src/librustc_mir/borrow_check/conflict_errors.rs

   1 use rustc::hir;
   2 use rustc::hir::def_id::DefId;
   3 use rustc::mir::{
   4     self, AggregateKind, BindingForm, BorrowKind, ClearCrossCrate, ConstraintCategory, Local,
   5     LocalDecl, LocalKind, Location, Operand, Place, PlaceBase, Projection, PlaceRef,
   6     ProjectionElem, Rvalue, Statement, StatementKind, TerminatorKind, VarBindingForm,
   7 };
   8 use rustc::ty::{self, Ty};
   9 use rustc_data_structures::fx::FxHashSet;
  10 use rustc_data_structures::indexed_vec::Idx;
  11 use rustc_errors::{Applicability, DiagnosticBuilder};
  12 use syntax_pos::Span;
  13 use syntax::source_map::DesugaringKind;
  14
  15 use super::nll::explain_borrow::BorrowExplanation;
  16 use super::nll::region_infer::{RegionName, RegionNameSource};
  17 use super::prefixes::IsPrefixOf;
  18 use super::WriteKind;
  19 use super::borrow_set::BorrowData;
  20 use super::MirBorrowckCtxt;
  21 use super::{InitializationRequiringAction, PrefixSet};
  22 use super::error_reporting::{IncludingDowncast, UseSpans};
  23 use crate::dataflow::drop_flag_effects;
  24 use crate::dataflow::indexes::{MovePathIndex, MoveOutIndex};
  25 use crate::util::borrowck_errors;
  26
  27 #[derive(Debug)]
  28 struct MoveSite {
  29     /// Index of the "move out" that we found. The `MoveData` can
  30     /// then tell us where the move occurred.
  31     moi: MoveOutIndex,
  32
  33     /// `true` if we traversed a back edge while walking from the point
  34     /// of error to the move site.
  35     traversed_back_edge: bool
  36 }
  37
  38 /// Which case a StorageDeadOrDrop is for.
  39 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
  40 enum StorageDeadOrDrop<'tcx> {
  41     LocalStorageDead,
  42     BoxedStorageDead,
  43     Destructor(Ty<'tcx>),
  44 }
  45
  46 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
  47     pub(super) fn report_use_of_moved_or_uninitialized(
  48         &mut self,
  49         location: Location,
  50         desired_action: InitializationRequiringAction,
  51         (moved_place, used_place, span): (PlaceRef<'cx, 'tcx>, PlaceRef<'cx, 'tcx>, Span),
  52         mpi: MovePathIndex,
  53     ) {
  54         debug!(
  55             "report_use_of_moved_or_uninitialized: location={:?} desired_action={:?} \
  56              moved_place={:?} used_place={:?} span={:?} mpi={:?}",
  57             location, desired_action, moved_place, used_place, span, mpi
  58         );
  59
  60         let use_spans = self.move_spans(moved_place, location)
  61             .or_else(|| self.borrow_spans(span, location));
  62         let span = use_spans.args_or_use();
  63
  64         let move_site_vec = self.get_moved_indexes(location, mpi);
  65         debug!(
  66             "report_use_of_moved_or_uninitialized: move_site_vec={:?}",
  67             move_site_vec
  68         );
  69         let move_out_indices: Vec<_> = move_site_vec
  70             .iter()
  71             .map(|move_site| move_site.moi)
  72             .collect();
  73
  74         if move_out_indices.is_empty() {
  75             let root_place = self
  76                 .prefixes(used_place, PrefixSet::All)
  77                 .last()
  78                 .unwrap();
  79
  80             if self.uninitialized_error_reported.contains(&root_place) {
  81                 debug!(
  82                     "report_use_of_moved_or_uninitialized place: error about {:?} suppressed",
  83                     root_place
  84                 );
  85                 return;
  86             }
  87
  88             self.uninitialized_error_reported.insert(root_place);
  89
  90             let item_msg = match self.describe_place_with_options(used_place,
  91                                                                   IncludingDowncast(true)) {
  92                 Some(name) => format!("`{}`", name),
  93                 None => "value".to_owned(),
  94             };
  95             let mut err = self.cannot_act_on_uninitialized_variable(
  96                 span,
  97                 desired_action.as_noun(),
  98                 &self.describe_place_with_options(moved_place, IncludingDowncast(true))
