compiler/rustc_borrowck/src/diagnostics/region_errors.rs

   1 //! Error reporting machinery for lifetime errors.
   2
   3 use rustc_data_structures::fx::FxHashSet;
   4 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan};
   5 use rustc_hir::def_id::DefId;
   6 use rustc_hir::intravisit::Visitor;
   7 use rustc_hir::{self as hir, Item, ItemKind, Node};
   8 use rustc_infer::infer::{
   9     error_reporting::nice_region_error::{
  10         self, find_anon_type, find_param_with_region, suggest_adding_lifetime_params,
  11         HirTraitObjectVisitor, NiceRegionError, TraitObjectVisitor,
  12     },
  13     error_reporting::unexpected_hidden_region_diagnostic,
  14     NllRegionVariableOrigin, RelateParamBound,
  15 };
  16 use rustc_middle::hir::place::PlaceBase;
  17 use rustc_middle::mir::{ConstraintCategory, ReturnConstraint};
  18 use rustc_middle::ty::subst::InternalSubsts;
  19 use rustc_middle::ty::Region;
  20 use rustc_middle::ty::TypeVisitor;
  21 use rustc_middle::ty::{self, RegionVid, Ty};
  22 use rustc_span::symbol::{kw, sym, Ident};
  23 use rustc_span::Span;
  24
  25 use crate::borrowck_errors;
  26 use crate::session_diagnostics::GenericDoesNotLiveLongEnough;
  27
  28 use super::{OutlivesSuggestionBuilder, RegionName};
  29 use crate::region_infer::BlameConstraint;
  30 use crate::{
  31     nll::ConstraintDescription,
  32     region_infer::{values::RegionElement, TypeTest},
  33     universal_regions::DefiningTy,
  34     MirBorrowckCtxt,
  35 };
  36
  37 impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
  38     fn description(&self) -> &'static str {
  39         // Must end with a space. Allows for empty names to be provided.
  40         match self {
  41             ConstraintCategory::Assignment => "assignment ",
  42             ConstraintCategory::Return(_) => "returning this value ",
  43             ConstraintCategory::Yield => "yielding this value ",
  44             ConstraintCategory::UseAsConst => "using this value as a constant ",
  45             ConstraintCategory::UseAsStatic => "using this value as a static ",
  46             ConstraintCategory::Cast => "cast ",
  47             ConstraintCategory::CallArgument(_) => "argument ",
  48             ConstraintCategory::TypeAnnotation => "type annotation ",
  49             ConstraintCategory::ClosureBounds => "closure body ",
  50             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
  51             ConstraintCategory::CopyBound => "copying this value ",
  52             ConstraintCategory::OpaqueType => "opaque type ",
  53             ConstraintCategory::ClosureUpvar(_) => "closure capture ",
  54             ConstraintCategory::Usage => "this usage ",
  55             ConstraintCategory::Predicate(_)
  56             | ConstraintCategory::Boring
  57             | ConstraintCategory::BoringNoLocation
  58             | ConstraintCategory::Internal => "",
  59         }
  60     }
  61 }
  62
  63 /// A collection of errors encountered during region inference. This is needed to efficiently
  64 /// report errors after borrow checking.
  65 ///
  66 /// Usually we expect this to either be empty or contain a small number of items, so we can avoid
  67 /// allocation most of the time.
  68 pub(crate) type RegionErrors<'tcx> = Vec<RegionErrorKind<'tcx>>;
  69
  70 #[derive(Clone, Debug)]
  71 pub(crate) enum RegionErrorKind<'tcx> {
  72     /// A generic bound failure for a type test (`T: 'a`).
  73     TypeTestError { type_test: TypeTest<'tcx> },
  74
  75     /// An unexpected hidden region for an opaque type.
  76     UnexpectedHiddenRegion {
  77         /// The span for the member constraint.
  78         span: Span,
  79         /// The hidden type.
  80         hidden_ty: Ty<'tcx>,
  81         /// The unexpected region.
  82         member_region: ty::Region<'tcx>,
  83     },
  84
  85     /// Higher-ranked subtyping error.
