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, PlaceRef, ProjectionElem, Rvalue,
   6     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: box [],
 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                 for (i, elem) in projection.iter().enumerate().rev() {
 618                     let base_proj = &projection[..i];
 619
 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                         _ => {},
 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                 for (i, elem) in projection.iter().enumerate().rev() {
 641                     let proj_base = &projection[..i];
 642
 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                 None
 665             })
 666             .unwrap_or_else(|| {
 667                 // If we didn't find a field access into a union, or both places match, then
 668                 // only return the description of the first place.
 669                 (
 670                     describe_place(first_borrowed_place.as_ref()),
 671                     "".to_string(),
 672                     "".to_string(),
 673                     "".to_string(),
 674                 )
 675             })
 676     }
 677
 678     /// Reports StorageDeadOrDrop of `place` conflicts with `borrow`.
 679     ///
 680     /// This means that some data referenced by `borrow` needs to live
 681     /// past the point where the StorageDeadOrDrop of `place` occurs.
 682     /// This is usually interpreted as meaning that `place` has too
 683     /// short a lifetime. (But sometimes it is more useful to report
 684     /// it as a more direct conflict between the execution of a
 685     /// `Drop::drop` with an aliasing borrow.)
 686     pub(super) fn report_borrowed_value_does_not_live_long_enough(
 687         &mut self,
 688         location: Location,
 689         borrow: &BorrowData<'tcx>,
 690         place_span: (&Place<'tcx>, Span),
 691         kind: Option<WriteKind>,
 692     ) {
 693         debug!(
 694             "report_borrowed_value_does_not_live_long_enough(\
 695              {:?}, {:?}, {:?}, {:?}\
 696              )",
 697             location, borrow, place_span, kind
 698         );
 699
 700         let drop_span = place_span.1;
 701         let root_place = self.prefixes(borrow.borrowed_place.as_ref(), PrefixSet::All)
 702             .last()
 703             .unwrap();
 704
 705         let borrow_spans = self.retrieve_borrow_spans(borrow);
 706         let borrow_span = borrow_spans.var_or_use();
 707
 708         assert!(root_place.projection.is_empty());
 709         let proper_span = match root_place.base {
 710             PlaceBase::Local(local) => self.body.local_decls[*local].source_info.span,
 711             _ => drop_span,
 712         };
 713
 714         if self.access_place_error_reported
 715             .contains(&(Place {
 716                 base: root_place.base.clone(),
 717                 projection: root_place.projection.to_vec().into_boxed_slice(),
 718             }, borrow_span))
 719         {
 720             debug!(
 721                 "suppressing access_place error when borrow doesn't live long enough for {:?}",
 722                 borrow_span
 723             );
 724             return;
 725         }
 726
 727         self.access_place_error_reported
 728             .insert((Place {
 729                 base: root_place.base.clone(),
 730                 projection: root_place.projection.to_vec().into_boxed_slice(),
 731             }, borrow_span));
 732
 733         if let StorageDeadOrDrop::Destructor(dropped_ty) =
 734             self.classify_drop_access_kind(borrow.borrowed_place.as_ref())
 735         {
 736             // If a borrow of path `B` conflicts with drop of `D` (and
 737             // we're not in the uninteresting case where `B` is a
 738             // prefix of `D`), then report this as a more interesting
 739             // destructor conflict.
 740             if !borrow.borrowed_place.as_ref().is_prefix_of(place_span.0.as_ref()) {
 741                 self.report_borrow_conflicts_with_destructor(
 742                     location, borrow, place_span, kind, dropped_ty,
 743                 );
 744                 return;
 745             }
 746         }
 747
 748         let place_desc = self.describe_place(borrow.borrowed_place.as_ref());
 749
 750         let kind_place = kind.filter(|_| place_desc.is_some()).map(|k| (k, place_span.0));
 751         let explanation = self.explain_why_borrow_contains_point(location, &borrow, kind_place);
 752
 753         let err = match (place_desc, explanation) {
 754             (Some(_), _) if self.is_place_thread_local(root_place) => {
 755                 self.report_thread_local_value_does_not_live_long_enough(drop_span, borrow_span)
 756             }
 757             // If the outlives constraint comes from inside the closure,
 758             // for example:
 759             //
 760             // let x = 0;
 761             // let y = &x;
 762             // Box::new(|| y) as Box<Fn() -> &'static i32>
 763             //
 764             // then just use the normal error. The closure isn't escaping
 765             // and `move` will not help here.
 766             (
 767                 Some(ref name),
 768                 BorrowExplanation::MustBeValidFor {
 769                     category: category @ ConstraintCategory::Return,
 770                     from_closure: false,
 771                     ref region_name,
 772                     span,
 773                     ..
 774                 },
 775             )
 776             | (
 777                 Some(ref name),
 778                 BorrowExplanation::MustBeValidFor {
 779                     category: category @ ConstraintCategory::CallArgument,
 780                     from_closure: false,
 781                     ref region_name,
 782                     span,
 783                     ..
 784                 },
 785             ) if borrow_spans.for_closure() => self.report_escaping_closure_capture(
 786                 borrow_spans.args_or_use(),
 787                 borrow_span,
 788                 region_name,
 789                 category,
 790                 span,
 791                 &format!("`{}`", name),
 792             ),
 793             (
 794                 ref name,
 795                 BorrowExplanation::MustBeValidFor {
 796                     category: ConstraintCategory::Assignment,
 797                     from_closure: false,
 798                     region_name: RegionName {
 799                         source: RegionNameSource::AnonRegionFromUpvar(upvar_span, ref upvar_name),
 800                         ..
 801                     },
 802                     span,
 803                     ..
