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, format!("{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         let mut last_unexpected_hidden_region: Option<(Span, Ty<'_>, ty::OpaqueTypeKey<'tcx>)> =
 196             None;
 197
 198         for nll_error in nll_errors.into_iter() {
 199             match nll_error {
 200                 RegionErrorKind::TypeTestError { type_test } => {
 201                     // Try to convert the lower-bound region into something named we can print for the user.
 202                     let lower_bound_region = self.to_error_region(type_test.lower_bound);
 203
 204                     let type_test_span = type_test.span;
 205
 206                     if let Some(lower_bound_region) = lower_bound_region {
 207                         let generic_ty = type_test.generic_kind.to_ty(self.infcx.tcx);
 208                         let origin = RelateParamBound(type_test_span, generic_ty, None);
 209                         self.buffer_error(self.infcx.err_ctxt().construct_generic_bound_failure(
 210                             self.body.source.def_id().expect_local(),
 211                             type_test_span,
 212                             Some(origin),
 213                             type_test.generic_kind,
 214                             lower_bound_region,
 215                         ));
 216                     } else {
 217                         // FIXME. We should handle this case better. It
 218                         // indicates that we have e.g., some region variable
 219                         // whose value is like `'a+'b` where `'a` and `'b` are
 220                         // distinct unrelated universal regions that are not
 221                         // known to outlive one another. It'd be nice to have
 222                         // some examples where this arises to decide how best
 223                         // to report it; we could probably handle it by
 224                         // iterating over the universal regions and reporting
 225                         // an error that multiple bounds are required.
 226                         self.buffer_error(self.infcx.tcx.sess.create_err(
 227                             GenericDoesNotLiveLongEnough {
 228                                 kind: type_test.generic_kind.to_string(),
 229                                 span: type_test_span,
 230                             },
 231                         ));
 232                     }
 233                 }
 234
 235                 RegionErrorKind::UnexpectedHiddenRegion { span, hidden_ty, key, member_region } => {
 236                     let named_ty = self.regioncx.name_regions(self.infcx.tcx, hidden_ty);
 237                     let named_key = self.regioncx.name_regions(self.infcx.tcx, key);
 238                     let named_region = self.regioncx.name_regions(self.infcx.tcx, member_region);
 239                     let mut diag = unexpected_hidden_region_diagnostic(
 240                         self.infcx.tcx,
 241                         span,
 242                         named_ty,
 243                         named_region,
 244                         named_key,
 245                     );
 246                     if last_unexpected_hidden_region != Some((span, named_ty, named_key)) {
 247                         self.buffer_error(diag);
 248                         last_unexpected_hidden_region = Some((span, named_ty, named_key));
 249                     } else {
 250                         diag.delay_as_bug();
 251                     }
 252                 }
 253
 254                 RegionErrorKind::BoundUniversalRegionError {
 255                     longer_fr,
 256                     placeholder,
 257                     error_element,
 258                 } => {
 259                     let error_vid = self.regioncx.region_from_element(longer_fr, &error_element);
 260
 261                     // Find the code to blame for the fact that `longer_fr` outlives `error_fr`.
 262                     let (_, cause) = self.regioncx.find_outlives_blame_span(
 263                         longer_fr,
 264                         NllRegionVariableOrigin::Placeholder(placeholder),
 265                         error_vid,
 266                     );
 267
 268                     let universe = placeholder.universe;
 269                     let universe_info = self.regioncx.universe_info(universe);
 270
 271                     universe_info.report_error(self, placeholder, error_element, cause);
 272                 }
 273
 274                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
 275                     if is_reported {
 276                         self.report_region_error(
 277                             longer_fr,
 278                             fr_origin,
 279                             shorter_fr,
 280                             &mut outlives_suggestion,
 281                         );
 282                     } else {
 283                         // We only report the first error, so as not to overwhelm the user. See
 284                         // `RegRegionErrorKind` docs.