  86     BoundUniversalRegionError {
  87         /// The placeholder free region.
  88         longer_fr: RegionVid,
  89         /// The region element that erroneously must be outlived by `longer_fr`.
  90         error_element: RegionElement,
  91         /// The placeholder region.
  92         placeholder: ty::PlaceholderRegion,
  93     },
  94
  95     /// Any other lifetime error.
  96     RegionError {
  97         /// The origin of the region.
  98         fr_origin: NllRegionVariableOrigin,
  99         /// The region that should outlive `shorter_fr`.
 100         longer_fr: RegionVid,
 101         /// The region that should be shorter, but we can't prove it.
 102         shorter_fr: RegionVid,
 103         /// Indicates whether this is a reported error. We currently only report the first error
 104         /// encountered and leave the rest unreported so as not to overwhelm the user.
 105         is_reported: bool,
 106     },
 107 }
 108
 109 /// Information about the various region constraints involved in a borrow checker error.
 110 #[derive(Clone, Debug)]
 111 pub struct ErrorConstraintInfo<'tcx> {
 112     // fr: outlived_fr
 113     pub(super) fr: RegionVid,
 114     pub(super) fr_is_local: bool,
 115     pub(super) outlived_fr: RegionVid,
 116     pub(super) outlived_fr_is_local: bool,
 117
 118     // Category and span for best blame constraint
 119     pub(super) category: ConstraintCategory<'tcx>,
 120     pub(super) span: Span,
 121 }
 122
 123 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 124     /// Converts a region inference variable into a `ty::Region` that
 125     /// we can use for error reporting. If `r` is universally bound,
 126     /// then we use the name that we have on record for it. If `r` is
 127     /// existentially bound, then we check its inferred value and try
 128     /// to find a good name from that. Returns `None` if we can't find
 129     /// one (e.g., this is just some random part of the CFG).
 130     pub(super) fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
 131         self.to_error_region_vid(r).and_then(|r| self.regioncx.region_definition(r).external_name)
 132     }
 133
 134     /// Returns the `RegionVid` corresponding to the region returned by
 135     /// `to_error_region`.
 136     pub(super) fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
 137         if self.regioncx.universal_regions().is_universal_region(r) {
 138             Some(r)
 139         } else {
 140             // We just want something nameable, even if it's not
 141             // actually an upper bound.
 142             let upper_bound = self.regioncx.approx_universal_upper_bound(r);
 143
 144             if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
 145                 self.to_error_region_vid(upper_bound)
 146             } else {
 147                 None
 148             }
 149         }
 150     }
 151
 152     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 153     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
 154         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr).as_deref()
 155             && let ty::BoundRegionKind::BrEnv = free_region.bound_region
 156             && let DefiningTy::Closure(_, substs) = self.regioncx.universal_regions().defining_ty
 157         {
 158             return substs.as_closure().kind() == ty::ClosureKind::FnMut;
 159         }
 160
 161         false
 162     }
 163
 164     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
 165     pub(crate) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
 166         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
 167         // buffered in the `MirBorrowckCtxt`.
 168
 169         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();
 170
 171         for nll_error in nll_errors.into_iter() {
 172             match nll_error {
 173                 RegionErrorKind::TypeTestError { type_test } => {
 174                     // Try to convert the lower-bound region into something named we can print for the user.
 175                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 176
 177                     let type_test_span = type_test.locations.span(&self.body);
 178
 179                     if let Some(lower_bound_region) = lower_bound_region {
 180                         let generic_ty = type_test.generic_kind.to_ty(self.infcx.tcx);
 181                         let origin = RelateParamBound(type_test_span, generic_ty, None);
 182                         self.buffer_error(self.infcx.construct_generic_bound_failure(
 183                             self.body.source.def_id().expect_local(),
 184                             type_test_span,
 185                             Some(origin),
 186                             type_test.generic_kind,
 187                             lower_bound_region,
 188                         ));
 189                     } else {
 190                         // FIXME. We should handle this case better. It
 191                         // indicates that we have e.g., some region variable
 192                         // whose value is like `'a+'b` where `'a` and `'b` are
 193                         // distinct unrelated universal regions that are not
 194                         // known to outlive one another. It'd be nice to have
 195                         // some examples where this arises to decide how best
 196                         // to report it; we could probably handle it by
 197                         // iterating over the universal regions and reporting
 198                         // an error that multiple bounds are required.
