compiler/rustc_borrowck/src/diagnostics/region_errors.rs

   1 #![deny(rustc::untranslatable_diagnostic)]
   2 #![deny(rustc::diagnostic_outside_of_impl)]
   3 //! Error reporting machinery for lifetime errors.
   4
   5 use rustc_data_structures::fx::FxIndexSet;
   6 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan};
   7 use rustc_hir as hir;
   8 use rustc_hir::def::Res::Def;
   9 use rustc_hir::def_id::DefId;
  10 use rustc_hir::intravisit::Visitor;
  11 use rustc_hir::GenericBound::Trait;
  12 use rustc_hir::QPath::Resolved;
  13 use rustc_hir::WherePredicate::BoundPredicate;
  14 use rustc_hir::{PolyTraitRef, TyKind, WhereBoundPredicate};
  15 use rustc_infer::infer::{
  16     error_reporting::nice_region_error::{
  17         self, find_anon_type, find_param_with_region, suggest_adding_lifetime_params,
  18         HirTraitObjectVisitor, NiceRegionError, TraitObjectVisitor,
  19     },
  20     error_reporting::unexpected_hidden_region_diagnostic,
  21     NllRegionVariableOrigin, RelateParamBound,
  22 };
  23 use rustc_middle::hir::place::PlaceBase;
  24 use rustc_middle::mir::{ConstraintCategory, ReturnConstraint};
  25 use rustc_middle::ty::subst::InternalSubsts;
  26 use rustc_middle::ty::TypeVisitor;
  27 use rustc_middle::ty::{self, RegionVid, Ty};
  28 use rustc_middle::ty::{Region, TyCtxt};
  29 use rustc_span::symbol::{kw, Ident};
  30 use rustc_span::{Span, DUMMY_SP};
  31
  32 use crate::borrowck_errors;
  33 use crate::session_diagnostics::{
  34     FnMutError, FnMutReturnTypeErr, GenericDoesNotLiveLongEnough, LifetimeOutliveErr,
  35     LifetimeReturnCategoryErr, RequireStaticErr, VarHereDenote,
  36 };
  37
  38 use super::{OutlivesSuggestionBuilder, RegionName};
  39 use crate::region_infer::{BlameConstraint, ExtraConstraintInfo};
  40 use crate::{
  41     nll::ConstraintDescription,
  42     region_infer::{values::RegionElement, TypeTest},
  43     universal_regions::DefiningTy,
  44     MirBorrowckCtxt,
  45 };
  46
  47 impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
  48     fn description(&self) -> &'static str {
  49         // Must end with a space. Allows for empty names to be provided.
  50         match self {
  51             ConstraintCategory::Assignment => "assignment ",
  52             ConstraintCategory::Return(_) => "returning this value ",
  53             ConstraintCategory::Yield => "yielding this value ",
  54             ConstraintCategory::UseAsConst => "using this value as a constant ",
  55             ConstraintCategory::UseAsStatic => "using this value as a static ",
  56             ConstraintCategory::Cast => "cast ",
  57             ConstraintCategory::CallArgument(_) => "argument ",
  58             ConstraintCategory::TypeAnnotation => "type annotation ",
  59             ConstraintCategory::ClosureBounds => "closure body ",
  60             ConstraintCategory::SizedBound => "proving this value is `Sized` ",
  61             ConstraintCategory::CopyBound => "copying this value ",
  62             ConstraintCategory::OpaqueType => "opaque type ",
  63             ConstraintCategory::ClosureUpvar(_) => "closure capture ",
  64             ConstraintCategory::Usage => "this usage ",
  65             ConstraintCategory::Predicate(_)
  66             | ConstraintCategory::Boring
  67             | ConstraintCategory::BoringNoLocation
  68             | ConstraintCategory::Internal => "",
  69         }
  70     }
  71 }
  72
  73 /// A collection of errors encountered during region inference. This is needed to efficiently
  74 /// report errors after borrow checking.
  75 ///
  76 /// Usually we expect this to either be empty or contain a small number of items, so we can avoid
  77 /// allocation most of the time.
  78 pub(crate) struct RegionErrors<'tcx>(Vec<RegionErrorKind<'tcx>>, TyCtxt<'tcx>);
  79
  80 impl<'tcx> RegionErrors<'tcx> {
  81     pub fn new(tcx: TyCtxt<'tcx>) -> Self {
  82         Self(vec![], tcx)
  83     }
  84     #[track_caller]
  85     pub fn push(&mut self, val: impl Into<RegionErrorKind<'tcx>>) {
  86         let val = val.into();
  87         self.1.sess.delay_span_bug(DUMMY_SP, format!("{val:?}"));
  88         self.0.push(val);
  89     }
  90     pub fn is_empty(&self) -> bool {
  91         self.0.is_empty()
  92     }
  93     pub fn into_iter(self) -> impl Iterator<Item = RegionErrorKind<'tcx>> {
  94         self.0.into_iter()
  95     }
  96 }
  97
  98 #[derive(Clone, Debug)]
  99 pub(crate) enum RegionErrorKind<'tcx> {
 100     /// A generic bound failure for a type test (`T: 'a`).