 804                 },
 805             ) => self.report_escaping_data(borrow_span, name, upvar_span, upvar_name, span),
 806             (Some(name), explanation) => self.report_local_value_does_not_live_long_enough(
 807                 location,
 808                 &name,
 809                 &borrow,
 810                 drop_span,
 811                 borrow_spans,
 812                 explanation,
 813             ),
 814             (None, explanation) => self.report_temporary_value_does_not_live_long_enough(
 815                 location,
 816                 &borrow,
 817                 drop_span,
 818                 borrow_spans,
 819                 proper_span,
 820                 explanation,
 821             ),
 822         };
 823
 824         err.buffer(&mut self.errors_buffer);
 825     }
 826
 827     fn report_local_value_does_not_live_long_enough(
 828         &mut self,
 829         location: Location,
 830         name: &str,
 831         borrow: &BorrowData<'tcx>,
 832         drop_span: Span,
 833         borrow_spans: UseSpans,
 834         explanation: BorrowExplanation,
 835     ) -> DiagnosticBuilder<'cx> {
 836         debug!(
 837             "report_local_value_does_not_live_long_enough(\
 838              {:?}, {:?}, {:?}, {:?}, {:?}\
 839              )",
 840             location, name, borrow, drop_span, borrow_spans
 841         );
 842
 843         let borrow_span = borrow_spans.var_or_use();
 844         if let BorrowExplanation::MustBeValidFor {
 845             category,
 846             span,
 847             ref opt_place_desc,
 848             from_closure: false,
 849             ..
 850         } = explanation {
 851             if let Some(diag) = self.try_report_cannot_return_reference_to_local(
 852                 borrow,
 853                 borrow_span,
 854                 span,
 855                 category,
 856                 opt_place_desc.as_ref(),
 857             ) {
 858                 return diag;
 859             }
 860         }
 861
 862         let mut err = self.path_does_not_live_long_enough(
 863             borrow_span,
 864             &format!("`{}`", name),
 865         );
 866
 867         if let Some(annotation) = self.annotate_argument_and_return_for_borrow(borrow) {
 868             let region_name = annotation.emit(self, &mut err);
 869
 870             err.span_label(
 871                 borrow_span,
 872                 format!("`{}` would have to be valid for `{}`...", name, region_name),
 873             );
 874
 875             if let Some(fn_hir_id) = self.infcx.tcx.hir().as_local_hir_id(self.mir_def_id) {
 876                 err.span_label(
 877                     drop_span,
 878                     format!(
 879                         "...but `{}` will be dropped here, when the function `{}` returns",
 880                         name,
 881                         self.infcx.tcx.hir().name(fn_hir_id),
 882                     ),
 883                 );
 884
 885                 err.note(
 886                     "functions cannot return a borrow to data owned within the function's scope, \
 887                      functions can only return borrows to data passed as arguments",
 888                 );
 889                 err.note(
 890                     "to learn more, visit <https://doc.rust-lang.org/book/ch04-02-\
 891                      references-and-borrowing.html#dangling-references>",
 892                 );
 893             } else {
 894                 err.span_label(
 895                     drop_span,
 896                     format!("...but `{}` dropped here while still borrowed", name),
 897                 );
 898             }
 899
 900             if let BorrowExplanation::MustBeValidFor { .. } = explanation {
 901             } else {
 902                 explanation.add_explanation_to_diagnostic(
 903                     self.infcx.tcx,
 904                     self.body,
 905                     &mut err,
 906                     "",
 907                     None,
 908                 );
 909             }
 910         } else {
 911             err.span_label(borrow_span, "borrowed value does not live long enough");
 912             err.span_label(
 913                 drop_span,
 914                 format!("`{}` dropped here while still borrowed", name),
 915             );
 916
 917             let within = if borrow_spans.for_generator() {
 918                 " by generator"
 919             } else {
 920                 ""
 921             };
 922
 923             borrow_spans.args_span_label(
 924                 &mut err,
 925                 format!("value captured here{}", within),
 926             );
 927
 928             explanation.add_explanation_to_diagnostic(
 929                 self.infcx.tcx, self.body, &mut err, "", None);
 930         }
 931
 932         err
 933     }
 934
 935     fn report_borrow_conflicts_with_destructor(
 936         &mut self,
 937         location: Location,
 938         borrow: &BorrowData<'tcx>,
 939         (place, drop_span): (&Place<'tcx>, Span),
 940         kind: Option<WriteKind>,
 941         dropped_ty: Ty<'tcx>,
 942     ) {
 943         debug!(
 944             "report_borrow_conflicts_with_destructor(\
 945              {:?}, {:?}, ({:?}, {:?}), {:?}\
 946              )",
 947             location, borrow, place, drop_span, kind,
 948         );
 949
 950         let borrow_spans = self.retrieve_borrow_spans(borrow);
 951         let borrow_span = borrow_spans.var_or_use();
 952
 953         let mut err = self.cannot_borrow_across_destructor(borrow_span);
 954
 955         let what_was_dropped = match self.describe_place(place.as_ref()) {
 956             Some(name) => format!("`{}`", name.as_str()),
 957             None => String::from("temporary value"),
 958         };
 959
 960         let label = match self.describe_place(borrow.borrowed_place.as_ref()) {
 961             Some(borrowed) => format!(
 962                 "here, drop of {D} needs exclusive access to `{B}`, \
 963                  because the type `{T}` implements the `Drop` trait",
 964                 D = what_was_dropped,
 965                 T = dropped_ty,
 966                 B = borrowed
 967             ),
 968             None => format!(
 969                 "here is drop of {D}; whose type `{T}` implements the `Drop` trait",
 970                 D = what_was_dropped,
 971                 T = dropped_ty
 972             ),
 973         };
 974         err.span_label(drop_span, label);
 975
 976         // Only give this note and suggestion if they could be relevant.