 199                         self.buffer_error(self.infcx.tcx.sess.create_err(
 200                             GenericDoesNotLiveLongEnough {
 201                                 kind: type_test.generic_kind.to_string(),
 202                                 span: type_test_span,
 203                             },
 204                         ));
 205                     }
 206                 }
 207
 208                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, member_region } => {
 209                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 210                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 211                     self.buffer_error(unexpected_hidden_region_diagnostic(
 212                         self.infcx.tcx,
 213                         span,
 214                         named_ty,
 215                         named_region,
 216                     ));
 217                 }
 218
 219                 RegionErrorKind::BoundUniversalRegionError {
 220                     longer_fr,
 221                     placeholder,
 222                     error_element,
 223                 } => {
 224                     let error_vid = self.regioncx.region_from_element(longer_fr, &error_element);
 225
 226                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 227                     let (_, cause) = self.regioncx.find_outlives_blame_span(
 228                         &self.body,
 229                         longer_fr,
 230                         NllRegionVariableOrigin::Placeholder(placeholder),
 231                         error_vid,
 232                     );
 233
 234                     let universe = placeholder.universe;
 235                     let universe_info = self.regioncx.universe_info(universe);
 236
 237                     universe_info.report_error(self, placeholder, error_element, cause);
 238                 }
 239
 240                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 241                     if is_reported {
 242                         self.report_region_error(
 243                             longer_fr,
 244                             fr_origin,
 245                             shorter_fr,
 246                             &mut outlives_suggestion,
 247                         );
 248                     } else {
 249                         // We only report the first error, so as not to overwhelm the user. See
 250                         // `RegRegionErrorKind` docs.
 251                         //
 252                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 253                         // FIXME: try collecting these constraints on the outlives suggestion
 254                         // builder. Does it make the suggestions any better?
 255                         debug!(
 256                             "Unreported region error: can't prove that {:?}: {:?}",
 257                             longer_fr, shorter_fr
 258                         );
 259                     }
 260                 }
 261             }
 262         }
 263
 264         // Emit one outlives suggestions for each MIR def we borrowck
 265         outlives_suggestion.add_suggestion(self);
 266     }
 267
 268     fn get_impl_ident_and_self_ty_from_trait(
 269         &self,
 270         def_id: DefId,
 271         trait_objects: &FxHashSet<DefId>,
 272     ) -> Option<(Ident, &'tcx hir::Ty<'tcx>)> {
 273         let tcx = self.infcx.tcx;
 274         match tcx.hir().get_if_local(def_id) {
 275             Some(Node::ImplItem(impl_item)) => {
 276                 match tcx.hir().find_by_def_id(tcx.hir().get_parent_item(impl_item.hir_id())) {
 277                     Some(Node::Item(Item {
 278                         kind: ItemKind::Impl(hir::Impl { self_ty, .. }),
 279                         ..
 280                     })) => Some((impl_item.ident, self_ty)),
 281                     _ => None,
 282                 }
 283             }
 284             Some(Node::TraitItem(trait_item)) => {
 285                 let trait_did = tcx.hir().get_parent_item(trait_item.hir_id());
 286                 match tcx.hir().find_by_def_id(trait_did) {
 287                     Some(Node::Item(Item { kind: ItemKind::Trait(..), .. })) => {
 288                         // The method being called is defined in the `trait`, but the `'static`
 289                         // obligation comes from the `impl`. Find that `impl` so that we can point
 290                         // at it in the suggestion.
 291                         let trait_did = trait_did.to_def_id();
 292                         match tcx
 293                             .hir()
 294                             .trait_impls(trait_did)
 295                             .iter()
 296                             .filter_map(|&impl_did| {
 297                                 match tcx.hir().get_if_local(impl_did.to_def_id()) {
 298                                     Some(Node::Item(Item {
 299                                         kind: ItemKind::Impl(hir::Impl { self_ty, .. }),
 300                                         ..
