src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs

   1 use rustc::mir::{
   2     self, AggregateKind, BindingForm, BorrowKind, ClearCrossCrate, ConstraintCategory,
   3     FakeReadCause, Local, LocalDecl, LocalInfo, LocalKind, Location, Operand, Place, PlaceRef,
   4     ProjectionElem, Rvalue, Statement, StatementKind, TerminatorKind, VarBindingForm,
   5 };
   6 use rustc::traits::error_reporting::suggest_constraining_type_param;
   7 use rustc::ty::{self, Ty};
   8 use rustc_data_structures::fx::FxHashSet;
   9 use rustc_errors::{Applicability, DiagnosticBuilder};
  10 use rustc_hir as hir;
  11 use rustc_hir::def_id::DefId;
  12 use rustc_hir::{AsyncGeneratorKind, GeneratorKind};
  13 use rustc_index::vec::Idx;
  14 use rustc_span::source_map::DesugaringKind;
  15 use rustc_span::Span;
  16
  17 use crate::dataflow::drop_flag_effects;
  18 use crate::dataflow::indexes::{MoveOutIndex, MovePathIndex};
  19 use crate::util::borrowck_errors;
  20
  21 use crate::borrow_check::{
  22     borrow_set::BorrowData, prefixes::IsPrefixOf, InitializationRequiringAction, MirBorrowckCtxt,
  23     PrefixSet, WriteKind,
  24 };
  25
  26 use super::{
  27     explain_borrow::BorrowExplanation, IncludingDowncast, RegionName, RegionNameSource, UseSpans,
  28 };
  29
  30 #[derive(Debug)]
  31 struct MoveSite {
  32     /// Index of the "move out" that we found. The `MoveData` can
  33     /// then tell us where the move occurred.
  34     moi: MoveOutIndex,
  35
  36     /// `true` if we traversed a back edge while walking from the point
  37     /// of error to the move site.
  38     traversed_back_edge: bool,
  39 }
  40
  41 /// Which case a StorageDeadOrDrop is for.
  42 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  43 enum StorageDeadOrDrop<'tcx> {
  44     LocalStorageDead,
  45     BoxedStorageDead,
  46     Destructor(Ty<'tcx>),
  47 }
  48
  49 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
  50     pub(in crate::borrow_check) fn report_use_of_moved_or_uninitialized(
  51         &mut self,
  52         location: Location,
  53         desired_action: InitializationRequiringAction,
  54         (moved_place, used_place, span): (PlaceRef<'cx, 'tcx>, PlaceRef<'cx, 'tcx>, Span),
  55         mpi: MovePathIndex,
  56     ) {
  57         debug!(
  58             "report_use_of_moved_or_uninitialized: location={:?} desired_action={:?} \
  59              moved_place={:?} used_place={:?} span={:?} mpi={:?}",
  60             location, desired_action, moved_place, used_place, span, mpi
  61         );
  62
  63         let use_spans =
  64             self.move_spans(moved_place, location).or_else(|| self.borrow_spans(span, location));
  65         let span = use_spans.args_or_use();
  66
  67         let move_site_vec = self.get_moved_indexes(location, mpi);
  68         debug!("report_use_of_moved_or_uninitialized: move_site_vec={:?}", move_site_vec);
  69         let move_out_indices: Vec<_> =
  70             move_site_vec.iter().map(|move_site| move_site.moi).collect();
  71
  72         if move_out_indices.is_empty() {
  73             let root_place = PlaceRef { projection: &[], ..used_place };
  74
  75             if !self.uninitialized_error_reported.insert(root_place) {
  76                 debug!(
  77                     "report_use_of_moved_or_uninitialized place: error about {:?} suppressed",
  78                     root_place
  79                 );
  80                 return;
  81             }
  82
  83             let item_msg =
  84                 match self.describe_place_with_options(used_place, IncludingDowncast(true)) {
  85                     Some(name) => format!("`{}`", name),
  86                     None => "value".to_owned(),
  87                 };
  88             let mut err = self.cannot_act_on_uninitialized_variable(
  89                 span,
  90                 desired_action.as_noun(),
  91                 &self
  92                     .describe_place_with_options(moved_place, IncludingDowncast(true))
  93                     .unwrap_or_else(|| "_".to_owned()),
  94             );
  95             err.span_label(span, format!("use of possibly-uninitialized {}", item_msg));
  96
  97             use_spans.var_span_label(
  98                 &mut err,
  99                 format!("{} occurs due to use{}", desired_action.as_noun(), use_spans.describe()),
 100             );
 101
 102             err.buffer(&mut self.errors_buffer);
 103         } else {
 104             if let Some((reported_place, _)) = self.move_error_reported.get(&move_out_indices) {
 105                 if self.prefixes(*reported_place, PrefixSet::All).any(|p| p == used_place) {
 106                     debug!(
 107                         "report_use_of_moved_or_uninitialized place: error suppressed \
 108                          mois={:?}",
 109                         move_out_indices
 110                     );
 111                     return;
 112                 }
 113             }
 114
 115             let msg = ""; //FIXME: add "partially " or "collaterally "
 116
 117             let mut err = self.cannot_act_on_moved_value(
 118                 span,
 119                 desired_action.as_noun(),
 120                 msg,
 121                 self.describe_place_with_options(moved_place, IncludingDowncast(true)),
 122             );
 123
 124             self.add_moved_or_invoked_closure_note(location, used_place, &mut err);
 125
 126             let mut is_loop_move = false;
 127             let is_partial_move = move_site_vec.iter().any(|move_site| {
 128                 let move_out = self.move_data.moves[(*move_site).moi];
 129                 let moved_place = &self.move_data.move_paths[move_out.path].place;
 130                 used_place != moved_place.as_ref() && used_place.is_prefix_of(moved_place.as_ref())
 131             });
 132             for move_site in &move_site_vec {
 133                 let move_out = self.move_data.moves[(*move_site).moi];
 134                 let moved_place = &self.move_data.move_paths[move_out.path].place;
 135
 136                 let move_spans = self.move_spans(moved_place.as_ref(), move_out.source);
 137                 let move_span = move_spans.args_or_use();
 138
 139                 let move_msg = if move_spans.for_closure() { " into closure" } else { "" };
 140
 141                 if span == move_span {
 142                     err.span_label(
 143                         span,
 144                         format!("value moved{} here, in previous iteration of loop", move_msg),
 145                     );
 146                     is_loop_move = true;
 147                 } else if move_site.traversed_back_edge {
 148                     err.span_label(
 149                         move_span,
 150                         format!("value moved{} here, in previous iteration of loop", move_msg),
 151                     );
 152                 } else {
 153                     err.span_label(move_span, format!("value moved{} here", move_msg));
 154                     move_spans.var_span_label(
 155                         &mut err,
 156                         format!("variable moved due to use{}", move_spans.describe()),
 157                     );
 158                 }
 159                 if Some(DesugaringKind::ForLoop) == move_span.desugaring_kind() {
 160                     let sess = self.infcx.tcx.sess;
 161                     if let Ok(snippet) = sess.source_map().span_to_snippet(move_span) {
 162                         err.span_suggestion(
 163                             move_span,
 164                             "consider borrowing to avoid moving into the for loop",
 165                             format!("&{}", snippet),
 166                             Applicability::MaybeIncorrect,
 167                         );
 168                     }
 169                 }
 170             }
 171
 172             use_spans.var_span_label(
 173                 &mut err,
 174                 format!("{} occurs due to use{}", desired_action.as_noun(), use_spans.describe()),
 175             );
 176
 177             if !is_loop_move {
 178                 err.span_label(
 179                     span,
 180                     format!(
 181                         "value {} here {}",
 182                         desired_action.as_verb_in_past_tense(),
 183                         if is_partial_move { "after partial move" } else { "after move" },
 184                     ),
 185                 );
 186             }
 187
 188             let ty =
 189                 Place::ty_from(used_place.local, used_place.projection, *self.body, self.infcx.tcx)
 190                     .ty;
 191             let needs_note = match ty.kind {
 192                 ty::Closure(id, _) => {
 193                     let tables = self.infcx.tcx.typeck_tables_of(id);
 194                     let hir_id = self.infcx.tcx.hir().as_local_hir_id(id).unwrap();
 195
 196                     tables.closure_kind_origins().get(hir_id).is_none()
 197                 }
 198                 _ => true,
 199             };
 200
 201             if needs_note {
 202                 let mpi = self.move_data.moves[move_out_indices[0]].path;
 203                 let place = &self.move_data.move_paths[mpi].place;
 204
 205                 let ty = place.ty(*self.body, self.infcx.tcx).ty;
 206                 let opt_name =
 207                     self.describe_place_with_options(place.as_ref(), IncludingDowncast(true));
 208                 let note_msg = match opt_name {
 209                     Some(ref name) => format!("`{}`", name),
 210                     None => "value".to_owned(),
 211                 };
 212                 if let ty::Param(param_ty) = ty.kind {
 213                     let tcx = self.infcx.tcx;
 214                     let generics = tcx.generics_of(self.mir_def_id);
 215                     let param = generics.type_param(&param_ty, tcx);
 216                     if let Some(generics) =
 217                         tcx.hir().get_generics(tcx.closure_base_def_id(self.mir_def_id))
 218                     {
 219                         suggest_constraining_type_param(
 220                             generics,
 221                             &mut err,
 222                             &param.name.as_str(),
 223                             "Copy",
 224                             tcx.sess.source_map(),
 225                             span,
 226                         );
 227                     }
 228                 }
 229                 let span = if let Some(local) = place.as_local() {
 230                     let decl = &self.body.local_decls[local];
 231                     Some(decl.source_info.span)
 232                 } else {
 233                     None
 234                 };
 235                 self.note_type_does_not_implement_copy(&mut err, &note_msg, ty, span);
 236             }
 237
 238             if let Some((_, mut old_err)) =
 239                 self.move_error_reported.insert(move_out_indices, (used_place, err))
 240             {
 241                 // Cancel the old error so it doesn't ICE.