 977         let explanation =
 978             self.explain_why_borrow_contains_point(location, borrow, kind.map(|k| (k, place)));
 979         match explanation {
 980             BorrowExplanation::UsedLater { .. }
 981             | BorrowExplanation::UsedLaterWhenDropped { .. } => {
 982                 err.note("consider using a `let` binding to create a longer lived value");
 983             }
 984             _ => {}
 985         }
 986
 987         explanation.add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
 988
 989         err.buffer(&mut self.errors_buffer);
 990     }
 991
 992     fn report_thread_local_value_does_not_live_long_enough(
 993         &mut self,
 994         drop_span: Span,
 995         borrow_span: Span,
 996     ) -> DiagnosticBuilder<'cx> {
 997         debug!(
 998             "report_thread_local_value_does_not_live_long_enough(\
 999              {:?}, {:?}\
1000              )",
1001             drop_span, borrow_span
1002         );
1003
1004         let mut err = self.thread_local_value_does_not_live_long_enough(borrow_span);
1005
1006         err.span_label(
1007             borrow_span,
1008             "thread-local variables cannot be borrowed beyond the end of the function",
1009         );
1010         err.span_label(drop_span, "end of enclosing function is here");
1011
1012         err
1013     }
1014
1015     fn report_temporary_value_does_not_live_long_enough(
1016         &mut self,
1017         location: Location,
1018         borrow: &BorrowData<'tcx>,
1019         drop_span: Span,
1020         borrow_spans: UseSpans,
1021         proper_span: Span,
1022         explanation: BorrowExplanation,
1023     ) -> DiagnosticBuilder<'cx> {
1024         debug!(
1025             "report_temporary_value_does_not_live_long_enough(\
1026              {:?}, {:?}, {:?}, {:?}\
1027              )",
1028             location, borrow, drop_span, proper_span
1029         );
1030
1031         if let BorrowExplanation::MustBeValidFor {
1032             category,
1033             span,
1034             from_closure: false,
1035             ..
1036         } = explanation {
1037             if let Some(diag) = self.try_report_cannot_return_reference_to_local(
1038                 borrow,
1039                 proper_span,
1040                 span,
1041                 category,
1042                 None,
1043             ) {
1044                 return diag;
1045             }
1046         }
1047
1048         let mut err = self.temporary_value_borrowed_for_too_long(proper_span);
1049         err.span_label(
1050             proper_span,
1051             "creates a temporary which is freed while still in use",
1052         );
1053         err.span_label(
1054             drop_span,
1055             "temporary value is freed at the end of this statement",
1056         );
1057
1058         match explanation {
1059             BorrowExplanation::UsedLater(..)
1060             | BorrowExplanation::UsedLaterInLoop(..)
1061             | BorrowExplanation::UsedLaterWhenDropped { .. } => {
1062                 // Only give this note and suggestion if it could be relevant.
1063                 err.note("consider using a `let` binding to create a longer lived value");
1064             }
1065             _ => {}
1066         }
1067         explanation.add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
1068
1069         let within = if borrow_spans.for_generator() {
1070             " by generator"
1071         } else {
1072             ""
1073         };
1074
1075         borrow_spans.args_span_label(
1076             &mut err,
1077             format!("value captured here{}", within),
1078         );
1079
1080         err
1081     }
1082
1083     fn try_report_cannot_return_reference_to_local(
1084         &self,
1085         borrow: &BorrowData<'tcx>,
1086         borrow_span: Span,
1087         return_span: Span,
1088         category: ConstraintCategory,
1089         opt_place_desc: Option<&String>,
1090     ) -> Option<DiagnosticBuilder<'cx>> {
1091         let return_kind = match category {
1092             ConstraintCategory::Return => "return",
1093             ConstraintCategory::Yield => "yield",
1094             _ => return None,
1095         };
1096
1097         // FIXME use a better heuristic than Spans
1098         let reference_desc = if return_span == self.body.source_info(borrow.reserve_location).span {
1099             "reference to"
1100         } else {
1101             "value referencing"
1102         };
1103
1104         let (place_desc, note) = if let Some(place_desc) = opt_place_desc {
1105             let local_kind = match borrow.borrowed_place {
1106                 Place {
1107                     base: PlaceBase::Local(local),
1108                     projection: box [],
1109                 } => {
1110                     match self.body.local_kind(local) {
1111                         LocalKind::ReturnPointer
1112                         | LocalKind::Temp => bug!("temporary or return pointer with a name"),
1113                         LocalKind::Var => "local variable ",
1114                         LocalKind::Arg
1115                         if !self.upvars.is_empty()
1116                             && local == Local::new(1) => {
1117                             "variable captured by `move` "
1118                         }
1119                         LocalKind::Arg => {
1120                             "function parameter "
1121                         }
1122                     }
1123                 }
1124                 _ => "local data ",
1125             };
1126             (
1127                 format!("{}`{}`", local_kind, place_desc),
1128                 format!("`{}` is borrowed here", place_desc),
1129             )
1130         } else {
1131             let root_place = self.prefixes(borrow.borrowed_place.as_ref(),
1132                                            PrefixSet::All)
1133                 .last()
1134                 .unwrap();
1135             let local = if let PlaceRef {
1136                 base: PlaceBase::Local(local),
1137                 projection: [],
1138             } = root_place {
1139                 local
1140             } else {
1141                 bug!("try_report_cannot_return_reference_to_local: not a local")
1142             };
1143             match self.body.local_kind(*local) {
1144                 LocalKind::ReturnPointer | LocalKind::Temp => (
1145                     "temporary value".to_string(),
1146                     "temporary value created here".to_string(),
1147                 ),
1148                 LocalKind::Arg => (
1149                     "function parameter".to_string(),
1150                     "function parameter borrowed here".to_string(),
1151                 ),
1152                 LocalKind::Var => (
1153                     "local binding".to_string(),
1154                     "local binding introduced here".to_string(),
1155                 ),
1156             }
1157         };
1158
1159         let mut err = self.cannot_return_reference_to_local(
1160             return_span,
1161             return_kind,
1162             reference_desc,
1163             &place_desc,
1164         );
1165
1166         if return_span != borrow_span {
1167             err.span_label(borrow_span, note);
1168         }
1169
1170         Some(err)
1171     }
1172
1173     fn report_escaping_closure_capture(
1174         &mut self,
1175         args_span: Span,
1176         var_span: Span,
1177         fr_name: &RegionName,
1178         category: ConstraintCategory,
