src/librustc_mir/borrow_check/diagnostics/region_errors.rs

   1 //! Error reporting machinery for lifetime errors.
   2
   3 use rustc::mir::ConstraintCategory;
   4 use rustc::ty::{self, RegionVid, Ty};
   5 use rustc_errors::{Applicability, DiagnosticBuilder};
   6 use rustc_infer::infer::{
   7     error_reporting::nice_region_error::NiceRegionError,
   8     error_reporting::unexpected_hidden_region_diagnostic, NLLRegionVariableOrigin,
   9 };
  10 use rustc_span::symbol::kw;
  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::Boring
  41             | ConstraintCategory::BoringNoLocation
  42             | ConstraintCategory::Internal => "",
  43         }
  44     }
  45 }
  46
  47 /// A collection of errors encountered during region inference. This is needed to efficiently
  48 /// report errors after borrow checking.
  49 ///
  50 /// Usually we expect this to either be empty or contain a small number of items, so we can avoid
  51 /// allocation most of the time.
  52 crate type RegionErrors<'tcx> = Vec<RegionErrorKind<'tcx>>;
  53
  54 #[derive(Clone, Debug)]
  55 crate enum RegionErrorKind<'tcx> {
  56     /// A generic bound failure for a type test (`T: 'a`).
  57     TypeTestError { type_test: TypeTest<'tcx> },
  58
  59     /// An unexpected hidden region for an opaque type.
  60     UnexpectedHiddenRegion {
  61         /// The span for the member constraint.
  62         span: Span,
  63         /// The hidden type.
  64         hidden_ty: Ty<'tcx>,
  65         /// The unexpected region.
  66         member_region: ty::Region<'tcx>,
  67     },
  68
  69     /// Higher-ranked subtyping error.
  70     BoundUniversalRegionError {
  71         /// The placeholder free region.
  72         longer_fr: RegionVid,
  73         /// The region element that erroneously must be outlived by `longer_fr`.
  74         error_element: RegionElement,
  75         /// The origin of the placeholder region.
  76         fr_origin: NLLRegionVariableOrigin,
  77     },
  78
  79     /// Any other lifetime error.
  80     RegionError {
  81         /// The origin of the region.
  82         fr_origin: NLLRegionVariableOrigin,
  83         /// The region that should outlive `shorter_fr`.
  84         longer_fr: RegionVid,
  85         /// The region that should be shorter, but we can't prove it.
  86         shorter_fr: RegionVid,
  87         /// Indicates whether this is a reported error. We currently only report the first error
  88         /// encountered and leave the rest unreported so as not to overwhelm the user.
  89         is_reported: bool,
  90     },
  91 }
  92
  93 /// Information about the various region constraints involved in a borrow checker error.
  94 #[derive(Clone, Debug)]
  95 pub struct ErrorConstraintInfo {
  96     // fr: outlived_fr
  97     pub(super) fr: RegionVid,
  98     pub(super) fr_is_local: bool,
  99     pub(super) outlived_fr: RegionVid,
 100     pub(super) outlived_fr_is_local: bool,
 101
 102     // Category and span for best blame constraint
 103     pub(super) category: ConstraintCategory,
 104     pub(super) span: Span,
 105 }
 106
 107 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 108     /// Converts a region inference variable into a `ty::Region` that
 109     /// we can use for error reporting. If `r` is universally bound,
 110     /// then we use the name that we have on record for it. If `r` is
 111     /// existentially bound, then we check its inferred value and try
 112     /// to find a good name from that. Returns `None` if we can't find
 113     /// one (e.g., this is just some random part of the CFG).
 114     pub(super) fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
 115         self.to_error_region_vid(r).and_then(|r| self.regioncx.region_definition(r).external_name)
 116     }
 117
 118     /// Returns the `RegionVid` corresponding to the region returned by
 119     /// `to_error_region`.
