed to return data with lifetime `{}` but it is returning \
 604                          data with lifetime `{}`",
 605                         mir_def_name, outlived_fr_name, fr_name
 606                     ),
 607                 );
 608             }
 609             _ => {
 610                 diag.span_label(
 611                     *span,
 612                     format!(
 613                         "{}requires that `{}` must outlive `{}`",
 614                         category.description(),
 615                         fr_name,
 616                         outlived_fr_name,
 617                     ),
 618                 );
 619             }
 620         }
 621
 622         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 623
 624         diag
 625     }
 626
 627     /// Adds a suggestion to errors where an `impl Trait` is returned.
 628     ///
 629     /// ```text
 630     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 631     ///       a constraint
 632     ///    |
 633     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 634     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 635     /// ```
 636     fn add_static_impl_trait_suggestion(
 637         &self,
 638         diag: &mut Diagnostic,
 639         fr: RegionVid,
 640         // We need to pass `fr_name` - computing it again will label it twice.
 641         fr_name: RegionName,
 642         outlived_fr: RegionVid,
 643     ) {
 644         if let (Some(f), Some(ty::ReStatic)) =
 645             (self.to_error_region(fr), self.to_error_region(outlived_fr).as_deref())
 646         {
 647             if let Some(&ty::Opaque(did, substs)) = self
 648                 .infcx
 649                 .tcx
 650                 .is_suitable_region(f)
 651                 .map(|r| r.def_id)
 652                 .and_then(|id| self.infcx.tcx.return_type_impl_trait(id))
 653                 .map(|(ty, _)| ty.kind())
 654             {
 655                 // Check whether or not the impl trait return type is intended to capture
 656                 // data with the static lifetime.
 657                 //
 658                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 659                 let has_static_predicate = {
 660                     let bounds = self.infcx.tcx.explicit_item_bounds(did);
 661
 662                     let mut found = false;
 663                     for (bound, _) in bounds {
 664                         if let ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(_, r)) =
 665                             bound.kind().skip_binder()
 666                         {
 667                             let r = r.subst(self.infcx.tcx, substs);
 668                             if r.is_static() {
 669                                 found = true;
 670                                 break;
 671                             } else {
 672                                 // If there's already a lifetime bound, don't
 673                                 // suggest anything.
 674                                 return;
 675                             }
 676                         }
 677                     }
 678
 679                     found
 680                 };
 681
 682                 debug!(
 683                     "add_static_impl_trait_suggestion: has_static_predicate={:?}",
 684                     has_static_predicate
 685                 );
 686                 let static_str = kw::StaticLifetime;
 687                 // If there is a static predicate, then the only sensible suggestion is to replace
 688                 // fr with `'static`.
 689                 if has_static_predicate {
 690                     diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
 691                 } else {
 692                     // Otherwise, we should suggest adding a constraint on the return type.
 693                     let span = self.infcx.tcx.def_span(did);
 694                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 695                         let suggestable_fr_name = if fr_name.was_named() {
 696                             fr_name.to_string()
 697                         } else {
 698                             "'_".to_string()
 699                         };
 700                         let span = if snippet.ends_with(';') {
 701                             // `type X = impl Trait;`
 702                             span.with_hi(span.hi() - BytePos(1))
 703                         } else {
 704                             span
 705                         };
 706                         let suggestion = format!(" + {}", suggestable_fr_name);
 707                         let span = span.shrink_to_hi();
 708                         diag.span_suggestion(
 709                             span,
 710                             &format!(
 711                                 "to allow this `impl Trait` to capture borrowed data with lifetime \
 712                                  `{}`, add `{}` as a bound",
 713                                 fr_name, suggestable_fr_name,
 714                             ),
 715                             suggestion,
 716                             Applicability::MachineApplicable,
 717                         );
 718                     }
 719                 }
 720             }
 721         }
 722     }
 723 }