compiler/rustc_borrowck/src/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::{BytePos, Span};
  13
  14 use crate::borrowck_errors;
  15
  16 use super::{OutlivesSuggestionBuilder, RegionName};
  17 use crate::region_infer::BlameConstraint;
  18 use crate::{
  19     nll::ConstraintDescription,
  20     region_infer::{values::RegionElement, TypeTest},
  21     universal_regions::DefiningTy,
  22     MirBorrowckCtxt,
  23 };
  24
  25 impl ConstraintDescription for ConstraintCategory {
  26     fn description(&self) -> &'static str {
  27         // Must end with a space. Allows for empty names to be provided.
  28         match self {
  29             ConstraintCategory::Assignment => "assignment ",
  30             ConstraintCategory::Return(_) => "returning this value ",
  31             ConstraintCategory::Yield => "yielding this value ",
  32             ConstraintCategory::UseAsConst => "using this value as a constant ",
  33             ConstraintCategory::UseAsStatic => "using this value as a static ",
  34             ConstraintCategory::Cast => "cast ",
  35             ConstraintCategory::CallArgument => "argument ",
  36             ConstraintCategory::TypeAnnotation => "type annotation ",
  37             ConstraintCategory::ClosureBounds => "closure body ",
  38             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
  39             ConstraintCategory::CopyBound => "copying this value ",
  40             ConstraintCategory::OpaqueType => "opaque type ",
  41             ConstraintCategory::ClosureUpvar(_) => "closure capture ",
  42             ConstraintCategory::Usage => "this usage ",
  43             ConstraintCategory::Predicate(_)
  44             | ConstraintCategory::Boring
  45             | ConstraintCategory::BoringNoLocation
  46             | ConstraintCategory::Internal => "",
  47         }
  48     }
  49 }
  50
  51 /// A collection of errors encountered during region inference. This is needed to efficiently
  52 /// report errors after borrow checking.
  53 ///
  54 /// Usually we expect this to either be empty or contain a small number of items, so we can avoid
  55 /// allocation most of the time.
  56 crate type RegionErrors<'tcx> = Vec<RegionErrorKind<'tcx>>;
  57
  58 #[derive(Clone, Debug)]
  59 crate enum RegionErrorKind<'tcx> {
  60     /// A generic bound failure for a type test (`T: 'a`).
  61     TypeTestError { type_test: TypeTest<'tcx> },
  62
  63     /// An unexpected hidden region for an opaque type.
  64     UnexpectedHiddenRegion {
  65         /// The span for the member constraint.
  66         span: Span,
  67         /// The hidden type.
  68         hidden_ty: Ty<'tcx>,
  69         /// The unexpected region.
  70         member_region: ty::Region<'tcx>,
  71     },
  72
  73     /// Higher-ranked subtyping error.
  74     BoundUniversalRegionError {
  75         /// The placeholder free region.
  76         longer_fr: RegionVid,
  77         /// The region element that erroneously must be outlived by `longer_fr`.
  78         error_element: RegionElement,
  79         /// The placeholder region.
  80         placeholder: ty::PlaceholderRegion,
  81     },
  82
  83     /// Any other lifetime error.
  84     RegionError {
  85         /// The origin of the region.
  86         fr_origin: NllRegionVariableOrigin,
  87         /// The region that should outlive `shorter_fr`.
  88         longer_fr: RegionVid,
  89         /// The region that should be shorter, but we can't prove it.
  90         shorter_fr: RegionVid,
  91         /// Indicates whether this is a reported error. We currently only report the first error
  92         /// encountered and leave the rest unreported so as not to overwhelm the user.
  93         is_reported: bool,
  94     },
  95 }
  96
  97 /// Information about the various region constraints involved in a borrow checker error.
  98 #[derive(Clone, Debug)]
  99 pub struct ErrorConstraintInfo {
 100     // fr: outlived_fr
 101     pub(super) fr: RegionVid,
 102     pub(super) fr_is_local: bool,
 103     pub(super) outlived_fr: RegionVid,
 104     pub(super) outlived_fr_is_local: bool,
 105
 106     // Category and span for best blame constraint
 107     pub(super) category: ConstraintCategory,
 108     pub(super) span: Span,
 109 }
 110
 111 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 112     /// Converts a region inference variable into a `ty::Region` that
 113     /// we can use for error reporting. If `r` is universally bound,
 114     /// then we use the name that we have on record for it. If `r` is
 115     /// existentially bound, then we check its inferred value and try
 116     /// to find a good name from that. Returns `None` if we can't find
 117     /// one (e.g., this is just some random part of the CFG).
