src/librustc_mir/borrow_check/conflict_errors.rs

   1 use rustc::hir;
   2 use rustc::hir::def_id::DefId;
   3 use rustc::mir::{
   4     self, AggregateKind, BindingForm, BorrowKind, ClearCrossCrate, ConstraintCategory, Local,
   5     LocalDecl, LocalKind, Location, Operand, Place, PlaceBase, Projection, PlaceRef,
   6     ProjectionElem, Rvalue, Statement, StatementKind, TerminatorKind, VarBindingForm,
   7 };
   8 use rustc::ty::{self, Ty};
   9 use rustc_data_structures::fx::FxHashSet;
  10 use rustc_data_structures::indexed_vec::Idx;
  11 use rustc_errors::{Applicability, DiagnosticBuilder};
  12 use syntax_pos::Span;
  13 use syntax::source_map::DesugaringKind;
  14
  15 use super::nll::explain_borrow::BorrowExplanation;
  16 use super::nll::region_infer::{RegionName, RegionNameSource};
  17 use super::prefixes::IsPrefixOf;
  18 use super::WriteKind;
  19 use super::borrow_set::BorrowData;
  20 use super::MirBorrowckCtxt;
  21 use super::{InitializationRequiringAction, PrefixSet};
  22 use super::error_reporting::{IncludingDowncast, UseSpans};
  23 use crate::dataflow::drop_flag_effects;
  24 use crate::dataflow::indexes::{MovePathIndex, MoveOutIndex};
  25 use crate::util::borrowck_errors;
  26
  27 #[derive(Debug)]
  28 struct MoveSite {
  29     /// Index of the "move out" that we found. The `MoveData` can
  30     /// then tell us where the move occurred.
  31     moi: MoveOutIndex,
  32
  33     /// `true` if we traversed a back edge while walking from the point
  34     /// of error to the move site.
  35     traversed_back_edge: bool
  36 }
  37
  38 /// Which case a StorageDeadOrDrop is for.
  39 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  40 enum StorageDeadOrDrop<'tcx> {
  41     LocalStorageDead,
  42     BoxedStorageDead,
  43     Destructor(Ty<'tcx>),
  44 }
  45
  46 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
  47     pub(super) fn report_use_of_moved_or_uninitialized(
  48         &mut self,
  49         location: Location,
  50         desired_action: InitializationRequiringAction,
  51         (moved_place, used_place, span): (PlaceRef<'cx, 'tcx>, PlaceRef<'cx, 'tcx>, Span),
  52         mpi: MovePathIndex,
  53     ) {
  54         debug!(
  55             "report_use_of_moved_or_uninitialized: location={:?} desired_action={:?} \
  56              moved_place={:?} used_place={:?} span={:?} mpi={:?}",
  57             location, desired_action, moved_place, used_place, span, mpi
  58         );
  59
  60         let use_spans = self.move_spans(moved_place, location)
  61             .or_else(|| self.borrow_spans(span, location));
  62         let span = use_spans.args_or_use();
  63
  64         let move_site_vec = self.get_moved_indexes(location, mpi);
  65         debug!(
  66             "report_use_of_moved_or_uninitialized: move_site_vec={:?}",
  67             move_site_vec
  68         );
  69         let move_out_indices: Vec<_> = move_site_vec
  70             .iter()
  71             .map(|move_site| move_site.moi)
  72             .collect();
  73
  74         if move_out_indices.is_empty() {
  75             let root_place = self
  76                 .prefixes(used_place, PrefixSet::All)
  77                 .last()
  78                 .unwrap();
  79
  80             if self.uninitialized_error_reported.contains(&root_place) {
  81                 debug!(
  82                     "report_use_of_moved_or_uninitialized place: error about {:?} suppressed",
  83                     root_place
  84                 );
  85                 return;
  86             }
  87
  88             self.uninitialized_error_reported.insert(root_place);
  89
  90             let item_msg = match self.describe_place_with_options(used_place,
  91                                                                   IncludingDowncast(true)) {
  92                 Some(name) => format!("`{}`", name),
  93                 None => "value".to_owned(),
  94             };
  95             let mut err = self.cannot_act_on_uninitialized_variable(
  96                 span,
  97                 desired_action.as_noun(),
  98                 &self.describe_place_with_options(moved_place, IncludingDowncast(true))
  99                     .unwrap_or_else(|| "_".to_owned()),
 100             );
 101             err.span_label(span, format!("use of possibly-uninitialized {}", item_msg));
 102
 103             use_spans.var_span_label(
 104                 &mut err,
 105                 format!("{} occurs due to use{}", desired_action.as_noun(), use_spans.describe()),
 106             );
 107
 108             // This error should not be downgraded to a warning,
 109             // even in migrate mode.
 110             self.disable_error_downgrading();
 111             err.buffer(&mut self.errors_buffer);
 112         } else {
 113             if let Some((reported_place, _)) = self.move_error_reported.get(&move_out_indices) {
 114                 if self.prefixes(*reported_place, PrefixSet::All)
 115                     .any(|p| p == used_place)
 116                 {
 117                     debug!(
 118                         "report_use_of_moved_or_uninitialized place: error suppressed \
 119                          mois={:?}",
 120                         move_out_indices
 121                     );
 122                     return;
 123                 }
 124             }
 125
 126             let msg = ""; //FIXME: add "partially " or "collaterally "
 127
 128             let mut err = self.cannot_act_on_moved_value(
 129                 span,
 130                 desired_action.as_noun(),
 131                 msg,
 132                 self.describe_place_with_options(moved_place, IncludingDowncast(true)),
 133             );
 134
 135             self.add_moved_or_invoked_closure_note(
 136                 location,
 137                 used_place,
 138                 &mut err,
 139             );
 140
 141             let mut is_loop_move = false;
 142             let is_partial_move = move_site_vec.iter().any(|move_site| {
 143                 let move_out = self.move_data.moves[(*move_site).moi];
 144                 let moved_place = &self.move_data.move_paths[move_out.path].place;
 145                 used_place != moved_place.as_ref()
 146                     && used_place.is_prefix_of(moved_place.as_ref())
 147             });
 148             for move_site in &move_site_vec {
 149                 let move_out = self.move_data.moves[(*move_site).moi];
 150                 let moved_place = &self.move_data.move_paths[move_out.path].place;
 151
 152                 let move_spans = self.move_spans(moved_place.as_ref(), move_out.source);
 153                 let move_span = move_spans.args_or_use();
 154
 155                 let move_msg = if move_spans.for_closure() {
 156                     " into closure"
 157                 } else {
 158                     ""
 159                 };
 160
 161                 if span == move_span {
 162                     err.span_label(
 163                         span,
 164                         format!("value moved{} here, in previous iteration of loop", move_msg),
 165                     );
 166                     is_loop_move = true;
 167                 } else if move_site.traversed_back_edge {
 168                     err.span_label(
 169                         move_span,
 170                         format!(
 171                             "value moved{} here, in previous iteration of loop",
 172                             move_msg
 173                         ),
 174                     );
 175                 } else {
 176                     err.span_label(move_span, format!("value moved{} here", move_msg));
 177                     move_spans.var_span_label(
 178                         &mut err,
 179                         format!("variable moved due to use{}", move_spans.describe()),
 180                     );
 181                 }
 182                 if Some(DesugaringKind::ForLoop) == move_span.desugaring_kind() {
 183                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 184                         err.span_suggestion(
 185                             move_span,
 186                             "consider borrowing to avoid moving into the for loop",
 187                             format!("&{}", snippet),
 188                             Applicability::MaybeIncorrect,
 189                         );
 190                     }
 191                 }
 192             }
 193
 194             use_spans.var_span_label(
 195                 &mut err,
 196                 format!("{} occurs due to use{}", desired_action.as_noun(), use_spans.describe()),
 197             );
 198
 199             if !is_loop_move {
 200                 err.span_label(
 201                     span,
 202                     format!(
 203                         "value {} here {}",
 204                         desired_action.as_verb_in_past_tense(),
 205                         if is_partial_move { "after partial move" } else { "after move" },
 206                     ),
 207                 );
 208             }
 209
 210             let ty =
 211                 Place::ty_from(used_place.base, used_place.projection, self.body, self.infcx.tcx)
 212                     .ty;
 213             let needs_note = match ty.sty {
 214                 ty::Closure(id, _) => {
 215                     let tables = self.infcx.tcx.typeck_tables_of(id);
 216                     let hir_id = self.infcx.tcx.hir().as_local_hir_id(id).unwrap();
 217
 218                     tables.closure_kind_origins().get(hir_id).is_none()
 219                 }
 220                 _ => true,
 221             };
 222
 223             if needs_note {
 224                 let mpi = self.move_data.moves[move_out_indices[0]].path;
 225                 let place = &self.move_data.move_paths[mpi].place;
 226
 227                 let ty = place.ty(self.body, self.infcx.tcx).ty;
 228                 let opt_name =
 229                     self.describe_place_with_options(place.as_ref(), IncludingDowncast(true));
 230                 let note_msg = match opt_name {
 231                     Some(ref name) => format!("`{}`", name),
 232                     None => "value".to_owned(),
 233                 };
 234                 if let ty::Param(param_ty) = ty.sty {
 235                     let tcx = self.infcx.tcx;
 236                     let generics = tcx.generics_of(self.mir_def_id);
 237                     let def_id = generics.type_param(&param_ty, tcx).def_id;
 238                     if let Some(sp) = tcx.hir().span_if_local(def_id) {
 239                         err.span_label(
 240                             sp,
 241                             "consider adding a `Copy` constraint to this type argument",
 242                         );
 243                     }
 244                 }
 245                 let span = if let Place {
 246                     base: PlaceBase::Local(local),
 247                     projection: None,
 248                 } = place {
 249                     let decl = &self.body.local_decls[*local];
 250                     Some(decl.source_info.span)
 251                 } else {
 252                     None
 253                 };
 254                 self.note_type_does_not_implement_copy(
 255                     &mut err,
 256                     &note_msg,
 257                     ty,
 258                     span,
 259                 );
 260             }
 261
 262             if let Some((_, mut old_err)) = self.move_error_reported
 263                 .insert(move_out_indices, (used_place, err))
 264             {
 265                 // Cancel the old error so it doesn't ICE.
