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