  99                     .unwrap_or_else(|| "_".to_owned()),
 100             );
 101             err.span_label(span, format!("use of possibly uninitialized {}", item_msg));
 102
 103             use_spans.var_span_label(
 104                 &mut err,
 105                 format!("{} occurs due to use{}", desired_action.as_noun(), use_spans.describe()),
 106             );
 107
 108             err.buffer(&mut self.errors_buffer);
 109         } else {
 110             if let Some((reported_place, _)) = self.move_error_reported.get(&move_out_indices) {
 111                 if self.prefixes(*reported_place, PrefixSet::All)
 112                     .any(|p| p == used_place)
 113                 {
 114                     debug!(
 115                         "report_use_of_moved_or_uninitialized place: error suppressed \
 116                          mois={:?}",
 117                         move_out_indices
 118                     );
 119                     return;
 120                 }
 121             }
 122
 123             let msg = ""; //FIXME: add "partially " or "collaterally "
 124
 125             let mut err = self.cannot_act_on_moved_value(
 126                 span,
 127                 desired_action.as_noun(),
 128                 msg,
 129                 self.describe_place_with_options(moved_place, IncludingDowncast(true)),
 130             );
 131
 132             self.add_moved_or_invoked_closure_note(
 133                 location,
 134                 used_place,
 135                 &mut err,
 136             );
 137
 138             let mut is_loop_move = false;
 139             let is_partial_move = move_site_vec.iter().any(|move_site| {
 140                 let move_out = self.move_data.moves[(*move_site).moi];
 141                 let moved_place = &self.move_data.move_paths[move_out.path].place;
 142                 used_place != moved_place.as_ref()
 143                     && used_place.is_prefix_of(moved_place.as_ref())
 144             });
 145             for move_site in &move_site_vec {
 146                 let move_out = self.move_data.moves[(*move_site).moi];
 147                 let moved_place = &self.move_data.move_paths[move_out.path].place;
 148
 149                 let move_spans = self.move_spans(moved_place.as_ref(), move_out.source);
 150                 let move_span = move_spans.args_or_use();
 151
 152                 let move_msg = if move_spans.for_closure() {
 153                     " into closure"
 154                 } else {
 155                     ""
 156                 };
 157
 158                 if span == move_span {
 159                     err.span_label(
 160                         span,
 161                         format!("value moved{} here, in previous iteration of loop", move_msg),
 162                     );
 163                     is_loop_move = true;
 164                 } else if move_site.traversed_back_edge {
 165                     err.span_label(
 166                         move_span,
 167                         format!(
 168                             "value moved{} here, in previous iteration of loop",
 169                             move_msg
 170                         ),
 171                     );
 172                 } else {
 173                     err.span_label(move_span, format!("value moved{} here", move_msg));
 174                     move_spans.var_span_label(
 175                         &mut err,
 176                         format!("variable moved due to use{}", move_spans.describe()),
 177                     );
 178                 }
 179                 if Some(DesugaringKind::ForLoop) == move_span.desugaring_kind() {
 180                     if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
 181                         err.span_suggestion(
 182                             move_span,
 183                             "consider borrowing to avoid moving into the for loop",
 184                             format!("&{}", snippet),
 185                             Applicability::MaybeIncorrect,
 186                         );
 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 =
