compiler/rustc_mir/src/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};
  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             // We just want something nameable, even if it's not
 126             // actually an upper bound.
 127             let upper_bound = self.regioncx.approx_universal_upper_bound(r);
 128
 129             if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
 130                 self.to_error_region_vid(upper_bound)
 131             } else {
 132                 None
 133             }
 134         }
 135     }
 136
 137     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 138     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
 139         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
 140             if let ty::BoundRegion::BrEnv = free_region.bound_region {
 141                 if let DefiningTy::Closure(_, substs) =
 142                     self.regioncx.universal_regions().defining_ty
 143                 {
 144                     return substs.as_closure().kind() == ty::ClosureKind::FnMut;
 145                 }
 146             }
 147         }
 148
 149         false
 150     }
 151
 152     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
 153     pub(in crate::borrow_check) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
 154         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
 155         // buffered in the `MirBorrowckCtxt`.
 156
 157         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();
 158
 159         for nll_error in nll_errors.into_iter() {
 160             match nll_error {
 161                 RegionErrorKind::TypeTestError { type_test } => {
 162                     // Try to convert the lower-bound region into something named we can print for the user.
 163                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 164
 165                     let type_test_span = type_test.locations.span(&self.body);
 166
 167                     if let Some(lower_bound_region) = lower_bound_region {
 168                         self.infcx
 169                             .construct_generic_bound_failure(
 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 named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 199                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 200                     unexpected_hidden_region_diagnostic(
 201                         self.infcx.tcx,
 202                         span,
 203                         named_ty,
 204                         named_region,
 205                     )
 206                     .buffer(&mut self.errors_buffer);
 207                 }
 208
 209                 RegionErrorKind::BoundUniversalRegionError {
 210                     longer_fr,
 211                     fr_origin,
 212                     error_element,
 213                 } => {
 214                     let error_region = self.regioncx.region_from_element(longer_fr, error_element);
 215
 216                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 217                     let (_, span) = self.regioncx.find_outlives_blame_span(
 218                         &self.body,
 219                         longer_fr,
 220                         fr_origin,
 221                         error_region,
 222                     );
 223
 224                     // FIXME: improve this error message
 225                     self.infcx
 226                         .tcx
 227                         .sess
 228                         .struct_span_err(span, "higher-ranked subtype error")
 229                         .buffer(&mut self.errors_buffer);
 230                 }
 231
 232                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 233                     if is_reported {
 234                         self.report_region_error(
 235                             longer_fr,
 236                             fr_origin,
 237                             shorter_fr,
 238                             &mut outlives_suggestion,
 239                         );
 240                     } else {
 241                         // We only report the first error, so as not to overwhelm the user. See
 242                         // `RegRegionErrorKind` docs.
 243                         //
 244                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 245                         // FIXME: try collecting these constraints on the outlives suggestion
 246                         // builder. Does it make the suggestions any better?
 247                         debug!(
 248                             "Unreported region error: can't prove that {:?}: {:?}",
 249                             longer_fr, shorter_fr
 250                         );
 251                     }
 252                 }
 253             }
 254         }
 255
 256         // Emit one outlives suggestions for each MIR def we borrowck
 257         outlives_suggestion.add_suggestion(self);
 258     }
 259
 260     /// Report an error because the universal region `fr` was required to outlive
 261     /// `outlived_fr` but it is not known to do so. For example:
 262     ///
 263     /// ```
 264     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 265     /// ```
 266     ///
 267     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 268     pub(in crate::borrow_check) fn report_region_error(
 269         &mut self,
 270         fr: RegionVid,
 271         fr_origin: NLLRegionVariableOrigin,
 272         outlived_fr: RegionVid,
 273         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 274     ) {
 275         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 276
 277         let (category, _, span) =
 278             self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
 279                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 280             });
 281
 282         debug!("report_region_error: category={:?} {:?}", category, span);
 283         // Check if we can use one of the "nice region errors".
 284         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 285             let nice = NiceRegionError::new_from_span(self.infcx, span, o, f);
 286             if let Some(diag) = nice.try_report_from_nll() {
 287                 diag.buffer(&mut self.errors_buffer);
 288                 return;
 289             }
 290         }
 291
 292         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 293             self.regioncx.universal_regions().is_local_free_region(fr),
 294             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 295         );
 296
 297         debug!(
 298             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 299             fr_is_local, outlived_fr_is_local, category
 300         );
 301
 302         let errci = ErrorConstraintInfo {
 303             fr,
 304             outlived_fr,
 305             fr_is_local,
 306             outlived_fr_is_local,
 307             category,
 308             span,
 309         };
 310
 311         let diag = match (category, fr_is_local, outlived_fr_is_local) {
 312             (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
 313                 self.report_fnmut_error(&errci, kind)
 314             }
 315             (ConstraintCategory::Assignment, true, false)
 316             | (ConstraintCategory::CallArgument, true, false) => {
 317                 let mut db = self.report_escaping_data_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                 let mut db = self.report_general_error(&errci);
 326
 327                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 328                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 329
 330                 db
 331             }
 332         };
 333
 334         diag.buffer(&mut self.errors_buffer);
 335     }
 336
 337     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 338     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 339     ///
 340     /// ```text
 341     /// error: captured variable cannot escape `FnMut` closure body
 342     ///   --> $DIR/issue-53040.rs:15:8
 343     ///    |
 344     /// LL |     || &mut v;
 345     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 346     ///    |     |
 347     ///    |     inferred to be a `FnMut` closure
 348     ///    |
 349     ///    = note: `FnMut` closures only have access to their captured variables while they are
 350     ///            executing...