 242                 old_err.cancel();
 243             }
 244         }
 245     }
 246
 247     pub(in crate::borrow_check) fn report_move_out_while_borrowed(
 248         &mut self,
 249         location: Location,
 250         (place, span): (&Place<'tcx>, Span),
 251         borrow: &BorrowData<'tcx>,
 252     ) {
 253         debug!(
 254             "report_move_out_while_borrowed: location={:?} place={:?} span={:?} borrow={:?}",
 255             location, place, span, borrow
 256         );
 257         let value_msg = match self.describe_place(place.as_ref()) {
 258             Some(name) => format!("`{}`", name),
 259             None => "value".to_owned(),
 260         };
 261         let borrow_msg = match self.describe_place(borrow.borrowed_place.as_ref()) {
 262             Some(name) => format!("`{}`", name),
 263             None => "value".to_owned(),
 264         };
 265
 266         let borrow_spans = self.retrieve_borrow_spans(borrow);
 267         let borrow_span = borrow_spans.args_or_use();
 268
 269         let move_spans = self.move_spans(place.as_ref(), location);
 270         let span = move_spans.args_or_use();
 271
 272         let mut err = self.cannot_move_when_borrowed(
 273             span,
 274             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
 275         );
 276         err.span_label(borrow_span, format!("borrow of {} occurs here", borrow_msg));
 277         err.span_label(span, format!("move out of {} occurs here", value_msg));
 278
 279         borrow_spans.var_span_label(
 280             &mut err,
 281             format!("borrow occurs due to use{}", borrow_spans.describe()),
 282         );
 283
 284         move_spans
 285             .var_span_label(&mut err, format!("move occurs due to use{}", move_spans.describe()));
 286
 287         self.explain_why_borrow_contains_point(location, borrow, None)
 288             .add_explanation_to_diagnostic(
 289                 self.infcx.tcx,
 290                 &self.body,
 291                 &self.local_names,
 292                 &mut err,
 293                 "",
 294                 Some(borrow_span),
 295             );
 296         err.buffer(&mut self.errors_buffer);
 297     }
 298
 299     pub(in crate::borrow_check) fn report_use_while_mutably_borrowed(
 300         &mut self,
 301         location: Location,
 302         (place, _span): (&Place<'tcx>, Span),
 303         borrow: &BorrowData<'tcx>,
 304     ) -> DiagnosticBuilder<'cx> {
 305         let borrow_spans = self.retrieve_borrow_spans(borrow);
 306         let borrow_span = borrow_spans.args_or_use();
 307
 308         // Conflicting borrows are reported separately, so only check for move
 309         // captures.
 310         let use_spans = self.move_spans(place.as_ref(), location);
 311         let span = use_spans.var_or_use();
 312
 313         let mut err = self.cannot_use_when_mutably_borrowed(
 314             span,
 315             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
 316             borrow_span,
 317             &self.describe_place(borrow.borrowed_place.as_ref()).unwrap_or_else(|| "_".to_owned()),
 318         );
 319
 320         borrow_spans.var_span_label(&mut err, {
 321             let place = &borrow.borrowed_place;
 322             let desc_place = self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned());
 323
 324             format!("borrow occurs due to use of `{}`{}", desc_place, borrow_spans.describe())
 325         });
 326
 327         self.explain_why_borrow_contains_point(location, borrow, None)
 328             .add_explanation_to_diagnostic(
 329                 self.infcx.tcx,
 330                 &self.body,
 331                 &self.local_names,
 332                 &mut err,
 333                 "",
 334                 None,
 335             );
 336         err
 337     }
 338
 339     pub(in crate::borrow_check) fn report_conflicting_borrow(
 340         &mut self,
 341         location: Location,
 342         (place, span): (&Place<'tcx>, Span),
 343         gen_borrow_kind: BorrowKind,
 344         issued_borrow: &BorrowData<'tcx>,
 345     ) -> DiagnosticBuilder<'cx> {
 346         let issued_spans = self.retrieve_borrow_spans(issued_borrow);
 347         let issued_span = issued_spans.args_or_use();
 348
 349         let borrow_spans = self.borrow_spans(span, location);
 350         let span = borrow_spans.args_or_use();
 351
 352         let container_name = if issued_spans.for_generator() || borrow_spans.for_generator() {
 353             "generator"
 354         } else {
 355             "closure"
 356         };
 357
 358         let (desc_place, msg_place, msg_borrow, union_type_name) =
 359             self.describe_place_for_conflicting_borrow(place, &issued_borrow.borrowed_place);
 360
 361         let explanation = self.explain_why_borrow_contains_point(location, issued_borrow, None);
 362         let second_borrow_desc = if explanation.is_explained() { "second " } else { "" };
 363
 364         // FIXME: supply non-"" `opt_via` when appropriate
 365         let first_borrow_desc;
 366         let mut err = match (gen_borrow_kind, issued_borrow.kind) {
 367             (BorrowKind::Shared, BorrowKind::Mut { .. }) => {
 368                 first_borrow_desc = "mutable ";
 369                 self.cannot_reborrow_already_borrowed(
 370                     span,
 371                     &desc_place,
 372                     &msg_place,
 373                     "immutable",
 374                     issued_span,
 375                     "it",
 376                     "mutable",
 377                     &msg_borrow,
 378                     None,
 379                 )
 380             }
 381             (BorrowKind::Mut { .. }, BorrowKind::Shared) => {
 382                 first_borrow_desc = "immutable ";
 383                 self.cannot_reborrow_already_borrowed(
 384                     span,
 385                     &desc_place,
 386                     &msg_place,
 387                     "mutable",
 388                     issued_span,
 389                     "it",
 390                     "immutable",
 391                     &msg_borrow,
 392                     None,
 393                 )
 394             }
 395
 396             (BorrowKind::Mut { .. }, BorrowKind::Mut { .. }) => {
 397                 first_borrow_desc = "first ";
 398                 self.cannot_mutably_borrow_multiply(
 399                     span,
 400                     &desc_place,
 401                     &msg_place,
 402                     issued_span,
 403                     &msg_borrow,
 404                     None,
 405                 )
 406             }
 407
 408             (BorrowKind::Unique, BorrowKind::Unique) => {
 409                 first_borrow_desc = "first ";
 410                 self.cannot_uniquely_borrow_by_two_closures(span, &desc_place, issued_span, None)
 411             }
 412
 413             (BorrowKind::Mut { .. }, BorrowKind::Shallow)
 414             | (BorrowKind::Unique, BorrowKind::Shallow) => {
 415                 if let Some(immutable_section_description) =
 416                     self.classify_immutable_section(&issued_borrow.assigned_place)
 417                 {
 418                     let mut err = self.cannot_mutate_in_immutable_section(
 419                         span,
 420                         issued_span,
 421                         &desc_place,
 422                         immutable_section_description,
 423                         "mutably borrow",
 424                     );
 425                     borrow_spans.var_span_label(
 426                         &mut err,
 427                         format!(
 428                             "borrow occurs due to use of `{}`{}",
 429                             desc_place,
 430                             borrow_spans.describe(),
 431                         ),
 432                     );
 433
 434                     return err;
 435                 } else {
 436                     first_borrow_desc = "immutable ";
 437                     self.cannot_reborrow_already_borrowed(
 438                         span,
 439                         &desc_place,
 440                         &msg_place,
 441                         "mutable",
 442                         issued_span,
 443                         "it",
 444                         "immutable",
 445                         &msg_borrow,
 446                         None,
 447                     )
 448                 }
 449             }
 450
 451             (BorrowKind::Unique, _) => {
 452                 first_borrow_desc = "first ";
 453                 self.cannot_uniquely_borrow_by_one_closure(
 454                     span,
 455                     container_name,
 456                     &desc_place,
 457                     "",
 458                     issued_span,
 459                     "it",
 460                     "",
 461                     None,
 462                 )
 463             }
 464
 465             (BorrowKind::Shared, BorrowKind::Unique) => {
 466                 first_borrow_desc = "first ";
 467                 self.cannot_reborrow_already_uniquely_borrowed(
 468                     span,
 469                     container_name,
 470                     &desc_place,
 471                     "",
 472                     "immutable",
 473                     issued_span,
 474                     "",
 475                     None,
 476                     second_borrow_desc,
 477                 )
 478             }
 479
 480             (BorrowKind::Mut { .. }, BorrowKind::Unique) => {
 481                 first_borrow_desc = "first ";
 482                 self.cannot_reborrow_already_uniquely_borrowed(
 483                     span,
 484                     container_name,
 485                     &desc_place,
 486                     "",
 487                     "mutable",
 488                     issued_span,
 489                     "",
 490                     None,
 491                     second_borrow_desc,
 492                 )
 493             }
 494
 495             (BorrowKind::Shared, BorrowKind::Shared)
 496             | (BorrowKind::Shared, BorrowKind::Shallow)
 497             | (BorrowKind::Shallow, BorrowKind::Mut { .. })
 498             | (BorrowKind::Shallow, BorrowKind::Unique)
 499             | (BorrowKind::Shallow, BorrowKind::Shared)
 500             | (BorrowKind::Shallow, BorrowKind::Shallow) => unreachable!(),
 501         };
 502
 503         if issued_spans == borrow_spans {
 504             borrow_spans.var_span_label(
 505                 &mut err,
 506                 format!("borrows occur due to use of `{}`{}", desc_place, borrow_spans.describe()),
 507             );
 508         } else {
 509             let borrow_place = &issued_borrow.borrowed_place;
 510             let borrow_place_desc =
 511                 self.describe_place(borrow_place.as_ref()).unwrap_or_else(|| "_".to_owned());
 512             issued_spans.var_span_label(
 513                 &mut err,
 514                 format!(
 515                     "first borrow occurs due to use of `{}`{}",
 516                     borrow_place_desc,
 517                     issued_spans.describe(),
 518                 ),
 519             );
 520
 521             borrow_spans.var_span_label(
 522                 &mut err,
 523                 format!(
 524                     "second borrow occurs due to use of `{}`{}",
 525                     desc_place,
 526                     borrow_spans.describe(),
 527                 ),
 528             );
 529         }
 530
 531         if union_type_name != "" {
 532             err.note(&format!(
 533                 "`{}` is a field of the union `{}`, so it overlaps the field `{}`",
 534                 msg_place, union_type_name, msg_borrow,
 535             ));
 536         }
 537
 538         explanation.add_explanation_to_diagnostic(
 539             self.infcx.tcx,
 540             &self.body,
 541             &self.local_names,
 542             &mut err,
 543             first_borrow_desc,
 544             None,
 545         );
 546
 547         err
 548     }
 549
 550     /// Returns the description of the root place for a conflicting borrow and the full
 551     /// descriptions of the places that caused the conflict.