 208                 Place::ty_from(used_place.base, used_place.projection, self.body, self.infcx.tcx)
 209                     .ty;
 210             let needs_note = match ty.sty {
 211                 ty::Closure(id, _) => {
 212                     let tables = self.infcx.tcx.typeck_tables_of(id);
 213                     let hir_id = self.infcx.tcx.hir().as_local_hir_id(id).unwrap();
 214
 215                     tables.closure_kind_origins().get(hir_id).is_none()
 216                 }
 217                 _ => true,
 218             };
 219
 220             if needs_note {
 221                 let mpi = self.move_data.moves[move_out_indices[0]].path;
 222                 let place = &self.move_data.move_paths[mpi].place;
 223
 224                 let ty = place.ty(self.body, self.infcx.tcx).ty;
 225                 let opt_name =
 226                     self.describe_place_with_options(place.as_ref(), IncludingDowncast(true));
 227                 let note_msg = match opt_name {
 228                     Some(ref name) => format!("`{}`", name),
 229                     None => "value".to_owned(),
 230                 };
 231                 if let ty::Param(param_ty) = ty.sty {
 232                     let tcx = self.infcx.tcx;
 233                     let generics = tcx.generics_of(self.mir_def_id);
 234                     let def_id = generics.type_param(&param_ty, tcx).def_id;
 235                     if let Some(sp) = tcx.hir().span_if_local(def_id) {
 236                         err.span_label(
 237                             sp,
 238                             "consider adding a `Copy` constraint to this type argument",
 239                         );
 240                     }
 241                 }
 242                 let span = if let Place {
 243                     base: PlaceBase::Local(local),
 244                     projection: None,
 245                 } = place {
 246                     let decl = &self.body.local_decls[*local];
 247                     Some(decl.source_info.span)
 248                 } else {
 249                     None
 250                 };
 251                 self.note_type_does_not_implement_copy(
 252                     &mut err,
 253                     &note_msg,
 254                     ty,
 255                     span,
 256                 );
 257             }
 258
 259             if let Some((_, mut old_err)) = self.move_error_reported
 260                 .insert(move_out_indices, (used_place, err))
 261             {
 262                 // Cancel the old error so it doesn't ICE.
 263                 old_err.cancel();
 264             }
 265         }
 266     }
 267
 268     pub(super) fn report_move_out_while_borrowed(
 269         &mut self,
 270         location: Location,
 271         (place, span): (&Place<'tcx>, Span),
 272         borrow: &BorrowData<'tcx>,
 273     ) {
 274         debug!(
 275             "report_move_out_while_borrowed: location={:?} place={:?} span={:?} borrow={:?}",
 276             location, place, span, borrow
 277         );
 278         let value_msg = match self.describe_place(place.as_ref()) {
 279             Some(name) => format!("`{}`", name),
 280             None => "value".to_owned(),
 281         };
 282         let borrow_msg = match self.describe_place(borrow.borrowed_place.as_ref()) {
 283             Some(name) => format!("`{}`", name),
 284             None => "value".to_owned(),
 285         };
 286
 287         let borrow_spans = self.retrieve_borrow_spans(borrow);
 288         let borrow_span = borrow_spans.args_or_use();
 289
 290         let move_spans = self.move_spans(place.as_ref(), location);
 291         let span = move_spans.args_or_use();
 292
 293         let mut err = self.cannot_move_when_borrowed(
 294             span,
 295             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
 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.body, &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 borrow_spans = self.retrieve_borrow_spans(borrow);
 325         let borrow_span = borrow_spans.args_or_use();
 326
 327         // Conflicting borrows are reported separately, so only check for move
 328         // captures.