 101     TypeTestError { type_test: TypeTest<'tcx> },
 102
 103     /// An unexpected hidden region for an opaque type.
 104     UnexpectedHiddenRegion {
 105         /// The span for the member constraint.
 106         span: Span,
 107         /// The hidden type.
 108         hidden_ty: Ty<'tcx>,
 109         /// The opaque type.
 110         key: ty::OpaqueTypeKey<'tcx>,
 111         /// The unexpected region.
 112         member_region: ty::Region<'tcx>,
 113     },
 114
 115     /// Higher-ranked subtyping error.
 116     BoundUniversalRegionError {
 117         /// The placeholder free region.
 118         longer_fr: RegionVid,
 119         /// The region element that erroneously must be outlived by `longer_fr`.
 120         error_element: RegionElement,
 121         /// The placeholder region.
 122         placeholder: ty::PlaceholderRegion,
 123     },
 124
 125     /// Any other lifetime error.
 126     RegionError {
 127         /// The origin of the region.
 128         fr_origin: NllRegionVariableOrigin,
 129         /// The region that should outlive `shorter_fr`.
 130         longer_fr: RegionVid,
 131         /// The region that should be shorter, but we can't prove it.
 132         shorter_fr: RegionVid,
 133         /// Indicates whether this is a reported error. We currently only report the first error
 134         /// encountered and leave the rest unreported so as not to overwhelm the user.
 135         is_reported: bool,
 136     },
 137 }
 138
 139 /// Information about the various region constraints involved in a borrow checker error.
 140 #[derive(Clone, Debug)]
 141 pub struct ErrorConstraintInfo<'tcx> {
 142     // fr: outlived_fr
 143     pub(super) fr: RegionVid,
 144     pub(super) fr_is_local: bool,
 145     pub(super) outlived_fr: RegionVid,
 146     pub(super) outlived_fr_is_local: bool,
 147
 148     // Category and span for best blame constraint
 149     pub(super) category: ConstraintCategory<'tcx>,
 150     pub(super) span: Span,
 151 }
 152
 153 impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 154     /// Converts a region inference variable into a `ty::Region` that
 155     /// we can use for error reporting. If `r` is universally bound,
 156     /// then we use the name that we have on record for it. If `r` is
 157     /// existentially bound, then we check its inferred value and try
 158     /// to find a good name from that. Returns `None` if we can't find
 159     /// one (e.g., this is just some random part of the CFG).
 160     pub(super) fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
 161         self.to_error_region_vid(r).and_then(|r| self.regioncx.region_definition(r).external_name)
 162     }
 163
 164     /// Returns the `RegionVid` corresponding to the region returned by
 165     /// `to_error_region`.
 166     pub(super) fn to_error_region_vid(&self, r: RegionVid) -> Option<RegionVid> {
 167         if self.regioncx.universal_regions().is_universal_region(r) {
 168             Some(r)
 169         } else {
 170             // We just want something nameable, even if it's not
 171             // actually an upper bound.
 172             let upper_bound = self.regioncx.approx_universal_upper_bound(r);
 173
 174             if self.regioncx.upper_bound_in_region_scc(r, upper_bound) {
 175                 self.to_error_region_vid(upper_bound)
 176             } else {
 177                 None
 178             }
 179         }
 180     }
 181
 182     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
 183     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
 184         if let Some(ty::ReFree(free_region)) = self.to_error_region(fr).as_deref()
 185             && let ty::BoundRegionKind::BrEnv = free_region.bound_region
 186             && let DefiningTy::Closure(_, substs) = self.regioncx.universal_regions().defining_ty
 187         {
 188             return substs.as_closure().kind() == ty::ClosureKind::FnMut;
 189         }
 190
 191         false
 192     }
 193
 194     // For generic associated types (GATs) which implied 'static requirement
 195     // from higher-ranked trait bounds (HRTB). Try to locate span of the trait
 196     // and the span which bounded to the trait for adding 'static lifetime suggestion
 197     fn suggest_static_lifetime_for_gat_from_hrtb(
 198         &self,
 199         diag: &mut DiagnosticBuilder<'_, ErrorGuaranteed>,
 200         lower_bound: RegionVid,
 201     ) {
 202         let mut suggestions = vec![];
 203         let hir = self.infcx.tcx.hir();
 204
 205         // find generic associated types in the given region 'lower_bound'
 206         let gat_id_and_generics = self
 207             .regioncx
 208             .placeholders_contained_in(lower_bound)
 209             .map(|placeholder| {
 210                 if let Some(id) = placeholder.bound.kind.get_id()
 211                     && let Some(placeholder_id) = id.as_local()
 212                     && let gat_hir_id = hir.local_def_id_to_hir_id(placeholder_id)
 213                     && let Some(generics_impl) = hir.get_parent(gat_hir_id).generics()
 214                 {
 215                     Some((gat_hir_id, generics_impl))
 216                 } else {
 217                     None
 218                 }
 219             })
 220             .collect::<Vec<_>>();
 221         debug!(?gat_id_and_generics);
 222
 223         // find higher-ranked trait bounds bounded to the generic associated types
 224         let mut hrtb_bounds = vec![];
 225         gat_id_and_generics.iter().flatten().for_each(|(gat_hir_id, generics)| {
 226             for pred in generics.predicates {
 227                 let BoundPredicate(
 228                         WhereBoundPredicate {
 229                             bound_generic_params,
 230                             bounds,
 231                             ..