 552     ///
 553     /// In the simplest case, where there are no unions involved, if a mutable borrow of `x` is
 554     /// attempted while a shared borrow is live, then this function will return:
 555     ///
 556     ///     ("x", "", "")
 557     ///
 558     /// In the simple union case, if a mutable borrow of a union field `x.z` is attempted while
 559     /// a shared borrow of another field `x.y`, then this function will return:
 560     ///
 561     ///     ("x", "x.z", "x.y")
 562     ///
 563     /// In the more complex union case, where the union is a field of a struct, then if a mutable
 564     /// borrow of a union field in a struct `x.u.z` is attempted while a shared borrow of
 565     /// another field `x.u.y`, then this function will return:
 566     ///
 567     ///     ("x.u", "x.u.z", "x.u.y")
 568     ///
 569     /// This is used when creating error messages like below:
 570     ///
 571     /// >  cannot borrow `a.u` (via `a.u.z.c`) as immutable because it is also borrowed as
 572     /// >  mutable (via `a.u.s.b`) [E0502]
 573     pub(in crate::borrow_check) fn describe_place_for_conflicting_borrow(
 574         &self,
 575         first_borrowed_place: &Place<'tcx>,
 576         second_borrowed_place: &Place<'tcx>,
 577     ) -> (String, String, String, String) {
 578         // Define a small closure that we can use to check if the type of a place
 579         // is a union.
 580         let union_ty = |place_base, place_projection| {
 581             let ty = Place::ty_from(place_base, place_projection, *self.body, self.infcx.tcx).ty;
 582             ty.ty_adt_def().filter(|adt| adt.is_union()).map(|_| ty)
 583         };
 584         let describe_place = |place| self.describe_place(place).unwrap_or_else(|| "_".to_owned());
 585
 586         // Start with an empty tuple, so we can use the functions on `Option` to reduce some
 587         // code duplication (particularly around returning an empty description in the failure
 588         // case).
 589         Some(())
 590             .filter(|_| {
 591                 // If we have a conflicting borrow of the same place, then we don't want to add
 592                 // an extraneous "via x.y" to our diagnostics, so filter out this case.
 593                 first_borrowed_place != second_borrowed_place
 594             })
 595             .and_then(|_| {
 596                 // We're going to want to traverse the first borrowed place to see if we can find
 597                 // field access to a union. If we find that, then we will keep the place of the
 598                 // union being accessed and the field that was being accessed so we can check the
 599                 // second borrowed place for the same union and a access to a different field.
 600                 let Place { local, projection } = first_borrowed_place;
 601
 602                 let mut cursor = projection.as_ref();
 603                 while let [proj_base @ .., elem] = cursor {
 604                     cursor = proj_base;
 605
 606                     match elem {
 607                         ProjectionElem::Field(field, _) if union_ty(local, proj_base).is_some() => {
 608                             return Some((PlaceRef { local, projection: proj_base }, field));
 609                         }
 610                         _ => {}
 611                     }
 612                 }
 613                 None
 614             })
 615             .and_then(|(target_base, target_field)| {
 616                 // With the place of a union and a field access into it, we traverse the second
 617                 // borrowed place and look for a access to a different field of the same union.
 618                 let Place { local, projection } = second_borrowed_place;
 619
 620                 let mut cursor = projection.as_ref();
 621                 while let [proj_base @ .., elem] = cursor {
 622                     cursor = proj_base;
 623
 624                     if let ProjectionElem::Field(field, _) = elem {
 625                         if let Some(union_ty) = union_ty(local, proj_base) {
 626                             if field != target_field
 627                                 && local == target_base.local
 628                                 && proj_base == target_base.projection
 629                             {
 630                                 // FIXME when we avoid clone reuse describe_place closure
 631                                 let describe_base_place = self
 632                                     .describe_place(PlaceRef { local, projection: proj_base })
 633                                     .unwrap_or_else(|| "_".to_owned());
 634
 635                                 return Some((
 636                                     describe_base_place,
 637                                     describe_place(first_borrowed_place.as_ref()),
 638                                     describe_place(second_borrowed_place.as_ref()),
 639                                     union_ty.to_string(),
 640                                 ));
 641                             }
 642                         }
 643                     }
 644                 }
 645                 None
 646             })
 647             .unwrap_or_else(|| {
 648                 // If we didn't find a field access into a union, or both places match, then
 649                 // only return the description of the first place.
 650                 (
 651                     describe_place(first_borrowed_place.as_ref()),
 652                     "".to_string(),
 653                     "".to_string(),
 654                     "".to_string(),
 655                 )
 656             })
 657     }
 658
 659     /// Reports StorageDeadOrDrop of `place` conflicts with `borrow`.
 660     ///
 661     /// This means that some data referenced by `borrow` needs to live
 662     /// past the point where the StorageDeadOrDrop of `place` occurs.
 663     /// This is usually interpreted as meaning that `place` has too
 664     /// short a lifetime. (But sometimes it is more useful to report
 665     /// it as a more direct conflict between the execution of a
 666     /// `Drop::drop` with an aliasing borrow.)
 667     pub(in crate::borrow_check) fn report_borrowed_value_does_not_live_long_enough(
 668         &mut self,
 669         location: Location,
 670         borrow: &BorrowData<'tcx>,
 671         place_span: (&Place<'tcx>, Span),
 672         kind: Option<WriteKind>,
 673     ) {
 674         debug!(
 675             "report_borrowed_value_does_not_live_long_enough(\
 676              {:?}, {:?}, {:?}, {:?}\
 677              )",
 678             location, borrow, place_span, kind
 679         );
 680
 681         let drop_span = place_span.1;
 682         let root_place =
 683             self.prefixes(borrow.borrowed_place.as_ref(), PrefixSet::All).last().unwrap();
 684
 685         let borrow_spans = self.retrieve_borrow_spans(borrow);
 686         let borrow_span = borrow_spans.var_or_use();
 687
 688         assert!(root_place.projection.is_empty());
 689         let proper_span = self.body.local_decls[*root_place.local].source_info.span;
 690
 691         let root_place_projection = self.infcx.tcx.intern_place_elems(root_place.projection);
 692
 693         if self.access_place_error_reported.contains(&(
 694             Place { local: root_place.local.clone(), projection: root_place_projection },
 695             borrow_span,
 696         )) {
 697             debug!(
 698                 "suppressing access_place error when borrow doesn't live long enough for {:?}",
 699                 borrow_span
 700             );
 701             return;
 702         }
 703
 704         self.access_place_error_reported.insert((
 705             Place { local: root_place.local.clone(), projection: root_place_projection },
 706             borrow_span,
 707         ));
 708
 709         let borrowed_local = borrow.borrowed_place.local;
 710         if self.body.local_decls[borrowed_local].is_ref_to_thread_local() {
 711             let err =
 712                 self.report_thread_local_value_does_not_live_long_enough(drop_span, borrow_span);
 713             err.buffer(&mut self.errors_buffer);
 714             return;
 715         }
 716
 717         if let StorageDeadOrDrop::Destructor(dropped_ty) =
 718             self.classify_drop_access_kind(borrow.borrowed_place.as_ref())
 719         {
 720             // If a borrow of path `B` conflicts with drop of `D` (and
 721             // we're not in the uninteresting case where `B` is a
 722             // prefix of `D`), then report this as a more interesting
 723             // destructor conflict.