 329         let use_spans = self.move_spans(place.as_ref(), location);
 330         let span = use_spans.var_or_use();
 331
 332         let mut err = self.cannot_use_when_mutably_borrowed(
 333             span,
 334             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
 335             borrow_span,
 336             &self.describe_place(borrow.borrowed_place.as_ref())
 337                 .unwrap_or_else(|| "_".to_owned()),
 338         );
 339
 340         borrow_spans.var_span_label(&mut err, {
 341             let place = &borrow.borrowed_place;
 342             let desc_place =
 343                 self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned());
 344
 345             format!("borrow occurs due to use of `{}`{}", desc_place, borrow_spans.describe())
 346         });
 347
 348         self.explain_why_borrow_contains_point(location, borrow, None)
 349             .add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
 350         err
 351     }
 352
 353     pub(super) fn report_conflicting_borrow(
 354         &mut self,
 355         location: Location,
 356         (place, span): (&Place<'tcx>, Span),
 357         gen_borrow_kind: BorrowKind,
 358         issued_borrow: &BorrowData<'tcx>,
 359     ) -> DiagnosticBuilder<'cx> {
 360         let issued_spans = self.retrieve_borrow_spans(issued_borrow);
 361         let issued_span = issued_spans.args_or_use();
 362
 363         let borrow_spans = self.borrow_spans(span, location);
 364         let span = borrow_spans.args_or_use();
 365
 366         let container_name = if issued_spans.for_generator() || borrow_spans.for_generator() {
 367             "generator"
 368         } else {
 369             "closure"
 370         };
 371
 372         let (desc_place, msg_place, msg_borrow, union_type_name) =
 373             self.describe_place_for_conflicting_borrow(place, &issued_borrow.borrowed_place);
 374
 375         let explanation = self.explain_why_borrow_contains_point(location, issued_borrow, None);
 376         let second_borrow_desc = if explanation.is_explained() {
 377             "second "
 378         } else {
 379             ""
 380         };
 381
 382         // FIXME: supply non-"" `opt_via` when appropriate
 383         let first_borrow_desc;
 384         let mut err = match (
 385             gen_borrow_kind,
 386             "immutable",
 387             "mutable",
 388             issued_borrow.kind,
 389             "immutable",
 390             "mutable",
 391         ) {
 392             (BorrowKind::Shared, lft, _, BorrowKind::Mut { .. }, _, rgt) => {
 393                 first_borrow_desc = "mutable ";
 394                 self.cannot_reborrow_already_borrowed(
 395                     span,
 396                     &desc_place,
 397                     &msg_place,
 398                     lft,
 399                     issued_span,
 400                     "it",
 401                     rgt,
 402                     &msg_borrow,
 403                     None,
 404                 )
 405             }
 406             (BorrowKind::Mut { .. }, _, lft, BorrowKind::Shared, rgt, _) => {
 407                 first_borrow_desc = "immutable ";
 408                 self.cannot_reborrow_already_borrowed(
 409                     span,
 410                     &desc_place,
 411                     &msg_place,
 412                     lft,
 413                     issued_span,
 414                     "it",
 415                     rgt,
 416                     &msg_borrow,
 417                     None,
 418                 )
 419             }
 420
 421             (BorrowKind::Mut { .. }, _, _, BorrowKind::Mut { .. }, _, _) => {
 422                 first_borrow_desc = "first ";
 423                 self.cannot_mutably_borrow_multiply(
 424                     span,
 425                     &desc_place,
 426                     &msg_place,
 427                     issued_span,
 428                     &msg_borrow,
 429                     None,
 430                 )
 431             }
 432
 433             (BorrowKind::Unique, _, _, BorrowKind::Unique, _, _) => {
 434                 first_borrow_desc = "first ";
 435                 self.cannot_uniquely_borrow_by_two_closures(
 436                     span,
 437                     &desc_place,
 438                     issued_span,
 439                     None,
 440                 )
 441             }
 442
 443             (BorrowKind::Mut { .. }, _, _, BorrowKind::Shallow, _, _)
 444             | (BorrowKind::Unique, _, _, BorrowKind::Shallow, _, _) => {
 445                 let mut err = self.cannot_mutate_in_match_guard(
 446                     span,
 447                     issued_span,
 448                     &desc_place,
 449                     "mutably borrow",
 450                 );
 451                 borrow_spans.var_span_label(
 452                     &mut err,
 453                     format!(
 454                         "borrow occurs due to use of `{}`{}", desc_place, borrow_spans.describe()
 455                     ),
 456                 );
 457
 458                 return err;
 459             }
 460
 461             (BorrowKind::Unique, _, _, _, _, _) => {