 232                         }) = pred else { continue; };
 233                 if bound_generic_params
 234                     .iter()
 235                     .rfind(|bgp| hir.local_def_id_to_hir_id(bgp.def_id) == *gat_hir_id)
 236                     .is_some()
 237                 {
 238                     for bound in *bounds {
 239                         hrtb_bounds.push(bound);
 240                     }
 241                 }
 242             }
 243         });
 244         debug!(?hrtb_bounds);
 245
 246         hrtb_bounds.iter().for_each(|bound| {
 247             let Trait(PolyTraitRef { trait_ref, span: trait_span, .. }, _) = bound else { return; };
 248             diag.span_note(
 249                 *trait_span,
 250                 format!("due to current limitations in the borrow checker, this implies a `'static` lifetime")
 251             );
 252             let Some(generics_fn) = hir.get_generics(self.body.source.def_id().expect_local()) else { return; };
 253             let Def(_, trait_res_defid) = trait_ref.path.res else { return; };
 254             debug!(?generics_fn);
 255             generics_fn.predicates.iter().for_each(|predicate| {
 256                 let BoundPredicate(
 257                     WhereBoundPredicate {
 258                         span: bounded_span,
 259                         bounded_ty,
 260                         bounds,
 261                         ..
 262                     }
 263                 ) = predicate else { return; };
 264                 bounds.iter().for_each(|bd| {
 265                     if let Trait(PolyTraitRef { trait_ref: tr_ref, .. }, _) = bd
 266                         && let Def(_, res_defid) = tr_ref.path.res
 267                         && res_defid == trait_res_defid // trait id matches
 268                         && let TyKind::Path(Resolved(_, path)) = bounded_ty.kind
 269                         && let Def(_, defid) = path.res
 270                         && generics_fn.params
 271                             .iter()
 272                             .rfind(|param| param.def_id.to_def_id() == defid)
 273                             .is_some() {
 274                             suggestions.push((bounded_span.shrink_to_hi(), format!(" + 'static")));
 275                         }
 276                 });
 277             });
 278         });
 279         if suggestions.len() > 0 {
 280             suggestions.dedup();
 281             diag.multipart_suggestion_verbose(
 282                 format!("consider restricting the type parameter to the `'static` lifetime"),
 283                 suggestions,
 284                 Applicability::MaybeIncorrect,
 285             );
 286         }
 287     }
 288
 289     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.
 290     pub(crate) fn report_region_errors(&mut self, nll_errors: RegionErrors<'tcx>) {
 291         // Iterate through all the errors, producing a diagnostic for each one. The diagnostics are
 292         // buffered in the `MirBorrowckCtxt`.
 293
 294         let mut outlives_suggestion = OutlivesSuggestionBuilder::default();
 295         let mut last_unexpected_hidden_region: Option<(Span, Ty<'_>, ty::OpaqueTypeKey<'tcx>)> =
 296             None;
 297
 298         for nll_error in nll_errors.into_iter() {
 299             match nll_error {
 300                 RegionErrorKind::TypeTestError { type_test } => {
 301                     // Try to convert the lower-bound region into something named we can print for the user.
 302                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 303
 304                     let type_test_span = type_test.span;
 305
 306                     if let Some(lower_bound_region) = lower_bound_region {
 307                         let generic_ty = type_test.generic_kind.to_ty(self.infcx.tcx);
 308                         let origin = RelateParamBound(type_test_span, generic_ty, None);
 309                         self.buffer_error(self.infcx.err_ctxt().construct_generic_bound_failure(
 310                             self.body.source.def_id().expect_local(),
 311                             type_test_span,
 312                             Some(origin),
 313                             type_test.generic_kind,
 314                             lower_bound_region,
 315                         ));
 316                     } else {
 317                         // FIXME. We should handle this case better. It
 318                         // indicates that we have e.g., some region variable
 319                         // whose value is like `'a+'b` where `'a` and `'b` are
 320                         // distinct unrelated universal regions that are not
 321                         // known to outlive one another. It'd be nice to have
 322                         // some examples where this arises to decide how best
 323                         // to report it; we could probably handle it by
 324                         // iterating over the universal regions and reporting
 325                         // an error that multiple bounds are required.