 724             if !borrow.borrowed_place.as_ref().is_prefix_of(place_span.0.as_ref()) {
 725                 self.report_borrow_conflicts_with_destructor(
 726                     location, borrow, place_span, kind, dropped_ty,
 727                 );
 728                 return;
 729             }
 730         }
 731
 732         let place_desc = self.describe_place(borrow.borrowed_place.as_ref());
 733
 734         let kind_place = kind.filter(|_| place_desc.is_some()).map(|k| (k, place_span.0));
 735         let explanation = self.explain_why_borrow_contains_point(location, &borrow, kind_place);
 736
 737         debug!(
 738             "report_borrowed_value_does_not_live_long_enough(place_desc: {:?}, explanation: {:?})",
 739             place_desc, explanation
 740         );
 741         let err = match (place_desc, explanation) {
 742             // If the outlives constraint comes from inside the closure,
 743             // for example:
 744             //
 745             // let x = 0;
 746             // let y = &x;
 747             // Box::new(|| y) as Box<Fn() -> &'static i32>
 748             //
 749             // then just use the normal error. The closure isn't escaping
 750             // and `move` will not help here.
 751             (
 752                 Some(ref name),
 753                 BorrowExplanation::MustBeValidFor {
 754                     category: category @ ConstraintCategory::Return,
 755                     from_closure: false,
 756                     ref region_name,
 757                     span,
 758                     ..
 759                 },
 760             )
 761             | (
 762                 Some(ref name),
 763                 BorrowExplanation::MustBeValidFor {
 764                     category: category @ ConstraintCategory::CallArgument,
 765                     from_closure: false,
 766                     ref region_name,
 767                     span,
 768                     ..
 769                 },
 770             ) if borrow_spans.for_closure() => self.report_escaping_closure_capture(
 771                 borrow_spans,
 772                 borrow_span,
 773                 region_name,
 774                 category,
 775                 span,
 776                 &format!("`{}`", name),
 777             ),
 778             (
 779                 Some(ref name),
 780                 BorrowExplanation::MustBeValidFor {
 781                     category: category @ ConstraintCategory::OpaqueType,
 782                     from_closure: false,
 783                     ref region_name,
 784                     span,
 785                     ..
 786                 },
 787             ) if borrow_spans.for_generator() => self.report_escaping_closure_capture(
 788                 borrow_spans,
 789                 borrow_span,
 790                 region_name,
 791                 category,
 792                 span,
 793                 &format!("`{}`", name),
 794             ),
 795             (
 796                 ref name,
 797                 BorrowExplanation::MustBeValidFor {
 798                     category: ConstraintCategory::Assignment,
 799                     from_closure: false,
 800                     region_name:
 801                         RegionName {
 802                             source:
 803                                 RegionNameSource::AnonRegionFromUpvar(upvar_span, ref upvar_name),
 804                             ..
 805                         },
 806                     span,
 807                     ..
 808                 },
 809             ) => self.report_escaping_data(borrow_span, name, upvar_span, upvar_name, span),
 810             (Some(name), explanation) => self.report_local_value_does_not_live_long_enough(
 811                 location,
 812                 &name,
 813                 &borrow,
 814                 drop_span,
 815                 borrow_spans,
 816                 explanation,
 817             ),
 818             (None, explanation) => self.report_temporary_value_does_not_live_long_enough(
 819                 location,
 820                 &borrow,
 821                 drop_span,
 822                 borrow_spans,
 823                 proper_span,
 824                 explanation,
 825             ),
 826         };
 827
 828         err.buffer(&mut self.errors_buffer);
 829     }
 830
 831     fn report_local_value_does_not_live_long_enough(
 832         &mut self,
 833         location: Location,
 834         name: &str,
 835         borrow: &BorrowData<'tcx>,
 836         drop_span: Span,
 837         borrow_spans: UseSpans,
 838         explanation: BorrowExplanation,
 839     ) -> DiagnosticBuilder<'cx> {
 840         debug!(
 841             "report_local_value_does_not_live_long_enough(\
 842              {:?}, {:?}, {:?}, {:?}, {:?}\
 843              )",
 844             location, name, borrow, drop_span, borrow_spans
 845         );
 846
 847         let borrow_span = borrow_spans.var_or_use();
 848         if let BorrowExplanation::MustBeValidFor {
 849             category,
 850             span,
 851             ref opt_place_desc,
 852             from_closure: false,
 853             ..
 854         } = explanation
 855         {
 856             if let Some(diag) = self.try_report_cannot_return_reference_to_local(
 857                 borrow,
 858                 borrow_span,
 859                 span,
 860                 category,
 861                 opt_place_desc.as_ref(),
 862             ) {
 863                 return diag;
 864             }
 865         }
 866
 867         let mut err = self.path_does_not_live_long_enough(borrow_span, &format!("`{}`", name));
 868
 869         if let Some(annotation) = self.annotate_argument_and_return_for_borrow(borrow) {
 870             let region_name = annotation.emit(self, &mut err);
 871
 872             err.span_label(
 873                 borrow_span,
 874                 format!("`{}` would have to be valid for `{}`...", name, region_name),
 875             );
 876
 877             if let Some(fn_hir_id) = self.infcx.tcx.hir().as_local_hir_id(self.mir_def_id) {
 878                 err.span_label(
 879                     drop_span,
 880                     format!(
 881                         "...but `{}` will be dropped here, when the {} returns",
 882                         name,
 883                         self.infcx
 884                             .tcx
 885                             .hir()
 886                             .opt_name(fn_hir_id)
 887                             .map(|name| format!("function `{}`", name))
 888                             .unwrap_or_else(|| {
 889                                 match &self
 890                                     .infcx
 891                                     .tcx
 892                                     .typeck_tables_of(self.mir_def_id)
 893                                     .node_type(fn_hir_id)
 894                                     .kind
 895                                 {
 896                                     ty::Closure(..) => "enclosing closure",
 897                                     ty::Generator(..) => "enclosing generator",
 898                                     kind => bug!("expected closure or generator, found {:?}", kind),
 899                                 }
 900                                 .to_string()
 901                             })
 902                     ),
 903                 );
 904
 905                 err.note(
 906                     "functions cannot return a borrow to data owned within the function's scope, \
 907                      functions can only return borrows to data passed as arguments",
 908                 );
 909                 err.note(
 910                     "to learn more, visit <https://doc.rust-lang.org/book/ch04-02-\
 911                      references-and-borrowing.html#dangling-references>",
 912                 );
 913             } else {
 914                 err.span_label(
 915                     drop_span,
 916                     format!("...but `{}` dropped here while still borrowed", name),
 917                 );
 918             }
 919
 920             if let BorrowExplanation::MustBeValidFor { .. } = explanation {
 921             } else {
 922                 explanation.add_explanation_to_diagnostic(
 923                     self.infcx.tcx,
 924                     &self.body,
 925                     &self.local_names,
 926                     &mut err,
 927                     "",
 928                     None,
 929                 );
 930             }
 931         } else {
 932             err.span_label(borrow_span, "borrowed value does not live long enough");
 933             err.span_label(drop_span, format!("`{}` dropped here while still borrowed", name));
 934
 935             let within = if borrow_spans.for_generator() { " by generator" } else { "" };
 936
 937             borrow_spans.args_span_label(&mut err, format!("value captured here{}", within));
 938
 939             explanation.add_explanation_to_diagnostic(
 940                 self.infcx.tcx,
 941                 &self.body,
 942                 &self.local_names,
 943                 &mut err,
 944                 "",
 945                 None,
 946             );
 947         }
 948
 949         err
 950     }
 951
 952     fn report_borrow_conflicts_with_destructor(
 953         &mut self,
 954         location: Location,
 955         borrow: &BorrowData<'tcx>,
 956         (place, drop_span): (&Place<'tcx>, Span),
 957         kind: Option<WriteKind>,
 958         dropped_ty: Ty<'tcx>,
 959     ) {
 960         debug!(
 961             "report_borrow_conflicts_with_destructor(\
 962              {:?}, {:?}, ({:?}, {:?}), {:?}\
 963              )",
 964             location, borrow, place, drop_span, kind,
 965         );
 966
 967         let borrow_spans = self.retrieve_borrow_spans(borrow);
 968         let borrow_span = borrow_spans.var_or_use();
 969
 970         let mut err = self.cannot_borrow_across_destructor(borrow_span);
 971
 972         let what_was_dropped = match self.describe_place(place.as_ref()) {
 973             Some(name) => format!("`{}`", name),
 974             None => String::from("temporary value"),
 975         };
 976
 977         let label = match self.describe_place(borrow.borrowed_place.as_ref()) {
 978             Some(borrowed) => format!(
 979                 "here, drop of {D} needs exclusive access to `{B}`, \
 980                  because the type `{T}` implements the `Drop` trait",
 981                 D = what_was_dropped,
 982                 T = dropped_ty,
 983                 B = borrowed
 984             ),
 985             None => format!(
 986                 "here is drop of {D}; whose type `{T}` implements the `Drop` trait",
 987                 D = what_was_dropped,
 988                 T = dropped_ty
 989             ),
 990         };
 991         err.span_label(drop_span, label);
 992
 993         // Only give this note and suggestion if they could be relevant.