 462                 first_borrow_desc = "first ";
 463                 self.cannot_uniquely_borrow_by_one_closure(
 464                     span,
 465                     container_name,
 466                     &desc_place,
 467                     "",
 468                     issued_span,
 469                     "it",
 470                     "",
 471                     None,
 472                 )
 473             },
 474
 475             (BorrowKind::Shared, lft, _, BorrowKind::Unique, _, _) => {
 476                 first_borrow_desc = "first ";
 477                 self.cannot_reborrow_already_uniquely_borrowed(
 478                     span,
 479                     container_name,
 480                     &desc_place,
 481                     "",
 482                     lft,
 483                     issued_span,
 484                     "",
 485                     None,
 486                     second_borrow_desc,
 487                 )
 488             }
 489
 490             (BorrowKind::Mut { .. }, _, lft, BorrowKind::Unique, _, _) => {
 491                 first_borrow_desc = "first ";
 492                 self.cannot_reborrow_already_uniquely_borrowed(
 493                     span,
 494                     container_name,
 495                     &desc_place,
 496                     "",
 497                     lft,
 498                     issued_span,
 499                     "",
 500                     None,
 501                     second_borrow_desc,
 502                 )
 503             }
 504
 505             (BorrowKind::Shared, _, _, BorrowKind::Shared, _, _)
 506             | (BorrowKind::Shared, _, _, BorrowKind::Shallow, _, _)
 507             | (BorrowKind::Shallow, _, _, BorrowKind::Mut { .. }, _, _)
 508             | (BorrowKind::Shallow, _, _, BorrowKind::Unique, _, _)
 509             | (BorrowKind::Shallow, _, _, BorrowKind::Shared, _, _)
 510             | (BorrowKind::Shallow, _, _, BorrowKind::Shallow, _, _) => unreachable!(),
 511         };
 512
 513         if issued_spans == borrow_spans {
 514             borrow_spans.var_span_label(
 515                 &mut err,
 516                 format!("borrows occur due to use of `{}`{}", desc_place, borrow_spans.describe()),
 517             );
 518         } else {
 519             let borrow_place = &issued_borrow.borrowed_place;
 520             let borrow_place_desc = self.describe_place(borrow_place.as_ref())
 521                                         .unwrap_or_else(|| "_".to_owned());
 522             issued_spans.var_span_label(
 523                 &mut err,
 524                 format!(
 525                     "first borrow occurs due to use of `{}`{}",
 526                     borrow_place_desc,
 527                     issued_spans.describe(),
 528                 ),
 529             );
 530
 531             borrow_spans.var_span_label(
 532                 &mut err,
 533                 format!(
 534                     "second borrow occurs due to use of `{}`{}",
 535                     desc_place,
 536                     borrow_spans.describe(),
 537                 ),
 538             );
 539         }
 540
 541         if union_type_name != "" {
 542             err.note(&format!(
 543                 "`{}` is a field of the union `{}`, so it overlaps the field `{}`",
 544                 msg_place, union_type_name, msg_borrow,
 545             ));
 546         }
 547
 548         explanation.add_explanation_to_diagnostic(
 549             self.infcx.tcx,
 550             self.body,
 551             &mut err,
 552             first_borrow_desc,
 553             None,
 554         );
 555
 556         err
 557     }
 558
 559     /// Returns the description of the root place for a conflicting borrow and the full
 560     /// descriptions of the places that caused the conflict.
 561     ///
 562     /// In the simplest case, where there are no unions involved, if a mutable borrow of `x` is
 563     /// attempted while a shared borrow is live, then this function will return:
 564     ///
 565     ///     ("x", "", "")
 566     ///
 567     /// In the simple union case, if a mutable borrow of a union field `x.z` is attempted while
 568     /// a shared borrow of another field `x.y`, then this function will return:
 569     ///
 570     ///     ("x", "x.z", "x.y")
 571     ///
 572     /// In the more complex union case, where the union is a field of a struct, then if a mutable
 573     /// borrow of a union field in a struct `x.u.z` is attempted while a shared borrow of
 574     /// another field `x.u.y`, then this function will return:
 575     ///
 576     ///     ("x.u", "x.u.z", "x.u.y")
 577     ///
 578     /// This is used when creating error messages like below:
 579     ///
 580     /// >  cannot borrow `a.u` (via `a.u.z.c`) as immutable because it is also borrowed as
 581     /// >  mutable (via `a.u.s.b`) [E0502]
 582     pub(super) fn describe_place_for_conflicting_borrow(
 583         &self,
 584         first_borrowed_place: &Place<'tcx>,
 585         second_borrowed_place: &Place<'tcx>,
 586     ) -> (String, String, String, String) {
 587         // Define a small closure that we can use to check if the type of a place
 588         // is a union.