 326                         let mut diag =
 327                             self.infcx.tcx.sess.create_err(GenericDoesNotLiveLongEnough {
 328                                 kind: type_test.generic_kind.to_string(),
 329                                 span: type_test_span,
 330                             });
 331
 332                         // Add notes and suggestions for the case of 'static lifetime
 333                         // implied but not specified when a generic associated types
 334                         // are from higher-ranked trait bounds
 335                         self.suggest_static_lifetime_for_gat_from_hrtb(
 336                             &mut diag,
 337                             type_test.lower_bound,
 338                         );
 339
 340                         self.buffer_error(diag);
 341                     }
 342                 }
 343
 344                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, key, member_region } => {
 345                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 346                     let named_key = self.regioncx.name_regions(self.infcx.tcx, key);
 347                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 348                     let mut diag = unexpected_hidden_region_diagnostic(
 349                         self.infcx.tcx,
 350                         span,
 351                         named_ty,
 352                         named_region,
 353                         named_key,
 354                     );
 355                     if last_unexpected_hidden_region != Some((span, named_ty, named_key)) {
 356                         self.buffer_error(diag);
 357                         last_unexpected_hidden_region = Some((span, named_ty, named_key));
 358                     } else {
 359                         diag.delay_as_bug();
 360                     }
 361                 }
 362
 363                 RegionErrorKind::BoundUniversalRegionError {
 364                     longer_fr,
 365                     placeholder,
 366                     error_element,
 367                 } => {
 368                     let error_vid = self.regioncx.region_from_element(longer_fr, &error_element);
 369
 370                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 371                     let (_, cause) = self.regioncx.find_outlives_blame_span(
 372                         longer_fr,
 373                         NllRegionVariableOrigin::Placeholder(placeholder),
 374                         error_vid,
 375                     );
 376
 377                     let universe = placeholder.universe;
 378                     let universe_info = self.regioncx.universe_info(universe);
 379
 380                     universe_info.report_error(self, placeholder, error_element, cause);
 381                 }
 382
 383                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 384                     if is_reported {
 385                         self.report_region_error(
 386                             longer_fr,
 387                             fr_origin,
 388                             shorter_fr,
 389                             &mut outlives_suggestion,
 390                         );
 391                     } else {
 392                         // We only report the first error, so as not to overwhelm the user. See
 393                         // `RegRegionErrorKind` docs.
 394                         //
 395                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 396                         // FIXME: try collecting these constraints on the outlives suggestion
 397                         // builder. Does it make the suggestions any better?
 398                         debug!(
 399                             "Unreported region error: can't prove that {:?}: {:?}",
 400                             longer_fr, shorter_fr
 401                         );
 402                     }
 403                 }
 404             }
 405         }
 406
 407         // Emit one outlives suggestions for each MIR def we borrowck
 408         outlives_suggestion.add_suggestion(self);
 409     }
 410
 411     /// Report an error because the universal region `fr` was required to outlive
 412     /// `outlived_fr` but it is not known to do so. For example:
 413     ///
 414     /// ```compile_fail
 415     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 416     /// ```
 417     ///
 418     /// Here we would be invoked with `fr = 'a` and `outlived_fr = 'b`.
 419     pub(crate) fn report_region_error(
 420         &mut self,
 421         fr: RegionVid,
 422         fr_origin: NllRegionVariableOrigin,
 423         outlived_fr: RegionVid,
 424         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 425     ) {
 426         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 427
 428         let (blame_constraint, extra_info) =
 429             self.regioncx.best_blame_constraint(fr, fr_origin, |r| {
 430                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 431             });
 432         let BlameConstraint { category, cause, variance_info, .. } = blame_constraint;
 433
 434         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);
 435
 436         // Check if we can use one of the "nice region errors".
 437         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 438             let infer_err = self.infcx.err_ctxt();
 439             let nice = NiceRegionError::new_from_span(&infer_err, cause.span, o, f);
 440             if let Some(diag) = nice.try_report_from_nll() {
 441                 self.buffer_error(diag);
 442                 return;
 443             }
 444         }
 445
 446         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 447             self.regioncx.universal_regions().is_local_free_region(fr),
 448             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 449         );
 450
 451         debug!(
 452             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 453             fr_is_local, outlived_fr_is_local, category
 454         );
 455
 456         let errci = ErrorConstraintInfo {
 457             fr,
 458             outlived_fr,
 459             fr_is_local,
 460             outlived_fr_is_local,
 461             category,
 462             span: cause.span,
 463         };
 464
 465         let mut diag = match (category, fr_is_local, outlived_fr_is_local) {
 466             (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
 467                 self.report_fnmut_error(&errci, kind)
 468             }
 469             (ConstraintCategory::Assignment, true, false)
 470             | (ConstraintCategory::CallArgument(_), true, false) => {
 471                 let mut db = self.report_escaping_data_error(&errci);
 472
 473                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 474                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 475
 476                 db
 477             }
 478             _ => {
 479                 let mut db = self.report_general_error(&errci);
 480