 266                 old_err.cancel();
 267             }
 268         }
 269     }
 270
 271     pub(super) fn report_move_out_while_borrowed(
 272         &mut self,
 273         location: Location,
 274         (place, span): (&Place<'tcx>, Span),
 275         borrow: &BorrowData<'tcx>,
 276     ) {
 277         debug!(
 278             "report_move_out_while_borrowed: location={:?} place={:?} span={:?} borrow={:?}",
 279             location, place, span, borrow
 280         );
 281         let value_msg = match self.describe_place(place.as_ref()) {
 282             Some(name) => format!("`{}`", name),
 283             None => "value".to_owned(),
 284         };
 285         let borrow_msg = match self.describe_place(borrow.borrowed_place.as_ref()) {
 286             Some(name) => format!("`{}`", name),
 287             None => "value".to_owned(),
 288         };
 289
 290         let borrow_spans = self.retrieve_borrow_spans(borrow);
 291         let borrow_span = borrow_spans.args_or_use();
 292
 293         let move_spans = self.move_spans(place.as_ref(), location);
 294         let span = move_spans.args_or_use();
 295
 296         let mut err = self.cannot_move_when_borrowed(
 297             span,
 298             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
 299         );
 300         err.span_label(borrow_span, format!("borrow of {} occurs here", borrow_msg));
 301         err.span_label(span, format!("move out of {} occurs here", value_msg));
 302
 303         borrow_spans.var_span_label(
 304             &mut err,
 305             format!("borrow occurs due to use{}", borrow_spans.describe())
 306         );
 307
 308         move_spans.var_span_label(
 309             &mut err,
 310             format!("move occurs due to use{}", move_spans.describe())
 311         );
 312
 313         self.explain_why_borrow_contains_point(
 314             location,
 315             borrow,
 316             None,
 317         ).add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", Some(borrow_span));
 318         err.buffer(&mut self.errors_buffer);
 319     }
 320
 321     pub(super) fn report_use_while_mutably_borrowed(
 322         &mut self,
 323         location: Location,
 324         (place, _span): (&Place<'tcx>, Span),
 325         borrow: &BorrowData<'tcx>,
 326     ) -> DiagnosticBuilder<'cx> {
 327         let borrow_spans = self.retrieve_borrow_spans(borrow);
 328         let borrow_span = borrow_spans.args_or_use();
 329
 330         // Conflicting borrows are reported separately, so only check for move
 331         // captures.
 332         let use_spans = self.move_spans(place.as_ref(), location);
 333         let span = use_spans.var_or_use();
 334
 335         let mut err = self.cannot_use_when_mutably_borrowed(
 336             span,
 337             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
 338             borrow_span,
 339             &self.describe_place(borrow.borrowed_place.as_ref())
 340                 .unwrap_or_else(|| "_".to_owned()),
 341         );
 342
 343         borrow_spans.var_span_label(&mut err, {
 344             let place = &borrow.borrowed_place;
 345             let desc_place =
 346                 self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned());
 347
 348             format!("borrow occurs due to use of `{}`{}", desc_place, borrow_spans.describe())
 349         });
 350
 351         self.explain_why_borrow_contains_point(location, borrow, None)
 352             .add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
 353         err
 354     }
 355
 356     pub(super) fn report_conflicting_borrow(
 357         &mut self,
 358         location: Location,
 359         (place, span): (&Place<'tcx>, Span),
 360         gen_borrow_kind: BorrowKind,
 361         issued_borrow: &BorrowData<'tcx>,
 362     ) -> DiagnosticBuilder<'cx> {
 363         let issued_spans = self.retrieve_borrow_spans(issued_borrow);
 364         let issued_span = issued_spans.args_or_use();
 365
 366         let borrow_spans = self.borrow_spans(span, location);
 367         let span = borrow_spans.args_or_use();
 368
 369         let container_name = if issued_spans.for_generator() || borrow_spans.for_generator() {
 370             "generator"
 371         } else {
 372             "closure"
 373         };
 374
 375         let (desc_place, msg_place, msg_borrow, union_type_name) =
 376             self.describe_place_for_conflicting_borrow(place, &issued_borrow.borrowed_place);
 377
 378         let explanation = self.explain_why_borrow_contains_point(location, issued_borrow, None);
 379         let second_borrow_desc = if explanation.is_explained() {
 380             "second "
 381         } else {
 382             ""
 383         };
 384
 385         // FIXME: supply non-"" `opt_via` when appropriate
 386         let first_borrow_desc;
 387         let mut err = match (
 388             gen_borrow_kind,
 389             "immutable",
 390             "mutable",
 391             issued_borrow.kind,
 392             "immutable",
 393             "mutable",
 394         ) {
 395             (BorrowKind::Shared, lft, _, BorrowKind::Mut { .. }, _, rgt) => {
 396                 first_borrow_desc = "mutable ";
 397                 self.cannot_reborrow_already_borrowed(
 398                     span,
 399                     &desc_place,
 400                     &msg_place,
 401                     lft,
 402                     issued_span,
 403                     "it",
 404                     rgt,
 405                     &msg_borrow,
 406                     None,
 407                 )
 408             }
 409             (BorrowKind::Mut { .. }, _, lft, BorrowKind::Shared, rgt, _) => {
 410                 first_borrow_desc = "immutable ";
 411                 self.cannot_reborrow_already_borrowed(
 412                     span,
 413                     &desc_place,
 414                     &msg_place,
 415                     lft,
 416                     issued_span,
 417                     "it",
 418                     rgt,
 419                     &msg_borrow,
 420                     None,
 421                 )
 422             }
 423
 424             (BorrowKind::Mut { .. }, _, _, BorrowKind::Mut { .. }, _, _) => {
 425                 first_borrow_desc = "first ";
 426                 self.cannot_mutably_borrow_multiply(
 427                     span,
 428                     &desc_place,
 429                     &msg_place,
 430                     issued_span,
 431                     &msg_borrow,
 432                     None,
 433                 )
 434             }
 435
 436             (BorrowKind::Unique, _, _, BorrowKind::Unique, _, _) => {
 437                 first_borrow_desc = "first ";
 438                 self.cannot_uniquely_borrow_by_two_closures(
 439                     span,
 440                     &desc_place,
 441                     issued_span,
 442                     None,
 443                 )
 444             }
 445
 446             (BorrowKind::Mut { .. }, _, _, BorrowKind::Shallow, _, _)
 447             | (BorrowKind::Unique, _, _, BorrowKind::Shallow, _, _) => {
 448                 let mut err = self.cannot_mutate_in_match_guard(
 449                     span,
 450                     issued_span,
 451                     &desc_place,
 452                     "mutably borrow",
 453                 );
 454                 borrow_spans.var_span_label(
 455                     &mut err,
 456                     format!(
 457                         "borrow occurs due to use of `{}`{}", desc_place, borrow_spans.describe()
 458                     ),
 459                 );
 460
 461                 return err;
 462             }
 463
 464             (BorrowKind::Unique, _, _, _, _, _) => {
 465                 first_borrow_desc = "first ";
 466                 self.cannot_uniquely_borrow_by_one_closure(
 467                     span,
 468                     container_name,
 469                     &desc_place,
 470                     "",
 471                     issued_span,
 472                     "it",
 473                     "",
 474                     None,
 475                 )
 476             },
 477
 478             (BorrowKind::Shared, lft, _, BorrowKind::Unique, _, _) => {
 479                 first_borrow_desc = "first ";
 480                 self.cannot_reborrow_already_uniquely_borrowed(
 481                     span,
 482                     container_name,
 483                     &desc_place,
 484                     "",
 485                     lft,
 486                     issued_span,
 487                     "",
 488                     None,
 489                     second_borrow_desc,
 490                 )
 491             }
 492
 493             (BorrowKind::Mut { .. }, _, lft, BorrowKind::Unique, _, _) => {
 494                 first_borrow_desc = "first ";
 495                 self.cannot_reborrow_already_uniquely_borrowed(
 496                     span,
 497                     container_name,