 994         let explanation =
 995             self.explain_why_borrow_contains_point(location, borrow, kind.map(|k| (k, place)));
 996         match explanation {
 997             BorrowExplanation::UsedLater { .. }
 998             | BorrowExplanation::UsedLaterWhenDropped { .. } => {
 999                 err.note("consider using a `let` binding to create a longer lived value");
1000             }
1001             _ => {}
1002         }
1003
1004         explanation.add_explanation_to_diagnostic(
1005             self.infcx.tcx,
1006             &self.body,
1007             &self.local_names,
1008             &mut err,
1009             "",
1010             None,
1011         );
1012
1013         err.buffer(&mut self.errors_buffer);
1014     }
1015
1016     fn report_thread_local_value_does_not_live_long_enough(
1017         &mut self,
1018         drop_span: Span,
1019         borrow_span: Span,
1020     ) -> DiagnosticBuilder<'cx> {
1021         debug!(
1022             "report_thread_local_value_does_not_live_long_enough(\
1023              {:?}, {:?}\
1024              )",
1025             drop_span, borrow_span
1026         );
1027
1028         let mut err = self.thread_local_value_does_not_live_long_enough(borrow_span);
1029
1030         err.span_label(
1031             borrow_span,
1032             "thread-local variables cannot be borrowed beyond the end of the function",
1033         );
1034         err.span_label(drop_span, "end of enclosing function is here");
1035
1036         err
1037     }
1038
1039     fn report_temporary_value_does_not_live_long_enough(
1040         &mut self,
1041         location: Location,
1042         borrow: &BorrowData<'tcx>,
1043         drop_span: Span,
1044         borrow_spans: UseSpans,
1045         proper_span: Span,
1046         explanation: BorrowExplanation,
1047     ) -> DiagnosticBuilder<'cx> {
1048         debug!(
1049             "report_temporary_value_does_not_live_long_enough(\
1050              {:?}, {:?}, {:?}, {:?}\
1051              )",
1052             location, borrow, drop_span, proper_span
1053         );
1054
1055         if let BorrowExplanation::MustBeValidFor { category, span, from_closure: false, .. } =
1056             explanation
1057         {
1058             if let Some(diag) = self.try_report_cannot_return_reference_to_local(
1059                 borrow,
1060                 proper_span,
1061                 span,
1062                 category,
1063                 None,
1064             ) {
1065                 return diag;
1066             }
1067         }
1068
1069         let mut err = self.temporary_value_borrowed_for_too_long(proper_span);
1070         err.span_label(proper_span, "creates a temporary which is freed while still in use");
1071         err.span_label(drop_span, "temporary value is freed at the end of this statement");
1072
1073         match explanation {
1074             BorrowExplanation::UsedLater(..)
1075             | BorrowExplanation::UsedLaterInLoop(..)
1076             | BorrowExplanation::UsedLaterWhenDropped { .. } => {
1077                 // Only give this note and suggestion if it could be relevant.
1078                 err.note("consider using a `let` binding to create a longer lived value");
1079             }
1080             _ => {}
1081         }
1082         explanation.add_explanation_to_diagnostic(
1083             self.infcx.tcx,
1084             &self.body,
1085             &self.local_names,
1086             &mut err,
1087             "",
1088             None,
1089         );
1090
1091         let within = if borrow_spans.for_generator() { " by generator" } else { "" };
1092
1093         borrow_spans.args_span_label(&mut err, format!("value captured here{}", within));
1094
1095         err
1096     }
1097
1098     fn try_report_cannot_return_reference_to_local(
1099         &self,
1100         borrow: &BorrowData<'tcx>,
1101         borrow_span: Span,
1102         return_span: Span,
1103         category: ConstraintCategory,
1104         opt_place_desc: Option<&String>,
1105     ) -> Option<DiagnosticBuilder<'cx>> {
1106         let return_kind = match category {
1107             ConstraintCategory::Return => "return",
1108             ConstraintCategory::Yield => "yield",
1109             _ => return None,
1110         };
1111
1112         // FIXME use a better heuristic than Spans
1113         let reference_desc = if return_span == self.body.source_info(borrow.reserve_location).span {
1114             "reference to"
1115         } else {
1116             "value referencing"
1117         };
1118
1119         let (place_desc, note) = if let Some(place_desc) = opt_place_desc {
1120             let local_kind = if let Some(local) = borrow.borrowed_place.as_local() {
1121                 match self.body.local_kind(local) {
1122                     LocalKind::ReturnPointer | LocalKind::Temp => {
1123                         bug!("temporary or return pointer with a name")
1124                     }
1125                     LocalKind::Var => "local variable ",
1126                     LocalKind::Arg if !self.upvars.is_empty() && local == Local::new(1) => {
1127                         "variable captured by `move` "
1128                     }
1129                     LocalKind::Arg => "function parameter ",
1130                 }
1131             } else {
1132                 "local data "
1133             };
1134             (
1135                 format!("{}`{}`", local_kind, place_desc),
1136                 format!("`{}` is borrowed here", place_desc),
1137             )
1138         } else {
1139             let root_place =
1140                 self.prefixes(borrow.borrowed_place.as_ref(), PrefixSet::All).last().unwrap();
1141             let local = root_place.local;
1142             match self.body.local_kind(*local) {
1143                 LocalKind::ReturnPointer | LocalKind::Temp => {
1144                     ("temporary value".to_string(), "temporary value created here".to_string())
1145                 }
1146                 LocalKind::Arg => (
1147                     "function parameter".to_string(),
1148                     "function parameter borrowed here".to_string(),
1149                 ),
1150                 LocalKind::Var => {
1151                     ("local binding".to_string(), "local binding introduced here".to_string())
1152                 }
1153             }
1154         };
1155
1156         let mut err = self.cannot_return_reference_to_local(
1157             return_span,
1158             return_kind,
1159             reference_desc,
1160             &place_desc,
1161         );
1162
1163         if return_span != borrow_span {
1164             err.span_label(borrow_span, note);
1165         }
1166
1167         Some(err)
1168     }
1169
1170     fn report_escaping_closure_capture(
1171         &mut self,
1172         use_span: UseSpans,
1173         var_span: Span,
1174         fr_name: &RegionName,
1175         category: ConstraintCategory,
1176         constraint_span: Span,
1177         captured_var: &str,
1178     ) -> DiagnosticBuilder<'cx> {
1179         let tcx = self.infcx.tcx;
1180         let args_span = use_span.args_or_use();
1181         let mut err = self.cannot_capture_in_long_lived_closure(args_span, captured_var, var_span);
1182
1183         let suggestion = match tcx.sess.source_map().span_to_snippet(args_span) {
1184             Ok(mut string) => {
1185                 if string.starts_with("async ") {
1186                     string.insert_str(6, "move ");
1187                 } else if string.starts_with("async|") {
1188                     string.insert_str(5, " move");
1189                 } else {
1190                     string.insert_str(0, "move ");
1191                 };
1192                 string
1193             }
1194             Err(_) => "move |<args>| <body>".to_string(),
1195         };
1196         let kind = match use_span.generator_kind() {
1197             Some(generator_kind) => match generator_kind {
1198                 GeneratorKind::Async(async_kind) => match async_kind {
1199                     AsyncGeneratorKind::Block => "async block",
1200                     AsyncGeneratorKind::Closure => "async closure",
1201                     _ => bug!("async block/closure expected, but async funtion found."),
1202                 },
1203                 GeneratorKind::Gen => "generator",
1204             },
1205             None => "closure",
1206         };
1207         err.span_suggestion(
1208             args_span,
1209             &format!(
1210                 "to force the {} to take ownership of {} (and any \
1211                  other referenced variables), use the `move` keyword",
1212                 kind, captured_var
1213             ),
1214             suggestion,
1215             Applicability::MachineApplicable,
1216         );
1217
1218         let msg = match category {
1219             ConstraintCategory::Return => "closure is returned here".to_string(),
1220             ConstraintCategory::OpaqueType => "generator is returned here".to_string(),
1221             ConstraintCategory::CallArgument => {
1222                 fr_name.highlight_region_name(&mut err);
1223                 format!("function requires argument type to outlive `{}`", fr_name)
1224             }
1225             _ => bug!(
1226                 "report_escaping_closure_capture called with unexpected constraint \
1227                        category: `{:?}`",
1228                 category
1229             ),
1230         };
1231         err.span_note(constraint_span, &msg);
1232         err
1233     }
1234
1235     fn report_escaping_data(
1236         &mut self,
1237         borrow_span: Span,
1238         name: &Option<String>,
1239         upvar_span: Span,
1240         upvar_name: &str,
1241         escape_span: Span,
1242     ) -> DiagnosticBuilder<'cx> {
1243         let tcx = self.infcx.tcx;
1244
1245         let escapes_from = if tcx.is_closure(self.mir_def_id) {
1246             let tables = tcx.typeck_tables_of(self.mir_def_id);
1247             let mir_hir_id = tcx.hir().def_index_to_hir_id(self.mir_def_id.index);
1248             match tables.node_type(mir_hir_id).kind {
1249                 ty::Closure(..) => "closure",
1250                 ty::Generator(..) => "generator",
1251                 _ => bug!("Closure body doesn't have a closure or generator type"),
1252             }
1253         } else {
1254             "function"
1255         };
1256
1257         let mut err =
1258             borrowck_errors::borrowed_data_escapes_closure(tcx, escape_span, escapes_from);
1259
1260         err.span_label(
1261             upvar_span,
1262             format!("`{}` is declared here, outside of the {} body", upvar_name, escapes_from),
1263         );
1264
1265         err.span_label(borrow_span, format!("borrow is only valid in the {} body", escapes_from));
1266
1267         if let Some(name) = name {
1268             err.span_label(
1269                 escape_span,
1270                 format!("reference to `{}` escapes the {} body here", name, escapes_from),
1271             );
1272         } else {
1273             err.span_label(
1274                 escape_span,
1275                 format!("reference escapes the {} body here", escapes_from),
1276             );
1277         }
1278
1279         err
1280     }
1281
1282     fn get_moved_indexes(&mut self, location: Location, mpi: MovePathIndex) -> Vec<MoveSite> {
1283         let mut stack = Vec::new();
1284         stack.extend(self.body.predecessor_locations(location).map(|predecessor| {
1285             let is_back_edge = location.dominates(predecessor, &self.dominators);
1286             (predecessor, is_back_edge)
1287         }));
1288
1289         let mut visited = FxHashSet::default();
1290         let mut result = vec![];
1291
1292         'dfs: while let Some((location, is_back_edge)) = stack.pop() {
1293             debug!(
1294                 "report_use_of_moved_or_uninitialized: (current_location={:?}, back_edge={})",
1295                 location, is_back_edge
1296             );
1297
1298             if !visited.insert(location) {
1299                 continue;
1300             }
1301
1302             // check for moves
1303             let stmt_kind =
1304                 self.body[location.block].statements.get(location.statement_index).map(|s| &s.kind);
1305             if let Some(StatementKind::StorageDead(..)) = stmt_kind {
1306                 // this analysis only tries to find moves explicitly
1307                 // written by the user, so we ignore the move-outs
1308                 // created by `StorageDead` and at the beginning
1309                 // of a function.
