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;
   9 use rustc_middle::ty::{self, RegionVid, Ty};
  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(_, substs) =
 139                     self.regioncx.universal_regions().defining_ty
 140                 {
 141                     return substs.as_closure().kind() == ty::ClosureKind::FnMut;
 142                 }
 143             }
 144         }
 145
 146         false
 147     }
 148
 149     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
 150     pub(in crate::borrow_check) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
 151         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
 152         // buffered in the `MirBorrowckCtxt`.
 153
 154         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();
 155
 156         for nll_error in nll_errors.into_iter() {
 157             match nll_error {
 158                 RegionErrorKind::TypeTestError { type_test } => {
 159                     // Try to convert the lower-bound region into something named we can print for the user.
 160                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 161
 162                     let type_test_span = type_test.locations.span(&self.body);
 163
 164                     if let Some(lower_bound_region) = lower_bound_region {
 165                         self.infcx
 166                             .construct_generic_bound_failure(
 167                                 type_test_span,
 168                                 None,
 169                                 type_test.generic_kind,
 170                                 lower_bound_region,
 171                             )
 172                             .buffer(&mut self.errors_buffer);
 173                     } else {
 174                         // FIXME. We should handle this case better. It
 175                         // indicates that we have e.g., some region variable
 176                         // whose value is like `'a+'b` where `'a` and `'b` are
 177                         // distinct unrelated univesal regions that are not
 178                         // known to outlive one another. It'd be nice to have
 179                         // some examples where this arises to decide how best
 180                         // to report it; we could probably handle it by
 181                         // iterating over the universal regions and reporting
 182                         // an error that multiple bounds are required.
 183                         self.infcx
 184                             .tcx
 185                             .sess
 186                             .struct_span_err(
 187                                 type_test_span,
 188                                 &format!("`{}` does not live long enough", type_test.generic_kind),
 189                             )
 190                             .buffer(&mut self.errors_buffer);
 191                     }
 192                 }
 193
 194                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
 195                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 196                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 197                     unexpected_hidden_region_diagnostic(
 198                         self.infcx.tcx,
 199                         span,
 200                         named_ty,
 201                         named_region,
 202                     )
 203                     .buffer(&mut self.errors_buffer);
 204                 }
 205
 206                 RegionErrorKind::BoundUniversalRegionError {
 207                     longer_fr,
 208                     fr_origin,
 209                     error_element,
 210                 } => {
 211                     let error_region = self.regioncx.region_from_element(longer_fr, error_element);
 212
 213                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 214                     let (_, span) = self.regioncx.find_outlives_blame_span(
 215                         &self.body,
 216                         longer_fr,
 217                         fr_origin,
 218                         error_region,
 219                     );
 220
 221                     // FIXME: improve this error message
 222                     self.infcx
 223                         .tcx
 224                         .sess
 225                         .struct_span_err(span, "higher-ranked subtype error")
 226                         .buffer(&mut self.errors_buffer);
 227                 }
 228
 229                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 230                     if is_reported {
 231                         self.report_region_error(
 232                             longer_fr,
 233                             fr_origin,
 234                             shorter_fr,
 235                             &mut outlives_suggestion,
 236                         );
 237                     } else {
 238                         // We only report the first error, so as not to overwhelm the user. See
 239                         // `RegRegionErrorKind` docs.
 240                         //
 241                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 242                         // FIXME: try collecting these constraints on the outlives suggestion
 243                         // builder. Does it make the suggestions any better?