1179         constraint_span: Span,
1180         captured_var: &str,
1181     ) -> DiagnosticBuilder<'cx> {
1182         let tcx = self.infcx.tcx;
1183
1184         let mut err = self.cannot_capture_in_long_lived_closure(
1185             args_span,
1186             captured_var,
1187             var_span,
1188         );
1189
1190         let suggestion = match tcx.sess.source_map().span_to_snippet(args_span) {
1191             Ok(mut string) => {
1192                 if string.starts_with("async ") {
1193                     string.insert_str(6, "move ");
1194                 } else if string.starts_with("async|") {
1195                     string.insert_str(5, " move");
1196                 } else {
1197                     string.insert_str(0, "move ");
1198                 };
1199                 string
1200             },
1201             Err(_) => "move |<args>| <body>".to_string()
1202         };
1203
1204         err.span_suggestion(
1205             args_span,
1206             &format!("to force the closure to take ownership of {} (and any \
1207                       other referenced variables), use the `move` keyword",
1208                       captured_var),
1209             suggestion,
1210             Applicability::MachineApplicable,
1211         );
1212
1213         match category {
1214             ConstraintCategory::Return => {
1215                 err.span_note(constraint_span, "closure is returned here");
1216             }
1217             ConstraintCategory::CallArgument => {
1218                 fr_name.highlight_region_name(&mut err);
1219                 err.span_note(
1220                     constraint_span,
1221                     &format!("function requires argument type to outlive `{}`", fr_name),
1222                 );
1223             }
1224             _ => bug!("report_escaping_closure_capture called with unexpected constraint \
1225                        category: `{:?}`", category),
1226         }
1227         err
1228     }
1229
1230     fn report_escaping_data(
1231         &mut self,
1232         borrow_span: Span,
1233         name: &Option<String>,
1234         upvar_span: Span,
1235         upvar_name: &str,
1236         escape_span: Span,
1237     ) -> DiagnosticBuilder<'cx> {
1238         let tcx = self.infcx.tcx;
1239
1240         let escapes_from = if tcx.is_closure(self.mir_def_id) {
1241             let tables = tcx.typeck_tables_of(self.mir_def_id);
1242             let mir_hir_id = tcx.hir().def_index_to_hir_id(self.mir_def_id.index);
1243             match tables.node_type(mir_hir_id).sty {
1244                 ty::Closure(..) => "closure",
1245                 ty::Generator(..) => "generator",
1246                 _ => bug!("Closure body doesn't have a closure or generator type"),
1247             }
1248         } else {
1249             "function"
1250         };
1251
1252         let mut err = borrowck_errors::borrowed_data_escapes_closure(
1253             tcx,
1254             escape_span,
1255             escapes_from,
1256         );
1257
1258         err.span_label(
1259             upvar_span,
1260             format!(
1261                 "`{}` is declared here, outside of the {} body",
1262                 upvar_name, escapes_from
1263             ),
1264         );
1265
1266         err.span_label(
1267             borrow_span,
1268             format!(
1269                 "borrow is only valid in the {} body",
1270                 escapes_from
1271             ),
1272         );
1273
1274         if let Some(name) = name {
1275             err.span_label(
1276                 escape_span,
1277                 format!("reference to `{}` escapes the {} body here", name, escapes_from),
1278             );
1279         } else {
1280             err.span_label(
1281                 escape_span,
1282                 format!("reference escapes the {} body here", escapes_from),
1283             );
1284         }
1285
1286         err
1287     }
1288
1289     fn get_moved_indexes(&mut self, location: Location, mpi: MovePathIndex) -> Vec<MoveSite> {
1290         let body = self.body;
1291
1292         let mut stack = Vec::new();
1293         stack.extend(body.predecessor_locations(location).map(|predecessor| {
1294             let is_back_edge = location.dominates(predecessor, &self.dominators);
1295             (predecessor, is_back_edge)
1296         }));
1297
1298         let mut visited = FxHashSet::default();
1299         let mut result = vec![];
1300
1301         'dfs: while let Some((location, is_back_edge)) = stack.pop() {
1302             debug!(
1303                 "report_use_of_moved_or_uninitialized: (current_location={:?}, back_edge={})",
1304                 location, is_back_edge
1305             );
1306
1307             if !visited.insert(location) {
1308                 continue;
1309             }
1310
1311             // check for moves
1312             let stmt_kind = body[location.block]
1313                 .statements
1314                 .get(location.statement_index)
1315                 .map(|s| &s.kind);
1316             if let Some(StatementKind::StorageDead(..)) = stmt_kind {
1317                 // this analysis only tries to find moves explicitly
1318                 // written by the user, so we ignore the move-outs
1319                 // created by `StorageDead` and at the beginning
1320                 // of a function.
1321             } else {
1322                 // If we are found a use of a.b.c which was in error, then we want to look for
1323                 // moves not only of a.b.c but also a.b and a.
1324                 //
1325                 // Note that the moves data already includes "parent" paths, so we don't have to
1326                 // worry about the other case: that is, if there is a move of a.b.c, it is already
1327                 // marked as a move of a.b and a as well, so we will generate the correct errors
1328                 // there.
1329                 let mut mpis = vec![mpi];
1330                 let move_paths = &self.move_data.move_paths;
1331                 mpis.extend(move_paths[mpi].parents(move_paths));
1332
1333                 for moi in &self.move_data.loc_map[location] {
1334                     debug!("report_use_of_moved_or_uninitialized: moi={:?}", moi);
1335                     if mpis.contains(&self.move_data.moves[*moi].path) {
1336                         debug!("report_use_of_moved_or_uninitialized: found");
1337                         result.push(MoveSite {
1338                             moi: *moi,
1339                             traversed_back_edge: is_back_edge,
1340                         });
1341
1342                         // Strictly speaking, we could continue our DFS here. There may be
1343                         // other moves that can reach the point of error. But it is kind of
1344                         // confusing to highlight them.
1345                         //
1346                         // Example:
1347                         //
1348                         // ```
1349                         // let a = vec![];
1350                         // let b = a;
1351                         // let c = a;
1352                         // drop(a); // <-- current point of error
1353                         // ```
1354                         //
1355                         // Because we stop the DFS here, we only highlight `let c = a`,
1356                         // and not `let b = a`. We will of course also report an error at
1357                         // `let c = a` which highlights `let b = a` as the move.