 301                                     })) if trait_objects.iter().all(|did| {
 302                                         // FIXME: we should check `self_ty` against the receiver
 303                                         // type in the `UnifyReceiver` context, but for now, use
 304                                         // this imperfect proxy. This will fail if there are
 305                                         // multiple `impl`s for the same trait like
 306                                         // `impl Foo for Box<dyn Bar>` and `impl Foo for dyn Bar`.
 307                                         // In that case, only the first one will get suggestions.
 308                                         let mut traits = vec![];
 309                                         let mut hir_v = HirTraitObjectVisitor(&mut traits, *did);
 310                                         hir_v.visit_ty(self_ty);
 311                                         !traits.is_empty()
 312                                     }) =>
 313                                     {
 314                                         Some(self_ty)
 315                                     }
 316                                     _ => None,
 317                                 }
 318                             })
 319                             .next()
 320                         {
 321                             Some(self_ty) => Some((trait_item.ident, self_ty)),
 322                             _ => None,
 323                         }
 324                     }
 325                     _ => None,
 326                 }
 327             }
 328             _ => None,
 329         }
 330     }
 331
 332     /// Report an error because the universal region `fr` was required to outlive
 333     /// `outlived_fr` but it is not known to do so. For example:
 334     ///
 335     /// ```compile_fail,E0312
 336     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 337     /// ```
 338     ///
 339     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 340     pub(crate) fn report_region_error(
 341         &mut self,
 342         fr: RegionVid,
 343         fr_origin: NllRegionVariableOrigin,
 344         outlived_fr: RegionVid,
 345         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 346     ) {
 347         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 348
 349         let BlameConstraint { category, cause, variance_info, from_closure: _ } =
 350             self.regioncx.best_blame_constraint(&self.body, fr, fr_origin, |r| {
 351                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 352             });
 353
 354         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);
 355
 356         // Check if we can use one of the "nice region errors".
 357         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 358             let nice = NiceRegionError::new_from_span(self.infcx, cause.span, o, f);
 359             if let Some(diag) = nice.try_report_from_nll() {
 360                 self.buffer_error(diag);
 361                 return;
 362             }
 363         }
 364
 365         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 366             self.regioncx.universal_regions().is_local_free_region(fr),
 367             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 368         );
 369
 370         debug!(
 371             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 372             fr_is_local, outlived_fr_is_local, category
 373         );
 374
 375         let errci = ErrorConstraintInfo {
 376             fr,
 377             outlived_fr,
 378             fr_is_local,
 379             outlived_fr_is_local,
 380             category,
 381             span: cause.span,
 382         };
 383
 384         let mut diag = match (category, fr_is_local, outlived_fr_is_local) {
 385             (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
 386                 self.report_fnmut_error(&errci, kind)
 387             }
 388             (ConstraintCategory::Assignment, true, false)
 389             | (ConstraintCategory::CallArgument(_), true, false) => {
 390                 let mut db = self.report_escaping_data_error(&errci);
 391
 392                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 393                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 394
 395                 db
 396             }
 397             _ => {
 398                 let mut db = self.report_general_error(&errci);
 399
 400                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 401                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 402
 403                 db
 404             }
 405         };
 406
 407         match variance_info {
 408             ty::VarianceDiagInfo::None => {}
 409             ty::VarianceDiagInfo::Invariant { ty, param_index } => {
 410                 let (desc, note) = match ty.kind() {
 411                     ty::RawPtr(ty_mut) => {
 412                         assert_eq!(ty_mut.mutbl, rustc_hir::Mutability::Mut);
 413                         (
 414                             format!("a mutable pointer to `{}`", ty_mut.ty),
 415                             "mutable pointers are invariant over their type parameter".to_string(),
 416                         )
 417                     }
 418                     ty::Ref(_, inner_ty, mutbl) => {
 419                         assert_eq!(*mutbl, rustc_hir::Mutability::Mut);
 420                         (
 421                             format!("a mutable reference to `{inner_ty}`"),
 422                             "mutable references are invariant over their type parameter"
 423                                 .to_string(),
 424                         )
 425                     }
 426                     ty::Adt(adt, substs) => {