 285                         //
 286                         // FIXME: currently we do nothing with these, but perhaps we can do better?
 287                         // FIXME: try collecting these constraints on the outlives suggestion
 288                         // builder. Does it make the suggestions any better?
 289                         debug!(
 290                             "Unreported region error: can't prove that {:?}: {:?}",
 291                             longer_fr, shorter_fr
 292                         );
 293                     }
 294                 }
 295             }
 296         }
 297
 298         // Emit one outlives suggestions for each MIR def we borrowck
 299         outlives_suggestion.add_suggestion(self);
 300     }
 301
 302     /// Report an error because the universal region `fr` was required to outlive
 303     /// `outlived_fr` but it is not known to do so. For example:
 304     ///
 305     /// ```compile_fail
 306     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
 307     /// ```
 308     ///
 309     /// Here we would be invoked with `fr = 'a` and `outlived_fr = `'b`.
 310     pub(crate) fn report_region_error(
 311         &mut self,
 312         fr: RegionVid,
 313         fr_origin: NllRegionVariableOrigin,
 314         outlived_fr: RegionVid,
 315         outlives_suggestion: &mut OutlivesSuggestionBuilder,
 316     ) {
 317         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 318
 319         let (blame_constraint, extra_info) =
 320             self.regioncx.best_blame_constraint(fr, fr_origin, |r| {
 321                 self.regioncx.provides_universal_region(r, fr, outlived_fr)
 322             });
 323         let BlameConstraint { category, cause, variance_info, .. } = blame_constraint;
 324
 325         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);
 326
 327         // Check if we can use one of the "nice region errors".
 328         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
 329             let infer_err = self.infcx.err_ctxt();
 330             let nice = NiceRegionError::new_from_span(&infer_err, cause.span, o, f);
 331             if let Some(diag) = nice.try_report_from_nll() {
 332                 self.buffer_error(diag);
 333                 return;
 334             }
 335         }
 336
 337         let (fr_is_local, outlived_fr_is_local): (bool, bool) = (
 338             self.regioncx.universal_regions().is_local_free_region(fr),
 339             self.regioncx.universal_regions().is_local_free_region(outlived_fr),
 340         );
 341
 342         debug!(
 343             "report_region_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
 344             fr_is_local, outlived_fr_is_local, category
 345         );
 346
 347         let errci = ErrorConstraintInfo {
 348             fr,
 349             outlived_fr,
 350             fr_is_local,
 351             outlived_fr_is_local,
 352             category,
 353             span: cause.span,
 354         };
 355
 356         let mut diag = match (category, fr_is_local, outlived_fr_is_local) {
 357             (ConstraintCategory::Return(kind), true, false) if self.is_closure_fn_mut(fr) => {
 358                 self.report_fnmut_error(&errci, kind)
 359             }
 360             (ConstraintCategory::Assignment, true, false)
 361             | (ConstraintCategory::CallArgument(_), true, false) => {
 362                 let mut db = self.report_escaping_data_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                 let mut db = self.report_general_error(&errci);
 371
 372                 outlives_suggestion.intermediate_suggestion(self, &errci, &mut db);
 373                 outlives_suggestion.collect_constraint(fr, outlived_fr);
 374
 375                 db
 376             }
 377         };
 378
 379         match variance_info {
 380             ty::VarianceDiagInfo::None => {}
 381             ty::VarianceDiagInfo::Invariant { ty, param_index } => {
 382                 let (desc, note) = match ty.kind() {
 383                     ty::RawPtr(ty_mut) => {
 384                         assert_eq!(ty_mut.mutbl, rustc_hir::Mutability::Mut);
 385                         (
 386                             format!("a mutable pointer to `{}`", ty_mut.ty),
 387                             "mutable pointers are invariant over their type parameter".to_string(),
 388                         )
 389                     }
 390                     ty::Ref(_, inner_ty, mutbl) => {
 391                         assert_eq!(*mutbl, rustc_hir::Mutability::Mut);
 392                         (
 393                             format!("a mutable reference to `{inner_ty}`"),