1358                         continue 'dfs;
1359                     }
1360                 }
1361             }
1362
1363             // check for inits
1364             let mut any_match = false;
1365             drop_flag_effects::for_location_inits(
1366                 self.infcx.tcx,
1367                 self.body,
1368                 self.move_data,
1369                 location,
1370                 |m| {
1371                     if m == mpi {
1372                         any_match = true;
1373                     }
1374                 },
1375             );
1376             if any_match {
1377                 continue 'dfs;
1378             }
1379
1380             stack.extend(body.predecessor_locations(location).map(|predecessor| {
1381                 let back_edge = location.dominates(predecessor, &self.dominators);
1382                 (predecessor, is_back_edge || back_edge)
1383             }));
1384         }
1385
1386         result
1387     }
1388
1389     pub(super) fn report_illegal_mutation_of_borrowed(
1390         &mut self,
1391         location: Location,
1392         (place, span): (&Place<'tcx>, Span),
1393         loan: &BorrowData<'tcx>,
1394     ) {
1395         let loan_spans = self.retrieve_borrow_spans(loan);
1396         let loan_span = loan_spans.args_or_use();
1397
1398         if loan.kind == BorrowKind::Shallow {
1399             let mut err = self.cannot_mutate_in_match_guard(
1400                 span,
1401                 loan_span,
1402                 &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
1403                 "assign",
1404             );
1405             loan_spans.var_span_label(
1406                 &mut err,
1407                 format!("borrow occurs due to use{}", loan_spans.describe()),
1408             );
1409
1410             err.buffer(&mut self.errors_buffer);
1411
1412             return;
1413         }
1414
1415         let mut err = self.cannot_assign_to_borrowed(
1416             span,
1417             loan_span,
1418             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
1419         );
1420
1421         loan_spans.var_span_label(
1422             &mut err,
1423             format!("borrow occurs due to use{}", loan_spans.describe()),
1424         );
1425
1426         self.explain_why_borrow_contains_point(location, loan, None)
1427             .add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
1428
1429         err.buffer(&mut self.errors_buffer);
1430     }
1431
1432     /// Reports an illegal reassignment; for example, an assignment to
1433     /// (part of) a non-`mut` local that occurs potentially after that
1434     /// local has already been initialized. `place` is the path being
1435     /// assigned; `err_place` is a place providing a reason why
1436     /// `place` is not mutable (e.g., the non-`mut` local `x` in an
1437     /// assignment to `x.f`).
1438     pub(super) fn report_illegal_reassignment(
1439         &mut self,
1440         _location: Location,
1441         (place, span): (&Place<'tcx>, Span),
1442         assigned_span: Span,
1443         err_place: &Place<'tcx>,
1444     ) {
1445         let (from_arg, local_decl) = if let Place {
1446             base: PlaceBase::Local(local),
1447             projection: box [],
1448         } = *err_place {
1449             if let LocalKind::Arg = self.body.local_kind(local) {
1450                 (true, Some(&self.body.local_decls[local]))
1451             } else {
1452                 (false, Some(&self.body.local_decls[local]))
1453             }
1454         } else {
1455             (false, None)
1456         };
1457
1458         // If root local is initialized immediately (everything apart from let
1459         // PATTERN;) then make the error refer to that local, rather than the
1460         // place being assigned later.
1461         let (place_description, assigned_span) = match local_decl {
1462             Some(LocalDecl {
1463                 is_user_variable: Some(ClearCrossCrate::Clear),
1464                 ..
1465             })
1466             | Some(LocalDecl {
1467                 is_user_variable:
1468                     Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
1469                         opt_match_place: None,
1470                         ..
1471                     }))),
1472                 ..
1473             })
1474             | Some(LocalDecl {
1475                 is_user_variable: None,
1476                 ..
1477             })
1478             | None => (self.describe_place(place.as_ref()), assigned_span),
1479             Some(decl) => (self.describe_place(err_place.as_ref()), decl.source_info.span),
1480         };
1481
1482         let mut err = self.cannot_reassign_immutable(
1483             span,
1484             place_description.as_ref().map(AsRef::as_ref).unwrap_or("_"),
1485             from_arg,
1486         );
1487         let msg = if from_arg {
1488             "cannot assign to immutable argument"
1489         } else {
1490             "cannot assign twice to immutable variable"
1491         };
1492         if span != assigned_span {
1493             if !from_arg {
1494                 let value_msg = match place_description {
1495                     Some(name) => format!("`{}`", name),
1496                     None => "value".to_owned(),
1497                 };
1498                 err.span_label(assigned_span, format!("first assignment to {}", value_msg));
1499             }
1500         }
1501         if let Some(decl) = local_decl {
1502             if let Some(name) = decl.name {
1503                 if decl.can_be_made_mutable() {
1504                     err.span_suggestion(
1505                         decl.source_info.span,
1506                         "make this binding mutable",
1507                         format!("mut {}", name),
1508                         Applicability::MachineApplicable,
1509                     );
1510                 }
1511             }
1512         }
1513         err.span_label(span, msg);
1514         err.buffer(&mut self.errors_buffer);
1515     }
1516
1517     fn classify_drop_access_kind(&self, place: PlaceRef<'cx, 'tcx>) -> StorageDeadOrDrop<'tcx> {
1518         let tcx = self.infcx.tcx;
1519         match place.projection {
1520             [] => {
1521                 StorageDeadOrDrop::LocalStorageDead
1522             }
1523             [.., elem] => {
1524                 // FIXME(spastorino) revisit when we get rid of Box
1525                 let base = &place.projection[..place.projection.len() - 1];
1526
1527                 // FIXME(spastorino) make this iterate
1528                 let base_access = self.classify_drop_access_kind(PlaceRef {
1529                     base: place.base,
1530                     projection: base,
1531                 });
1532                 match elem {
1533                     ProjectionElem::Deref => match base_access {
1534                         StorageDeadOrDrop::LocalStorageDead
1535                         | StorageDeadOrDrop::BoxedStorageDead => {
1536                             assert!(
1537                                 Place::ty_from(&place.base, base, self.body, tcx).ty.is_box(),
1538                                 "Drop of value behind a reference or raw pointer"
1539                             );
1540                             StorageDeadOrDrop::BoxedStorageDead
1541                         }
1542                         StorageDeadOrDrop::Destructor(_) => base_access,
1543                     },
1544                     ProjectionElem::Field(..) | ProjectionElem::Downcast(..) => {
1545                         let base_ty = Place::ty_from(&place.base, base, self.body, tcx).ty;
1546                         match base_ty.sty {
1547                             ty::Adt(def, _) if def.has_dtor(tcx) => {
1548                                 // Report the outermost adt with a destructor
1549                                 match base_access {
1550                                     StorageDeadOrDrop::Destructor(_) => base_access,
1551                                     StorageDeadOrDrop::LocalStorageDead
1552                                     | StorageDeadOrDrop::BoxedStorageDead => {
1553                                         StorageDeadOrDrop::Destructor(base_ty)
1554                                     }
1555                                 }
1556                             }
1557                             _ => base_access,
1558                         }
1559                     }
1560
1561                     ProjectionElem::ConstantIndex { .. }
1562                     | ProjectionElem::Subslice { .. }
1563                     | ProjectionElem::Index(_) => base_access,
1564                 }
1565             }
1566         }
1567     }
1568
1569     /// Annotate argument and return type of function and closure with (synthesized) lifetime for
1570     /// borrow of local value that does not live long enough.