 427                         let generic_arg = substs[param_index as usize];
 428                         let identity_substs =
 429                             InternalSubsts::identity_for_item(self.infcx.tcx, adt.did());
 430                         let base_ty = self.infcx.tcx.mk_adt(*adt, identity_substs);
 431                         let base_generic_arg = identity_substs[param_index as usize];
 432                         let adt_desc = adt.descr();
 433
 434                         let desc = format!(
 435                             "the type `{ty}`, which makes the generic argument `{generic_arg}` invariant"
 436                         );
 437                         let note = format!(
 438                             "the {adt_desc} `{base_ty}` is invariant over the parameter `{base_generic_arg}`"
 439                         );
 440                         (desc, note)
 441                     }
 442                     ty::FnDef(def_id, _) => {
 443                         let name = self.infcx.tcx.item_name(*def_id);
 444                         let identity_substs =
 445                             InternalSubsts::identity_for_item(self.infcx.tcx, *def_id);
 446                         let desc = format!("a function pointer to `{name}`");
 447                         let note = format!(
 448                             "the function `{name}` is invariant over the parameter `{}`",
 449                             identity_substs[param_index as usize]
 450                         );
 451                         (desc, note)
 452                     }
 453                     _ => panic!("Unexpected type {:?}", ty),
 454                 };
 455                 diag.note(&format!("requirement occurs because of {desc}",));
 456                 diag.note(&note);
 457                 diag.help("see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance");
 458             }
 459         }
 460
 461         self.buffer_error(diag);
 462     }
 463
 464     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 465     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 466     ///
 467     /// ```text
 468     /// error: captured variable cannot escape `FnMut` closure body
 469     ///   --> $DIR/issue-53040.rs:15:8
 470     ///    |
 471     /// LL |     || &mut v;
 472     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 473     ///    |     |
 474     ///    |     inferred to be a `FnMut` closure
 475     ///    |
 476     ///    = note: `FnMut` closures only have access to their captured variables while they are
 477     ///            executing...
 478     ///    = note: ...therefore, returned references to captured variables will escape the closure
 479     /// ```
 480     fn report_fnmut_error(
 481         &self,
 482         errci: &ErrorConstraintInfo<'tcx>,
 483         kind: ReturnConstraint,
 484     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 485         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 486
 487         let mut diag = self
 488             .infcx
 489             .tcx
 490             .sess
 491             .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
 492
 493         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 494         if let ty::Opaque(def_id, _) = *output_ty.kind() {
 495             output_ty = self.infcx.tcx.type_of(def_id)
 496         };
 497
 498         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 499
 500         let message = match output_ty.kind() {
 501             ty::Closure(_, _) => {
 502                 "returns a closure that contains a reference to a captured variable, which then \
 503                  escapes the closure body"
 504             }
 505             ty::Adt(def, _) if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did()) => {
 506                 "returns an `async` block that contains a reference to a captured variable, which then \
 507                  escapes the closure body"
 508             }
 509             _ => "returns a reference to a captured variable which escapes the closure body",
 510         };
 511
 512         diag.span_label(*span, message);
 513
 514         if let ReturnConstraint::ClosureUpvar(upvar_field) = kind {
 515             let def_id = match self.regioncx.universal_regions().defining_ty {
 516                 DefiningTy::Closure(def_id, _) => def_id,
 517                 ty => bug!("unexpected DefiningTy {:?}", ty),
 518             };
 519
 520             let captured_place = &self.upvars[upvar_field.index()].place;
 521             let defined_hir = match captured_place.place.base {
 522                 PlaceBase::Local(hirid) => Some(hirid),
 523                 PlaceBase::Upvar(upvar) => Some(upvar.var_path.hir_id),
 524                 _ => None,
 525             };
 526
 527             if let Some(def_hir) = defined_hir {
 528                 let upvars_map = self.infcx.tcx.upvars_mentioned(def_id).unwrap();
 529                 let upvar_def_span = self.infcx.tcx.hir().span(def_hir);
 530                 let upvar_span = upvars_map.get(&def_hir).unwrap().span;
 531                 diag.span_label(upvar_def_span, "variable defined here");
 532                 diag.span_label(upvar_span, "variable captured here");
 533             }
 534         }
 535
 536         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
 537             diag.span_label(fr_span, "inferred to be a `FnMut` closure");
 538         }
 539
 540         diag.note(
 541             "`FnMut` closures only have access to their captured variables while they are \
 542              executing...",
 543         );
 544         diag.note("...therefore, they cannot allow references to captured variables to escape");
 545
 546         diag
 547     }
 548
 549     /// Reports an error specifically for when data is escaping a closure.