 244                         debug!(
 245                             "Unreported region error: can't prove that {:?}: {:?}",
 246                             longer_fr, shorter_fr
 247                         );
 248                     }
 249                 }
 250             }
 251         }
 252
 253         // Emit one outlives suggestions for each MIR def we borrowck
 254         outlives_suggestion.add_suggestion(self);
 255     }
 256
 257     /// Report an error because the universal region `fr` was required to outlive
 258     /// `outlived_fr` but it is not known to do so. For example:
 259     ///
 260     /// ```
 261     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 262     /// ```
 263     ///
 264     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 265     pub(in crate::borrow_check) fn report_region_error(
 266         &mut self,
 267         fr: RegionVid,
 268         fr_origin: NLLRegionVariableOrigin,
 269         outlived_fr: RegionVid,
 270         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 271     ) {
 272         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 273
 274         let (category, _, span) =
 275             self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
 276                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 277             });
 278
 279         debug!("report_region_error: category={:?} {:?}", category, span);
 280         // Check if we can use one of the "nice region errors".
 281         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 282             let nice = NiceRegionError::new_from_span(self.infcx, span, o, f);
 283             if let Some(diag) = nice.try_report_from_nll() {
 284                 diag.buffer(&mut self.errors_buffer);
 285                 return;
 286             }
 287         }
 288
 289         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 290             self.regioncx.universal_regions().is_local_free_region(fr),
 291             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 292         );
 293
 294         debug!(
 295             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 296             fr_is_local, outlived_fr_is_local, category
 297         );
 298
 299         let errci = ErrorConstraintInfo {
 300             fr,
 301             outlived_fr,
 302             fr_is_local,
 303             outlived_fr_is_local,
 304             category,
 305             span,
 306         };
 307
 308         let diag = match (category, fr_is_local, outlived_fr_is_local) {
 309             (ConstraintCategory::Return, true, false) if self.is_closure_fn_mut(fr) => {
 310                 self.report_fnmut_error(&errci)
 311             }
 312             (ConstraintCategory::Assignment, true, false)
 313             | (ConstraintCategory::CallArgument, true, false) => {
 314                 let mut db = self.report_escaping_data_error(&errci);
 315
 316                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 317                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 318
 319                 db
 320             }
 321             _ => {
 322                 let mut db = self.report_general_error(&errci);
 323
 324                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 325                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 326
 327                 db
 328             }
 329         };
 330
 331         diag.buffer(&mut self.errors_buffer);
 332     }
 333
 334     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 335     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 336     ///
 337     /// ```text
 338     /// error: captured variable cannot escape `FnMut` closure body
 339     ///   --> $DIR/issue-53040.rs:15:8
 340     ///    |
 341     /// LL |     || &mut v;
 342     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 343     ///    |     |
 344     ///    |     inferred to be a `FnMut` closure
 345     ///    |
 346     ///    = note: `FnMut` closures only have access to their captured variables while they are
 347     ///            executing...
 348     ///    = note: ...therefore, returned references to captured variables will escape the closure
 349     /// ```
 350     fn report_fnmut_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 351         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 352
 353         let mut diag = self
 354             .infcx
 355             .tcx
 356             .sess
 357             .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
 358
 359         // We should check if the return type of this closure is in fact a closure - in that
 360         // case, we can special case the error further.
 361         let return_type_is_closure =
 362             self.regioncx.universal_regions().unnormalized_output_ty.is_closure();
 363         let message = if return_type_is_closure {
 364             "returns a closure that contains a reference to a captured variable, which then \
 365              escapes the closure body"
 366         } else {
 367             "returns a reference to a captured variable which escapes the closure body"
 368         };
 369
 370         diag.span_label(*span, message);
 371
 372         match self.give_region_a_name(*outlived_fr).unwrap().source {
 373             RegionNameSource::NamedEarlyBoundRegion(fr_span)
 374             | RegionNameSource::NamedFreeRegion(fr_span)
 375             | RegionNameSource::SynthesizedFreeEnvRegion(fr_span, _)
 376             | RegionNameSource::CannotMatchHirTy(fr_span, _)
 377             | RegionNameSource::MatchedHirTy(fr_span)
 378             | RegionNameSource::MatchedAdtAndSegment(fr_span)
 379             | RegionNameSource::AnonRegionFromUpvar(fr_span, _)
 380             | RegionNameSource::AnonRegionFromOutput(fr_span, _, _) => {
 381                 diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 382             }
 383             _ => {}
 384         }
 385
 386         diag.note(
 387             "`FnMut` closures only have access to their captured variables while they are \
 388              executing...",
 389         );
 390         diag.note("...therefore, they cannot allow references to captured variables to escape");
 391
 392         diag
 393     }
 394
 395     /// Reports a error specifically for when data is escaping a closure.