 481                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 482                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 483
 484                 db
 485             }
 486         };
 487
 488         match variance_info {
 489             ty::VarianceDiagInfo::None => {}
 490             ty::VarianceDiagInfo::Invariant { ty, param_index } => {
 491                 let (desc, note) = match ty.kind() {
 492                     ty::RawPtr(ty_mut) => {
 493                         assert_eq!(ty_mut.mutbl, rustc_hir::Mutability::Mut);
 494                         (
 495                             format!("a mutable pointer to `{}`", ty_mut.ty),
 496                             "mutable pointers are invariant over their type parameter".to_string(),
 497                         )
 498                     }
 499                     ty::Ref(_, inner_ty, mutbl) => {
 500                         assert_eq!(*mutbl, rustc_hir::Mutability::Mut);
 501                         (
 502                             format!("a mutable reference to `{inner_ty}`"),
 503                             "mutable references are invariant over their type parameter"
 504                                 .to_string(),
 505                         )
 506                     }
 507                     ty::Adt(adt, substs) => {
 508                         let generic_arg = substs[param_index as usize];
 509                         let identity_substs =
 510                             InternalSubsts::identity_for_item(self.infcx.tcx, adt.did());
 511                         let base_ty = self.infcx.tcx.mk_adt(*adt, identity_substs);
 512                         let base_generic_arg = identity_substs[param_index as usize];
 513                         let adt_desc = adt.descr();
 514
 515                         let desc = format!(
 516                             "the type `{ty}`, which makes the generic argument `{generic_arg}` invariant"
 517                         );
 518                         let note = format!(
 519                             "the {adt_desc} `{base_ty}` is invariant over the parameter `{base_generic_arg}`"
 520                         );
 521                         (desc, note)
 522                     }
 523                     ty::FnDef(def_id, _) => {
 524                         let name = self.infcx.tcx.item_name(*def_id);
 525                         let identity_substs =
 526                             InternalSubsts::identity_for_item(self.infcx.tcx, *def_id);
 527                         let desc = format!("a function pointer to `{name}`");
 528                         let note = format!(
 529                             "the function `{name}` is invariant over the parameter `{}`",
 530                             identity_substs[param_index as usize]
 531                         );
 532                         (desc, note)
 533                     }
 534                     _ => panic!("Unexpected type {ty:?}"),
 535                 };
 536                 diag.note(&format!("requirement occurs because of {desc}",));
 537                 diag.note(&note);
 538                 diag.help("see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance");
 539             }
 540         }
 541
 542         for extra in extra_info {
 543             match extra {
 544                 ExtraConstraintInfo::PlaceholderFromPredicate(span) => {
 545                     diag.span_note(span, "due to current limitations in the borrow checker, this implies a `'static` lifetime");
 546                 }
 547             }
 548         }
 549
 550         self.buffer_error(diag);
 551     }
 552
 553     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 554     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 555     ///
 556     /// ```text
 557     /// error: captured variable cannot escape `FnMut` closure body
 558     ///   --> $DIR/issue-53040.rs:15:8
 559     ///    |
 560     /// LL |     || &mut v;
 561     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 562     ///    |     |
 563     ///    |     inferred to be a `FnMut` closure
 564     ///    |
 565     ///    = note: `FnMut` closures only have access to their captured variables while they are
 566     ///            executing...
 567     ///    = note: ...therefore, returned references to captured variables will escape the closure
 568     /// ```
 569     fn report_fnmut_error(
 570         &self,
 571         errci: &ErrorConstraintInfo<'tcx>,
 572         kind: ReturnConstraint,
 573     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 574         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 575
 576         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 577         if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) = *output_ty.kind() {
 578             output_ty = self.infcx.tcx.type_of(def_id).subst_identity()
 579         };
 580
 581         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 582
 583         let err = FnMutError {
 584             span: *span,
 585             ty_err: match output_ty.kind() {
 586                 ty::Generator(def, ..) if self.infcx.tcx.generator_is_async(*def) => {
 587                     FnMutReturnTypeErr::ReturnAsyncBlock { span: *span }
 588                 }
 589                 _ if output_ty.contains_closure() => {
 590                     FnMutReturnTypeErr::ReturnClosure { span: *span }
 591                 }
 592                 _ => FnMutReturnTypeErr::ReturnRef { span: *span },
 593             },
 594         };
 595
 596         let mut diag = self.infcx.tcx.sess.create_err(err);
 597
 598         if let ReturnConstraint::ClosureUpvar(upvar_field) = kind {
 599             let def_id = match self.regioncx.universal_regions().defining_ty {
 600                 DefiningTy::Closure(def_id, _) => def_id,
 601                 ty => bug!("unexpected DefiningTy {:?}", ty),
 602             };
 603
 604             let captured_place = &self.upvars[upvar_field.index()].place;
 605             let defined_hir = match captured_place.place.base {
 606                 PlaceBase::Local(hirid) => Some(hirid),
 607                 PlaceBase::Upvar(upvar) => Some(upvar.var_path.hir_id),
 608                 _ => None,
 609             };
 610
 611             if let Some(def_hir) = defined_hir {
 612                 let upvars_map = self.infcx.tcx.upvars_mentioned(def_id).unwrap();
 613                 let upvar_def_span = self.infcx.tcx.hir().span(def_hir);
 614                 let upvar_span = upvars_map.get(&def_hir).unwrap().span;
 615                 diag.subdiagnostic(VarHereDenote::Defined { span: upvar_def_span });
 616                 diag.subdiagnostic(VarHereDenote::Captured { span: upvar_span });
 617             }
 618         }
 619
 620         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
 621             diag.subdiagnostic(VarHereDenote::FnMutInferred { span: fr_span });
 622         }
 623
 624         self.suggest_move_on_borrowing_closure(&mut diag);
 625
 626         diag
 627     }
 628
 629     /// Reports an error specifically for when data is escaping a closure.