 550     ///
 551     /// ```text
 552     /// error: borrowed data escapes outside of function
 553     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 554     ///    |
 555     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 556     ///    |              - `x` is a reference that is only valid in the function body
 557     /// LL |     // but ref_obj will not, so warn.
 558     /// LL |     ref_obj(x)
 559     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 560     /// ```
 561     fn report_escaping_data_error(
 562         &self,
 563         errci: &ErrorConstraintInfo<'tcx>,
 564     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 565         let ErrorConstraintInfo { span, category, .. } = errci;
 566
 567         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 568             self.infcx.tcx,
 569             &self.body,
 570             &self.local_names,
 571             &self.upvars,
 572             errci.fr,
 573         );
 574         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 575             self.infcx.tcx,
 576             &self.body,
 577             &self.local_names,
 578             &self.upvars,
 579             errci.outlived_fr,
 580         );
 581
 582         let (_, escapes_from) = self
 583             .infcx
 584             .tcx
 585             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());
 586
 587         // Revert to the normal error in these cases.
 588         // Assignments aren't "escapes" in function items.
 589         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 590             || (*category == ConstraintCategory::Assignment
 591                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 592             || self.regioncx.universal_regions().defining_ty.is_const()
 593         {
 594             return self.report_general_error(&ErrorConstraintInfo {
 595                 fr_is_local: true,
 596                 outlived_fr_is_local: false,
 597                 ..*errci
 598             });
 599         }
 600
 601         let mut diag =
 602             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 603
 604         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 605             diag.span_label(
 606                 outlived_fr_span,
 607                 format!("`{outlived_fr_name}` declared here, outside of the {escapes_from} body",),
 608             );
 609         }
 610
 611         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 612             diag.span_label(
 613                 fr_span,
 614                 format!(
 615                     "`{fr_name}` is a reference that is only valid in the {escapes_from} body",
 616                 ),
 617             );
 618
 619             diag.span_label(*span, format!("`{fr_name}` escapes the {escapes_from} body here"));
 620         }
 621
 622         // Only show an extra note if we can find an 'error region' for both of the region
 623         // variables. This avoids showing a noisy note that just mentions 'synthetic' regions
 624         // that don't help the user understand the error.
 625         match (self.to_error_region(errci.fr), self.to_error_region(errci.outlived_fr)) {
 626             (Some(f), Some(o)) => {
 627                 self.maybe_suggest_constrain_dyn_trait_impl(&mut diag, f, o, category);
 628
 629                 let fr_region_name = self.give_region_a_name(errci.fr).unwrap();
 630                 fr_region_name.highlight_region_name(&mut diag);
 631                 let outlived_fr_region_name = self.give_region_a_name(errci.outlived_fr).unwrap();
 632                 outlived_fr_region_name.highlight_region_name(&mut diag);
 633
 634                 diag.span_label(
 635                     *span,
 636                     format!(
 637                         "{}requires that `{}` must outlive `{}`",
 638                         category.description(),
 639                         fr_region_name,
 640                         outlived_fr_region_name,
 641                     ),
 642                 );
 643             }
 644             _ => {}
 645         }
 646
 647         diag
 648     }
 649
 650     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 651     /// explain what is happening.
