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