 630     ///
 631     /// ```text
 632     /// error: borrowed data escapes outside of function
 633     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 634     ///    |
 635     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 636     ///    |              - `x` is a reference that is only valid in the function body
 637     /// LL |     // but ref_obj will not, so warn.
 638     /// LL |     ref_obj(x)
 639     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 640     /// ```
 641     #[instrument(level = "debug", skip(self))]
 642     fn report_escaping_data_error(
 643         &self,
 644         errci: &ErrorConstraintInfo<'tcx>,
 645     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 646         let ErrorConstraintInfo { span, category, .. } = errci;
 647
 648         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 649             self.infcx.tcx,
 650             &self.body,
 651             &self.local_names,
 652             &self.upvars,
 653             errci.fr,
 654         );
 655         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 656             self.infcx.tcx,
 657             &self.body,
 658             &self.local_names,
 659             &self.upvars,
 660             errci.outlived_fr,
 661         );
 662
 663         let escapes_from =
 664             self.infcx.tcx.def_descr(self.regioncx.universal_regions().defining_ty.def_id());
 665
 666         // Revert to the normal error in these cases.
 667         // Assignments aren't "escapes" in function items.
 668         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 669             || (*category == ConstraintCategory::Assignment
 670                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 671             || self.regioncx.universal_regions().defining_ty.is_const()
 672         {
 673             return self.report_general_error(&ErrorConstraintInfo {
 674                 fr_is_local: true,
 675                 outlived_fr_is_local: false,
 676                 ..*errci
 677             });
 678         }
 679
 680         let mut diag =
 681             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 682
 683         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 684             diag.span_label(
 685                 outlived_fr_span,
 686                 format!("`{outlived_fr_name}` declared here, outside of the {escapes_from} body",),
 687             );
 688         }
 689
 690         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 691             diag.span_label(
 692                 fr_span,
 693                 format!(
 694                     "`{fr_name}` is a reference that is only valid in the {escapes_from} body",
 695                 ),
 696             );
 697
 698             diag.span_label(*span, format!("`{fr_name}` escapes the {escapes_from} body here"));
 699         }
 700
 701         // Only show an extra note if we can find an 'error region' for both of the region
 702         // variables. This avoids showing a noisy note that just mentions 'synthetic' regions
 703         // that don't help the user understand the error.
 704         match (self.to_error_region(errci.fr), self.to_error_region(errci.outlived_fr)) {
 705             (Some(f), Some(o)) => {
 706                 self.maybe_suggest_constrain_dyn_trait_impl(&mut diag, f, o, category);
 707
 708                 let fr_region_name = self.give_region_a_name(errci.fr).unwrap();
 709                 fr_region_name.highlight_region_name(&mut diag);
 710                 let outlived_fr_region_name = self.give_region_a_name(errci.outlived_fr).unwrap();
 711                 outlived_fr_region_name.highlight_region_name(&mut diag);
 712
 713                 diag.span_label(
 714                     *span,
 715                     format!(
 716                         "{}requires that `{}` must outlive `{}`",
 717                         category.description(),
 718                         fr_region_name,
 719                         outlived_fr_region_name,
 720                     ),
 721                 );
 722             }
 723             _ => {}
 724         }
 725
 726         diag
 727     }
 728
 729     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 730     /// explain what is happening.
 731     ///
 732     /// ```text
 733     /// error: unsatisfied lifetime constraints
 734     ///   --> $DIR/regions-creating-enums3.rs:17:5
 735     ///    |
 736     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 737     ///    |                -- -- lifetime `'b` defined here
 738     ///    |                |
 739     ///    |                lifetime `'a` defined here
 740     /// LL |     ast::add(x, y)
 741     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 742     ///    |                    is returning data with lifetime `'b`
 743     /// ```
 744     fn report_general_error(
 745         &self,
 746         errci: &ErrorConstraintInfo<'tcx>,
 747     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 748         let ErrorConstraintInfo {
 749             fr,
 750             fr_is_local,
 751             outlived_fr,
 752             outlived_fr_is_local,
 753             span,
 754             category,
 755             ..
