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::subst::Subst;
  10 use rustc_middle::ty::{self, RegionVid, Ty};
  11 use rustc_span::symbol::{kw, sym};
  12 use rustc_span::Span;
  13
  14 use crate::util::borrowck_errors;
  15
  16 use crate::borrow_check::region_infer::BlameConstraint;
  17 use crate::borrow_check::{
  18     nll::ConstraintDescription,
  19     region_infer::{values::RegionElement, TypeTest},
  20     universal_regions::DefiningTy,
  21     MirBorrowckCtxt,
  22 };
  23
  24 use super::{OutlivesSuggestionBuilder, RegionName};
  25
  26 impl ConstraintDescription for ConstraintCategory {
  27     fn description(&self) -> &'static str {
  28         // Must end with a space. Allows for empty names to be provided.
  29         match self {
  30             ConstraintCategory::Assignment => "assignment ",
  31             ConstraintCategory::Return(_) => "returning this value ",
  32             ConstraintCategory::Yield => "yielding this value ",
  33             ConstraintCategory::UseAsConst => "using this value as a constant ",
  34             ConstraintCategory::UseAsStatic => "using this value as a static ",
  35             ConstraintCategory::Cast => "cast ",
  36             ConstraintCategory::CallArgument => "argument ",
  37             ConstraintCategory::TypeAnnotation => "type annotation ",
  38             ConstraintCategory::ClosureBounds => "closure body ",
  39             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
  40             ConstraintCategory::CopyBound => "copying this value ",
  41             ConstraintCategory::OpaqueType => "opaque type ",
  42             ConstraintCategory::ClosureUpvar(_) => "closure capture ",
  43             ConstraintCategory::Boring
  44             | ConstraintCategory::BoringNoLocation
  45             | ConstraintCategory::Internal => "",
  46         }
  47     }
  48 }
  49
  50 /// A collection of errors encountered during region inference. This is needed to efficiently
  51 /// report errors after borrow checking.
  52 ///
  53 /// Usually we expect this to either be empty or contain a small number of items, so we can avoid
  54 /// allocation most of the time.
  55 crate type RegionErrors<'tcx> = Vec<RegionErrorKind<'tcx>>;
  56
  57 #[derive(Clone, Debug)]
  58 crate enum RegionErrorKind<'tcx> {
  59     /// A generic bound failure for a type test (`T: 'a`).
  60     TypeTestError { type_test: TypeTest<'tcx> },
  61
  62     /// An unexpected hidden region for an opaque type.
  63     UnexpectedHiddenRegion {
  64         /// The span for the member constraint.
  65         span: Span,
  66         /// The hidden type.
  67         hidden_ty: Ty<'tcx>,
  68         /// The unexpected region.
  69         member_region: ty::Region<'tcx>,
  70     },
  71
  72     /// Higher-ranked subtyping error.
  73     BoundUniversalRegionError {
  74         /// The placeholder free region.
  75         longer_fr: RegionVid,
  76         /// The region element that erroneously must be outlived by `longer_fr`.
  77         error_element: RegionElement,
  78         /// The origin of the placeholder region.
  79         fr_origin: NllRegionVariableOrigin,
  80     },
  81
  82     /// Any other lifetime error.
  83     RegionError {
  84         /// The origin of the region.
  85         fr_origin: NllRegionVariableOrigin,
  86         /// The region that should outlive `shorter_fr`.
  87         longer_fr: RegionVid,
  88         /// The region that should be shorter, but we can't prove it.
  89         shorter_fr: RegionVid,
  90         /// Indicates whether this is a reported error. We currently only report the first error
  91         /// encountered and leave the rest unreported so as not to overwhelm the user.
  92         is_reported: bool,
  93     },
  94 }
  95
  96 /// Information about the various region constraints involved in a borrow checker error.
  97 #[derive(Clone, Debug)]
  98 pub struct ErrorConstraintInfo {
  99     // fr: outlived_fr
 100     pub(super) fr: RegionVid,
 101     pub(super) fr_is_local: bool,
 102     pub(super) outlived_fr: RegionVid,
 103     pub(super) outlived_fr_is_local: bool,
 104
 105     // Category and span for best blame constraint
 106     pub(super) category: ConstraintCategory,
 107     pub(super) span: Span,
 108 }
 109
 110 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 111     /// Converts a region inference variable into a `ty::Region` that
 112     /// we can use for error reporting. If `r` is universally bound,
 113     /// then we use the name that we have on record for it. If `r` is
 114     /// existentially bound, then we check its inferred value and try
 115     /// to find a good name from that. Returns `None` if we can't find
 116     /// one (e.g., this is just some random part of the CFG).