 396     ///
 397     /// ```text
 398     /// error: borrowed data escapes outside of function
 399     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 400     ///    |
 401     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 402     ///    |              - `x` is a reference that is only valid in the function body
 403     /// LL |     // but ref_obj will not, so warn.
 404     /// LL |     ref_obj(x)
 405     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 406     /// ```
 407     fn report_escaping_data_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 408         let ErrorConstraintInfo { span, category, .. } = errci;
 409
 410         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 411             self.infcx.tcx,
 412             &self.body,
 413             &self.local_names,
 414             &self.upvars,
 415             errci.fr,
 416         );
 417         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 418             self.infcx.tcx,
 419             &self.body,
 420             &self.local_names,
 421             &self.upvars,
 422             errci.outlived_fr,
 423         );
 424
 425         let (_, escapes_from) = self
 426             .infcx
 427             .tcx
 428             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());
 429
 430         // Revert to the normal error in these cases.
 431         // Assignments aren't "escapes" in function items.
 432         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 433             || (*category == ConstraintCategory::Assignment
 434                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 435             || self.regioncx.universal_regions().defining_ty.is_const()
 436         {
 437             return self.report_general_error(&ErrorConstraintInfo {
 438                 fr_is_local: true,
 439                 outlived_fr_is_local: false,
 440                 ..*errci
 441             });
 442         }
 443
 444         let mut diag =
 445             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 446
 447         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 448             diag.span_label(
 449                 outlived_fr_span,
 450                 format!(
 451                     "`{}` declared here, outside of the {} body",
 452                     outlived_fr_name, escapes_from
 453                 ),
 454             );
 455         }
 456
 457         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 458             diag.span_label(
 459                 fr_span,
 460                 format!(
 461                     "`{}` is a reference that is only valid in the {} body",
 462                     fr_name, escapes_from
 463                 ),
 464             );
 465
 466             diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
 467         }
 468
 469         diag
 470     }
 471
 472     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 473     /// explain what is happening.
 474     ///
 475     /// ```text
 476     /// error: unsatisfied lifetime constraints
 477     ///   --> $DIR/regions-creating-enums3.rs:17:5
 478     ///    |
 479     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 480     ///    |                -- -- lifetime `'b` defined here
 481     ///    |                |
 482     ///    |                lifetime `'a` defined here
 483     /// LL |     ast::add(x, y)
 484     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 485     ///    |                    is returning data with lifetime `'b`
 486     /// ```
 487     fn report_general_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 488         let ErrorConstraintInfo {
 489             fr,
 490             fr_is_local,
 491             outlived_fr,
 492             outlived_fr_is_local,
 493             span,
 494             category,
 495             ..
 496         } = errci;
 497
 498         let mut diag =
 499             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
 500
 501         let (_, mir_def_name) = self.infcx.tcx.article_and_description(self.mir_def_id);
 502
 503         let fr_name = self.give_region_a_name(*fr).unwrap();
 504         fr_name.highlight_region_name(&mut diag);
 505         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 506         outlived_fr_name.highlight_region_name(&mut diag);
 507
 508         match (category, outlived_fr_is_local, fr_is_local) {
 509             (ConstraintCategory::Return, true, _) => {
 510                 diag.span_label(
 511                     *span,
 512                     format!(
 513                         "{} was supposed to return data with lifetime `{}` but it is returning \
 514                          data with lifetime `{}`",
 515                         mir_def_name, outlived_fr_name, fr_name
 516                     ),
 517                 );
 518             }
 519             _ => {
 520                 diag.span_label(
 521                     *span,
 522                     format!(
 523                         "{}requires that `{}` must outlive `{}`",
 524                         category.description(),
 525                         fr_name,
 526                         outlived_fr_name,
 527                     ),
 528                 );
 529             }
 530         }
 531
 532         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 533
 534         diag
 535     }
 536
 537     /// Adds a suggestion to errors where a `impl Trait` is returned.