 498                     &desc_place,
 499                     "",
 500                     lft,
 501                     issued_span,
 502                     "",
 503                     None,
 504                     second_borrow_desc,
 505                 )
 506             }
 507
 508             (BorrowKind::Shared, _, _, BorrowKind::Shared, _, _)
 509             | (BorrowKind::Shared, _, _, BorrowKind::Shallow, _, _)
 510             | (BorrowKind::Shallow, _, _, BorrowKind::Mut { .. }, _, _)
 511             | (BorrowKind::Shallow, _, _, BorrowKind::Unique, _, _)
 512             | (BorrowKind::Shallow, _, _, BorrowKind::Shared, _, _)
 513             | (BorrowKind::Shallow, _, _, BorrowKind::Shallow, _, _) => unreachable!(),
 514         };
 515
 516         if issued_spans == borrow_spans {
 517             borrow_spans.var_span_label(
 518                 &mut err,
 519                 format!("borrows occur due to use of `{}`{}", desc_place, borrow_spans.describe()),
 520             );
 521         } else {
 522             let borrow_place = &issued_borrow.borrowed_place;
 523             let borrow_place_desc = self.describe_place(borrow_place.as_ref())
 524                                         .unwrap_or_else(|| "_".to_owned());
 525             issued_spans.var_span_label(
 526                 &mut err,
 527                 format!(
 528                     "first borrow occurs due to use of `{}`{}",
 529                     borrow_place_desc,
 530                     issued_spans.describe(),
 531                 ),
 532             );
 533
 534             borrow_spans.var_span_label(
 535                 &mut err,
 536                 format!(
 537                     "second borrow occurs due to use of `{}`{}",
 538                     desc_place,
 539                     borrow_spans.describe(),
 540                 ),
 541             );
 542         }
 543
 544         if union_type_name != "" {
 545             err.note(&format!(
 546                 "`{}` is a field of the union `{}`, so it overlaps the field `{}`",
 547                 msg_place, union_type_name, msg_borrow,
 548             ));
 549         }
 550
 551         explanation.add_explanation_to_diagnostic(
 552             self.infcx.tcx,
 553             self.body,
 554             &mut err,
 555             first_borrow_desc,
 556             None,
 557         );
 558
 559         err
 560     }
 561
 562     /// Returns the description of the root place for a conflicting borrow and the full
 563     /// descriptions of the places that caused the conflict.
 564     ///
 565     /// In the simplest case, where there are no unions involved, if a mutable borrow of `x` is
 566     /// attempted while a shared borrow is live, then this function will return:
 567     ///
 568     ///     ("x", "", "")
 569     ///
 570     /// In the simple union case, if a mutable borrow of a union field `x.z` is attempted while
 571     /// a shared borrow of another field `x.y`, then this function will return:
 572     ///
 573     ///     ("x", "x.z", "x.y")
 574     ///
 575     /// In the more complex union case, where the union is a field of a struct, then if a mutable
 576     /// borrow of a union field in a struct `x.u.z` is attempted while a shared borrow of
 577     /// another field `x.u.y`, then this function will return:
 578     ///
 579     ///     ("x.u", "x.u.z", "x.u.y")
 580     ///
 581     /// This is used when creating error messages like below:
 582     ///
 583     /// >  cannot borrow `a.u` (via `a.u.z.c`) as immutable because it is also borrowed as
 584     /// >  mutable (via `a.u.s.b`) [E0502]
 585     pub(super) fn describe_place_for_conflicting_borrow(
 586         &self,
 587         first_borrowed_place: &Place<'tcx>,
 588         second_borrowed_place: &Place<'tcx>,
 589     ) -> (String, String, String, String) {
 590         // Define a small closure that we can use to check if the type of a place
 591         // is a union.
 592         let union_ty = |place_base, place_projection| {
 593             let ty = Place::ty_from(place_base, place_projection, self.body, self.infcx.tcx).ty;
 594             ty.ty_adt_def().filter(|adt| adt.is_union()).map(|_| ty)
 595         };
 596         let describe_place = |place| self.describe_place(place).unwrap_or_else(|| "_".to_owned());
 597
 598         // Start with an empty tuple, so we can use the functions on `Option` to reduce some
 599         // code duplication (particularly around returning an empty description in the failure
 600         // case).
 601         Some(())
 602             .filter(|_| {
 603                 // If we have a conflicting borrow of the same place, then we don't want to add
 604                 // an extraneous "via x.y" to our diagnostics, so filter out this case.
 605                 first_borrowed_place != second_borrowed_place
 606             })
 607             .and_then(|_| {
 608                 // We're going to want to traverse the first borrowed place to see if we can find
 609                 // field access to a union. If we find that, then we will keep the place of the
 610                 // union being accessed and the field that was being accessed so we can check the
 611                 // second borrowed place for the same union and a access to a different field.
 612                 let Place {
 613                     base,
 614                     projection,
 615                 } = first_borrowed_place;
 616
 617                 let mut current = projection;
 618
 619                 while let Some(box Projection { base: base_proj, elem }) = current {
 620                     match elem {
 621                         ProjectionElem::Field(field, _) if union_ty(base, base_proj).is_some() => {
 622                             return Some((PlaceRef {
 623                                 base: base,
 624                                 projection: base_proj,
 625                             }, field));
 626                         },
 627                         _ => current = base_proj,
 628                     }
 629                 }
 630                 None
 631             })
 632             .and_then(|(target_base, target_field)| {
 633                 // With the place of a union and a field access into it, we traverse the second
 634                 // borrowed place and look for a access to a different field of the same union.
 635                 let Place {
 636                     base,
 637                     projection,
 638                 } = second_borrowed_place;
 639
 640                 let mut current = projection;
 641
 642                 while let Some(box Projection { base: proj_base, elem }) = current {
 643                     if let ProjectionElem::Field(field, _) = elem {
 644                         if let Some(union_ty) = union_ty(base, proj_base) {
 645                             if field != target_field
 646                                 && base == target_base.base
 647                                 && proj_base == target_base.projection {
 648                                 // FIXME when we avoid clone reuse describe_place closure
 649                                 let describe_base_place =  self.describe_place(PlaceRef {
 650                                     base: base,
 651                                     projection: proj_base,
 652                                 }).unwrap_or_else(|| "_".to_owned());
 653
 654                                 return Some((
 655                                     describe_base_place,
 656                                     describe_place(first_borrowed_place.as_ref()),
 657                                     describe_place(second_borrowed_place.as_ref()),
 658                                     union_ty.to_string(),
 659                                 ));
 660                             }
 661                         }
 662                     }
 663
 664                     current = proj_base;
 665                 }
 666                 None
 667             })
 668             .unwrap_or_else(|| {
 669                 // If we didn't find a field access into a union, or both places match, then
 670                 // only return the description of the first place.
 671                 (
 672                     describe_place(first_borrowed_place.as_ref()),
 673                     "".to_string(),
 674                     "".to_string(),
 675                     "".to_string(),
 676                 )
 677             })
 678     }
 679
 680     /// Reports StorageDeadOrDrop of `place` conflicts with `borrow`.
 681     ///
 682     /// This means that some data referenced by `borrow` needs to live
 683     /// past the point where the StorageDeadOrDrop of `place` occurs.
 684     /// This is usually interpreted as meaning that `place` has too
 685     /// short a lifetime. (But sometimes it is more useful to report
 686     /// it as a more direct conflict between the execution of a
 687     /// `Drop::drop` with an aliasing borrow.)