1310             } else {
1311                 // If we are found a use of a.b.c which was in error, then we want to look for
1312                 // moves not only of a.b.c but also a.b and a.
1313                 //
1314                 // Note that the moves data already includes "parent" paths, so we don't have to
1315                 // worry about the other case: that is, if there is a move of a.b.c, it is already
1316                 // marked as a move of a.b and a as well, so we will generate the correct errors
1317                 // there.
1318                 let mut mpis = vec![mpi];
1319                 let move_paths = &self.move_data.move_paths;
1320                 mpis.extend(move_paths[mpi].parents(move_paths));
1321
1322                 for moi in &self.move_data.loc_map[location] {
1323                     debug!("report_use_of_moved_or_uninitialized: moi={:?}", moi);
1324                     if mpis.contains(&self.move_data.moves[*moi].path) {
1325                         debug!("report_use_of_moved_or_uninitialized: found");
1326                         result.push(MoveSite { moi: *moi, traversed_back_edge: is_back_edge });
1327
1328                         // Strictly speaking, we could continue our DFS here. There may be
1329                         // other moves that can reach the point of error. But it is kind of
1330                         // confusing to highlight them.
1331                         //
1332                         // Example:
1333                         //
1334                         // ```
1335                         // let a = vec![];
1336                         // let b = a;
1337                         // let c = a;
1338                         // drop(a); // <-- current point of error
1339                         // ```
1340                         //
1341                         // Because we stop the DFS here, we only highlight `let c = a`,
1342                         // and not `let b = a`. We will of course also report an error at
1343                         // `let c = a` which highlights `let b = a` as the move.
1344                         continue 'dfs;
1345                     }
1346                 }
1347             }
1348
1349             // check for inits
1350             let mut any_match = false;
1351             drop_flag_effects::for_location_inits(
1352                 self.infcx.tcx,
1353                 &self.body,
1354                 self.move_data,
1355                 location,
1356                 |m| {
1357                     if m == mpi {
1358                         any_match = true;
1359                     }
1360                 },
1361             );
1362             if any_match {
1363                 continue 'dfs;
1364             }
1365
1366             stack.extend(self.body.predecessor_locations(location).map(|predecessor| {
1367                 let back_edge = location.dominates(predecessor, &self.dominators);
1368                 (predecessor, is_back_edge || back_edge)
1369             }));
1370         }
1371
1372         result
1373     }
1374
1375     pub(in crate::borrow_check) fn report_illegal_mutation_of_borrowed(
1376         &mut self,
1377         location: Location,
1378         (place, span): (&Place<'tcx>, Span),
1379         loan: &BorrowData<'tcx>,
1380     ) {
1381         let loan_spans = self.retrieve_borrow_spans(loan);
1382         let loan_span = loan_spans.args_or_use();
1383
1384         if loan.kind == BorrowKind::Shallow {
1385             if let Some(section) = self.classify_immutable_section(&loan.assigned_place) {
1386                 let mut err = self.cannot_mutate_in_immutable_section(
1387                     span,
1388                     loan_span,
1389                     &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
1390                     section,
1391                     "assign",
1392                 );
1393                 loan_spans.var_span_label(
1394                     &mut err,
1395                     format!("borrow occurs due to use{}", loan_spans.describe()),
1396                 );
1397
1398                 err.buffer(&mut self.errors_buffer);
1399
1400                 return;
1401             }
1402         }
1403
1404         let mut err = self.cannot_assign_to_borrowed(
1405             span,
1406             loan_span,
1407             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
1408         );
1409
1410         loan_spans
1411             .var_span_label(&mut err, format!("borrow occurs due to use{}", loan_spans.describe()));
1412
1413         self.explain_why_borrow_contains_point(location, loan, None).add_explanation_to_diagnostic(
1414             self.infcx.tcx,
1415             &self.body,
1416             &self.local_names,
1417             &mut err,
1418             "",
1419             None,
1420         );
1421
1422         err.buffer(&mut self.errors_buffer);
1423     }
1424
1425     /// Reports an illegal reassignment; for example, an assignment to
1426     /// (part of) a non-`mut` local that occurs potentially after that
1427     /// local has already been initialized. `place` is the path being
1428     /// assigned; `err_place` is a place providing a reason why
1429     /// `place` is not mutable (e.g., the non-`mut` local `x` in an
1430     /// assignment to `x.f`).
1431     pub(in crate::borrow_check) fn report_illegal_reassignment(
1432         &mut self,
1433         _location: Location,
1434         (place, span): (&Place<'tcx>, Span),
1435         assigned_span: Span,
1436         err_place: &Place<'tcx>,
1437     ) {
1438         let (from_arg, local_decl, local_name) = match err_place.as_local() {
1439             Some(local) => (
1440                 self.body.local_kind(local) == LocalKind::Arg,
1441                 Some(&self.body.local_decls[local]),
1442                 self.local_names[local],
1443             ),
1444             None => (false, None, None),
1445         };
1446
1447         // If root local is initialized immediately (everything apart from let
1448         // PATTERN;) then make the error refer to that local, rather than the
1449         // place being assigned later.
1450         let (place_description, assigned_span) = match local_decl {
1451             Some(LocalDecl { local_info: LocalInfo::User(ClearCrossCrate::Clear), .. })
1452             | Some(LocalDecl {
1453                 local_info:
1454                     LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
1455                         opt_match_place: None,
1456                         ..
1457                     }))),
1458                 ..
1459             })
1460             | Some(LocalDecl { local_info: LocalInfo::StaticRef { .. }, .. })
1461             | Some(LocalDecl { local_info: LocalInfo::Other, .. })
1462             | None => (self.describe_place(place.as_ref()), assigned_span),
1463             Some(decl) => (self.describe_place(err_place.as_ref()), decl.source_info.span),
1464         };
1465
1466         let mut err = self.cannot_reassign_immutable(
1467             span,
1468             place_description.as_ref().map(AsRef::as_ref).unwrap_or("_"),
1469             from_arg,
1470         );
1471         let msg = if from_arg {
1472             "cannot assign to immutable argument"
1473         } else {
1474             "cannot assign twice to immutable variable"
1475         };
1476         if span != assigned_span {
1477             if !from_arg {
1478                 let value_msg = match place_description {
1479                     Some(name) => format!("`{}`", name),
1480                     None => "value".to_owned(),
1481                 };
1482                 err.span_label(assigned_span, format!("first assignment to {}", value_msg));
1483             }
1484         }
1485         if let Some(decl) = local_decl {
1486             if let Some(name) = local_name {
1487                 if decl.can_be_made_mutable() {
1488                     err.span_suggestion(
1489                         decl.source_info.span,
1490                         "make this binding mutable",
1491                         format!("mut {}", name),
1492                         Applicability::MachineApplicable,
1493                     );
1494                 }
1495             }
1496         }
1497         err.span_label(span, msg);
1498         err.buffer(&mut self.errors_buffer);
1499     }
1500
1501     fn classify_drop_access_kind(&self, place: PlaceRef<'cx, 'tcx>) -> StorageDeadOrDrop<'tcx> {
1502         let tcx = self.infcx.tcx;
1503         match place.projection {
1504             [] => StorageDeadOrDrop::LocalStorageDead,
1505             [proj_base @ .., elem] => {
1506                 // FIXME(spastorino) make this iterate
1507                 let base_access = self.classify_drop_access_kind(PlaceRef {
1508                     local: place.local,
1509                     projection: proj_base,
1510                 });
1511                 match elem {
1512                     ProjectionElem::Deref => match base_access {
1513                         StorageDeadOrDrop::LocalStorageDead
1514                         | StorageDeadOrDrop::BoxedStorageDead => {
1515                             assert!(
1516                                 Place::ty_from(&place.local, proj_base, *self.body, tcx)
1517                                     .ty
1518                                     .is_box(),
1519                                 "Drop of value behind a reference or raw pointer"
1520                             );
1521                             StorageDeadOrDrop::BoxedStorageDead
1522                         }
1523                         StorageDeadOrDrop::Destructor(_) => base_access,
1524                     },
1525                     ProjectionElem::Field(..) | ProjectionElem::Downcast(..) => {
1526                         let base_ty = Place::ty_from(&place.local, proj_base, *self.body, tcx).ty;
1527                         match base_ty.kind {
1528                             ty::Adt(def, _) if def.has_dtor(tcx) => {
1529                                 // Report the outermost adt with a destructor
1530                                 match base_access {
1531                                     StorageDeadOrDrop::Destructor(_) => base_access,
1532                                     StorageDeadOrDrop::LocalStorageDead
1533                                     | StorageDeadOrDrop::BoxedStorageDead => {
1534                                         StorageDeadOrDrop::Destructor(base_ty)
1535                                     }
1536                                 }
1537                             }
1538                             _ => base_access,
1539                         }
1540                     }
1541
1542                     ProjectionElem::ConstantIndex { .. }
1543                     | ProjectionElem::Subslice { .. }
1544                     | ProjectionElem::Index(_) => base_access,
1545                 }
1546             }
1547         }
1548     }
1549
1550     /// Describe the reason for the fake borrow that was assigned to `place`.