 351     ///    = note: ...therefore, returned references to captured variables will escape the closure
 352     /// ```
 353     fn report_fnmut_error(
 354         &self,
 355         errci: &ErrorConstraintInfo,
 356         kind: ReturnConstraint,
 357     ) -> DiagnosticBuilder<'tcx> {
 358         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 359
 360         let mut diag = self
 361             .infcx
 362             .tcx
 363             .sess
 364             .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
 365
 366         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 367         if let ty::Opaque(def_id, _) = *output_ty.kind() {
 368             output_ty = self.infcx.tcx.type_of(def_id)
 369         };
 370
 371         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 372
 373         let message = match output_ty.kind() {
 374             ty::Closure(_, _) => {
 375                 "returns a closure that contains a reference to a captured variable, which then \
 376                  escapes the closure body"
 377             }
 378             ty::Adt(def, _) if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did) => {
 379                 "returns an `async` block that contains a reference to a captured variable, which then \
 380                  escapes the closure body"
 381             }
 382             _ => "returns a reference to a captured variable which escapes the closure body",
 383         };
 384
 385         diag.span_label(*span, message);
 386
 387         if let ReturnConstraint::ClosureUpvar(upvar) = kind {
 388             let def_id = match self.regioncx.universal_regions().defining_ty {
 389                 DefiningTy::Closure(def_id, _) => def_id,
 390                 ty => bug!("unexpected DefiningTy {:?}", ty),
 391             };
 392
 393             let upvar_def_span = self.infcx.tcx.hir().span(upvar);
 394             let upvar_span = self.infcx.tcx.upvars_mentioned(def_id).unwrap()[&upvar].span;
 395             diag.span_label(upvar_def_span, "variable defined here");
 396             diag.span_label(upvar_span, "variable captured here");
 397         }
 398
 399         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
 400             diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 401         }
 402
 403         diag.note(
 404             "`FnMut` closures only have access to their captured variables while they are \
 405              executing...",
 406         );
 407         diag.note("...therefore, they cannot allow references to captured variables to escape");
 408
 409         diag
 410     }
 411
 412     /// Reports a error specifically for when data is escaping a closure.
 413     ///
 414     /// ```text
 415     /// error: borrowed data escapes outside of function
 416     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 417     ///    |
 418     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 419     ///    |              - `x` is a reference that is only valid in the function body
 420     /// LL |     // but ref_obj will not, so warn.
 421     /// LL |     ref_obj(x)
 422     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 423     /// ```
 424     fn report_escaping_data_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 425         let ErrorConstraintInfo { span, category, .. } = errci;
 426
 427         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 428             self.infcx.tcx,
 429             &self.body,
 430             &self.local_names,
 431             &self.upvars,
 432             errci.fr,
 433         );
 434         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 435             self.infcx.tcx,
 436             &self.body,
 437             &self.local_names,
 438             &self.upvars,
 439             errci.outlived_fr,
 440         );
 441
 442         let (_, escapes_from) = self
 443             .infcx
 444             .tcx
 445             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());
 446
 447         // Revert to the normal error in these cases.
 448         // Assignments aren't "escapes" in function items.
 449         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 450             || (*category == ConstraintCategory::Assignment
 451                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 452             || self.regioncx.universal_regions().defining_ty.is_const()
 453         {
 454             return self.report_general_error(&ErrorConstraintInfo {
 455                 fr_is_local: true,
 456                 outlived_fr_is_local: false,
 457                 ..*errci
 458             });
 459         }
 460
 461         let mut diag =
 462             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 463
 464         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 465             diag.span_label(
 466                 outlived_fr_span,
 467                 format!(
 468                     "`{}` declared here, outside of the {} body",
 469                     outlived_fr_name, escapes_from
 470                 ),
 471             );
 472         }
 473
 474         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 475             diag.span_label(
 476                 fr_span,
 477                 format!(
 478                     "`{}` is a reference that is only valid in the {} body",
 479                     fr_name, escapes_from
 480                 ),
 481             );
 482
 483             diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
 484         }
 485
 486         diag
 487     }
 488
 489     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 490     /// explain what is happening.