1571     fn annotate_argument_and_return_for_borrow(
1572         &self,
1573         borrow: &BorrowData<'tcx>,
1574     ) -> Option<AnnotatedBorrowFnSignature<'tcx>> {
1575         // Define a fallback for when we can't match a closure.
1576         let fallback = || {
1577             let is_closure = self.infcx.tcx.is_closure(self.mir_def_id);
1578             if is_closure {
1579                 None
1580             } else {
1581                 let ty = self.infcx.tcx.type_of(self.mir_def_id);
1582                 match ty.sty {
1583                     ty::FnDef(_, _) | ty::FnPtr(_) => self.annotate_fn_sig(
1584                         self.mir_def_id,
1585                         self.infcx.tcx.fn_sig(self.mir_def_id),
1586                     ),
1587                     _ => None,
1588                 }
1589             }
1590         };
1591
1592         // In order to determine whether we need to annotate, we need to check whether the reserve
1593         // place was an assignment into a temporary.
1594         //
1595         // If it was, we check whether or not that temporary is eventually assigned into the return
1596         // place. If it was, we can add annotations about the function's return type and arguments
1597         // and it'll make sense.
1598         let location = borrow.reserve_location;
1599         debug!(
1600             "annotate_argument_and_return_for_borrow: location={:?}",
1601             location
1602         );
1603         if let Some(&Statement { kind: StatementKind::Assign(ref reservation, _), ..})
1604              = &self.body[location.block].statements.get(location.statement_index)
1605         {
1606             debug!(
1607                 "annotate_argument_and_return_for_borrow: reservation={:?}",
1608                 reservation
1609             );
1610             // Check that the initial assignment of the reserve location is into a temporary.
1611             let mut target = *match reservation {
1612                 Place {
1613                     base: PlaceBase::Local(local),
1614                     projection: box [],
1615                 } if self.body.local_kind(*local) == LocalKind::Temp => local,
1616                 _ => return None,
1617             };
1618
1619             // Next, look through the rest of the block, checking if we are assigning the
1620             // `target` (that is, the place that contains our borrow) to anything.
1621             let mut annotated_closure = None;
1622             for stmt in &self.body[location.block].statements[location.statement_index + 1..] {
1623                 debug!(
1624                     "annotate_argument_and_return_for_borrow: target={:?} stmt={:?}",
1625                     target, stmt
1626                 );
1627                 if let StatementKind::Assign(
1628                     Place {
1629                         base: PlaceBase::Local(assigned_to),
1630                         projection: box [],
1631                     },
1632                     box rvalue
1633                 ) = &stmt.kind {
1634                     debug!(
1635                         "annotate_argument_and_return_for_borrow: assigned_to={:?} \
1636                          rvalue={:?}",
1637                         assigned_to, rvalue
1638                     );
1639                     // Check if our `target` was captured by a closure.
1640                     if let Rvalue::Aggregate(
1641                         box AggregateKind::Closure(def_id, substs),
1642                         operands,
1643                     ) = rvalue
1644                     {
1645                         for operand in operands {
1646                             let assigned_from = match operand {
1647                                 Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1648                                     assigned_from
1649                                 }
1650                                 _ => continue,
1651                             };
1652                             debug!(
1653                                 "annotate_argument_and_return_for_borrow: assigned_from={:?}",
1654                                 assigned_from
1655                             );
1656
1657                             // Find the local from the operand.
1658                             let assigned_from_local = match assigned_from.local_or_deref_local() {
1659                                 Some(local) => local,
1660                                 None => continue,
1661                             };
1662
1663                             if assigned_from_local != target {
1664                                 continue;
1665                             }
1666
1667                             // If a closure captured our `target` and then assigned
1668                             // into a place then we should annotate the closure in
1669                             // case it ends up being assigned into the return place.
1670                             annotated_closure = self.annotate_fn_sig(
1671                                 *def_id,
1672                                 self.infcx.closure_sig(*def_id, *substs),
1673                             );
1674                             debug!(
1675                                 "annotate_argument_and_return_for_borrow: \
1676                                  annotated_closure={:?} assigned_from_local={:?} \
1677                                  assigned_to={:?}",
1678                                 annotated_closure, assigned_from_local, assigned_to
1679                             );
1680
1681                             if *assigned_to == mir::RETURN_PLACE {
1682                                 // If it was assigned directly into the return place, then
1683                                 // return now.
1684                                 return annotated_closure;
1685                             } else {
1686                                 // Otherwise, update the target.
1687                                 target = *assigned_to;
1688                             }
1689                         }
1690
1691                         // If none of our closure's operands matched, then skip to the next
1692                         // statement.
1693                         continue;
1694                     }
1695
1696                     // Otherwise, look at other types of assignment.