 394                             "mutable references are invariant over their type parameter"
 395                                 .to_string(),
 396                         )
 397                     }
 398                     ty::Adt(adt, substs) => {
 399                         let generic_arg = substs[param_index as usize];
 400                         let identity_substs =
 401                             InternalSubsts::identity_for_item(self.infcx.tcx, adt.did());
 402                         let base_ty = self.infcx.tcx.mk_adt(*adt, identity_substs);
 403                         let base_generic_arg = identity_substs[param_index as usize];
 404                         let adt_desc = adt.descr();
 405
 406                         let desc = format!(
 407                             "the type `{ty}`, which makes the generic argument `{generic_arg}` invariant"
 408                         );
 409                         let note = format!(
 410                             "the {adt_desc} `{base_ty}` is invariant over the parameter `{base_generic_arg}`"
 411                         );
 412                         (desc, note)
 413                     }
 414                     ty::FnDef(def_id, _) => {
 415                         let name = self.infcx.tcx.item_name(*def_id);
 416                         let identity_substs =
 417                             InternalSubsts::identity_for_item(self.infcx.tcx, *def_id);
 418                         let desc = format!("a function pointer to `{name}`");
 419                         let note = format!(
 420                             "the function `{name}` is invariant over the parameter `{}`",
 421                             identity_substs[param_index as usize]
 422                         );
 423                         (desc, note)
 424                     }
 425                     _ => panic!("Unexpected type {ty:?}"),
 426                 };
 427                 diag.note(&format!("requirement occurs because of {desc}",));
 428                 diag.note(&note);
 429                 diag.help("see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance");
 430             }
 431         }
 432
 433         for extra in extra_info {
 434             match extra {
 435                 ExtraConstraintInfo::PlaceholderFromPredicate(span) => {
 436                     diag.span_note(span, "due to current limitations in the borrow checker, this implies a `'static` lifetime");
 437                 }
 438             }
 439         }
 440
 441         self.buffer_error(diag);
 442     }
 443
 444     /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
 445     /// This function expects `fr` to be local and `outlived_fr` to not be local.
 446     ///
 447     /// ```text
 448     /// error: captured variable cannot escape `FnMut` closure body
 449     ///   --> $DIR/issue-53040.rs:15:8
 450     ///    |
 451     /// LL |     || &mut v;
 452     ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
 453     ///    |     |
 454     ///    |     inferred to be a `FnMut` closure
 455     ///    |
 456     ///    = note: `FnMut` closures only have access to their captured variables while they are
 457     ///            executing...
 458     ///    = note: ...therefore, returned references to captured variables will escape the closure
 459     /// ```
 460     fn report_fnmut_error(
 461         &self,
 462         errci: &ErrorConstraintInfo<'tcx>,
 463         kind: ReturnConstraint,
 464     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 465         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 466
 467         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
 468         if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) = *output_ty.kind() {
 469             output_ty = self.infcx.tcx.type_of(def_id)
 470         };
 471
 472         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 473
 474         let err = FnMutError {
 475             span: *span,
 476             ty_err: match output_ty.kind() {
 477                 ty::Closure(_, _) => FnMutReturnTypeErr::ReturnClosure { span: *span },
 478                 ty::Generator(def, ..) if self.infcx.tcx.generator_is_async(*def) => {
 479                     FnMutReturnTypeErr::ReturnAsyncBlock { span: *span }
 480                 }
 481                 _ => FnMutReturnTypeErr::ReturnRef { span: *span },
 482             },
 483         };
 484
 485         let mut diag = self.infcx.tcx.sess.create_err(err);
 486
 487         if let ReturnConstraint::ClosureUpvar(upvar_field) = kind {
 488             let def_id = match self.regioncx.universal_regions().defining_ty {