 120     pub(super) fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
 121         if self.regioncx.universal_regions().is_universal_region(r) {
 122             Some(r)
 123         } else {
 124             let upper_bound = self.regioncx.universal_upper_bound(r);
 125
 126             if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
 127                 self.to_error_region_vid(upper_bound)
 128             } else {
 129                 None
 130             }
 131         }
 132     }
 133
 134     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 135     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
 136         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
 137             if let ty::BoundRegion::BrEnv = free_region.bound_region {
 138                 if let DefiningTy::Closure(def_id, substs) =
 139                     self.regioncx.universal_regions().defining_ty
 140                 {
 141                     return substs.as_closure().kind(def_id, self.infcx.tcx)
 142                         == 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                         let region_scope_tree = &self.infcx.tcx.region_scope_tree(self.mir_def_id);
 167                         self.infcx
 168                             .construct_generic_bound_failure(
 169                                 region_scope_tree,
 170                                 type_test_span,
 171                                 None,
 172                                 type_test.generic_kind,
 173                                 lower_bound_region,
 174                             )
 175                             .buffer(&mut self.errors_buffer);
 176                     } else {
 177                         // FIXME. We should handle this case better. It
 178                         // indicates that we have e.g., some region variable
 179                         // whose value is like `'a+'b` where `'a` and `'b` are
 180                         // distinct unrelated univesal regions that are not
 181                         // known to outlive one another. It'd be nice to have
 182                         // some examples where this arises to decide how best
 183                         // to report it; we could probably handle it by
 184                         // iterating over the universal regions and reporting
 185                         // an error that multiple bounds are required.
 186                         self.infcx
 187                             .tcx
 188                             .sess
 189                             .struct_span_err(
 190                                 type_test_span,
 191                                 &format!("`{}` does not live long enough", type_test.generic_kind),
 192                             )
 193                             .buffer(&mut self.errors_buffer);
 194                     }
 195                 }
 196
 197                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
 198                     let region_scope_tree = &self.infcx.tcx.region_scope_tree(self.mir_def_id);
 199                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 200                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 201                     unexpected_hidden_region_diagnostic(
 202                         self.infcx.tcx,
 203                         Some(region_scope_tree),
 204                         span,
 205                         named_ty,
 206                         named_region,
 207                     )
 208                     .buffer(&mut self.errors_buffer);
 209                 }
 210
 211                 RegionErrorKind::BoundUniversalRegionError {
 212                     longer_fr,
 213                     fr_origin,
 214                     error_element,
 215                 } => {
 216                     let error_region = self.regioncx.region_from_element(longer_fr, error_element);
 217
 218                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 219                     let (_, span) = self.regioncx.find_outlives_blame_span(
 220                         &self.body,
 221                         longer_fr,
 222                         fr_origin,
 223                         error_region,
 224                     );
 225
 226                     // FIXME: improve this error message
 227                     self.infcx
 228                         .tcx
 229                         .sess
 230                         .struct_span_err(span, "higher-ranked subtype error")
 231                         .buffer(&mut self.errors_buffer);
 232                 }
 233
 234                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 235                     if is_reported {
 236                         self.report_region_error(
 237                             longer_fr,
 238                             fr_origin,
 239                             shorter_fr,
 240                             &mut outlives_suggestion,
 241                         );
 242                     } else {
 243                         // We only report the first error, so as not to overwhelm the user. See
 244                         // `RegRegionErrorKind` docs.
 245                         //
 246                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 247                         // FIXME: try collecting these constraints on the outlives suggestion
 248                         // builder. Does it make the suggestions any better?