 118     pub(super) fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
 119         self.to_error_region_vid(r).and_then(|r| self.regioncx.region_definition(r).external_name)
 120     }
 121
 122     /// Returns the `RegionVid` corresponding to the region returned by
 123     /// `to_error_region`.
 124     pub(super) fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
 125         if self.regioncx.universal_regions().is_universal_region(r) {
 126             Some(r)
 127         } else {
 128             // We just want something nameable, even if it's not
 129             // actually an upper bound.
 130             let upper_bound = self.regioncx.approx_universal_upper_bound(r);
 131
 132             if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
 133                 self.to_error_region_vid(upper_bound)
 134             } else {
 135                 None
 136             }
 137         }
 138     }
 139
 140     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 141     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
 142         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
 143             if let ty::BoundRegionKind::BrEnv = free_region.bound_region {
 144                 if let DefiningTy::Closure(_, substs) =
 145                     self.regioncx.universal_regions().defining_ty
 146                 {
 147                     return substs.as_closure().kind() == ty::ClosureKind::FnMut;
 148                 }
 149             }
 150         }
 151
 152         false
 153     }
 154
 155     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
 156     pub(crate) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
 157         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
 158         // buffered in the `MirBorrowckCtxt`.
 159
 160         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();
 161
 162         for nll_error in nll_errors.into_iter() {
 163             match nll_error {
 164                 RegionErrorKind::TypeTestError { type_test } => {
 165                     // Try to convert the lower-bound region into something named we can print for the user.
 166                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 167
 168                     let type_test_span = type_test.locations.span(&self.body);
 169
 170                     if let Some(lower_bound_region) = lower_bound_region {
 171                         self.infcx
 172                             .construct_generic_bound_failure(
 173                                 type_test_span,
 174                                 None,
 175                                 type_test.generic_kind,
 176                                 lower_bound_region,
 177                             )
 178                             .buffer(&mut self.errors_buffer);
 179                     } else {
 180                         // FIXME. We should handle this case better. It
 181                         // indicates that we have e.g., some region variable
 182                         // whose value is like `'a+'b` where `'a` and `'b` are
 183                         // distinct unrelated univesal regions that are not
 184                         // known to outlive one another. It'd be nice to have
 185                         // some examples where this arises to decide how best
 186                         // to report it; we could probably handle it by
 187                         // iterating over the universal regions and reporting
 188                         // an error that multiple bounds are required.
 189                         self.infcx
 190                             .tcx
 191                             .sess
 192                             .struct_span_err(
 193                                 type_test_span,
 194                                 &format!("`{}` does not live long enough", type_test.generic_kind),
 195                             )
 196                             .buffer(&mut self.errors_buffer);
 197                     }
 198                 }
 199
 200                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
 201                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 202                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 203                     unexpected_hidden_region_diagnostic(
 204                         self.infcx.tcx,
 205                         span,
 206                         named_ty,
 207                         named_region,
 208                     )
 209                     .buffer(&mut self.errors_buffer);
 210                 }
 211
 212                 RegionErrorKind::BoundUniversalRegionError {
 213                     longer_fr,
 214                     placeholder,
 215                     error_element,
 216                 } => {
 217                     let error_vid = self.regioncx.region_from_element(longer_fr, &error_element);
 218
 219                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 220                     let (_, cause) = self.regioncx.find_outlives_blame_span(
 221                         &self.body,
 222                         longer_fr,
 223                         NllRegionVariableOrigin::Placeholder(placeholder),
 224                         error_vid,
 225                     );
 226
 227                     let universe = placeholder.universe;
 228                     let universe_info = self.regioncx.universe_info(universe);
 229
 230                     universe_info.report_error(self, placeholder, error_element, cause);
 231                 }
 232
 233                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 234                     if is_reported {
 235                         self.report_region_error(
 236                             longer_fr,
 237                             fr_origin,
 238                             shorter_fr,
 239                             &mut outlives_suggestion,
 240                         );
 241                     } else {
 242                         // We only report the first error, so as not to overwhelm the user. See
 243                         // `RegRegionErrorKind` docs.
 244                         //
 245                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 246                         // FIXME: try collecting these constraints on the outlives suggestion
 247                         // builder. Does it make the suggestions any better?