 489                 DefiningTy::Closure(def_id, _) => def_id,
 490                 ty => bug!("unexpected DefiningTy {:?}", ty),
 491             };
 492
 493             let captured_place = &self.upvars[upvar_field.index()].place;
 494             let defined_hir = match captured_place.place.base {
 495                 PlaceBase::Local(hirid) => Some(hirid),
 496                 PlaceBase::Upvar(upvar) => Some(upvar.var_path.hir_id),
 497                 _ => None,
 498             };
 499
 500             if let Some(def_hir) = defined_hir {
 501                 let upvars_map = self.infcx.tcx.upvars_mentioned(def_id).unwrap();
 502                 let upvar_def_span = self.infcx.tcx.hir().span(def_hir);
 503                 let upvar_span = upvars_map.get(&def_hir).unwrap().span;
 504                 diag.subdiagnostic(VarHereDenote::Defined { span: upvar_def_span });
 505                 diag.subdiagnostic(VarHereDenote::Captured { span: upvar_span });
 506             }
 507         }
 508
 509         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
 510             diag.subdiagnostic(VarHereDenote::FnMutInferred { span: fr_span });
 511         }
 512
 513         self.suggest_move_on_borrowing_closure(&mut diag);
 514
 515         diag
 516     }
 517
 518     /// Reports an error specifically for when data is escaping a closure.
 519     ///
 520     /// ```text
 521     /// error: borrowed data escapes outside of function
 522     ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
 523     ///    |
 524     /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
 525     ///    |              - `x` is a reference that is only valid in the function body
 526     /// LL |     // but ref_obj will not, so warn.
 527     /// LL |     ref_obj(x)
 528     ///    |     ^^^^^^^^^^ `x` escapes the function body here
 529     /// ```
 530     #[instrument(level = "debug", skip(self))]
 531     fn report_escaping_data_error(
 532         &self,
 533         errci: &ErrorConstraintInfo<'tcx>,
 534     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 535         let ErrorConstraintInfo { span, category, .. } = errci;
 536
 537         let fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 538             self.infcx.tcx,
 539             &self.body,
 540             &self.local_names,
 541             &self.upvars,
 542             errci.fr,
 543         );
 544         let outlived_fr_name_and_span = self.regioncx.get_var_name_and_span_for_region(
 545             self.infcx.tcx,
 546             &self.body,
 547             &self.local_names,
 548             &self.upvars,
 549             errci.outlived_fr,
 550         );
 551
 552         let (_, escapes_from) = self
 553             .infcx
 554             .tcx
 555             .article_and_description(self.regioncx.universal_regions().defining_ty.def_id());
 556
 557         // Revert to the normal error in these cases.
 558         // Assignments aren't "escapes" in function items.
 559         if (fr_name_and_span.is_none() && outlived_fr_name_and_span.is_none())
 560             || (*category == ConstraintCategory::Assignment
 561                 && self.regioncx.universal_regions().defining_ty.is_fn_def())
 562             || self.regioncx.universal_regions().defining_ty.is_const()
 563         {
 564             return self.report_general_error(&ErrorConstraintInfo {
 565                 fr_is_local: true,
 566                 outlived_fr_is_local: false,
 567                 ..*errci
 568             });
 569         }
 570
 571         let mut diag =
 572             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 573
 574         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
 575             diag.span_label(
 576                 outlived_fr_span,
 577                 format!("`{outlived_fr_name}` declared here, outside of the {escapes_from} body",),
 578             );
 579         }
 580
 581         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
 582             diag.span_label(
 583                 fr_span,
 584                 format!(
 585                     "`{fr_name}` is a reference that is only valid in the {escapes_from} body",
 586                 ),
 587             );
 588
 589             diag.span_label(*span, format!("`{fr_name}` escapes the {escapes_from} body here"));
 590         }
 591
 592         // Only show an extra note if we can find an 'error region' for both of the region
 593         // variables. This avoids showing a noisy note that just mentions 'synthetic' regions
 594         // that don't help the user understand the error.