 652     ///
 653     /// ```text
 654     /// error: unsatisfied lifetime constraints
 655     ///   --> $DIR/regions-creating-enums3.rs:17:5
 656     ///    |
 657     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 658     ///    |                -- -- lifetime `'b` defined here
 659     ///    |                |
 660     ///    |                lifetime `'a` defined here
 661     /// LL |     ast::add(x, y)
 662     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 663     ///    |                    is returning data with lifetime `'b`
 664     /// ```
 665     fn report_general_error(
 666         &self,
 667         errci: &ErrorConstraintInfo<'tcx>,
 668     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 669         let ErrorConstraintInfo {
 670             fr,
 671             fr_is_local,
 672             outlived_fr,
 673             outlived_fr_is_local,
 674             span,
 675             category,
 676             ..
 677         } = errci;
 678
 679         let mut diag =
 680             self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
 681
 682         let (_, mir_def_name) =
 683             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());
 684
 685         let fr_name = self.give_region_a_name(*fr).unwrap();
 686         fr_name.highlight_region_name(&mut diag);
 687         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 688         outlived_fr_name.highlight_region_name(&mut diag);
 689
 690         match (category, outlived_fr_is_local, fr_is_local) {
 691             (ConstraintCategory::Return(_), true, _) => {
 692                 diag.span_label(
 693                     *span,
 694                     format!(
 695                         "{mir_def_name} was supposed to return data with lifetime `{outlived_fr_name}` but it is returning \
 696                          data with lifetime `{fr_name}`",
 697                     ),
 698                 );
 699             }
 700             _ => {
 701                 diag.span_label(
 702                     *span,
 703                     format!(
 704                         "{}requires that `{}` must outlive `{}`",
 705                         category.description(),
 706                         fr_name,
 707                         outlived_fr_name,
 708                     ),
 709                 );
 710             }
 711         }
 712
 713         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 714         self.suggest_adding_lifetime_params(&mut diag, *fr, *outlived_fr);
 715
 716         diag
 717     }
 718
 719     /// Adds a suggestion to errors where an `impl Trait` is returned.
 720     ///
 721     /// ```text
 722     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 723     ///       a constraint
 724     ///    |
 725     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 726     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 727     /// ```
 728     fn add_static_impl_trait_suggestion(
 729         &self,
 730         diag: &mut Diagnostic,
 731         fr: RegionVid,
 732         // We need to pass `fr_name` - computing it again will label it twice.
 733         fr_name: RegionName,
 734         outlived_fr: RegionVid,
 735     ) {
 736         if let (Some(f), Some(outlived_f)) =
 737             (self.to_error_region(fr), self.to_error_region(outlived_fr))
 738         {
 739             if *outlived_f != ty::ReStatic {
 740                 return;
 741             }
 742
 743             let fn_returns = self
 744                 .infcx
 745                 .tcx
 746                 .is_suitable_region(f)
 747                 .map(|r| self.infcx.tcx.return_type_impl_or_dyn_traits(r.def_id))
 748                 .unwrap_or_default();
 749
 750             if fn_returns.is_empty() {
 751                 return;
 752             }
 753
 754             let param = if let Some(param) = find_param_with_region(self.infcx.tcx, f, outlived_f) {
 755                 param
 756             } else {
 757                 return;
 758             };
 759
 760             let lifetime = if f.has_name() { fr_name.name } else { kw::UnderscoreLifetime };
 761
 762             let arg = match param.param.pat.simple_ident() {
 763                 Some(simple_ident) => format!("argument `{}`", simple_ident),
 764                 None => "the argument".to_string(),
 765             };
 766             let captures = format!("captures data from {}", arg);
 767
 768             return nice_region_error::suggest_new_region_bound(
 769                 self.infcx.tcx,
 770                 diag,
 771                 fn_returns,
 772                 lifetime.to_string(),
 773                 Some(arg),
 774                 captures,
