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