 595         match (self.to_error_region(errci.fr), self.to_error_region(errci.outlived_fr)) {
 596             (Some(f), Some(o)) => {
 597                 self.maybe_suggest_constrain_dyn_trait_impl(&mut diag, f, o, category);
 598
 599                 let fr_region_name = self.give_region_a_name(errci.fr).unwrap();
 600                 fr_region_name.highlight_region_name(&mut diag);
 601                 let outlived_fr_region_name = self.give_region_a_name(errci.outlived_fr).unwrap();
 602                 outlived_fr_region_name.highlight_region_name(&mut diag);
 603
 604                 diag.span_label(
 605                     *span,
 606                     format!(
 607                         "{}requires that `{}` must outlive `{}`",
 608                         category.description(),
 609                         fr_region_name,
 610                         outlived_fr_region_name,
 611                     ),
 612                 );
 613             }
 614             _ => {}
 615         }
 616
 617         diag
 618     }
 619
 620     /// Reports a region inference error for the general case with named/synthesized lifetimes to
 621     /// explain what is happening.
 622     ///
 623     /// ```text
 624     /// error: unsatisfied lifetime constraints
 625     ///   --> $DIR/regions-creating-enums3.rs:17:5
 626     ///    |
 627     /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
 628     ///    |                -- -- lifetime `'b` defined here
 629     ///    |                |
 630     ///    |                lifetime `'a` defined here
 631     /// LL |     ast::add(x, y)
 632     ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
 633     ///    |                    is returning data with lifetime `'b`
 634     /// ```
 635     fn report_general_error(
 636         &self,
 637         errci: &ErrorConstraintInfo<'tcx>,
 638     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 639         let ErrorConstraintInfo {
 640             fr,
 641             fr_is_local,
 642             outlived_fr,
 643             outlived_fr_is_local,
 644             span,
 645             category,
 646             ..
 647         } = errci;
 648
 649         let (_, mir_def_name) =
 650             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());
 651
 652         let err = LifetimeOutliveErr { span: *span };
 653         let mut diag = self.infcx.tcx.sess.create_err(err);
 654
 655         let fr_name = self.give_region_a_name(*fr).unwrap();
 656         fr_name.highlight_region_name(&mut diag);
 657         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
 658         outlived_fr_name.highlight_region_name(&mut diag);
 659
 660         let err_category = match (category, outlived_fr_is_local, fr_is_local) {
 661             (ConstraintCategory::Return(_), true, _) => LifetimeReturnCategoryErr::WrongReturn {
 662                 span: *span,
 663                 mir_def_name,
 664                 outlived_fr_name,
 665                 fr_name: &fr_name,
 666             },
 667             _ => LifetimeReturnCategoryErr::ShortReturn {
 668                 span: *span,
 669                 category_desc: category.description(),
 670                 free_region_name: &fr_name,
 671                 outlived_fr_name,
 672             },
 673         };
 674
 675         diag.subdiagnostic(err_category);
 676
 677         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
 678         self.suggest_adding_lifetime_params(&mut diag, *fr, *outlived_fr);
 679         self.suggest_move_on_borrowing_closure(&mut diag);
 680
 681         diag
 682     }
 683
 684     /// Adds a suggestion to errors where an `impl Trait` is returned.
 685     ///
 686     /// ```text
 687     /// help: to allow this `impl Trait` to capture borrowed data with lifetime `'1`, add `'_` as
 688     ///       a constraint
 689     ///    |
 690     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
 691     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 692     /// ```
 693     fn add_static_impl_trait_suggestion(
 694         &self,
 695         diag: &mut Diagnostic,
 696         fr: RegionVid,
 697         // We need to pass `fr_name` - computing it again will label it twice.