 775                 Some((param.param_ty_span, param.param_ty.to_string())),
 776             );
 777         }
 778     }
 779
 780     fn maybe_suggest_constrain_dyn_trait_impl(
 781         &self,
 782         diag: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
 783         f: Region<'tcx>,
 784         o: Region<'tcx>,
 785         category: &ConstraintCategory<'tcx>,
 786     ) {
 787         if !o.is_static() {
 788             return;
 789         }
 790
 791         let tcx = self.infcx.tcx;
 792
 793         let instance = if let ConstraintCategory::CallArgument(Some(func_ty)) = category {
 794             let (fn_did, substs) = match func_ty.kind() {
 795                 ty::FnDef(fn_did, substs) => (fn_did, substs),
 796                 _ => return,
 797             };
 798             debug!(?fn_did, ?substs);
 799
 800             // Only suggest this on function calls, not closures
 801             let ty = tcx.type_of(fn_did);
 802             debug!("ty: {:?}, ty.kind: {:?}", ty, ty.kind());
 803             if let ty::Closure(_, _) = ty.kind() {
 804                 return;
 805             }
 806
 807             if let Ok(Some(instance)) = ty::Instance::resolve(
 808                 tcx,
 809                 self.param_env,
 810                 *fn_did,
 811                 self.infcx.resolve_vars_if_possible(substs),
 812             ) {
 813                 instance
 814             } else {
 815                 return;
 816             }
 817         } else {
 818             return;
 819         };
 820
 821         let param = match find_param_with_region(tcx, f, o) {
 822             Some(param) => param,
 823             None => return,
 824         };
 825         debug!(?param);
 826
 827         let mut visitor = TraitObjectVisitor(FxHashSet::default());
 828         visitor.visit_ty(param.param_ty);
 829
 830         let Some((ident, self_ty)) =
 831             self.get_impl_ident_and_self_ty_from_trait(instance.def_id(), &visitor.0) else {return};
 832
 833         self.suggest_constrain_dyn_trait_in_impl(diag, &visitor.0, ident, self_ty);
 834     }
 835
 836     #[instrument(skip(self, err), level = "debug")]
 837     fn suggest_constrain_dyn_trait_in_impl(
 838         &self,
 839         err: &mut Diagnostic,
 840         found_dids: &FxHashSet<DefId>,
 841         ident: Ident,
 842         self_ty: &hir::Ty<'_>,
 843     ) -> bool {
 844         debug!("err: {:#?}", err);
 845         let mut suggested = false;
 846         for found_did in found_dids {
 847             let mut traits = vec![];
 848             let mut hir_v = HirTraitObjectVisitor(&mut traits, *found_did);
 849             hir_v.visit_ty(&self_ty);
 850             debug!("trait spans found: {:?}", traits);
 851             for span in &traits {
 852                 let mut multi_span: MultiSpan = vec![*span].into();
 853                 multi_span.push_span_label(
 854                     *span,
 855                     "this has an implicit `'static` lifetime requirement".to_string(),
 856                 );
 857                 multi_span.push_span_label(
 858                     ident.span,
 859                     "calling this method introduces the `impl`'s 'static` requirement".to_string(),
 860                 );
 861                 err.span_note(multi_span, "the used `impl` has a `'static` requirement");
 862                 err.span_suggestion_verbose(
 863                     span.shrink_to_hi(),
 864                     "consider relaxing the implicit `'static` requirement",
 865                     " + '_",
 866                     Applicability::MaybeIncorrect,
 867                 );
 868                 suggested = true;
 869             }
 870         }
 871         suggested
 872     }
 873
 874     fn suggest_adding_lifetime_params(
 875         &self,
 876         diag: &mut Diagnostic,
 877         sub: RegionVid,
 878         sup: RegionVid,
 879     ) {
 880         let (Some(sub), Some(sup)) = (self.to_error_region(sub), self.to_error_region(sup)) else {
 881             return
 882         };
 883
 884         let Some((ty_sub, _)) = self
 885             .infcx
 886             .tcx
 887             .is_suitable_region(sub)
 888             .and_then(|anon_reg| find_anon_type(self.infcx.tcx, sub, &anon_reg.boundregion)) else {
 889             return
 890         };
 891
 892         let Some((ty_sup, _)) = self
 893             .infcx
 894             .tcx
 895             .is_suitable_region(sup)
 896             .and_then(|anon_reg| find_anon_type(self.infcx.tcx, sup, &anon_reg.boundregion)) else {
 897             return
 898         };
 899
 900         suggest_adding_lifetime_params(self.infcx.tcx, sub, ty_sup, ty_sub, diag);
 901     }
 902 }