 117     pub(super) fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
 118         self.to_error_region_vid(r).and_then(|r| self.regioncx.region_definition(r).external_name)
 119     }
 120
 121     /// Returns the `RegionVid` corresponding to the region returned by
 122     /// `to_error_region`.
 123     pub(super) fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
 124         if self.regioncx.universal_regions().is_universal_region(r) {
 125             Some(r)
 126         } else {
 127             // We just want something nameable, even if it's not
 128             // actually an upper bound.
 129             let upper_bound = self.regioncx.approx_universal_upper_bound(r);
 130
 131             if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
 132                 self.to_error_region_vid(upper_bound)
 133             } else {
 134                 None
 135             }
 136         }
 137     }
 138
 139     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 140     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
 141         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
 142             if let ty::BoundRegionKind::BrEnv = free_region.bound_region {
 143                 if let DefiningTy::Closure(_, substs) =
 144                     self.regioncx.universal_regions().defining_ty
 145                 {
 146                     return substs.as_closure().kind() == ty::ClosureKind::FnMut;
 147                 }
 148             }
 149         }
 150
 151         false
 152     }
 153
 154     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
 155     pub(in crate::borrow_check) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
 156         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
 157         // buffered in the `MirBorrowckCtxt`.
 158
 159         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();
 160
 161         for nll_error in nll_errors.into_iter() {
 162             match nll_error {
 163                 RegionErrorKind::TypeTestError { type_test } => {
 164                     // Try to convert the lower-bound region into something named we can print for the user.
 165                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 166
 167                     let type_test_span = type_test.locations.span(&self.body);
 168
 169                     if let Some(lower_bound_region) = lower_bound_region {
 170                         self.infcx
 171                             .construct_generic_bound_failure(
 172                                 type_test_span,
 173                                 None,
 174                                 type_test.generic_kind,
 175                                 lower_bound_region,
 176                             )
 177                             .buffer(&mut self.errors_buffer);
 178                     } else {
 179                         // FIXME. We should handle this case better. It
 180                         // indicates that we have e.g., some region variable
 181                         // whose value is like `'a+'b` where `'a` and `'b` are
 182                         // distinct unrelated univesal regions that are not
 183                         // known to outlive one another. It'd be nice to have
 184                         // some examples where this arises to decide how best
 185                         // to report it; we could probably handle it by
 186                         // iterating over the universal regions and reporting
 187                         // an error that multiple bounds are required.
 188                         self.infcx
 189                             .tcx
 190                             .sess
 191                             .struct_span_err(
 192                                 type_test_span,
 193                                 &format!("`{}` does not live long enough", type_test.generic_kind),
 194                             )
 195                             .buffer(&mut self.errors_buffer);
 196                     }
 197                 }
 198
 199                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
 200                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 201                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 202                     unexpected_hidden_region_diagnostic(
 203                         self.infcx.tcx,
 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 BlameConstraint { category, span, variance_info, from_closure: _ } =
 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, variance_info);
 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 mut diag = match (category, fr_is_local, outlived_fr_is_local) {
 314             (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
 315                 self.report_fnmut_error(&errci, kind)
 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         match variance_info {
 337             ty::VarianceDiagInfo::None => {}
 338             ty::VarianceDiagInfo::Mut { kind, ty } => {
 339                 let kind_name = match kind {
 340                     ty::VarianceDiagMutKind::Ref => "reference",
 341                     ty::VarianceDiagMutKind::RawPtr => "pointer",
 342                 };
 343                 diag.note(&format!("requirement occurs because of a mutable {kind_name} to {ty}",));
 344                 diag.note(&format!("mutable {kind_name}s are invariant over their type parameter"));
 345                 diag.help("see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance");
 346             }
 347         }
 348
 349         diag.buffer(&mut self.errors_buffer);
 350     }
 351
 352     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 353     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 354     ///
 355     /// ```text
 356     /// error: captured variable cannot escape `FnMut` closure body
 357     ///   --> $DIR/issue-53040.rs:15:8
 358     ///    |
 359     /// LL |     || &mut v;
 360     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 361     ///    |     |
 362     ///    |     inferred to be a `FnMut` closure
 363     ///    |
 364     ///    = note: `FnMut` closures only have access to their captured variables while they are
 365     ///            executing...