 698         fr_name: RegionName,
 699         outlived_fr: RegionVid,
 700     ) {
 701         if let (Some(f), Some(outlived_f)) =
 702             (self.to_error_region(fr), self.to_error_region(outlived_fr))
 703         {
 704             if *outlived_f != ty::ReStatic {
 705                 return;
 706             }
 707
 708             let fn_returns = self
 709                 .infcx
 710                 .tcx
 711                 .is_suitable_region(f)
 712                 .map(|r| self.infcx.tcx.return_type_impl_or_dyn_traits(r.def_id))
 713                 .unwrap_or_default();
 714
 715             if fn_returns.is_empty() {
 716                 return;
 717             }
 718
 719             let param = if let Some(param) = find_param_with_region(self.infcx.tcx, f, outlived_f) {
 720                 param
 721             } else {
 722                 return;
 723             };
 724
 725             let lifetime = if f.has_name() { fr_name.name } else { kw::UnderscoreLifetime };
 726
 727             let arg = match param.param.pat.simple_ident() {
 728                 Some(simple_ident) => format!("argument `{simple_ident}`"),
 729                 None => "the argument".to_string(),
 730             };
 731             let captures = format!("captures data from {arg}");
 732
 733             return nice_region_error::suggest_new_region_bound(
 734                 self.infcx.tcx,
 735                 diag,
 736                 fn_returns,
 737                 lifetime.to_string(),
 738                 Some(arg),
 739                 captures,
 740                 Some((param.param_ty_span, param.param_ty.to_string())),
 741                 self.infcx.tcx.is_suitable_region(f).map(|r| r.def_id),
 742             );
 743         }
 744     }
 745
 746     fn maybe_suggest_constrain_dyn_trait_impl(
 747         &self,
 748         diag: &mut Diagnostic,
 749         f: Region<'tcx>,
 750         o: Region<'tcx>,
 751         category: &ConstraintCategory<'tcx>,
 752     ) {
 753         if !o.is_static() {
 754             return;
 755         }
 756
 757         let tcx = self.infcx.tcx;
 758
 759         let instance = if let ConstraintCategory::CallArgument(Some(func_ty)) = category {
 760             let (fn_did, substs) = match func_ty.kind() {
 761                 ty::FnDef(fn_did, substs) => (fn_did, substs),
 762                 _ => return,
 763             };
 764             debug!(?fn_did, ?substs);
 765
 766             // Only suggest this on function calls, not closures
 767             let ty = tcx.type_of(fn_did);
 768             debug!("ty: {:?}, ty.kind: {:?}", ty, ty.kind());
 769             if let ty::Closure(_, _) = ty.kind() {
 770                 return;
 771             }
 772
 773             if let Ok(Some(instance)) = ty::Instance::resolve(
 774                 tcx,
 775                 self.param_env,
 776                 *fn_did,
 777                 self.infcx.resolve_vars_if_possible(substs),
 778             ) {
 779                 instance
 780             } else {
 781                 return;
 782             }
 783         } else {
 784             return;
 785         };
 786
 787         let param = match find_param_with_region(tcx, f, o) {
 788             Some(param) => param,
 789             None => return,
 790         };
 791         debug!(?param);
 792
 793         let mut visitor = TraitObjectVisitor(FxIndexSet::default());
 794         visitor.visit_ty(param.param_ty);
 795
 796         let Some((ident, self_ty)) =
 797             NiceRegionError::get_impl_ident_and_self_ty_from_trait(tcx, instance.def_id(), &visitor.0) else { return; };
 798
 799         self.suggest_constrain_dyn_trait_in_impl(diag, &visitor.0, ident, self_ty);
 800     }
 801
 802     #[instrument(skip(self, err), level = "debug")]
 803     fn suggest_constrain_dyn_trait_in_impl(
 804         &self,
 805         err: &mut Diagnostic,
 806         found_dids: &FxIndexSet<DefId>,
 807         ident: Ident,
 808         self_ty: &hir::Ty<'_>,
 809     ) -> bool {
 810         debug!("err: {:#?}", err);
 811         let mut suggested = false;
 812         for found_did in found_dids {