 248                         debug!(
 249                             "Unreported region error: can't prove that {:?}: {:?}",
 250                             longer_fr, shorter_fr
 251                         );
 252                     }
 253                 }
 254             }
 255         }
 256
 257         // Emit one outlives suggestions for each MIR def we borrowck
 258         outlives_suggestion.add_suggestion(self);
 259     }
 260
 261     /// Report an error because the universal region `fr` was required to outlive
 262     /// `outlived_fr` but it is not known to do so. For example:
 263     ///
 264     /// ```
 265     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 266     /// ```
 267     ///
 268     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 269     pub(crate) fn report_region_error(
 270         &mut self,
 271         fr: RegionVid,
 272         fr_origin: NllRegionVariableOrigin,
 273         outlived_fr: RegionVid,
 274         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 275     ) {
 276         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 277
 278         let BlameConstraint { category, cause, variance_info, from_closure: _ } =
 279             self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
 280                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 281             });
 282
 283         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);
 284         // Check if we can use one of the "nice region errors".
 285         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 286             let nice = NiceRegionError::new_from_span(self.infcx, cause.span, o, f);
 287             if let Some(diag) = nice.try_report_from_nll() {
 288                 diag.buffer(&mut self.errors_buffer);
 289                 return;
 290             }
 291         }
 292
 293         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 294             self.regioncx.universal_regions().is_local_free_region(fr),
 295             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 296         );
 297
 298         debug!(
 299             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 300             fr_is_local, outlived_fr_is_local, category
 301         );
 302
 303         let errci = ErrorConstraintInfo {
 304             fr,
 305             outlived_fr,
 306             fr_is_local,
 307             outlived_fr_is_local,
 308             category,
 309             span: cause.span,
 310         };
 311
 312         let mut diag = match (category, fr_is_local, outlived_fr_is_local) {
 313             (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
 314                 self.report_fnmut_error(&errci, kind)
 315             }
 316             (ConstraintCategory::Assignment, true, false)
 317             | (ConstraintCategory::CallArgument, true, false) => {
 318                 let mut db = self.report_escaping_data_error(&errci);
 319
 320                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 321                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 322
 323                 db
 324             }
 325             _ => {
 326                 let mut db = self.report_general_error(&errci);
 327
 328                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 329                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 330
 331                 db
 332             }
 333         };
 334
 335         match variance_info {
 336             ty::VarianceDiagInfo::None => {}
 337             ty::VarianceDiagInfo::Mut { kind, ty } => {
 338                 let kind_name = match kind {
 339                     ty::VarianceDiagMutKind::Ref => "reference",
 340                     ty::VarianceDiagMutKind::RawPtr => "pointer",
 341                 };
 342                 diag.note(&format!("requirement occurs because of a mutable {kind_name} to {ty}",));
 343                 diag.note(&format!("mutable {kind_name}s are invariant over their type parameter"));
 344                 diag.help("see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance");
 345             }
 346         }
 347
 348         diag.buffer(&mut self.errors_buffer);
 349     }
 350
 351     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 352     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 353     ///
 354     /// ```text
 355     /// error: captured variable cannot escape `FnMut` closure body
 356     ///   --> $DIR/issue-53040.rs:15:8
 357     ///    |
 358     /// LL |     || &mut v;
 359     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 360     ///    |     |
 361     ///    |     inferred to be a `FnMut` closure
 362     ///    |
 363     ///    = note: `FnMut` closures only have access to their captured variables while they are
 364     ///            executing...
 365     ///    = note: ...therefore, returned references to captured variables will escape the closure
 366     /// ```
 367     fn report_fnmut_error(
 368         &self,
 369         errci: &ErrorConstraintInfo,
 370         kind: ReturnConstraint,
 371     ) -> DiagnosticBuilder<'tcx> {
 372         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 373
 374         let mut diag = self
 375             .infcx
 376             .tcx
 377             .sess
 378             .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
 379
 380         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 381         if let ty::Opaque(def_id, _) = *output_ty.kind() {
 382             output_ty = self.infcx.tcx.type_of(def_id)
 383         };
 384
 385         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 386
 387         let message = match output_ty.kind() {
 388             ty::Closure(_, _) => {
 389                 "returns a closure that contains a reference to a captured variable, which then \
 390                  escapes the closure body"
 391             }
 392             ty::Adt(def, _) if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did) => {
 393                 "returns an `async` block that contains a reference to a captured variable, which then \
 394                  escapes the closure body"
 395             }
 396             _ => "returns a reference to a captured variable which escapes the closure body",
 397         };
 398
 399         diag.span_label(*span, message);
 400
 401         // FIXME(project-rfc-2229#48): This should store a captured_place not a hir id
 402         if let ReturnConstraint::ClosureUpvar(upvar) = kind {
 403             let def_id = match self.regioncx.universal_regions().defining_ty {
 404                 DefiningTy::Closure(def_id, _) => def_id,
 405                 ty => bug!("unexpected DefiningTy {:?}", ty),
 406             };
 407
 408             let upvar_def_span = self.infcx.tcx.hir().span(upvar);
 409             let upvar_span = self.infcx.tcx.upvars_mentioned(def_id).unwrap()[&upvar].span;
 410             diag.span_label(upvar_def_span, "variable defined here");
 411             diag.span_label(upvar_span, "variable captured here");
 412         }
 413
 414         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
 415             diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 416         }
 417
 418         diag.note(
 419             "`FnMut` closures only have access to their captured variables while they are \
 420              executing...",
 421         );
 422         diag.note("...therefore, they cannot allow references to captured variables to escape");
 423
 424         diag
 425     }
 426
 427     /// Reports an error specifically for when data is escaping a closure.