 366     ///    = note: ...therefore, returned references to captured variables will escape the closure
 367     /// ```
 368     fn report_fnmut_error(
 369         &self,
 370         errci: &ErrorConstraintInfo,
 371         kind: ReturnConstraint,
 372     ) -> DiagnosticBuilder<'tcx> {
 373         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 374
 375         let mut diag = self
 376             .infcx
 377             .tcx
 378             .sess
 379             .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
 380
 381         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 382         if let ty::Opaque(def_id, _) = *output_ty.kind() {
 383             output_ty = self.infcx.tcx.type_of(def_id)
 384         };
 385
 386         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 387
 388         let message = match output_ty.kind() {
 389             ty::Closure(_, _) => {
 390                 "returns a closure that contains a reference to a captured variable, which then \
 391                  escapes the closure body"
 392             }
 393             ty::Adt(def, _) if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did) => {
 394                 "returns an `async` block that contains a reference to a captured variable, which then \
 395                  escapes the closure body"
 396             }
 397             _ => "returns a reference to a captured variable which escapes the closure body",
 398         };
 399
 400         diag.span_label(*span, message);
 401
 402         // FIXME(project-rfc-2229#48): This should store a captured_place not a hir id
 403         if let ReturnConstraint::ClosureUpvar(upvar) = kind {
 404             let def_id = match self.regioncx.universal_regions().defining_ty {
 405                 DefiningTy::Closure(def_id, _) => def_id,
 406                 ty => bug!("unexpected DefiningTy {:?}", ty),
 407             };
 408
 409             let upvar_def_span = self.infcx.tcx.hir().span(upvar);
 410             let upvar_span = self.infcx.tcx.upvars_mentioned(def_id).unwrap()[&upvar].span;
 411             diag.span_label(upvar_def_span, "variable defined here");
 412             diag.span_label(upvar_span, "variable captured here");
 413         }
 414
 415         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
 416             diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 417         }
 418
 419         diag.note(
 420             "`FnMut` closures only have access to their captured variables while they are \
 421              executing...",
 422         );
 423         diag.note("...therefore, they cannot allow references to captured variables to escape");
 424
 425         diag
 426     }
 427
 428     /// Reports a error specifically for when data is escaping a closure.
 429     ///
 430     /// ```text
 431     /// error: borrowed data escapes outside of function
 432     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 433     ///    |
 434     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 435     ///    |              - `x` is a reference that is only valid in the function body
 436     /// LL |     // but ref_obj will not, so warn.
 437     /// LL |     ref_obj(x)
 438     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 439     /// ```
 440     fn report_escaping_data_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 441         let ErrorConstraintInfo { span, category, .. } = errci;
 442
 443         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 444             self.infcx.tcx,
 445             &self.body,
 446             &self.local_names,
 447             &self.upvars,
 448             errci.fr,
 449         );
 450         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 451             self.infcx.tcx,
 452             &self.body,
 453             &self.local_names,
 454             &self.upvars,
 455             errci.outlived_fr,
 456         );
 457
 458         let (_, escapes_from) = self
 459             .infcx
 460             .tcx
 461             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());
 462
 463         // Revert to the normal error in these cases.
 464         // Assignments aren't "escapes" in function items.
 465         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 466             || (*category == ConstraintCategory::Assignment
 467                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 468             || self.regioncx.universal_regions().defining_ty.is_const()
 469         {
 470             return self.report_general_error(&ErrorConstraintInfo {
 471                 fr_is_local: true,
 472                 outlived_fr_is_local: false,
 473                 ..*errci
 474             });
 475         }
 476
 477         let mut diag =
 478             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 479
 480         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 481             diag.span_label(
 482                 outlived_fr_span,
 483                 format!(
 484                     "`{}` declared here, outside of the {} body",
 485                     outlived_fr_name, escapes_from
 486                 ),
 487             );
 488         }
 489
 490         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 491             diag.span_label(
 492                 fr_span,
 493                 format!(
 494                     "`{}` is a reference that is only valid in the {} body",
 495                     fr_name, escapes_from
 496                 ),
 497             );
 498
 499             diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
 500         }
 501
 502         diag
 503     }
 504
 505     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 506     /// explain what is happening.
 507     ///
 508     /// ```text
 509     /// error: unsatisfied lifetime constraints
 510     ///   --> $DIR/regions-creating-enums3.rs:17:5
 511     ///    |
 512     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 513     ///    |                -- -- lifetime `'b` defined here
 514     ///    |                |
 515     ///    |                lifetime `'a` defined here
 516     /// LL |     ast::add(x, y)
 517     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 518     ///    |                    is returning data with lifetime `'b`
 519     /// ```
 520     fn report_general_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 521         let ErrorConstraintInfo {
 522             fr,
 523             fr_is_local,
 524             outlived_fr,
 525             outlived_fr_is_local,
 526             span,
 527             category,
 528             ..