 538     ///
 539     /// ```text
 540     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 541     ///       a constraint
 542     ///    |
 543     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 544     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 545     /// ```
 546     fn add_static_impl_trait_suggestion(
 547         &self,
 548         diag: &mut DiagnosticBuilder<'tcx>,
 549         fr: RegionVid,
 550         // We need to pass `fr_name` - computing it again will label it twice.
 551         fr_name: RegionName,
 552         outlived_fr: RegionVid,
 553     ) {
 554         if let (Some(f), Some(ty::RegionKind::ReStatic)) =
 555             (self.to_error_region(fr), self.to_error_region(outlived_fr))
 556         {
 557             if let Some((ty::TyS { kind: ty::Opaque(did, substs), .. }, _)) = self
 558                 .infcx
 559                 .tcx
 560                 .is_suitable_region(f)
 561                 .map(|r| r.def_id)
 562                 .map(|id| self.infcx.tcx.return_type_impl_trait(id))
 563                 .unwrap_or(None)
 564             {
 565                 // Check whether or not the impl trait return type is intended to capture
 566                 // data with the static lifetime.
 567                 //
 568                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 569                 let has_static_predicate = {
 570                     let predicates_of = self.infcx.tcx.predicates_of(*did);
 571                     let bounds = predicates_of.instantiate(self.infcx.tcx, substs);
 572
 573                     let mut found = false;
 574                     for predicate in bounds.predicates {
 575                         if let ty::PredicateKind::TypeOutlives(binder) = predicate.kind() {
 576                             if let ty::OutlivesPredicate(_, ty::RegionKind::ReStatic) =
 577                                 binder.skip_binder()
 578                             {
 579                                 found = true;
 580                                 break;
 581                             } else {
 582                                 // If there's already a lifetime bound, don't
 583                                 // suggest anything.
 584                                 return;
 585                             }
 586                         }
 587                     }
 588
 589                     found
 590                 };
 591
 592                 debug!(
 593                     "add_static_impl_trait_suggestion: has_static_predicate={:?}",
 594                     has_static_predicate
 595                 );
 596                 let static_str = kw::StaticLifetime;
 597                 // If there is a static predicate, then the only sensible suggestion is to replace
 598                 // fr with `'static`.
 599                 if has_static_predicate {
 600                     diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
 601                 } else {
 602                     // Otherwise, we should suggest adding a constraint on the return type.
 603                     let span = self.infcx.tcx.def_span(*did);
 604                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 605                         let suggestable_fr_name = if fr_name.was_named() {
 606                             fr_name.to_string()
 607                         } else {
 608                             "'_".to_string()
 609                         };
 610                         let suggestion = if snippet.ends_with(';') {
 611                             // `type X = impl Trait;`
 612                             format!("{} + {};", &snippet[..snippet.len() - 1], suggestable_fr_name)
 613                         } else {
 614                             format!("{} + {}", snippet, suggestable_fr_name)
 615                         };
 616                         diag.span_suggestion(
 617                             span,
 618                             &format!(
 619                                 "to allow this `impl Trait` to capture borrowed data with lifetime \
 620                                  `{}`, add `{}` as a bound",
 621                                 fr_name, suggestable_fr_name,
 622                             ),
 623                             suggestion,
 624                             Applicability::MachineApplicable,
 625                         );
 626                     }
 627                 }
 628             }
 629         }
 630     }
 631 }