 756         } = errci;
 757
 758         let mir_def_name = self.infcx.tcx.def_descr(self.mir_def_id().to_def_id());
 759
 760         let err = LifetimeOutliveErr { span: *span };
 761         let mut diag = self.infcx.tcx.sess.create_err(err);
 762
 763         let fr_name = self.give_region_a_name(*fr).unwrap();
 764         fr_name.highlight_region_name(&mut diag);
 765         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 766         outlived_fr_name.highlight_region_name(&mut diag);
 767
 768         let err_category = match (category, outlived_fr_is_local, fr_is_local) {
 769             (ConstraintCategory::Return(_), true, _) => LifetimeReturnCategoryErr::WrongReturn {
 770                 span: *span,
 771                 mir_def_name,
 772                 outlived_fr_name,
 773                 fr_name: &fr_name,
 774             },
 775             _ => LifetimeReturnCategoryErr::ShortReturn {
 776                 span: *span,
 777                 category_desc: category.description(),
 778                 free_region_name: &fr_name,
 779                 outlived_fr_name,
 780             },
 781         };
 782
 783         diag.subdiagnostic(err_category);
 784
 785         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 786         self.suggest_adding_lifetime_params(&mut diag, *fr, *outlived_fr);
 787         self.suggest_move_on_borrowing_closure(&mut diag);
 788
 789         diag
 790     }
 791
 792     /// Adds a suggestion to errors where an `impl Trait` is returned.
 793     ///
 794     /// ```text
 795     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 796     ///       a constraint
 797     ///    |
 798     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 799     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 800     /// ```
 801     fn add_static_impl_trait_suggestion(
 802         &self,
 803         diag: &mut Diagnostic,
 804         fr: RegionVid,
 805         // We need to pass `fr_name` - computing it again will label it twice.
 806         fr_name: RegionName,
 807         outlived_fr: RegionVid,
 808     ) {
 809         if let (Some(f), Some(outlived_f)) =
 810             (self.to_error_region(fr), self.to_error_region(outlived_fr))
 811         {
 812             if *outlived_f != ty::ReStatic {
 813                 return;
 814             }
 815             let suitable_region = self.infcx.tcx.is_suitable_region(f);
 816             let Some(suitable_region) = suitable_region else { return; };
 817
 818             let fn_returns = self.infcx.tcx.return_type_impl_or_dyn_traits(suitable_region.def_id);
 819
 820             let param = if let Some(param) = find_param_with_region(self.infcx.tcx, f, outlived_f) {
 821                 param
 822             } else {
 823                 return;
 824             };
 825
 826             let lifetime = if f.has_name() { fr_name.name } else { kw::UnderscoreLifetime };
 827
 828             let arg = match param.param.pat.simple_ident() {
 829                 Some(simple_ident) => format!("argument `{simple_ident}`"),
 830                 None => "the argument".to_string(),
 831             };
 832             let captures = format!("captures data from {arg}");
 833
 834             if !fn_returns.is_empty() {
 835                 nice_region_error::suggest_new_region_bound(
 836                     self.infcx.tcx,
 837                     diag,
 838                     fn_returns,
 839                     lifetime.to_string(),
 840                     Some(arg),
 841                     captures,
 842                     Some((param.param_ty_span, param.param_ty.to_string())),
 843                     Some(suitable_region.def_id),
 844                 );
 845                 return;
 846             }
 847
 848             let Some((alias_tys, alias_span)) = self
 849                 .infcx
 850                 .tcx
 851                 .return_type_impl_or_dyn_traits_with_type_alias(suitable_region.def_id) else { return; };
 852
 853             // in case the return type of the method is a type alias
 854             let mut spans_suggs: Vec<_> = Vec::new();
 855             for alias_ty in alias_tys {
 856                 if alias_ty.span.desugaring_kind().is_some() {
 857                     // Skip `async` desugaring `impl Future`.
 858                     ()
 859                 }
 860                 if let TyKind::TraitObject(_, lt, _) = alias_ty.kind {
 861                     spans_suggs.push((lt.ident.span.shrink_to_hi(), " + 'a".to_string()));
 862                 }
 863             }
 864             spans_suggs.push((alias_span.shrink_to_hi(), "<'a>".to_string()));
 865             diag.multipart_suggestion_verbose(
 866                 &format!(
 867                     "to declare that the trait object {captures}, you can add a lifetime parameter `'a` in the type alias"
 868                 ),
 869                 spans_suggs,
 870                 Applicability::MaybeIncorrect,
 871             );
 872         }
 873     }
 874
 875     fn maybe_suggest_constrain_dyn_trait_impl(
 876         &self,
 877         diag: &mut Diagnostic,
 878         f: Region<'tcx>,
 879         o: Region<'tcx>,
 880         category: &ConstraintCategory<'tcx>,
 881     ) {
 882         if !o.is_static() {
 883             return;
 884         }
 885
 886         let tcx = self.infcx.tcx;
 887
 888         let instance = if let ConstraintCategory::CallArgument(Some(func_ty)) = category {
 889             let (fn_did, substs) = match func_ty.kind() {
 890                 ty::FnDef(fn_did, substs) => (fn_did, substs),
 891                 _ => return,