 688     pub(super) fn report_borrowed_value_does_not_live_long_enough(
 689         &mut self,
 690         location: Location,
 691         borrow: &BorrowData<'tcx>,
 692         place_span: (&Place<'tcx>, Span),
 693         kind: Option<WriteKind>,
 694     ) {
 695         debug!(
 696             "report_borrowed_value_does_not_live_long_enough(\
 697              {:?}, {:?}, {:?}, {:?}\
 698              )",
 699             location, borrow, place_span, kind
 700         );
 701
 702         let drop_span = place_span.1;
 703         let root_place = self.prefixes(borrow.borrowed_place.as_ref(), PrefixSet::All)
 704             .last()
 705             .unwrap();
 706
 707         let borrow_spans = self.retrieve_borrow_spans(borrow);
 708         let borrow_span = borrow_spans.var_or_use();
 709
 710         assert!(root_place.projection.is_none());
 711         let proper_span = match root_place.base {
 712             PlaceBase::Local(local) => self.body.local_decls[*local].source_info.span,
 713             _ => drop_span,
 714         };
 715
 716         if self.access_place_error_reported
 717             .contains(&(Place {
 718                 base: root_place.base.clone(),
 719                 projection: root_place.projection.clone(),
 720             }, borrow_span))
 721         {
 722             debug!(
 723                 "suppressing access_place error when borrow doesn't live long enough for {:?}",
 724                 borrow_span
 725             );
 726             return;
 727         }
 728
 729         self.access_place_error_reported
 730             .insert((Place {
 731                 base: root_place.base.clone(),
 732                 projection: root_place.projection.clone(),
 733             }, borrow_span));
 734
 735         if let StorageDeadOrDrop::Destructor(dropped_ty) =
 736             self.classify_drop_access_kind(borrow.borrowed_place.as_ref())
 737         {
 738             // If a borrow of path `B` conflicts with drop of `D` (and
 739             // we're not in the uninteresting case where `B` is a
 740             // prefix of `D`), then report this as a more interesting
 741             // destructor conflict.
 742             if !borrow.borrowed_place.as_ref().is_prefix_of(place_span.0.as_ref()) {
 743                 self.report_borrow_conflicts_with_destructor(
 744                     location, borrow, place_span, kind, dropped_ty,
 745                 );
 746                 return;
 747             }
 748         }
 749
 750         let place_desc = self.describe_place(borrow.borrowed_place.as_ref());
 751
 752         let kind_place = kind.filter(|_| place_desc.is_some()).map(|k| (k, place_span.0));
 753         let explanation = self.explain_why_borrow_contains_point(location, &borrow, kind_place);
 754
 755         let err = match (place_desc, explanation) {
 756             (Some(_), _) if self.is_place_thread_local(root_place) => {
 757                 self.report_thread_local_value_does_not_live_long_enough(drop_span, borrow_span)
 758             }
 759             // If the outlives constraint comes from inside the closure,
 760             // for example:
 761             //
 762             // let x = 0;
 763             // let y = &x;
 764             // Box::new(|| y) as Box<Fn() -> &'static i32>
 765             //
 766             // then just use the normal error. The closure isn't escaping
 767             // and `move` will not help here.
 768             (
 769                 Some(ref name),
 770                 BorrowExplanation::MustBeValidFor {
 771                     category: category @ ConstraintCategory::Return,
 772                     from_closure: false,
 773                     ref region_name,
 774                     span,
 775                     ..
 776                 },
 777             )
 778             | (
 779                 Some(ref name),
 780                 BorrowExplanation::MustBeValidFor {
 781                     category: category @ ConstraintCategory::CallArgument,
 782                     from_closure: false,
 783                     ref region_name,
 784                     span,
 785                     ..
 786                 },
 787             ) if borrow_spans.for_closure() => self.report_escaping_closure_capture(
 788                 borrow_spans.args_or_use(),
 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: RegionName {
 801                         source: RegionNameSource::AnonRegionFromUpvar(upvar_span, ref upvar_name),
 802                         ..
 803                     },
 804                     span,
 805                     ..
 806                 },
 807             ) => self.report_escaping_data(borrow_span, name, upvar_span, upvar_name, span),
 808             (Some(name), explanation) => self.report_local_value_does_not_live_long_enough(
 809                 location,
 810                 &name,
 811                 &borrow,
 812                 drop_span,
 813                 borrow_spans,
 814                 explanation,
 815             ),
 816             (None, explanation) => self.report_temporary_value_does_not_live_long_enough(
 817                 location,
 818                 &borrow,
 819                 drop_span,
 820                 borrow_spans,
 821                 proper_span,
 822                 explanation,
 823             ),
 824         };
 825
 826         err.buffer(&mut self.errors_buffer);
 827     }
 828
 829     fn report_local_value_does_not_live_long_enough(
 830         &mut self,
 831         location: Location,
 832         name: &str,
 833         borrow: &BorrowData<'tcx>,
 834         drop_span: Span,
 835         borrow_spans: UseSpans,
 836         explanation: BorrowExplanation,
 837     ) -> DiagnosticBuilder<'cx> {
 838         debug!(
 839             "report_local_value_does_not_live_long_enough(\
 840              {:?}, {:?}, {:?}, {:?}, {:?}\
 841              )",
 842             location, name, borrow, drop_span, borrow_spans
 843         );
 844
 845         let borrow_span = borrow_spans.var_or_use();
 846         if let BorrowExplanation::MustBeValidFor {
 847             category,
 848             span,
 849             ref opt_place_desc,
 850             from_closure: false,
 851             ..
 852         } = explanation {
 853             if let Some(diag) = self.try_report_cannot_return_reference_to_local(
 854                 borrow,
 855                 borrow_span,
 856                 span,
 857                 category,
 858                 opt_place_desc.as_ref(),
 859             ) {
 860                 return diag;
 861             }
 862         }
 863
 864         let mut err = self.path_does_not_live_long_enough(
 865             borrow_span,
 866             &format!("`{}`", name),
 867         );
 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 function `{}` returns",
 882                         name,
 883                         self.infcx.tcx.hir().name(fn_hir_id),
 884                     ),
 885                 );
 886
 887                 err.note(
 888                     "functions cannot return a borrow to data owned within the function's scope, \
 889                      functions can only return borrows to data passed as arguments",
 890                 );
 891                 err.note(
 892                     "to learn more, visit <https://doc.rust-lang.org/book/ch04-02-\
 893                      references-and-borrowing.html#dangling-references>",
 894                 );
 895             } else {
 896                 err.span_label(
 897                     drop_span,
 898                     format!("...but `{}` dropped here while still borrowed", name),
 899                 );
 900             }
 901
 902             if let BorrowExplanation::MustBeValidFor { .. } = explanation {
 903             } else {
 904                 explanation.add_explanation_to_diagnostic(
 905                     self.infcx.tcx,
 906                     self.body,
 907                     &mut err,
 908                     "",
 909                     None,
 910                 );
 911             }
 912         } else {
 913             err.span_label(borrow_span, "borrowed value does not live long enough");
 914             err.span_label(
 915                 drop_span,
 916                 format!("`{}` dropped here while still borrowed", name),
 917             );
 918
 919             let within = if borrow_spans.for_generator() {
 920                 " by generator"
 921             } else {
 922                 ""
 923             };
 924
 925             borrow_spans.args_span_label(
 926                 &mut err,
 927                 format!("value captured here{}", within),
 928             );
 929
 930             explanation.add_explanation_to_diagnostic(
 931                 self.infcx.tcx, self.body, &mut err, "", None);
 932         }
 933
 934         err
 935     }
 936
 937     fn report_borrow_conflicts_with_destructor(
 938         &mut self,
 939         location: Location,
 940         borrow: &BorrowData<'tcx>,
 941         (place, drop_span): (&Place<'tcx>, Span),
 942         kind: Option<WriteKind>,
 943         dropped_ty: Ty<'tcx>,
 944     ) {
 945         debug!(
 946             "report_borrow_conflicts_with_destructor(\
 947              {:?}, {:?}, ({:?}, {:?}), {:?}\
 948              )",
 949             location, borrow, place, drop_span, kind,
 950         );
 951
 952         let borrow_spans = self.retrieve_borrow_spans(borrow);
 953         let borrow_span = borrow_spans.var_or_use();
 954
 955         let mut err = self.cannot_borrow_across_destructor(borrow_span);
 956
 957         let what_was_dropped = match self.describe_place(place.as_ref()) {
 958             Some(name) => format!("`{}`", name.as_str()),
 959             None => String::from("temporary value"),
 960         };
 961
 962         let label = match self.describe_place(borrow.borrowed_place.as_ref()) {
 963             Some(borrowed) => format!(
 964                 "here, drop of {D} needs exclusive access to `{B}`, \
 965                  because the type `{T}` implements the `Drop` trait",
 966                 D = what_was_dropped,
 967                 T = dropped_ty,
 968                 B = borrowed
 969             ),
 970             None => format!(
 971                 "here is drop of {D}; whose type `{T}` implements the `Drop` trait",
 972                 D = what_was_dropped,
 973                 T = dropped_ty
 974             ),
 975         };
 976         err.span_label(drop_span, label);
 977
 978         // Only give this note and suggestion if they could be relevant.