 529         } = errci;
 530
 531         let mut diag =
 532             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
 533
 534         let (_, mir_def_name) =
 535             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());
 536
 537         let fr_name = self.give_region_a_name(*fr).unwrap();
 538         fr_name.highlight_region_name(&mut diag);
 539         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 540         outlived_fr_name.highlight_region_name(&mut diag);
 541
 542         match (category, outlived_fr_is_local, fr_is_local) {
 543             (ConstraintCategory::Return(_), true, _) => {
 544                 diag.span_label(
 545                     *span,
 546                     format!(
 547                         "{} was supposed to return data with lifetime `{}` but it is returning \
 548                          data with lifetime `{}`",
 549                         mir_def_name, outlived_fr_name, fr_name
 550                     ),
 551                 );
 552             }
 553             _ => {
 554                 diag.span_label(
 555                     *span,
 556                     format!(
 557                         "{}requires that `{}` must outlive `{}`",
 558                         category.description(),
 559                         fr_name,
 560                         outlived_fr_name,
 561                     ),
 562                 );
 563             }
 564         }
 565
 566         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 567
 568         diag
 569     }
 570
 571     /// Adds a suggestion to errors where a `impl Trait` is returned.
 572     ///
 573     /// ```text
 574     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 575     ///       a constraint
 576     ///    |
 577     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 578     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 579     /// ```
 580     fn add_static_impl_trait_suggestion(
 581         &self,
 582         diag: &mut DiagnosticBuilder<'tcx>,
 583         fr: RegionVid,
 584         // We need to pass `fr_name` - computing it again will label it twice.
 585         fr_name: RegionName,
 586         outlived_fr: RegionVid,
 587     ) {
 588         if let (Some(f), Some(ty::RegionKind::ReStatic)) =
 589             (self.to_error_region(fr), self.to_error_region(outlived_fr))
 590         {
 591             if let Some(&ty::Opaque(did, substs)) = self
 592                 .infcx
 593                 .tcx
 594                 .is_suitable_region(f)
 595                 .map(|r| r.def_id)
 596                 .and_then(|id| self.infcx.tcx.return_type_impl_trait(id))
 597                 .map(|(ty, _)| ty.kind())
 598             {
 599                 // Check whether or not the impl trait return type is intended to capture
 600                 // data with the static lifetime.
 601                 //
 602                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 603                 let has_static_predicate = {
 604                     let bounds = self.infcx.tcx.explicit_item_bounds(did);
 605
 606                     let mut found = false;
 607                     for (bound, _) in bounds {
 608                         if let ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(_, r)) =
 609                             bound.kind().skip_binder()
 610                         {
 611                             let r = r.subst(self.infcx.tcx, substs);
 612                             if let ty::RegionKind::ReStatic = r {
 613                                 found = true;
 614                                 break;
 615                             } else {
 616                                 // If there's already a lifetime bound, don't
 617                                 // suggest anything.
 618                                 return;
 619                             }
 620                         }
 621                     }
 622
 623                     found
 624                 };
 625
 626                 debug!(
 627                     "add_static_impl_trait_suggestion: has_static_predicate={:?}",
 628                     has_static_predicate
 629                 );
 630                 let static_str = kw::StaticLifetime;
 631                 // If there is a static predicate, then the only sensible suggestion is to replace
 632                 // fr with `'static`.
 633                 if has_static_predicate {
 634                     diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
 635                 } else {
 636                     // Otherwise, we should suggest adding a constraint on the return type.
 637                     let span = self.infcx.tcx.def_span(did);
 638                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 639                         let suggestable_fr_name = if fr_name.was_named() {
 640                             fr_name.to_string()
 641                         } else {
 642                             "'_".to_string()
 643                         };
 644                         let suggestion = if snippet.ends_with(';') {
 645                             // `type X = impl Trait;`
 646                             format!("{} + {};", &snippet[..snippet.len() - 1], suggestable_fr_name)
 647                         } else {
 648                             format!("{} + {}", snippet, suggestable_fr_name)
 649                         };
 650                         diag.span_suggestion(
 651                             span,
 652                             &format!(
 653                                 "to allow this `impl Trait` to capture borrowed data with lifetime \
 654                                  `{}`, add `{}` as a bound",
 655                                 fr_name, suggestable_fr_name,
 656                             ),
 657                             suggestion,
 658                             Applicability::MachineApplicable,
 659                         );
 660                     }
 661                 }
 662             }
 663         }
 664     }
 665 }