1697                     let assigned_from = match rvalue {
1698                         Rvalue::Ref(_, _, assigned_from) => assigned_from,
1699                         Rvalue::Use(operand) => match operand {
1700                             Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1701                                 assigned_from
1702                             }
1703                             _ => continue,
1704                         },
1705                         _ => continue,
1706                     };
1707                     debug!(
1708                         "annotate_argument_and_return_for_borrow: \
1709                          assigned_from={:?}",
1710                         assigned_from,
1711                     );
1712
1713                     // Find the local from the rvalue.
1714                     let assigned_from_local = match assigned_from.local_or_deref_local() {
1715                         Some(local) => local,
1716                         None => continue,
1717                     };
1718                     debug!(
1719                         "annotate_argument_and_return_for_borrow: \
1720                          assigned_from_local={:?}",
1721                         assigned_from_local,
1722                     );
1723
1724                     // Check if our local matches the target - if so, we've assigned our
1725                     // borrow to a new place.
1726                     if assigned_from_local != target {
1727                         continue;
1728                     }
1729
1730                     // If we assigned our `target` into a new place, then we should
1731                     // check if it was the return place.
1732                     debug!(
1733                         "annotate_argument_and_return_for_borrow: \
1734                          assigned_from_local={:?} assigned_to={:?}",
1735                         assigned_from_local, assigned_to
1736                     );
1737                     if *assigned_to == mir::RETURN_PLACE {
1738                         // If it was then return the annotated closure if there was one,
1739                         // else, annotate this function.
1740                         return annotated_closure.or_else(fallback);
1741                     }
1742
1743                     // If we didn't assign into the return place, then we just update
1744                     // the target.
1745                     target = *assigned_to;
1746                 }
1747             }
1748
1749             // Check the terminator if we didn't find anything in the statements.
1750             let terminator = &self.body[location.block].terminator();
1751             debug!(
1752                 "annotate_argument_and_return_for_borrow: target={:?} terminator={:?}",
1753                 target, terminator
1754             );
1755             if let TerminatorKind::Call {
1756                 destination: Some((Place {
1757                     base: PlaceBase::Local(assigned_to),
1758                     projection: box [],
1759                 }, _)),
1760                 args,
1761                 ..
1762             } = &terminator.kind
1763             {
1764                 debug!(
1765                     "annotate_argument_and_return_for_borrow: assigned_to={:?} args={:?}",
1766                     assigned_to, args
1767                 );
1768                 for operand in args {
1769                     let assigned_from = match operand {
1770                         Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1771                             assigned_from
1772                         }
1773                         _ => continue,
1774                     };
1775                     debug!(
1776                         "annotate_argument_and_return_for_borrow: assigned_from={:?}",
1777                         assigned_from,
1778                     );
1779
1780                     if let Some(assigned_from_local) = assigned_from.local_or_deref_local() {
1781                         debug!(
1782                             "annotate_argument_and_return_for_borrow: assigned_from_local={:?}",
1783                             assigned_from_local,
1784                         );
1785
1786                         if *assigned_to == mir::RETURN_PLACE && assigned_from_local == target {
1787                             return annotated_closure.or_else(fallback);
1788                         }
1789                     }
1790                 }
1791             }
1792         }
1793
1794         // If we haven't found an assignment into the return place, then we need not add
1795         // any annotations.
1796         debug!("annotate_argument_and_return_for_borrow: none found");
1797         None
1798     }
1799
1800     /// Annotate the first argument and return type of a function signature if they are
1801     /// references.
1802     fn annotate_fn_sig(
1803         &self,
1804         did: DefId,
1805         sig: ty::PolyFnSig<'tcx>,
1806     ) -> Option<AnnotatedBorrowFnSignature<'tcx>> {
1807         debug!("annotate_fn_sig: did={:?} sig={:?}", did, sig);
1808         let is_closure = self.infcx.tcx.is_closure(did);
1809         let fn_hir_id = self.infcx.tcx.hir().as_local_hir_id(did)?;
1810         let fn_decl = self.infcx.tcx.hir().fn_decl_by_hir_id(fn_hir_id)?;
1811
1812         // We need to work out which arguments to highlight. We do this by looking
1813         // at the return type, where there are three cases:
1814         //
1815         // 1. If there are named arguments, then we should highlight the return type and
1816         //    highlight any of the arguments that are also references with that lifetime.
1817         //    If there are no arguments that have the same lifetime as the return type,
1818         //    then don't highlight anything.
1819         // 2. The return type is a reference with an anonymous lifetime. If this is
1820         //    the case, then we can take advantage of (and teach) the lifetime elision
1821         //    rules.
1822         //
1823         //    We know that an error is being reported. So the arguments and return type
1824         //    must satisfy the elision rules. Therefore, if there is a single argument
1825         //    then that means the return type and first (and only) argument have the same
1826         //    lifetime and the borrow isn't meeting that, we can highlight the argument
1827         //    and return type.
1828         //
1829         //    If there are multiple arguments then the first argument must be self (else
1830         //    it would not satisfy the elision rules), so we can highlight self and the
1831         //    return type.
1832         // 3. The return type is not a reference. In this case, we don't highlight
1833         //    anything.
1834         let return_ty = sig.output();
1835         match return_ty.skip_binder().sty {
1836             ty::Ref(return_region, _, _) if return_region.has_name() && !is_closure => {
1837                 // This is case 1 from above, return type is a named reference so we need to
1838                 // search for relevant arguments.
1839                 let mut arguments = Vec::new();
1840                 for (index, argument) in sig.inputs().skip_binder().iter().enumerate() {
1841                     if let ty::Ref(argument_region, _, _) = argument.sty {
1842                         if argument_region == return_region {
1843                             // Need to use the `rustc::ty` types to compare against the
1844                             // `return_region`. Then use the `rustc::hir` type to get only
1845                             // the lifetime span.
1846                             if let hir::TyKind::Rptr(lifetime, _) = &fn_decl.inputs[index].node {
1847                                 // With access to the lifetime, we can get
1848                                 // the span of it.