 813             let mut traits = vec![];
 814             let mut hir_v = HirTraitObjectVisitor(&mut traits, *found_did);
 815             hir_v.visit_ty(&self_ty);
 816             debug!("trait spans found: {:?}", traits);
 817             for span in &traits {
 818                 let mut multi_span: MultiSpan = vec![*span].into();
 819                 multi_span
 820                     .push_span_label(*span, "this has an implicit `'static` lifetime requirement");
 821                 multi_span.push_span_label(
 822                     ident.span,
 823                     "calling this method introduces the `impl`'s 'static` requirement",
 824                 );
 825                 err.subdiagnostic(RequireStaticErr::UsedImpl { multi_span });
 826                 err.span_suggestion_verbose(
 827                     span.shrink_to_hi(),
 828                     "consider relaxing the implicit `'static` requirement",
 829                     " + '_",
 830                     Applicability::MaybeIncorrect,
 831                 );
 832                 suggested = true;
 833             }
 834         }
 835         suggested
 836     }
 837
 838     fn suggest_adding_lifetime_params(
 839         &self,
 840         diag: &mut Diagnostic,
 841         sub: RegionVid,
 842         sup: RegionVid,
 843     ) {
 844         let (Some(sub), Some(sup)) = (self.to_error_region(sub), self.to_error_region(sup)) else {
 845             return
 846         };
 847
 848         let Some((ty_sub, _)) = self
 849             .infcx
 850             .tcx
 851             .is_suitable_region(sub)
 852             .and_then(|anon_reg| find_anon_type(self.infcx.tcx, sub, &anon_reg.boundregion)) else {
 853             return
 854         };
 855
 856         let Some((ty_sup, _)) = self
 857             .infcx
 858             .tcx
 859             .is_suitable_region(sup)
 860             .and_then(|anon_reg| find_anon_type(self.infcx.tcx, sup, &anon_reg.boundregion)) else {
 861             return
 862         };
 863
 864         suggest_adding_lifetime_params(self.infcx.tcx, sub, ty_sup, ty_sub, diag);
 865     }
 866
 867     fn suggest_move_on_borrowing_closure(&self, diag: &mut Diagnostic) {
 868         let map = self.infcx.tcx.hir();
 869         let body_id = map.body_owned_by(self.mir_def_id());
 870         let expr = &map.body(body_id).value;
 871         let mut closure_span = None::<rustc_span::Span>;
 872         match expr.kind {
 873             hir::ExprKind::MethodCall(.., args, _) => {
 874                 for arg in args {
 875                     if let hir::ExprKind::Closure(hir::Closure {
 876                         capture_clause: hir::CaptureBy::Ref,
 877                         ..
 878                     }) = arg.kind
 879                     {
 880                         closure_span = Some(arg.span.shrink_to_lo());
 881                         break;
 882                     }
 883                 }
 884             }
 885             hir::ExprKind::Block(blk, _) => {
 886                 if let Some(expr) = blk.expr {
 887                     // only when the block is a closure
 888                     if let hir::ExprKind::Closure(hir::Closure {
 889                         capture_clause: hir::CaptureBy::Ref,
 890                         body,
 891                         ..
 892                     }) = expr.kind
 893                     {
 894                         let body = map.body(*body);
 895                         if !matches!(body.generator_kind, Some(hir::GeneratorKind::Async(..))) {
 896                             closure_span = Some(expr.span.shrink_to_lo());
 897                         }
 898                     }
 899                 }
 900             }
 901             _ => {}
 902         }
 903         if let Some(closure_span) = closure_span {
 904             diag.span_suggestion_verbose(
 905                 closure_span,
 906                 "consider adding 'move' keyword before the nested closure",
 907                 "move ",
 908                 Applicability::MaybeIncorrect,
 909             );
 910         }
 911     }
 912 }