 979         let explanation =
 980             self.explain_why_borrow_contains_point(location, borrow, kind.map(|k| (k, place)));
 981         match explanation {
 982             BorrowExplanation::UsedLater { .. }
 983             | BorrowExplanation::UsedLaterWhenDropped { .. } => {
 984                 err.note("consider using a `let` binding to create a longer lived value");
 985             }
 986             _ => {}
 987         }
 988
 989         explanation.add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
 990
 991         err.buffer(&mut self.errors_buffer);
 992     }
 993
 994     fn report_thread_local_value_does_not_live_long_enough(
 995         &mut self,
 996         drop_span: Span,
 997         borrow_span: Span,
 998     ) -> DiagnosticBuilder<'cx> {
 999         debug!(
1000             "report_thread_local_value_does_not_live_long_enough(\
1001              {:?}, {:?}\
1002              )",
1003             drop_span, borrow_span
1004         );
1005
1006         let mut err = self.thread_local_value_does_not_live_long_enough(borrow_span);
1007
1008         err.span_label(
1009             borrow_span,
1010             "thread-local variables cannot be borrowed beyond the end of the function",
1011         );
1012         err.span_label(drop_span, "end of enclosing function is here");
1013
1014         err
1015     }
1016
1017     fn report_temporary_value_does_not_live_long_enough(
1018         &mut self,
1019         location: Location,
1020         borrow: &BorrowData<'tcx>,
1021         drop_span: Span,
1022         borrow_spans: UseSpans,
1023         proper_span: Span,
1024         explanation: BorrowExplanation,
1025     ) -> DiagnosticBuilder<'cx> {
1026         debug!(
1027             "report_temporary_value_does_not_live_long_enough(\
1028              {:?}, {:?}, {:?}, {:?}\
1029              )",
1030             location, borrow, drop_span, proper_span
1031         );
1032
1033         if let BorrowExplanation::MustBeValidFor {
1034             category,
1035             span,
1036             from_closure: false,
1037             ..
1038         } = explanation {
1039             if let Some(diag) = self.try_report_cannot_return_reference_to_local(
1040                 borrow,
1041                 proper_span,
1042                 span,
1043                 category,
1044                 None,
1045             ) {
1046                 return diag;
1047             }
1048         }
1049
1050         let mut err = self.temporary_value_borrowed_for_too_long(proper_span);
1051         err.span_label(
1052             proper_span,
1053             "creates a temporary which is freed while still in use",
1054         );
1055         err.span_label(
1056             drop_span,
1057             "temporary value is freed at the end of this statement",
1058         );
1059
1060         match explanation {
1061             BorrowExplanation::UsedLater(..)
1062             | BorrowExplanation::UsedLaterInLoop(..)
1063             | BorrowExplanation::UsedLaterWhenDropped { .. } => {
1064                 // Only give this note and suggestion if it could be relevant.
1065                 err.note("consider using a `let` binding to create a longer lived value");
1066             }
1067             _ => {}
1068         }
1069         explanation.add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
1070
1071         let within = if borrow_spans.for_generator() {
1072             " by generator"
1073         } else {
1074             ""
1075         };
1076
1077         borrow_spans.args_span_label(
1078             &mut err,
1079             format!("value captured here{}", within),
1080         );
1081
1082         err
1083     }
1084
1085     fn try_report_cannot_return_reference_to_local(
1086         &self,
1087         borrow: &BorrowData<'tcx>,
1088         borrow_span: Span,
1089         return_span: Span,
1090         category: ConstraintCategory,
1091         opt_place_desc: Option<&String>,
1092     ) -> Option<DiagnosticBuilder<'cx>> {
1093         let return_kind = match category {
1094             ConstraintCategory::Return => "return",
1095             ConstraintCategory::Yield => "yield",
1096             _ => return None,
1097         };
1098
1099         // FIXME use a better heuristic than Spans
1100         let reference_desc = if return_span == self.body.source_info(borrow.reserve_location).span {
1101             "reference to"
1102         } else {
1103             "value referencing"
1104         };
1105
1106         let (place_desc, note) = if let Some(place_desc) = opt_place_desc {
1107             let local_kind = match borrow.borrowed_place {
1108                 Place {
1109                     base: PlaceBase::Local(local),
1110                     projection: None,
1111                 } => {
1112                     match self.body.local_kind(local) {
1113                         LocalKind::ReturnPointer
1114                         | LocalKind::Temp => bug!("temporary or return pointer with a name"),
1115                         LocalKind::Var => "local variable ",
1116                         LocalKind::Arg
1117                         if !self.upvars.is_empty()
1118                             && local == Local::new(1) => {
1119                             "variable captured by `move` "
1120                         }
1121                         LocalKind::Arg => {
1122                             "function parameter "
1123                         }
1124                     }
1125                 }
1126                 _ => "local data ",
1127             };
1128             (
1129                 format!("{}`{}`", local_kind, place_desc),
1130                 format!("`{}` is borrowed here", place_desc),
1131             )
1132         } else {
1133             let root_place = self.prefixes(borrow.borrowed_place.as_ref(),
1134                                            PrefixSet::All)
1135                 .last()
1136                 .unwrap();
1137             let local = if let PlaceRef {
1138                 base: PlaceBase::Local(local),
1139                 projection: None,
1140             } = root_place {
1141                 local
1142             } else {
1143                 bug!("try_report_cannot_return_reference_to_local: not a local")
1144             };
1145             match self.body.local_kind(*local) {
1146                 LocalKind::ReturnPointer | LocalKind::Temp => (
1147                     "temporary value".to_string(),
1148                     "temporary value created here".to_string(),
1149                 ),
1150                 LocalKind::Arg => (
1151                     "function parameter".to_string(),
1152                     "function parameter borrowed here".to_string(),
1153                 ),
1154                 LocalKind::Var => (
1155                     "local binding".to_string(),
1156                     "local binding introduced here".to_string(),
1157                 ),
1158             }
1159         };
1160
1161         let mut err = self.cannot_return_reference_to_local(
1162             return_span,
1163             return_kind,
1164             reference_desc,
1165             &place_desc,
1166         );
1167
1168         if return_span != borrow_span {
1169             err.span_label(borrow_span, note);
1170         }
1171
1172         Some(err)
1173     }
1174
1175     fn report_escaping_closure_capture(
1176         &mut self,
1177         args_span: Span,
1178         var_span: Span,
1179         fr_name: &RegionName,
1180         category: ConstraintCategory,
1181         constraint_span: Span,
1182         captured_var: &str,
1183     ) -> DiagnosticBuilder<'cx> {
1184         let tcx = self.infcx.tcx;
1185
1186         let mut err = self.cannot_capture_in_long_lived_closure(
1187             args_span,
1188             captured_var,
1189             var_span,
1190         );
1191
1192         let suggestion = match tcx.sess.source_map().span_to_snippet(args_span) {
1193             Ok(mut string) => {
1194                 if string.starts_with("async ") {
1195                     string.insert_str(6, "move ");
1196                 } else if string.starts_with("async|") {
1197                     string.insert_str(5, " move");
1198                 } else {
1199                     string.insert_str(0, "move ");
1200                 };
1201                 string
1202             },
1203             Err(_) => "move |<args>| <body>".to_string()
1204         };
1205
1206         err.span_suggestion(
1207             args_span,
1208             &format!("to force the closure to take ownership of {} (and any \
1209                       other referenced variables), use the `move` keyword",
1210                       captured_var),
1211             suggestion,
1212             Applicability::MachineApplicable,
1213         );
1214
1215         match category {
1216             ConstraintCategory::Return => {
1217                 err.span_note(constraint_span, "closure is returned here");
1218             }
1219             ConstraintCategory::CallArgument => {
1220                 fr_name.highlight_region_name(&mut err);
1221                 err.span_note(
1222                     constraint_span,
1223                     &format!("function requires argument type to outlive `{}`", fr_name),
1224                 );
1225             }
1226             _ => bug!("report_escaping_closure_capture called with unexpected constraint \
1227                        category: `{:?}`", category),
1228         }
1229         err
1230     }
1231
1232     fn report_escaping_data(
1233         &mut self,
1234         borrow_span: Span,
1235         name: &Option<String>,
1236         upvar_span: Span,
1237         upvar_name: &str,
1238         escape_span: Span,
1239     ) -> DiagnosticBuilder<'cx> {
1240         let tcx = self.infcx.tcx;
1241
1242         let escapes_from = if tcx.is_closure(self.mir_def_id) {
1243             let tables = tcx.typeck_tables_of(self.mir_def_id);
1244             let mir_hir_id = tcx.hir().def_index_to_hir_id(self.mir_def_id.index);
1245             match tables.node_type(mir_hir_id).sty {
1246                 ty::Closure(..) => "closure",
1247                 ty::Generator(..) => "generator",
1248                 _ => bug!("Closure body doesn't have a closure or generator type"),
1249             }
1250         } else {
1251             "function"
1252         };
1253
1254         let mut err = borrowck_errors::borrowed_data_escapes_closure(
1255             tcx,
1256             escape_span,
1257             escapes_from,
1258         );
1259
1260         err.span_label(
1261             upvar_span,
1262             format!(
1263                 "`{}` is declared here, outside of the {} body",
1264                 upvar_name, escapes_from
1265             ),
1266         );
1267
1268         err.span_label(
1269             borrow_span,
1270             format!(
1271                 "borrow is only valid in the {} body",
1272                 escapes_from
1273             ),
1274         );
1275
1276         if let Some(name) = name {
1277             err.span_label(
1278                 escape_span,
1279                 format!("reference to `{}` escapes the {} body here", name, escapes_from),
1280             );
1281         } else {
1282             err.span_label(
1283                 escape_span,
1284                 format!("reference escapes the {} body here", escapes_from),
1285             );
1286         }
1287
1288         err
1289     }
1290
1291     fn get_moved_indexes(&mut self, location: Location, mpi: MovePathIndex) -> Vec<MoveSite> {
1292         let body = self.body;
1293
1294         let mut stack = Vec::new();
1295         stack.extend(body.predecessor_locations(location).map(|predecessor| {
1296             let is_back_edge = location.dominates(predecessor, &self.dominators);
1297             (predecessor, is_back_edge)
1298         }));
1299
1300         let mut visited = FxHashSet::default();
1301         let mut result = vec![];
1302
1303         'dfs: while let Some((location, is_back_edge)) = stack.pop() {
1304             debug!(
1305                 "report_use_of_moved_or_uninitialized: (current_location={:?}, back_edge={})",
1306                 location, is_back_edge
1307             );
1308
1309             if !visited.insert(location) {
1310                 continue;
1311             }
1312
1313             // check for moves
1314             let stmt_kind = body[location.block]
1315                 .statements
1316                 .get(location.statement_index)
1317                 .map(|s| &s.kind);
1318             if let Some(StatementKind::StorageDead(..)) = stmt_kind {
1319                 // this analysis only tries to find moves explicitly
1320                 // written by the user, so we ignore the move-outs
1321                 // created by `StorageDead` and at the beginning
1322                 // of a function.