 428     ///
 429     /// ```text
 430     /// error: borrowed data escapes outside of function
 431     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 432     ///    |
 433     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 434     ///    |              - `x` is a reference that is only valid in the function body
 435     /// LL |     // but ref_obj will not, so warn.
 436     /// LL |     ref_obj(x)
 437     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 438     /// ```
 439     fn report_escaping_data_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 440         let ErrorConstraintInfo { span, category, .. } = errci;
 441
 442         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 443             self.infcx.tcx,
 444             &self.body,
 445             &self.local_names,
 446             &self.upvars,
 447             errci.fr,
 448         );
 449         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 450             self.infcx.tcx,
 451             &self.body,
 452             &self.local_names,
 453             &self.upvars,
 454             errci.outlived_fr,
 455         );
 456
 457         let (_, escapes_from) = self
 458             .infcx
 459             .tcx
 460             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());
 461
 462         // Revert to the normal error in these cases.
 463         // Assignments aren't "escapes" in function items.
 464         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 465             || (*category == ConstraintCategory::Assignment
 466                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 467             || self.regioncx.universal_regions().defining_ty.is_const()
 468         {
 469             return self.report_general_error(&ErrorConstraintInfo {
 470                 fr_is_local: true,
 471                 outlived_fr_is_local: false,
 472                 ..*errci
 473             });
 474         }
 475
 476         let mut diag =
 477             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 478
 479         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 480             diag.span_label(
 481                 outlived_fr_span,
 482                 format!(
 483                     "`{}` declared here, outside of the {} body",
 484                     outlived_fr_name, escapes_from
 485                 ),
 486             );
 487         }
 488
 489         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 490             diag.span_label(
 491                 fr_span,
 492                 format!(
 493                     "`{}` is a reference that is only valid in the {} body",
 494                     fr_name, escapes_from
 495                 ),
 496             );
 497
 498             diag.span_label(*span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
 499         }
 500
 501         // Only show an extra note if we can find an 'error region' for both of the region
 502         // variables. This avoids showing a noisy note that just mentions 'synthetic' regions
 503         // that don't help the user understand the error.
 504         if self.to_error_region(errci.fr).is_some()
 505             && self.to_error_region(errci.outlived_fr).is_some()
 506         {
 507             let fr_region_name = self.give_region_a_name(errci.fr).unwrap();
 508             fr_region_name.highlight_region_name(&mut diag);
 509             let outlived_fr_region_name = self.give_region_a_name(errci.outlived_fr).unwrap();
 510             outlived_fr_region_name.highlight_region_name(&mut diag);
 511
 512             diag.span_label(
 513                 *span,
 514                 format!(
 515                     "{}requires that `{}` must outlive `{}`",
 516                     category.description(),
 517                     fr_region_name,
 518                     outlived_fr_region_name,
 519                 ),
 520             );
 521         }
 522         diag
 523     }
 524
 525     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 526     /// explain what is happening.
 527     ///
 528     /// ```text
 529     /// error: unsatisfied lifetime constraints
 530     ///   --> $DIR/regions-creating-enums3.rs:17:5
 531     ///    |
 532     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 533     ///    |                -- -- lifetime `'b` defined here
 534     ///    |                |
 535     ///    |                lifetime `'a` defined here
 536     /// LL |     ast::add(x, y)
 537     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 538     ///    |                    is returning data with lifetime `'b`
 539     /// ```
 540     fn report_general_error(&self, errci: &ErrorConstraintInfo) -> DiagnosticBuilder<'tcx> {
 541         let ErrorConstraintInfo {
 542             fr,
 543             fr_is_local,
 544             outlived_fr,
 545             outlived_fr_is_local,
 546             span,
 547             category,
 548             ..