 589         let union_ty = |place_base, place_projection| {
 590             let ty = Place::ty_from(place_base, place_projection, self.body, self.infcx.tcx).ty;
 591             ty.ty_adt_def().filter(|adt| adt.is_union()).map(|_| ty)
 592         };
 593         let describe_place = |place| self.describe_place(place).unwrap_or_else(|| "_".to_owned());
 594
 595         // Start with an empty tuple, so we can use the functions on `Option` to reduce some
 596         // code duplication (particularly around returning an empty description in the failure
 597         // case).
 598         Some(())
 599             .filter(|_| {
 600                 // If we have a conflicting borrow of the same place, then we don't want to add
 601                 // an extraneous "via x.y" to our diagnostics, so filter out this case.
 602                 first_borrowed_place != second_borrowed_place
 603             })
 604             .and_then(|_| {
 605                 // We're going to want to traverse the first borrowed place to see if we can find
 606                 // field access to a union. If we find that, then we will keep the place of the
 607                 // union being accessed and the field that was being accessed so we can check the
 608                 // second borrowed place for the same union and a access to a different field.
 609                 let Place {
 610                     base,
 611                     projection,
 612                 } = first_borrowed_place;
 613
 614                 let mut current = projection;
 615
 616                 while let Some(box Projection { base: base_proj, elem }) = current {
 617                     match elem {
 618                         ProjectionElem::Field(field, _) if union_ty(base, base_proj).is_some() => {
 619                             return Some((PlaceRef {
 620                                 base: base,
 621                                 projection: base_proj,
 622                             }, field));
 623                         },
 624                         _ => current = base_proj,
 625                     }
 626                 }
 627                 None
 628             })
 629             .and_then(|(target_base, target_field)| {
 630                 // With the place of a union and a field access into it, we traverse the second
 631                 // borrowed place and look for a access to a different field of the same union.
 632                 let Place {
 633                     base,
 634                     projection,
 635                 } = second_borrowed_place;
 636
 637                 let mut current = projection;
 638
 639                 while let Some(box Projection { base: proj_base, elem }) = current {
 640                     if let ProjectionElem::Field(field, _) = elem {
 641                         if let Some(union_ty) = union_ty(base, proj_base) {
 642                             if field != target_field
 643                                 && base == target_base.base
 644                                 && proj_base == target_base.projection {
 645                                 // FIXME when we avoid clone reuse describe_place closure
 646                                 let describe_base_place =  self.describe_place(PlaceRef {
 647                                     base: base,
 648                                     projection: proj_base,
 649                                 }).unwrap_or_else(|| "_".to_owned());
 650
 651                                 return Some((
 652                                     describe_base_place,
 653                                     describe_place(first_borrowed_place.as_ref()),
 654                                     describe_place(second_borrowed_place.as_ref()),
 655                                     union_ty.to_string(),
 656                                 ));
 657                             }
 658                         }
 659                     }
 660
 661                     current = proj_base;
 662                 }
 663                 None
 664             })
 665             .unwrap_or_else(|| {
 666                 // If we didn't find a field access into a union, or both places match, then
 667                 // only return the description of the first place.
 668                 (
 669                     describe_place(first_borrowed_place.as_ref()),
 670                     "".to_string(),
 671                     "".to_string(),
 672                     "".to_string(),
 673                 )
 674             })
 675     }
 676
 677     /// Reports StorageDeadOrDrop of `place` conflicts with `borrow`.
 678     ///
 679     /// This means that some data referenced by `borrow` needs to live
 680     /// past the point where the StorageDeadOrDrop of `place` occurs.
 681     /// This is usually interpreted as meaning that `place` has too
 682     /// short a lifetime. (But sometimes it is more useful to report
 683     /// it as a more direct conflict between the execution of a
 684     /// `Drop::drop` with an aliasing borrow.)