1323             } else {
1324                 // If we are found a use of a.b.c which was in error, then we want to look for
1325                 // moves not only of a.b.c but also a.b and a.
1326                 //
1327                 // Note that the moves data already includes "parent" paths, so we don't have to
1328                 // worry about the other case: that is, if there is a move of a.b.c, it is already
1329                 // marked as a move of a.b and a as well, so we will generate the correct errors
1330                 // there.
1331                 let mut mpis = vec![mpi];
1332                 let move_paths = &self.move_data.move_paths;
1333                 mpis.extend(move_paths[mpi].parents(move_paths));
1334
1335                 for moi in &self.move_data.loc_map[location] {
1336                     debug!("report_use_of_moved_or_uninitialized: moi={:?}", moi);
1337                     if mpis.contains(&self.move_data.moves[*moi].path) {
1338                         debug!("report_use_of_moved_or_uninitialized: found");
1339                         result.push(MoveSite {
1340                             moi: *moi,
1341                             traversed_back_edge: is_back_edge,
1342                         });
1343
1344                         // Strictly speaking, we could continue our DFS here. There may be
1345                         // other moves that can reach the point of error. But it is kind of
1346                         // confusing to highlight them.
1347                         //
1348                         // Example:
1349                         //
1350                         // ```
1351                         // let a = vec![];
1352                         // let b = a;
1353                         // let c = a;
1354                         // drop(a); // <-- current point of error
1355                         // ```
1356                         //
1357                         // Because we stop the DFS here, we only highlight `let c = a`,
1358                         // and not `let b = a`. We will of course also report an error at
1359                         // `let c = a` which highlights `let b = a` as the move.
1360                         continue 'dfs;
1361                     }
1362                 }
1363             }
1364
1365             // check for inits
1366             let mut any_match = false;
1367             drop_flag_effects::for_location_inits(
1368                 self.infcx.tcx,
1369                 self.body,
1370                 self.move_data,
1371                 location,
1372                 |m| {
1373                     if m == mpi {
1374                         any_match = true;
1375                     }
1376                 },
1377             );
1378             if any_match {
1379                 continue 'dfs;
1380             }
1381
1382             stack.extend(body.predecessor_locations(location).map(|predecessor| {
1383                 let back_edge = location.dominates(predecessor, &self.dominators);
1384                 (predecessor, is_back_edge || back_edge)
1385             }));
1386         }
1387
1388         result
1389     }
1390
1391     pub(super) fn report_illegal_mutation_of_borrowed(
1392         &mut self,
1393         location: Location,
1394         (place, span): (&Place<'tcx>, Span),
1395         loan: &BorrowData<'tcx>,
1396     ) {
1397         let loan_spans = self.retrieve_borrow_spans(loan);
1398         let loan_span = loan_spans.args_or_use();
1399
1400         if loan.kind == BorrowKind::Shallow {
1401             let mut err = self.cannot_mutate_in_match_guard(
1402                 span,
1403                 loan_span,
1404                 &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
1405                 "assign",
1406             );
1407             loan_spans.var_span_label(
1408                 &mut err,
1409                 format!("borrow occurs due to use{}", loan_spans.describe()),
1410             );
1411
1412             err.buffer(&mut self.errors_buffer);
1413
1414             return;
1415         }
1416
1417         let mut err = self.cannot_assign_to_borrowed(
1418             span,
1419             loan_span,
1420             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
1421         );
1422
1423         loan_spans.var_span_label(
1424             &mut err,
1425             format!("borrow occurs due to use{}", loan_spans.describe()),
1426         );
1427
1428         self.explain_why_borrow_contains_point(location, loan, None)
1429             .add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
1430
1431         err.buffer(&mut self.errors_buffer);
1432     }
1433
1434     /// Reports an illegal reassignment; for example, an assignment to
1435     /// (part of) a non-`mut` local that occurs potentially after that
1436     /// local has already been initialized. `place` is the path being
1437     /// assigned; `err_place` is a place providing a reason why
1438     /// `place` is not mutable (e.g., the non-`mut` local `x` in an
1439     /// assignment to `x.f`).
1440     pub(super) fn report_illegal_reassignment(
1441         &mut self,
1442         _location: Location,
1443         (place, span): (&Place<'tcx>, Span),
1444         assigned_span: Span,
1445         err_place: &Place<'tcx>,
1446     ) {
1447         let (from_arg, local_decl) = if let Place {
1448             base: PlaceBase::Local(local),
1449             projection: None,
1450         } = *err_place {
1451             if let LocalKind::Arg = self.body.local_kind(local) {
1452                 (true, Some(&self.body.local_decls[local]))
1453             } else {
1454                 (false, Some(&self.body.local_decls[local]))
1455             }
1456         } else {
1457             (false, None)
1458         };
1459
1460         // If root local is initialized immediately (everything apart from let
1461         // PATTERN;) then make the error refer to that local, rather than the
1462         // place being assigned later.
1463         let (place_description, assigned_span) = match local_decl {
1464             Some(LocalDecl {
1465                 is_user_variable: Some(ClearCrossCrate::Clear),
1466                 ..
1467             })
1468             | Some(LocalDecl {
1469                 is_user_variable:
1470                     Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
1471                         opt_match_place: None,
1472                         ..
1473                     }))),
1474                 ..
1475             })
1476             | Some(LocalDecl {
1477                 is_user_variable: None,
1478                 ..
1479             })
1480             | None => (self.describe_place(place.as_ref()), assigned_span),
1481             Some(decl) => (self.describe_place(err_place.as_ref()), decl.source_info.span),
1482         };
1483
1484         let mut err = self.cannot_reassign_immutable(
1485             span,
1486             place_description.as_ref().map(AsRef::as_ref).unwrap_or("_"),
1487             from_arg,
1488         );
1489         let msg = if from_arg {
1490             "cannot assign to immutable argument"
1491         } else {
1492             "cannot assign twice to immutable variable"
1493         };
1494         if span != assigned_span {
1495             if !from_arg {
1496                 let value_msg = match place_description {
1497                     Some(name) => format!("`{}`", name),
1498                     None => "value".to_owned(),
1499                 };
1500                 err.span_label(assigned_span, format!("first assignment to {}", value_msg));
1501             }
1502         }
1503         if let Some(decl) = local_decl {
1504             if let Some(name) = decl.name {
1505                 if decl.can_be_made_mutable() {
1506                     err.span_suggestion(
1507                         decl.source_info.span,
1508                         "make this binding mutable",
1509                         format!("mut {}", name),
1510                         Applicability::MachineApplicable,
1511                     );
1512                 }
1513             }
1514         }
1515         err.span_label(span, msg);
1516         err.buffer(&mut self.errors_buffer);
1517     }
1518
1519     fn classify_drop_access_kind(&self, place: PlaceRef<'cx, 'tcx>) -> StorageDeadOrDrop<'tcx> {
1520         let tcx = self.infcx.tcx;
1521         match place.projection {
1522             None => {
1523                 StorageDeadOrDrop::LocalStorageDead
1524             }
1525             Some(box Projection { ref base, ref elem }) => {
1526                 let base_access = self.classify_drop_access_kind(PlaceRef {
1527                     base: place.base,
1528                     projection: base,
1529                 });
1530                 match elem {
1531                     ProjectionElem::Deref => match base_access {
1532                         StorageDeadOrDrop::LocalStorageDead
1533                         | StorageDeadOrDrop::BoxedStorageDead => {
1534                             assert!(
1535                                 Place::ty_from(&place.base, base, self.body, tcx).ty.is_box(),
1536                                 "Drop of value behind a reference or raw pointer"
1537                             );
1538                             StorageDeadOrDrop::BoxedStorageDead
1539                         }
1540                         StorageDeadOrDrop::Destructor(_) => base_access,
1541                     },
1542                     ProjectionElem::Field(..) | ProjectionElem::Downcast(..) => {
1543                         let base_ty = Place::ty_from(&place.base, base, self.body, tcx).ty;
1544                         match base_ty.sty {
1545                             ty::Adt(def, _) if def.has_dtor(tcx) => {
1546                                 // Report the outermost adt with a destructor
1547                                 match base_access {
1548                                     StorageDeadOrDrop::Destructor(_) => base_access,
1549                                     StorageDeadOrDrop::LocalStorageDead
1550                                     | StorageDeadOrDrop::BoxedStorageDead => {
1551                                         StorageDeadOrDrop::Destructor(base_ty)
1552                                     }
1553                                 }
1554                             }
1555                             _ => base_access,
1556                         }
1557                     }
1558
1559                     ProjectionElem::ConstantIndex { .. }
1560                     | ProjectionElem::Subslice { .. }
1561                     | ProjectionElem::Index(_) => base_access,
1562                 }
1563             }
1564         }
1565     }
1566
1567     /// Annotate argument and return type of function and closure with (synthesized) lifetime for
1568     /// borrow of local value that does not live long enough.