 549         } = errci;
 550
 551         let mut diag =
 552             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
 553
 554         let (_, mir_def_name) =
 555             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());
 556
 557         let fr_name = self.give_region_a_name(*fr).unwrap();
 558         fr_name.highlight_region_name(&mut diag);
 559         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 560         outlived_fr_name.highlight_region_name(&mut diag);
 561
 562         match (category, outlived_fr_is_local, fr_is_local) {
 563             (ConstraintCategory::Return(_), true, _) => {
 564                 diag.span_label(
 565                     *span,
 566                     format!(
 567                         "{} was supposed to return data with lifetime `{}` but it is returning \
 568                          data with lifetime `{}`",
 569                         mir_def_name, outlived_fr_name, fr_name
 570                     ),
 571                 );
 572             }
 573             _ => {
 574                 diag.span_label(
 575                     *span,
 576                     format!(
 577                         "{}requires that `{}` must outlive `{}`",
 578                         category.description(),
 579                         fr_name,
 580                         outlived_fr_name,
 581                     ),
 582                 );
 583             }
 584         }
 585
 586         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 587
 588         diag
 589     }
 590
 591     /// Adds a suggestion to errors where an `impl Trait` is returned.
 592     ///
 593     /// ```text
 594     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 595     ///       a constraint
 596     ///    |
 597     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 598     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 599     /// ```
 600     fn add_static_impl_trait_suggestion(
 601         &self,
 602         diag: &mut DiagnosticBuilder<'tcx>,
 603         fr: RegionVid,
 604         // We need to pass `fr_name` - computing it again will label it twice.
 605         fr_name: RegionName,
 606         outlived_fr: RegionVid,
 607     ) {
 608         if let (Some(f), Some(ty::RegionKind::ReStatic)) =
 609             (self.to_error_region(fr), self.to_error_region(outlived_fr))
 610         {
 611             if let Some(&ty::Opaque(did, substs)) = self
 612                 .infcx
 613                 .tcx
 614                 .is_suitable_region(f)
 615                 .map(|r| r.def_id)
 616                 .and_then(|id| self.infcx.tcx.return_type_impl_trait(id))
 617                 .map(|(ty, _)| ty.kind())
 618             {
 619                 // Check whether or not the impl trait return type is intended to capture
 620                 // data with the static lifetime.
 621                 //
 622                 // eg. check for `impl Trait + 'static` instead of `impl Trait`.
 623                 let has_static_predicate = {
 624                     let bounds = self.infcx.tcx.explicit_item_bounds(did);
 625
 626                     let mut found = false;
 627                     for (bound, _) in bounds {
 628                         if let ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(_, r)) =
 629                             bound.kind().skip_binder()
 630                         {
 631                             let r = r.subst(self.infcx.tcx, substs);
 632                             if let ty::RegionKind::ReStatic = r {
 633                                 found = true;
 634                                 break;
 635                             } else {
 636                                 // If there's already a lifetime bound, don't
 637                                 // suggest anything.
 638                                 return;
 639                             }
 640                         }
 641                     }
 642
 643                     found
 644                 };
 645
 646                 debug!(
 647                     "add_static_impl_trait_suggestion: has_static_predicate={:?}",
 648                     has_static_predicate
 649                 );
 650                 let static_str = kw::StaticLifetime;
 651                 // If there is a static predicate, then the only sensible suggestion is to replace
 652                 // fr with `'static`.
 653                 if has_static_predicate {
 654                     diag.help(&format!("consider replacing `{}` with `{}`", fr_name, static_str));
 655                 } else {
 656                     // Otherwise, we should suggest adding a constraint on the return type.
 657                     let span = self.infcx.tcx.def_span(did);
 658                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 659                         let suggestable_fr_name = if fr_name.was_named() {
 660                             fr_name.to_string()
 661                         } else {
 662                             "'_".to_string()
 663                         };
 664                         let span = if snippet.ends_with(';') {
 665                             // `type X = impl Trait;`
 666                             span.with_hi(span.hi() - BytePos(1))
 667                         } else {
 668                             span
 669                         };
 670                         let suggestion = format!(" + {}", suggestable_fr_name);
 671                         let span = span.shrink_to_hi();
 672                         diag.span_suggestion(
 673                             span,
 674                             &format!(
 675                                 "to allow this `impl Trait` to capture borrowed data with lifetime \
 676                                  `{}`, add `{}` as a bound",
 677                                 fr_name, suggestable_fr_name,
 678                             ),
 679                             suggestion,
 680                             Applicability::MachineApplicable,
 681                         );
 682                     }
 683                 }
 684             }
 685         }
 686     }
 687 }