 249                         debug!(
 250                             "Unreported region error: can't prove that {:?}: {:?}",
 251                             longer_fr, shorter_fr
 252                         );
 253                     }
 254                 }
 255             }
 256         }
 257
 258         // Emit one outlives suggestions for each MIR def we borrowck
 259         outlives_suggestion.add_suggestion(self);
 260     }
 261
 262     /// Report an error because the universal region `fr` was required to outlive
 263     /// `outlived_fr` but it is not known to do so. For example:
 264     ///
 265     /// ```
 266     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 267     /// ```
 268     ///
 269     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 270     pub(in crate::borrow_check) fn report_region_error(
 271         &mut self,
 272         fr: RegionVid,
 273         fr_origin: NLLRegionVariableOrigin,
 274         outlived_fr: RegionVid,
 275         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 276     ) {
 277         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 278
 279         let (category, _, span) =
 280             self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
 281                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 282             });
 283
 284         debug!("report_region_error: category={:?} {:?}", category, span);
 285         // Check if we can use one of the "nice region errors".
 286         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 287             let nice = NiceRegionError::new_from_span(self.infcx, span, o, f);
 288             if let Some(diag) = nice.try_report_from_nll() {
 289                 diag.buffer(&mut self.errors_buffer);
 290                 return;
 291             }
 292         }
 293
 294         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 295             self.regioncx.universal_regions().is_local_free_region(fr),
 296             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 297         );
 298
 299         debug!(
 300             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 301             fr_is_local, outlived_fr_is_local, category
 302         );
 303
 304         let errci = ErrorConstraintInfo {
 305             fr,
 306             outlived_fr,
 307             fr_is_local,
 308             outlived_fr_is_local,
 309             category,
 310             span,
 311         };
 312
 313         let diag = match (category, fr_is_local, outlived_fr_is_local) {
 314             (ConstraintCategory::Return, true, false) if self.is_closure_fn_mut(fr) => {
 315                 self.report_fnmut_error(&errci)
 316             }
 317             (ConstraintCategory::Assignment, true, false)
 318             | (ConstraintCategory::CallArgument, true, false) => {
 319                 let mut db = self.report_escaping_data_error(&errci);
 320
 321                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 322                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 323
 324                 db
 325             }
 326             _ => {
 327                 let mut db = self.report_general_error(&errci);
 328
 329                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 330                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 331
 332                 db
 333             }
 334         };
 335
 336         diag.buffer(&mut self.errors_buffer);
 337     }
 338
 339     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 340     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 341     ///
 342     /// ```text
 343     /// error: captured variable cannot escape `FnMut` closure body
 344     ///   --> $DIR/issue-53040.rs:15:8
 345     ///    |
 346     /// LL |     || &mut v;
 347     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 348     ///    |     |
 349     ///    |     inferred to be a `FnMut` closure
 350     ///    |
 351     ///    = note: `FnMut` closures only have access to their captured variables while they are
 352     ///            executing...
 353     ///    = note: ...therefore, returned references to captured variables will escape the closure
 354     /// ```
 355     fn report_fnmut_error(&self, errci: &ErrorConstraintInfo) -> 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         // We should check if the return type of this closure is in fact a closure - in that
 365         // case, we can special case the error further.
 366         let return_type_is_closure =
 367             self.regioncx.universal_regions().unnormalized_output_ty.is_closure();
 368         let message = if return_type_is_closure {
 369             "returns a closure that contains a reference to a captured variable, which then \
 370              escapes the closure body"
 371         } else {
 372             "returns a reference to a captured variable which escapes the closure body"
 373         };
 374
 375         diag.span_label(*span, message);
 376
 377         match self.give_region_a_name(*outlived_fr).unwrap().source {
 378             RegionNameSource::NamedEarlyBoundRegion(fr_span)
 379             | RegionNameSource::NamedFreeRegion(fr_span)
 380             | RegionNameSource::SynthesizedFreeEnvRegion(fr_span, _)
 381             | RegionNameSource::CannotMatchHirTy(fr_span, _)
 382             | RegionNameSource::MatchedHirTy(fr_span)
 383             | RegionNameSource::MatchedAdtAndSegment(fr_span)
 384             | RegionNameSource::AnonRegionFromUpvar(fr_span, _)
 385             | RegionNameSource::AnonRegionFromOutput(fr_span, _, _) => {
 386                 diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 387             }
 388             _ => {}
 389         }
 390
 391         diag.note(
 392             "`FnMut` closures only have access to their captured variables while they are \
 393              executing...",
 394         );
 395         diag.note("...therefore, they cannot allow references to captured variables to escape");
 396
 397         diag
 398     }
 399
 400     /// Reports a error specifically for when data is escaping a closure.