 892             };
 893             debug!(?fn_did, ?substs);
 894
 895             // Only suggest this on function calls, not closures
 896             let ty = tcx.type_of(fn_did).subst_identity();
 897             debug!("ty: {:?}, ty.kind: {:?}", ty, ty.kind());
 898             if let ty::Closure(_, _) = ty.kind() {
 899                 return;
 900             }
 901
 902             if let Ok(Some(instance)) = ty::Instance::resolve(
 903                 tcx,
 904                 self.param_env,
 905                 *fn_did,
 906                 self.infcx.resolve_vars_if_possible(substs),
 907             ) {
 908                 instance
 909             } else {
 910                 return;
 911             }
 912         } else {
 913             return;
 914         };
 915
 916         let param = match find_param_with_region(tcx, f, o) {
 917             Some(param) => param,
 918             None => return,
 919         };
 920         debug!(?param);
 921
 922         let mut visitor = TraitObjectVisitor(FxIndexSet::default());
 923         visitor.visit_ty(param.param_ty);
 924
 925         let Some((ident, self_ty)) =
 926             NiceRegionError::get_impl_ident_and_self_ty_from_trait(tcx, instance.def_id(), &visitor.0) else { return; };
 927
 928         self.suggest_constrain_dyn_trait_in_impl(diag, &visitor.0, ident, self_ty);
 929     }
 930
 931     #[instrument(skip(self, err), level = "debug")]
 932     fn suggest_constrain_dyn_trait_in_impl(
 933         &self,
 934         err: &mut Diagnostic,
 935         found_dids: &FxIndexSet<DefId>,
 936         ident: Ident,
 937         self_ty: &hir::Ty<'_>,
 938     ) -> bool {
 939         debug!("err: {:#?}", err);
 940         let mut suggested = false;
 941         for found_did in found_dids {
 942             let mut traits = vec![];
 943             let mut hir_v = HirTraitObjectVisitor(&mut traits, *found_did);
 944             hir_v.visit_ty(&self_ty);
 945             debug!("trait spans found: {:?}", traits);
 946             for span in &traits {
 947                 let mut multi_span: MultiSpan = vec![*span].into();
 948                 multi_span
 949                     .push_span_label(*span, "this has an implicit `'static` lifetime requirement");
 950                 multi_span.push_span_label(
 951                     ident.span,
 952                     "calling this method introduces the `impl`'s `'static` requirement",
 953                 );
 954                 err.subdiagnostic(RequireStaticErr::UsedImpl { multi_span });
 955                 err.span_suggestion_verbose(
 956                     span.shrink_to_hi(),
 957                     "consider relaxing the implicit `'static` requirement",
 958                     " + '_",
 959                     Applicability::MaybeIncorrect,
 960                 );
 961                 suggested = true;
 962             }
 963         }
 964         suggested
 965     }
 966
 967     fn suggest_adding_lifetime_params(
 968         &self,
 969         diag: &mut Diagnostic,
 970         sub: RegionVid,
 971         sup: RegionVid,
 972     ) {
 973         let (Some(sub), Some(sup)) = (self.to_error_region(sub), self.to_error_region(sup)) else {
 974             return
 975         };
 976
 977         let Some((ty_sub, _)) = self
 978             .infcx
 979             .tcx
 980             .is_suitable_region(sub)
 981             .and_then(|anon_reg| find_anon_type(self.infcx.tcx, sub, &anon_reg.boundregion)) else {
 982             return
 983         };
 984
 985         let Some((ty_sup, _)) = self
 986             .infcx
 987             .tcx
 988             .is_suitable_region(sup)
 989             .and_then(|anon_reg| find_anon_type(self.infcx.tcx, sup, &anon_reg.boundregion)) else {
 990             return
 991         };
 992
 993         suggest_adding_lifetime_params(self.infcx.tcx, sub, ty_sup, ty_sub, diag);
 994     }
 995
 996     fn suggest_move_on_borrowing_closure(&self, diag: &mut Diagnostic) {
 997         let map = self.infcx.tcx.hir();
 998         let body_id = map.body_owned_by(self.mir_def_id());
 999         let expr = &map.body(body_id).value.peel_blocks();
1000         let mut closure_span = None::<rustc_span::Span>;
1001         match expr.kind {
1002             hir::ExprKind::MethodCall(.., args, _) => {
1003                 for arg in args {
1004                     if let hir::ExprKind::Closure(hir::Closure {
1005                         capture_clause: hir::CaptureBy::Ref,
1006                         ..
1007                     }) = arg.kind
1008                     {
1009                         closure_span = Some(arg.span.shrink_to_lo());
1010                         break;
1011                     }
1012                 }
1013             }
1014             hir::ExprKind::Closure(hir::Closure {
1015                 capture_clause: hir::CaptureBy::Ref,
1016                 body,
1017                 ..
1018             }) => {
1019                 let body = map.body(*body);
1020                 if !matches!(body.generator_kind, Some(hir::GeneratorKind::Async(..))) {
1021                     closure_span = Some(expr.span.shrink_to_lo());
1022                 }
1023             }
1024             _ => {}
1025         }
1026         if let Some(closure_span) = closure_span {
1027             diag.span_suggestion_verbose(
1028                 closure_span,
1029                 "consider adding 'move' keyword before the nested closure",
1030                 "move ",
1031                 Applicability::MaybeIncorrect,
1032             );
1033         }
1034     }
1035 }