1849                                 arguments.push((*argument, lifetime.span));
1850                             } else {
1851                                 bug!("ty type is a ref but hir type is not");
1852                             }
1853                         }
1854                     }
1855                 }
1856
1857                 // We need to have arguments. This shouldn't happen, but it's worth checking.
1858                 if arguments.is_empty() {
1859                     return None;
1860                 }
1861
1862                 // We use a mix of the HIR and the Ty types to get information
1863                 // as the HIR doesn't have full types for closure arguments.
1864                 let return_ty = *sig.output().skip_binder();
1865                 let mut return_span = fn_decl.output.span();
1866                 if let hir::FunctionRetTy::Return(ty) = &fn_decl.output {
1867                     if let hir::TyKind::Rptr(lifetime, _) = ty.node {
1868                         return_span = lifetime.span;
1869                     }
1870                 }
1871
1872                 Some(AnnotatedBorrowFnSignature::NamedFunction {
1873                     arguments,
1874                     return_ty,
1875                     return_span,
1876                 })
1877             }
1878             ty::Ref(_, _, _) if is_closure => {
1879                 // This is case 2 from above but only for closures, return type is anonymous
1880                 // reference so we select
1881                 // the first argument.
1882                 let argument_span = fn_decl.inputs.first()?.span;
1883                 let argument_ty = sig.inputs().skip_binder().first()?;
1884
1885                 // Closure arguments are wrapped in a tuple, so we need to get the first
1886                 // from that.
1887                 if let ty::Tuple(elems) = argument_ty.sty {
1888                     let argument_ty = elems.first()?.expect_ty();
1889                     if let ty::Ref(_, _, _) = argument_ty.sty {
1890                         return Some(AnnotatedBorrowFnSignature::Closure {
1891                             argument_ty,
1892                             argument_span,
1893                         });
1894                     }
1895                 }
1896
1897                 None
1898             }
1899             ty::Ref(_, _, _) => {
1900                 // This is also case 2 from above but for functions, return type is still an
1901                 // anonymous reference so we select the first argument.
1902                 let argument_span = fn_decl.inputs.first()?.span;
1903                 let argument_ty = sig.inputs().skip_binder().first()?;
1904
1905                 let return_span = fn_decl.output.span();
1906                 let return_ty = *sig.output().skip_binder();
1907
1908                 // We expect the first argument to be a reference.
1909                 match argument_ty.sty {
1910                     ty::Ref(_, _, _) => {}
1911                     _ => return None,
1912                 }
1913
1914                 Some(AnnotatedBorrowFnSignature::AnonymousFunction {
1915                     argument_ty,
1916                     argument_span,
1917                     return_ty,
1918                     return_span,
1919                 })
1920             }
1921             _ => {
1922                 // This is case 3 from above, return type is not a reference so don't highlight
1923                 // anything.
1924                 None
1925             }
1926         }
1927     }
1928 }
1929
1930 #[derive(Debug)]
1931 enum AnnotatedBorrowFnSignature<'tcx> {
1932     NamedFunction {
1933         arguments: Vec<(Ty<'tcx>, Span)>,
1934         return_ty: Ty<'tcx>,
1935         return_span: Span,
1936     },
1937     AnonymousFunction {
1938         argument_ty: Ty<'tcx>,
1939         argument_span: Span,
1940         return_ty: Ty<'tcx>,
1941         return_span: Span,
1942     },
1943     Closure {
1944         argument_ty: Ty<'tcx>,
1945         argument_span: Span,
1946     },
1947 }
1948
1949 impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
1950     /// Annotate the provided diagnostic with information about borrow from the fn signature that
1951     /// helps explain.
1952     pub(super) fn emit(
1953         &self,
1954         cx: &mut MirBorrowckCtxt<'_, 'tcx>,
1955         diag: &mut DiagnosticBuilder<'_>,
1956     ) -> String {
1957         match self {
1958             AnnotatedBorrowFnSignature::Closure {
1959                 argument_ty,
1960                 argument_span,
1961             } => {
1962                 diag.span_label(
1963                     *argument_span,
1964                     format!("has type `{}`", cx.get_name_for_ty(argument_ty, 0)),
1965                 );
1966
1967                 cx.get_region_name_for_ty(argument_ty, 0)
1968             }
1969             AnnotatedBorrowFnSignature::AnonymousFunction {
1970                 argument_ty,
1971                 argument_span,
1972                 return_ty,
1973                 return_span,
1974             } => {
1975                 let argument_ty_name = cx.get_name_for_ty(argument_ty, 0);
1976                 diag.span_label(*argument_span, format!("has type `{}`", argument_ty_name));
1977
1978                 let return_ty_name = cx.get_name_for_ty(return_ty, 0);
1979                 let types_equal = return_ty_name == argument_ty_name;
1980                 diag.span_label(
1981                     *return_span,
1982                     format!(
1983                         "{}has type `{}`",
1984                         if types_equal { "also " } else { "" },
1985                         return_ty_name,
1986                     ),
1987                 );
1988
1989                 diag.note(
1990                     "argument and return type have the same lifetime due to lifetime elision rules",
1991                 );
1992                 diag.note(
1993                     "to learn more, visit <https://doc.rust-lang.org/book/ch10-03-\
1994                      lifetime-syntax.html#lifetime-elision>",
1995                 );
1996
1997                 cx.get_region_name_for_ty(return_ty, 0)
1998             }
1999             AnnotatedBorrowFnSignature::NamedFunction {
2000                 arguments,
2001                 return_ty,
2002                 return_span,
2003             } => {
2004                 // Region of return type and arguments checked to be the same earlier.
2005                 let region_name = cx.get_region_name_for_ty(return_ty, 0);
2006                 for (_, argument_span) in arguments {
2007                     diag.span_label(*argument_span, format!("has lifetime `{}`", region_name));
2008                 }
2009
2010                 diag.span_label(
2011                     *return_span,
2012                     format!("also has lifetime `{}`", region_name,),
2013                 );
2014
2015                 diag.help(&format!(
2016                     "use data from the highlighted arguments which match the `{}` lifetime of \
2017                      the return type",
2018                     region_name,
2019                 ));
2020
2021                 region_name
2022             }
2023         }
2024     }
2025 }