1551     fn classify_immutable_section(&self, place: &Place<'tcx>) -> Option<&'static str> {
1552         use rustc::mir::visit::Visitor;
1553         struct FakeReadCauseFinder<'a, 'tcx> {
1554             place: &'a Place<'tcx>,
1555             cause: Option<FakeReadCause>,
1556         }
1557         impl<'tcx> Visitor<'tcx> for FakeReadCauseFinder<'_, 'tcx> {
1558             fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) {
1559                 match statement {
1560                     Statement { kind: StatementKind::FakeRead(cause, box ref place), .. }
1561                         if *place == *self.place =>
1562                     {
1563                         self.cause = Some(*cause);
1564                     }
1565                     _ => (),
1566                 }
1567             }
1568         }
1569         let mut visitor = FakeReadCauseFinder { place, cause: None };
1570         visitor.visit_body(self.body);
1571         match visitor.cause {
1572             Some(FakeReadCause::ForMatchGuard) => Some("match guard"),
1573             Some(FakeReadCause::ForIndex) => Some("indexing expression"),
1574             _ => None,
1575         }
1576     }
1577
1578     /// Annotate argument and return type of function and closure with (synthesized) lifetime for
1579     /// borrow of local value that does not live long enough.
1580     fn annotate_argument_and_return_for_borrow(
1581         &self,
1582         borrow: &BorrowData<'tcx>,
1583     ) -> Option<AnnotatedBorrowFnSignature<'tcx>> {
1584         // Define a fallback for when we can't match a closure.
1585         let fallback = || {
1586             let is_closure = self.infcx.tcx.is_closure(self.mir_def_id);
1587             if is_closure {
1588                 None
1589             } else {
1590                 let ty = self.infcx.tcx.type_of(self.mir_def_id);
1591                 match ty.kind {
1592                     ty::FnDef(_, _) | ty::FnPtr(_) => self
1593                         .annotate_fn_sig(self.mir_def_id, self.infcx.tcx.fn_sig(self.mir_def_id)),
1594                     _ => None,
1595                 }
1596             }
1597         };
1598
1599         // In order to determine whether we need to annotate, we need to check whether the reserve
1600         // place was an assignment into a temporary.
1601         //
1602         // If it was, we check whether or not that temporary is eventually assigned into the return
1603         // place. If it was, we can add annotations about the function's return type and arguments
1604         // and it'll make sense.
1605         let location = borrow.reserve_location;
1606         debug!("annotate_argument_and_return_for_borrow: location={:?}", location);
1607         if let Some(&Statement { kind: StatementKind::Assign(box (ref reservation, _)), .. }) =
1608             &self.body[location.block].statements.get(location.statement_index)
1609         {
1610             debug!("annotate_argument_and_return_for_borrow: reservation={:?}", reservation);
1611             // Check that the initial assignment of the reserve location is into a temporary.
1612             let mut target = match reservation.as_local() {
1613                 Some(local) if self.body.local_kind(local) == LocalKind::Temp => local,
1614                 _ => return None,
1615             };
1616
1617             // Next, look through the rest of the block, checking if we are assigning the
1618             // `target` (that is, the place that contains our borrow) to anything.
1619             let mut annotated_closure = None;
1620             for stmt in &self.body[location.block].statements[location.statement_index + 1..] {
1621                 debug!(
1622                     "annotate_argument_and_return_for_borrow: target={:?} stmt={:?}",
1623                     target, stmt
1624                 );
1625                 if let StatementKind::Assign(box (place, rvalue)) = &stmt.kind {
1626                     if let Some(assigned_to) = place.as_local() {
1627                         debug!(
1628                             "annotate_argument_and_return_for_borrow: assigned_to={:?} \
1629                              rvalue={:?}",
1630                             assigned_to, rvalue
1631                         );
1632                         // Check if our `target` was captured by a closure.
1633                         if let Rvalue::Aggregate(
1634                             box AggregateKind::Closure(def_id, substs),
1635                             operands,
1636                         ) = rvalue
1637                         {
1638                             for operand in operands {
1639                                 let assigned_from = match operand {
1640                                     Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1641                                         assigned_from
1642                                     }
1643                                     _ => continue,
1644                                 };
1645                                 debug!(
1646                                     "annotate_argument_and_return_for_borrow: assigned_from={:?}",
1647                                     assigned_from
1648                                 );
1649
1650                                 // Find the local from the operand.
1651                                 let assigned_from_local = match assigned_from.local_or_deref_local()
1652                                 {
1653                                     Some(local) => local,
1654                                     None => continue,
1655                                 };
1656
1657                                 if assigned_from_local != target {
1658                                     continue;
1659                                 }
1660
1661                                 // If a closure captured our `target` and then assigned
1662                                 // into a place then we should annotate the closure in
1663                                 // case it ends up being assigned into the return place.
1664                                 annotated_closure = self.annotate_fn_sig(
1665                                     *def_id,
1666                                     self.infcx.closure_sig(*def_id, *substs),
1667                                 );
1668                                 debug!(
1669                                     "annotate_argument_and_return_for_borrow: \
1670                                      annotated_closure={:?} assigned_from_local={:?} \
1671                                      assigned_to={:?}",
1672                                     annotated_closure, assigned_from_local, assigned_to
1673                                 );
1674
1675                                 if assigned_to == mir::RETURN_PLACE {
1676                                     // If it was assigned directly into the return place, then
1677                                     // return now.
1678                                     return annotated_closure;
1679                                 } else {
1680                                     // Otherwise, update the target.
1681                                     target = assigned_to;
1682                                 }
1683                             }
1684
1685                             // If none of our closure's operands matched, then skip to the next
1686                             // statement.
1687                             continue;
1688                         }
1689
1690                         // Otherwise, look at other types of assignment.
1691                         let assigned_from = match rvalue {
1692                             Rvalue::Ref(_, _, assigned_from) => assigned_from,
1693                             Rvalue::Use(operand) => match operand {
1694                                 Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1695                                     assigned_from
1696                                 }
1697                                 _ => continue,
1698                             },
1699                             _ => continue,
1700                         };
1701                         debug!(
1702                             "annotate_argument_and_return_for_borrow: \
1703                              assigned_from={:?}",
1704                             assigned_from,
1705                         );
1706
1707                         // Find the local from the rvalue.
1708                         let assigned_from_local = match assigned_from.local_or_deref_local() {
1709                             Some(local) => local,
1710                             None => continue,
1711                         };
1712                         debug!(
1713                             "annotate_argument_and_return_for_borrow: \
1714                              assigned_from_local={:?}",
1715                             assigned_from_local,
1716                         );
1717
1718                         // Check if our local matches the target - if so, we've assigned our
1719                         // borrow to a new place.
1720                         if assigned_from_local != target {
1721                             continue;
1722                         }
1723
1724                         // If we assigned our `target` into a new place, then we should
1725                         // check if it was the return place.
1726                         debug!(
1727                             "annotate_argument_and_return_for_borrow: \
1728                              assigned_from_local={:?} assigned_to={:?}",
1729                             assigned_from_local, assigned_to
1730                         );
1731                         if assigned_to == mir::RETURN_PLACE {
1732                             // If it was then return the annotated closure if there was one,
1733                             // else, annotate this function.
1734                             return annotated_closure.or_else(fallback);
1735                         }
1736
1737                         // If we didn't assign into the return place, then we just update
1738                         // the target.
1739                         target = assigned_to;
1740                     }
1741                 }
1742             }
1743
1744             // Check the terminator if we didn't find anything in the statements.
1745             let terminator = &self.body[location.block].terminator();
1746             debug!(
1747                 "annotate_argument_and_return_for_borrow: target={:?} terminator={:?}",
1748                 target, terminator
1749             );
1750             if let TerminatorKind::Call { destination: Some((place, _)), args, .. } =
1751                 &terminator.kind
1752             {
1753                 if let Some(assigned_to) = place.as_local() {
1754                     debug!(
1755                         "annotate_argument_and_return_for_borrow: assigned_to={:?} args={:?}",
1756                         assigned_to, args
1757                     );
1758                     for operand in args {
1759                         let assigned_from = match operand {
1760                             Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1761                                 assigned_from
1762                             }
1763                             _ => continue,
1764                         };
1765                         debug!(
1766                             "annotate_argument_and_return_for_borrow: assigned_from={:?}",
1767                             assigned_from,
1768                         );
1769
1770                         if let Some(assigned_from_local) = assigned_from.local_or_deref_local() {
1771                             debug!(
1772                                 "annotate_argument_and_return_for_borrow: assigned_from_local={:?}",
1773                                 assigned_from_local,
1774                             );
1775
1776                             if assigned_to == mir::RETURN_PLACE && assigned_from_local == target {
1777                                 return annotated_closure.or_else(fallback);
1778                             }
1779                         }
1780                     }
1781                 }
1782             }
1783         }
1784
1785         // If we haven't found an assignment into the return place, then we need not add
1786         // any annotations.