 491     ///
 492     /// ```text
 493     /// error: unsatisfied lifetime constraints
 494     ///   --> $DIR/regions-creating-enums3.rs:17:5
 495     ///    |
 496     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 497     ///    |                -- -- lifetime `'b` defined here
 498     ///    |                |
 499     ///    |                lifetime `'a` defined here
 500     /// LL |     ast::add(x, y)
 501     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 502     ///    |                    is returning data with lifetime `'b`
 503     /// ```
 504     fn report_general_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 505         let ErrorConstraintInfo {
 506             fr,
 507             fr_is_local,
 508             outlived_fr,
 509             outlived_fr_is_local,
 510             span,
 511             category,
 512             ..
 513         } = errci;
 514
 515         let mut diag =
 516             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
 517
 518         let (_, mir_def_name) =
 519             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());
 520
 521         let fr_name = self.give_region_a_name(*fr).unwrap();
 522         fr_name.highlight_region_name(&mut diag);
 523         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 524         outlived_fr_name.highlight_region_name(&mut diag);
 525
 526         match (category, outlived_fr_is_local, fr_is_local) {
 527             (ConstraintCategory::Return(_), true, _) => {
 528                 diag.span_label(
 529                     *span,
 530                     format!(
 531                         "{} was supposed to return data with lifetime `{}` but it is returning \
 532                          data with lifetime `{}`",
 533                         mir_def_name, outlived_fr_name, fr_name
 534                     ),
 535                 );
 536             }
 537             _ => {
 538                 diag.span_label(
 539                     *span,
 540                     format!(
 541                         "{}requires that `{}` must outlive `{}`",
 542                         category.description(),
 543                         fr_name,
 544                         outlived_fr_name,
 545                     ),
 546                 );
 547             }
 548         }
 549
 550         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 551
 552         diag
 553     }
 554
 555     /// Adds a suggestion to errors where a `impl Trait` is returned.
 556     ///
 557     /// ```text
 558     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 559     ///       a constraint
 560     ///    |
 561     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 562     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 563     /// ```
 564     fn add_static_impl_trait_suggestion(
 565         &self,
 566         diag: &mut DiagnosticBuilder<'tcx>,
 567         fr: RegionVid,
 568         // We need to pass `fr_name` - computing it again will label it twice.
 569         fr_name: RegionName,
 570         outlived_fr: RegionVid,
 571     ) {
 572         if let (Some(f), Some(ty::RegionKind::ReStatic)) =
 573             (self.to_error_region(fr), self.to_error_region(outlived_fr))
 574         {
 575             if let Some(&ty::Opaque(did, substs)) = self
 576                 .infcx
 577                 .tcx
 578                 .is_suitable_region(f)
 579                 .map(|r| r.def_id)
 580                 .and_then(|id| self.infcx.tcx.return_type_impl_trait(id))
 581                 .map(|(ty, _)| ty.kind())
 582             {
 583                 // Check whether or not the impl trait return type is intended to capture
 584                 // data with the static lifetime.
 585                 //
 586                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 587                 let has_static_predicate = {
 588                     let predicates_of = self.infcx.tcx.predicates_of(did);
 589                     let bounds = predicates_of.instantiate(self.infcx.tcx, substs);
 590
 591                     let mut found = false;
 592                     for predicate in bounds.predicates {
 593                         if let ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(_, r)) =
 594                             predicate.skip_binders()
 595                         {
 596                             if let ty::RegionKind::ReStatic = r {
 597                                 found = true;
 598                                 break;
 599                             } else {
 600                                 // If there's already a lifetime bound, don't
 601                                 // suggest anything.
 602                                 return;
 603                             }
 604                         }
 605                     }
 606
 607                     found
 608                 };
 609
 610                 debug!(
 611                     "add_static_impl_trait_suggestion: has_static_predicate={:?}",
 612                     has_static_predicate
 613                 );
 614                 let static_str = kw::StaticLifetime;
 615                 // If there is a static predicate, then the only sensible suggestion is to replace
 616                 // fr with `'static`.
 617                 if has_static_predicate {
 618                     diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
 619                 } else {
 620                     // Otherwise, we should suggest adding a constraint on the return type.
 621                     let span = self.infcx.tcx.def_span(did);
 622                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 623                         let suggestable_fr_name = if fr_name.was_named() {
 624                             fr_name.to_string()
 625                         } else {
 626                             "'_".to_string()
 627                         };
 628                         let suggestion = if snippet.ends_with(';') {
 629                             // `type X = impl Trait;`
 630                             format!("{} + {};", &snippet[..snippet.len() - 1], suggestable_fr_name)
 631                         } else {
 632                             format!("{} + {}", snippet, suggestable_fr_name)
 633                         };
 634                         diag.span_suggestion(
 635                             span,
 636                             &format!(
 637                                 "to allow this `impl Trait` to capture borrowed data with lifetime \
 638                                  `{}`, add `{}` as a bound",
 639                                 fr_name, suggestable_fr_name,
 640                             ),
 641                             suggestion,
 642                             Applicability::MachineApplicable,
 643                         );
 644                     }
 645                 }
 646             }
 647         }
 648     }
 649 }