 401     ///
 402     /// ```text
 403     /// error: borrowed data escapes outside of function
 404     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 405     ///    |
 406     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 407     ///    |              - `x` is a reference that is only valid in the function body
 408     /// LL |     // but ref_obj will not, so warn.
 409     /// LL |     ref_obj(x)
 410     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 411     /// ```
 412     fn report_escaping_data_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 413         let ErrorConstraintInfo { span, category, .. } = errci;
 414
 415         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 416             self.infcx.tcx,
 417             &self.body,
 418             &self.local_names,
 419             &self.upvars,
 420             errci.fr,
 421         );
 422         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 423             self.infcx.tcx,
 424             &self.body,
 425             &self.local_names,
 426             &self.upvars,
 427             errci.outlived_fr,
 428         );
 429
 430         let (_, escapes_from) = self
 431             .infcx
 432             .tcx
 433             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());
 434
 435         // Revert to the normal error in these cases.
 436         // Assignments aren't "escapes" in function items.
 437         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 438             || (*category == ConstraintCategory::Assignment
 439                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 440             || self.regioncx.universal_regions().defining_ty.is_const()
 441         {
 442             return self.report_general_error(&ErrorConstraintInfo {
 443                 fr_is_local: true,
 444                 outlived_fr_is_local: false,
 445                 ..*errci
 446             });
 447         }
 448
 449         let mut diag =
 450             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 451
 452         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 453             diag.span_label(
 454                 outlived_fr_span,
 455                 format!(
 456                     "`{}` declared here, outside of the {} body",
 457                     outlived_fr_name, escapes_from
 458                 ),
 459             );
 460         }
 461
 462         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 463             diag.span_label(
 464                 fr_span,
 465                 format!(
 466                     "`{}` is a reference that is only valid in the {} body",
 467                     fr_name, escapes_from
 468                 ),
 469             );
 470
 471             diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
 472         }
 473
 474         diag
 475     }
 476
 477     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 478     /// explain what is happening.
 479     ///
 480     /// ```text
 481     /// error: unsatisfied lifetime constraints
 482     ///   --> $DIR/regions-creating-enums3.rs:17:5
 483     ///    |
 484     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 485     ///    |                -- -- lifetime `'b` defined here
 486     ///    |                |
 487     ///    |                lifetime `'a` defined here
 488     /// LL |     ast::add(x, y)
 489     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 490     ///    |                    is returning data with lifetime `'b`
 491     /// ```
 492     fn report_general_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 493         let ErrorConstraintInfo {
 494             fr,
 495             fr_is_local,
 496             outlived_fr,
 497             outlived_fr_is_local,
 498             span,
 499             category,
 500             ..
 501         } = errci;
 502
 503         let mut diag =
 504             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
 505
 506         let (_, mir_def_name) = self.infcx.tcx.article_and_description(self.mir_def_id);
 507
 508         let fr_name = self.give_region_a_name(*fr).unwrap();
 509         fr_name.highlight_region_name(&mut diag);
 510         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 511         outlived_fr_name.highlight_region_name(&mut diag);
 512
 513         match (category, outlived_fr_is_local, fr_is_local) {
 514             (ConstraintCategory::Return, true, _) => {
 515                 diag.span_label(
 516                     *span,
 517                     format!(
 518                         "{} was supposed to return data with lifetime `{}` but it is returning \
 519                          data with lifetime `{}`",
 520                         mir_def_name, outlived_fr_name, fr_name
 521                     ),
 522                 );
 523             }
 524             _ => {
 525                 diag.span_label(
 526                     *span,
 527                     format!(
 528                         "{}requires that `{}` must outlive `{}`",
 529                         category.description(),
 530                         fr_name,
 531                         outlived_fr_name,
 532                     ),
 533                 );
 534             }
 535         }
 536
 537         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 538
 539         diag
 540     }
 541
 542     /// Adds a suggestion to errors where a `impl Trait` is returned.