1787         debug!("annotate_argument_and_return_for_borrow: none found");
1788         None
1789     }
1790
1791     /// Annotate the first argument and return type of a function signature if they are
1792     /// references.
1793     fn annotate_fn_sig(
1794         &self,
1795         did: DefId,
1796         sig: ty::PolyFnSig<'tcx>,
1797     ) -> Option<AnnotatedBorrowFnSignature<'tcx>> {
1798         debug!("annotate_fn_sig: did={:?} sig={:?}", did, sig);
1799         let is_closure = self.infcx.tcx.is_closure(did);
1800         let fn_hir_id = self.infcx.tcx.hir().as_local_hir_id(did)?;
1801         let fn_decl = self.infcx.tcx.hir().fn_decl_by_hir_id(fn_hir_id)?;
1802
1803         // We need to work out which arguments to highlight. We do this by looking
1804         // at the return type, where there are three cases:
1805         //
1806         // 1. If there are named arguments, then we should highlight the return type and
1807         //    highlight any of the arguments that are also references with that lifetime.
1808         //    If there are no arguments that have the same lifetime as the return type,
1809         //    then don't highlight anything.
1810         // 2. The return type is a reference with an anonymous lifetime. If this is
1811         //    the case, then we can take advantage of (and teach) the lifetime elision
1812         //    rules.
1813         //
1814         //    We know that an error is being reported. So the arguments and return type
1815         //    must satisfy the elision rules. Therefore, if there is a single argument
1816         //    then that means the return type and first (and only) argument have the same
1817         //    lifetime and the borrow isn't meeting that, we can highlight the argument
1818         //    and return type.
1819         //
1820         //    If there are multiple arguments then the first argument must be self (else
1821         //    it would not satisfy the elision rules), so we can highlight self and the
1822         //    return type.
1823         // 3. The return type is not a reference. In this case, we don't highlight
1824         //    anything.
1825         let return_ty = sig.output();
1826         match return_ty.skip_binder().kind {
1827             ty::Ref(return_region, _, _) if return_region.has_name() && !is_closure => {
1828                 // This is case 1 from above, return type is a named reference so we need to
1829                 // search for relevant arguments.
1830                 let mut arguments = Vec::new();
1831                 for (index, argument) in sig.inputs().skip_binder().iter().enumerate() {
1832                     if let ty::Ref(argument_region, _, _) = argument.kind {
1833                         if argument_region == return_region {
1834                             // Need to use the `rustc::ty` types to compare against the
1835                             // `return_region`. Then use the `rustc_hir` type to get only
1836                             // the lifetime span.
1837                             if let hir::TyKind::Rptr(lifetime, _) = &fn_decl.inputs[index].kind {
1838                                 // With access to the lifetime, we can get
1839                                 // the span of it.
1840                                 arguments.push((*argument, lifetime.span));
1841                             } else {
1842                                 bug!("ty type is a ref but hir type is not");
1843                             }
1844                         }
1845                     }
1846                 }
1847
1848                 // We need to have arguments. This shouldn't happen, but it's worth checking.
1849                 if arguments.is_empty() {
1850                     return None;
1851                 }
1852
1853                 // We use a mix of the HIR and the Ty types to get information
1854                 // as the HIR doesn't have full types for closure arguments.
1855                 let return_ty = *sig.output().skip_binder();
1856                 let mut return_span = fn_decl.output.span();
1857                 if let hir::FunctionRetTy::Return(ty) = &fn_decl.output {
1858                     if let hir::TyKind::Rptr(lifetime, _) = ty.kind {
1859                         return_span = lifetime.span;
1860                     }
1861                 }
1862
1863                 Some(AnnotatedBorrowFnSignature::NamedFunction {
1864                     arguments,
1865                     return_ty,
1866                     return_span,
1867                 })
1868             }
1869             ty::Ref(_, _, _) if is_closure => {
1870                 // This is case 2 from above but only for closures, return type is anonymous
1871                 // reference so we select
1872                 // the first argument.
1873                 let argument_span = fn_decl.inputs.first()?.span;
1874                 let argument_ty = sig.inputs().skip_binder().first()?;
1875
1876                 // Closure arguments are wrapped in a tuple, so we need to get the first
1877                 // from that.
1878                 if let ty::Tuple(elems) = argument_ty.kind {
1879                     let argument_ty = elems.first()?.expect_ty();
1880                     if let ty::Ref(_, _, _) = argument_ty.kind {
1881                         return Some(AnnotatedBorrowFnSignature::Closure {
1882                             argument_ty,
1883                             argument_span,
1884                         });
1885                     }
1886                 }
1887
1888                 None
1889             }
1890             ty::Ref(_, _, _) => {
1891                 // This is also case 2 from above but for functions, return type is still an
1892                 // anonymous reference so we select the first argument.
1893                 let argument_span = fn_decl.inputs.first()?.span;
1894                 let argument_ty = sig.inputs().skip_binder().first()?;
1895
1896                 let return_span = fn_decl.output.span();
1897                 let return_ty = *sig.output().skip_binder();
1898
1899                 // We expect the first argument to be a reference.
1900                 match argument_ty.kind {
1901                     ty::Ref(_, _, _) => {}
1902                     _ => return None,
1903                 }
1904
1905                 Some(AnnotatedBorrowFnSignature::AnonymousFunction {
1906                     argument_ty,
1907                     argument_span,
1908                     return_ty,
1909                     return_span,
1910                 })
1911             }
1912             _ => {
1913                 // This is case 3 from above, return type is not a reference so don't highlight
1914                 // anything.
1915                 None
1916             }
1917         }
1918     }
1919 }
1920
1921 #[derive(Debug)]
1922 enum AnnotatedBorrowFnSignature<'tcx> {
1923     NamedFunction {
1924         arguments: Vec<(Ty<'tcx>, Span)>,
1925         return_ty: Ty<'tcx>,
1926         return_span: Span,
1927     },
1928     AnonymousFunction {
1929         argument_ty: Ty<'tcx>,
1930         argument_span: Span,
1931         return_ty: Ty<'tcx>,
1932         return_span: Span,
1933     },
1934     Closure {
1935         argument_ty: Ty<'tcx>,
1936         argument_span: Span,
1937     },
1938 }
1939
1940 impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
1941     /// Annotate the provided diagnostic with information about borrow from the fn signature that
1942     /// helps explain.
1943     pub(in crate::borrow_check) fn emit(
1944         &self,
1945         cx: &mut MirBorrowckCtxt<'_, 'tcx>,
1946         diag: &mut DiagnosticBuilder<'_>,
1947     ) -> String {
1948         match self {
1949             AnnotatedBorrowFnSignature::Closure { argument_ty, argument_span } => {
1950                 diag.span_label(
1951                     *argument_span,
1952                     format!("has type `{}`", cx.get_name_for_ty(argument_ty, 0)),
1953                 );
1954
1955                 cx.get_region_name_for_ty(argument_ty, 0)
1956             }
1957             AnnotatedBorrowFnSignature::AnonymousFunction {
1958                 argument_ty,
1959                 argument_span,
1960                 return_ty,
1961                 return_span,
1962             } => {
1963                 let argument_ty_name = cx.get_name_for_ty(argument_ty, 0);
1964                 diag.span_label(*argument_span, format!("has type `{}`", argument_ty_name));
1965
1966                 let return_ty_name = cx.get_name_for_ty(return_ty, 0);
1967                 let types_equal = return_ty_name == argument_ty_name;
1968                 diag.span_label(
1969                     *return_span,
1970                     format!(
1971                         "{}has type `{}`",
1972                         if types_equal { "also " } else { "" },
1973                         return_ty_name,
1974                     ),
1975                 );
1976
1977                 diag.note(
1978                     "argument and return type have the same lifetime due to lifetime elision rules",
1979                 );
1980                 diag.note(
1981                     "to learn more, visit <https://doc.rust-lang.org/book/ch10-03-\
1982                      lifetime-syntax.html#lifetime-elision>",
1983                 );
1984
1985                 cx.get_region_name_for_ty(return_ty, 0)
1986             }
1987             AnnotatedBorrowFnSignature::NamedFunction { arguments, return_ty, return_span } => {
1988                 // Region of return type and arguments checked to be the same earlier.
1989                 let region_name = cx.get_region_name_for_ty(return_ty, 0);
1990                 for (_, argument_span) in arguments {
1991                     diag.span_label(*argument_span, format!("has lifetime `{}`", region_name));
1992                 }
1993
1994                 diag.span_label(*return_span, format!("also has lifetime `{}`", region_name,));
1995
1996                 diag.help(&format!(
1997                     "use data from the highlighted arguments which match the `{}` lifetime of \
1998                      the return type",
1999                     region_name,
2000                 ));
2001
2002                 region_name
2003             }
2004         }
2005     }
2006 }