compiler/rustc_borrowck/src/diagnostics/region_errors.rs

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