 543     ///
 544     /// ```text
 545     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 546     ///       a constraint
 547     ///    |
 548     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 549     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 550     /// ```
 551     fn add_static_impl_trait_suggestion(
 552         &self,
 553         diag: &mut DiagnosticBuilder<'tcx>,
 554         fr: RegionVid,
 555         // We need to pass `fr_name` - computing it again will label it twice.
 556         fr_name: RegionName,
 557         outlived_fr: RegionVid,
 558     ) {
 559         if let (Some(f), Some(ty::RegionKind::ReStatic)) =
 560             (self.to_error_region(fr), self.to_error_region(outlived_fr))
 561         {
 562             if let Some((ty::TyS { kind: ty::Opaque(did, substs), .. }, _)) = self
 563                 .infcx
 564                 .tcx
 565                 .is_suitable_region(f)
 566                 .map(|r| r.def_id)
 567                 .map(|id| self.infcx.tcx.return_type_impl_trait(id))
 568                 .unwrap_or(None)
 569             {
 570                 // Check whether or not the impl trait return type is intended to capture
 571                 // data with the static lifetime.
 572                 //
 573                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 574                 let has_static_predicate = {
 575                     let predicates_of = self.infcx.tcx.predicates_of(*did);
 576                     let bounds = predicates_of.instantiate(self.infcx.tcx, substs);
 577
 578                     let mut found = false;
 579                     for predicate in bounds.predicates {
 580                         if let ty::Predicate::TypeOutlives(binder) = predicate {
 581                             if let ty::OutlivesPredicate(_, ty::RegionKind::ReStatic) =
 582                                 binder.skip_binder()
 583                             {
 584                                 found = true;
 585                                 break;
 586                             } else {
 587                                 // If there's already a lifetime bound, don't
 588                                 // suggest anything.
 589                                 return;
 590                             }
 591                         }
 592                     }
 593
 594                     found
 595                 };
 596
 597                 debug!(
 598                     "add_static_impl_trait_suggestion: has_static_predicate={:?}",
 599                     has_static_predicate
 600                 );
 601                 let static_str = kw::StaticLifetime;
 602                 // If there is a static predicate, then the only sensible suggestion is to replace
 603                 // fr with `'static`.
 604                 if has_static_predicate {
 605                     diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
 606                 } else {
 607                     // Otherwise, we should suggest adding a constraint on the return type.
 608                     let span = self.infcx.tcx.def_span(*did);
 609                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 610                         let suggestable_fr_name = if fr_name.was_named() {
 611                             fr_name.to_string()
 612                         } else {
 613                             "'_".to_string()
 614                         };
 615                         let suggestion = if snippet.ends_with(';') {
 616                             // `type X = impl Trait;`
 617                             format!("{} + {};", &snippet[..snippet.len() - 1], suggestable_fr_name)
 618                         } else {
 619                             format!("{} + {}", snippet, suggestable_fr_name)
 620                         };
 621                         diag.span_suggestion(
 622                             span,
 623                             &format!(
 624                                 "to allow this `impl Trait` to capture borrowed data with lifetime \
 625                                  `{}`, add `{}` as a bound",
 626                                 fr_name, suggestable_fr_name,
 627                             ),
 628                             suggestion,
 629                             Applicability::MachineApplicable,
 630                         );
 631                     }
 632                 }
 633             }
 634         }
 635     }
 636 }