1569     fn annotate_argument_and_return_for_borrow(
1570         &self,
1571         borrow: &BorrowData<'tcx>,
1572     ) -> Option<AnnotatedBorrowFnSignature<'tcx>> {
1573         // Define a fallback for when we can't match a closure.
1574         let fallback = || {
1575             let is_closure = self.infcx.tcx.is_closure(self.mir_def_id);
1576             if is_closure {
1577                 None
1578             } else {
1579                 let ty = self.infcx.tcx.type_of(self.mir_def_id);
1580                 match ty.sty {
1581                     ty::FnDef(_, _) | ty::FnPtr(_) => self.annotate_fn_sig(
1582                         self.mir_def_id,
1583                         self.infcx.tcx.fn_sig(self.mir_def_id),
1584                     ),
1585                     _ => None,
1586                 }
1587             }
1588         };
1589
1590         // In order to determine whether we need to annotate, we need to check whether the reserve
1591         // place was an assignment into a temporary.
1592         //
1593         // If it was, we check whether or not that temporary is eventually assigned into the return
1594         // place. If it was, we can add annotations about the function's return type and arguments
1595         // and it'll make sense.
1596         let location = borrow.reserve_location;
1597         debug!(
1598             "annotate_argument_and_return_for_borrow: location={:?}",
1599             location
1600         );
1601         if let Some(&Statement { kind: StatementKind::Assign(ref reservation, _), ..})
1602              = &self.body[location.block].statements.get(location.statement_index)
1603         {
1604             debug!(
1605                 "annotate_argument_and_return_for_borrow: reservation={:?}",
1606                 reservation
1607             );
1608             // Check that the initial assignment of the reserve location is into a temporary.
1609             let mut target = *match reservation {
1610                 Place {
1611                     base: PlaceBase::Local(local),
1612                     projection: None,
1613                 } 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(
1626                     Place {
1627                         base: PlaceBase::Local(assigned_to),
1628                         projection: None,
1629                     },
1630                     box rvalue
1631                 ) = &stmt.kind {
1632                     debug!(
1633                         "annotate_argument_and_return_for_borrow: assigned_to={:?} \
1634                          rvalue={:?}",
1635                         assigned_to, rvalue
1636                     );
1637                     // Check if our `target` was captured by a closure.
1638                     if let Rvalue::Aggregate(
1639                         box AggregateKind::Closure(def_id, substs),
1640                         operands,
1641                     ) = rvalue
1642                     {
1643                         for operand in operands {
1644                             let assigned_from = match operand {
1645                                 Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1646                                     assigned_from
1647                                 }
1648                                 _ => continue,
1649                             };
1650                             debug!(
1651                                 "annotate_argument_and_return_for_borrow: assigned_from={:?}",
1652                                 assigned_from
1653                             );
1654
1655                             // Find the local from the operand.
1656                             let assigned_from_local = match assigned_from.local_or_deref_local() {
1657                                 Some(local) => local,
1658                                 None => continue,
1659                             };
1660
1661                             if assigned_from_local != target {
1662                                 continue;
1663                             }
1664
1665                             // If a closure captured our `target` and then assigned
1666                             // into a place then we should annotate the closure in
1667                             // case it ends up being assigned into the return place.
1668                             annotated_closure = self.annotate_fn_sig(
1669                                 *def_id,
1670                                 self.infcx.closure_sig(*def_id, *substs),
1671                             );
1672                             debug!(
1673                                 "annotate_argument_and_return_for_borrow: \
1674                                  annotated_closure={:?} assigned_from_local={:?} \
1675                                  assigned_to={:?}",
1676                                 annotated_closure, assigned_from_local, assigned_to
1677                             );
1678
1679                             if *assigned_to == mir::RETURN_PLACE {
1680                                 // If it was assigned directly into the return place, then
1681                                 // return now.
1682                                 return annotated_closure;
1683                             } else {
1684                                 // Otherwise, update the target.
1685                                 target = *assigned_to;
1686                             }
1687                         }
1688
1689                         // If none of our closure's operands matched, then skip to the next
1690                         // statement.
1691                         continue;
1692                     }
1693
1694                     // Otherwise, look at other types of assignment.
1695                     let assigned_from = match rvalue {
1696                         Rvalue::Ref(_, _, assigned_from) => assigned_from,
1697                         Rvalue::Use(operand) => match operand {
1698                             Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1699                                 assigned_from
1700                             }
1701                             _ => continue,
1702                         },
1703                         _ => continue,
1704                     };
1705                     debug!(
1706                         "annotate_argument_and_return_for_borrow: \
1707                          assigned_from={:?}",
1708                         assigned_from,
1709                     );
1710
1711                     // Find the local from the rvalue.
1712                     let assigned_from_local = match assigned_from.local_or_deref_local() {
1713                         Some(local) => local,
1714                         None => continue,
1715                     };
1716                     debug!(
1717                         "annotate_argument_and_return_for_borrow: \
1718                          assigned_from_local={:?}",
1719                         assigned_from_local,
1720                     );
1721
1722                     // Check if our local matches the target - if so, we've assigned our
1723                     // borrow to a new place.
1724                     if assigned_from_local != target {
1725                         continue;
1726                     }
1727
1728                     // If we assigned our `target` into a new place, then we should
1729                     // check if it was the return place.
1730                     debug!(
1731                         "annotate_argument_and_return_for_borrow: \
1732                          assigned_from_local={:?} assigned_to={:?}",
1733                         assigned_from_local, assigned_to
1734                     );
1735                     if *assigned_to == mir::RETURN_PLACE {
1736                         // If it was then return the annotated closure if there was one,
1737                         // else, annotate this function.
1738                         return annotated_closure.or_else(fallback);
1739                     }
1740
1741                     // If we didn't assign into the return place, then we just update
1742                     // the target.
1743                     target = *assigned_to;
1744                 }
1745             }
1746
1747             // Check the terminator if we didn't find anything in the statements.
1748             let terminator = &self.body[location.block].terminator();
1749             debug!(
1750                 "annotate_argument_and_return_for_borrow: target={:?} terminator={:?}",
1751                 target, terminator
1752             );
1753             if let TerminatorKind::Call {
1754                 destination: Some((Place {
1755                     base: PlaceBase::Local(assigned_to),
1756                     projection: None,
1757                 }, _)),
1758                 args,
1759                 ..
1760             } = &terminator.kind
1761             {
1762                 debug!(
1763                     "annotate_argument_and_return_for_borrow: assigned_to={:?} args={:?}",
1764                     assigned_to, args
1765                 );
1766                 for operand in args {
1767                     let assigned_from = match operand {
1768                         Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1769                             assigned_from
1770                         }
1771                         _ => continue,
1772                     };
1773                     debug!(
1774                         "annotate_argument_and_return_for_borrow: assigned_from={:?}",
1775                         assigned_from,
1776                     );
1777
1778                     if let Some(assigned_from_local) = assigned_from.local_or_deref_local() {
1779                         debug!(
1780                             "annotate_argument_and_return_for_borrow: assigned_from_local={:?}",
1781                             assigned_from_local,
1782                         );
1783
1784                         if *assigned_to == mir::RETURN_PLACE && assigned_from_local == target {
1785                             return annotated_closure.or_else(fallback);
1786                         }
1787                     }
1788                 }
1789             }
1790         }
1791
1792         // If we haven't found an assignment into the return place, then we need not add
1793         // any annotations.
1794         debug!("annotate_argument_and_return_for_borrow: none found");
1795         None
1796     }
1797
1798     /// Annotate the first argument and return type of a function signature if they are
1799     /// references.
1800     fn annotate_fn_sig(
1801         &self,
1802         did: DefId,
1803         sig: ty::PolyFnSig<'tcx>,
1804     ) -> Option<AnnotatedBorrowFnSignature<'tcx>> {
1805         debug!("annotate_fn_sig: did={:?} sig={:?}", did, sig);
1806         let is_closure = self.infcx.tcx.is_closure(did);
1807         let fn_hir_id = self.infcx.tcx.hir().as_local_hir_id(did)?;
1808         let fn_decl = self.infcx.tcx.hir().fn_decl_by_hir_id(fn_hir_id)?;
1809
1810         // We need to work out which arguments to highlight. We do this by looking
1811         // at the return type, where there are three cases:
1812         //
1813         // 1. If there are named arguments, then we should highlight the return type and
1814         //    highlight any of the arguments that are also references with that lifetime.
1815         //    If there are no arguments that have the same lifetime as the return type,
1816         //    then don't highlight anything.
1817         // 2. The return type is a reference with an anonymous lifetime. If this is
1818         //    the case, then we can take advantage of (and teach) the lifetime elision
1819         //    rules.
1820         //
1821         //    We know that an error is being reported. So the arguments and return type
1822         //    must satisfy the elision rules. Therefore, if there is a single argument
1823         //    then that means the return type and first (and only) argument have the same
1824         //    lifetime and the borrow isn't meeting that, we can highlight the argument
1825         //    and return type.
1826         //
1827         //    If there are multiple arguments then the first argument must be self (else
1828         //    it would not satisfy the elision rules), so we can highlight self and the
1829         //    return type.
1830         // 3. The return type is not a reference. In this case, we don't highlight
1831         //    anything.
1832         let return_ty = sig.output();
1833         match return_ty.skip_binder().sty {
1834             ty::Ref(return_region, _, _) if return_region.has_name() && !is_closure => {
1835                 // This is case 1 from above, return type is a named reference so we need to
1836                 // search for relevant arguments.
1837                 let mut arguments = Vec::new();
1838                 for (index, argument) in sig.inputs().skip_binder().iter().enumerate() {
1839                     if let ty::Ref(argument_region, _, _) = argument.sty {
1840                         if argument_region == return_region {
1841                             // Need to use the `rustc::ty` types to compare against the
1842                             // `return_region`. Then use the `rustc::hir` type to get only
1843                             // the lifetime span.
1844                             if let hir::TyKind::Rptr(lifetime, _) = &fn_decl.inputs[index].node {
1845                                 // With access to the lifetime, we can get
1846                                 // the span of it.
1847                                 arguments.push((*argument, lifetime.span));
1848                             } else {
1849                                 bug!("ty type is a ref but hir type is not");
1850                             }
1851                         }
1852                     }
1853                 }
1854
1855                 // We need to have arguments. This shouldn't happen, but it's worth checking.
1856                 if arguments.is_empty() {
1857                     return None;
1858                 }
1859
1860                 // We use a mix of the HIR and the Ty types to get information
1861                 // as the HIR doesn't have full types for closure arguments.
1862                 let return_ty = *sig.output().skip_binder();
1863                 let mut return_span = fn_decl.output.span();
1864                 if let hir::FunctionRetTy::Return(ty) = &fn_decl.output {
1865                     if let hir::TyKind::Rptr(lifetime, _) = ty.node {
1866                         return_span = lifetime.span;
1867                     }
1868                 }
1869
1870                 Some(AnnotatedBorrowFnSignature::NamedFunction {
1871                     arguments,
1872                     return_ty,
1873                     return_span,
1874                 })
1875             }
1876             ty::Ref(_, _, _) if is_closure => {
1877                 // This is case 2 from above but only for closures, return type is anonymous
1878                 // reference so we select
1879                 // the first argument.
1880                 let argument_span = fn_decl.inputs.first()?.span;
1881                 let argument_ty = sig.inputs().skip_binder().first()?;
1882
1883                 // Closure arguments are wrapped in a tuple, so we need to get the first
1884                 // from that.
1885                 if let ty::Tuple(elems) = argument_ty.sty {
1886                     let argument_ty = elems.first()?.expect_ty();
1887                     if let ty::Ref(_, _, _) = argument_ty.sty {
1888                         return Some(AnnotatedBorrowFnSignature::Closure {
1889                             argument_ty,
1890                             argument_span,
1891                         });
1892                     }
1893                 }
1894
1895                 None
1896             }
1897             ty::Ref(_, _, _) => {
1898                 // This is also case 2 from above but for functions, return type is still an
1899                 // anonymous reference so we select the first argument.
1900                 let argument_span = fn_decl.inputs.first()?.span;
1901                 let argument_ty = sig.inputs().skip_binder().first()?;
1902
1903                 let return_span = fn_decl.output.span();
1904                 let return_ty = *sig.output().skip_binder();
1905
1906                 // We expect the first argument to be a reference.
1907                 match argument_ty.sty {
1908                     ty::Ref(_, _, _) => {}
1909                     _ => return None,
1910                 }
1911
1912                 Some(AnnotatedBorrowFnSignature::AnonymousFunction {
1913                     argument_ty,
1914                     argument_span,
1915                     return_ty,
1916                     return_span,
1917                 })
1918             }
1919             _ => {
1920                 // This is case 3 from above, return type is not a reference so don't highlight
1921                 // anything.
1922                 None
1923             }
1924         }
1925     }
1926 }
1927
1928 #[derive(Debug)]
1929 enum AnnotatedBorrowFnSignature<'tcx> {
1930     NamedFunction {
1931         arguments: Vec<(Ty<'tcx>, Span)>,
1932         return_ty: Ty<'tcx>,
1933         return_span: Span,
1934     },
1935     AnonymousFunction {
1936         argument_ty: Ty<'tcx>,
1937         argument_span: Span,
1938         return_ty: Ty<'tcx>,
1939         return_span: Span,
1940     },
1941     Closure {
1942         argument_ty: Ty<'tcx>,
1943         argument_span: Span,
1944     },
1945 }
1946
1947 impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
1948     /// Annotate the provided diagnostic with information about borrow from the fn signature that
1949     /// helps explain.
1950     pub(super) fn emit(
1951         &self,
1952         cx: &mut MirBorrowckCtxt<'_, 'tcx>,
1953         diag: &mut DiagnosticBuilder<'_>,
1954     ) -> String {
1955         match self {
1956             AnnotatedBorrowFnSignature::Closure {
1957                 argument_ty,
1958                 argument_span,
1959             } => {
1960                 diag.span_label(
1961                     *argument_span,
1962                     format!("has type `{}`", cx.get_name_for_ty(argument_ty, 0)),
1963                 );
1964
1965                 cx.get_region_name_for_ty(argument_ty, 0)
1966             }
1967             AnnotatedBorrowFnSignature::AnonymousFunction {
1968                 argument_ty,
1969                 argument_span,
1970                 return_ty,
1971                 return_span,
1972             } => {
1973                 let argument_ty_name = cx.get_name_for_ty(argument_ty, 0);
1974                 diag.span_label(*argument_span, format!("has type `{}`", argument_ty_name));
1975
1976                 let return_ty_name = cx.get_name_for_ty(return_ty, 0);
1977                 let types_equal = return_ty_name == argument_ty_name;
1978                 diag.span_label(
1979                     *return_span,
1980                     format!(
1981                         "{}has type `{}`",
1982                         if types_equal { "also " } else { "" },
1983                         return_ty_name,
1984                     ),
1985                 );
1986
1987                 diag.note(
1988                     "argument and return type have the same lifetime due to lifetime elision rules",
1989                 );
1990                 diag.note(
1991                     "to learn more, visit <https://doc.rust-lang.org/book/ch10-03-\
1992                      lifetime-syntax.html#lifetime-elision>",
1993                 );
1994
1995                 cx.get_region_name_for_ty(return_ty, 0)
1996             }
1997             AnnotatedBorrowFnSignature::NamedFunction {
1998                 arguments,
1999                 return_ty,
2000                 return_span,
2001             } => {
2002                 // Region of return type and arguments checked to be the same earlier.
2003                 let region_name = cx.get_region_name_for_ty(return_ty, 0);
2004                 for (_, argument_span) in arguments {
2005                     diag.span_label(*argument_span, format!("has lifetime `{}`", region_name));
2006                 }
2007
2008                 diag.span_label(
2009                     *return_span,
2010                     format!("also has lifetime `{}`", region_name,),
2011                 );
2012
2013                 diag.help(&format!(
2014                     "use data from the highlighted arguments which match the `{}` lifetime of \
2015                      the return type",
2016                     region_name,
2017                 ));
2018
2019                 region_name
2020             }
2021         }
2022     }
2023 }