 685     pub(super) fn report_borrowed_value_does_not_live_long_enough(
 686         &mut self,
 687         location: Location,
 688         borrow: &BorrowData<'tcx>,
 689         place_span: (&Place<'tcx>, Span),
 690         kind: Option<WriteKind>,
 691     ) {
 692         debug!(
 693             "report_borrowed_value_does_not_live_long_enough(\
 694              {:?}, {:?}, {:?}, {:?}\
 695              )",
 696             location, borrow, place_span, kind
 697         );
 698
 699         let drop_span = place_span.1;
 700         let root_place = self.prefixes(borrow.borrowed_place.as_ref(), PrefixSet::All)
 701             .last()
 702             .unwrap();
 703
 704         let borrow_spans = self.retrieve_borrow_spans(borrow);
 705         let borrow_span = borrow_spans.var_or_use();
 706
 707         assert!(root_place.projection.is_none());
 708         let proper_span = match root_place.base {
 709             PlaceBase::Local(local) => self.body.local_decls[*local].source_info.span,
 710             _ => drop_span,
 711         };
 712
 713         if self.access_place_error_reported
 714             .contains(&(Place {
 715                 base: root_place.base.clone(),
 716                 projection: root_place.projection.clone(),
 717             }, borrow_span))
 718         {
 719             debug!(
 720                 "suppressing access_place error when borrow doesn't live long enough for {:?}",
 721                 borrow_span
 722             );
 723             return;
 724         }
 725
 726         self.access_place_error_reported
 727             .insert((Place {
 728                 base: root_place.base.clone(),
 729                 projection: root_place.projection.clone(),
 730             }, borrow_span));
 731
 732         if let StorageDeadOrDrop::Destructor(dropped_ty) =
 733             self.classify_drop_access_kind(borrow.borrowed_place.as_ref())
 734         {
 735             // If a borrow of path `B` conflicts with drop of `D` (and
 736             // we're not in the uninteresting case where `B` is a
 737             // prefix of `D`), then report this as a more interesting
 738             // destructor conflict.
 739             if !borrow.borrowed_place.as_ref().is_prefix_of(place_span.0.as_ref()) {
 740                 self.report_borrow_conflicts_with_destructor(
 741                     location, borrow, place_span, kind, dropped_ty,
 742                 );
 743                 return;
 744             }
 745         }
 746
 747         let place_desc = self.describe_place(borrow.borrowed_place.as_ref());
 748
 749         let kind_place = kind.filter(|_| place_desc.is_some()).map(|k| (k, place_span.0));
 750         let explanation = self.explain_why_borrow_contains_point(location, &borrow, kind_place);
 751
 752         let err = match (place_desc, explanation) {
 753             (Some(_), _) if self.is_place_thread_local(root_place) => {
 754                 self.report_thread_local_value_does_not_live_long_enough(drop_span, borrow_span)
 755             }
 756             // If the outlives constraint comes from inside the closure,
 757             // for example:
 758             //
 759             // let x = 0;
 760             // let y = &x;
 761             // Box::new(|| y) as Box<Fn() -> &'static i32>
 762             //
 763             // then just use the normal error. The closure isn't escaping
 764             // and `move` will not help here.
 765             (
 766                 Some(ref name),
 767                 BorrowExplanation::MustBeValidFor {
 768                     category: category @ ConstraintCategory::Return,
 769                     from_closure: false,
 770                     ref region_name,
 771                     span,
 772                     ..
 773                 },
 774             )
 775             | (
 776                 Some(ref name),
 777                 BorrowExplanation::MustBeValidFor {
 778                     category: category @ ConstraintCategory::CallArgument,
 779                     from_closure: false,
 780                     ref region_name,
 781                     span,
 782                     ..
 783                 },
 784             ) if borrow_spans.for_closure() => self.report_escaping_closure_capture(
 785                 borrow_spans.args_or_use(),
 786                 borrow_span,
 787                 region_name,
 788                 category,
 789                 span,
 790                 &format!("`{}`", name),
 791             ),
 792             (
 793                 ref name,
 794                 BorrowExplanation::MustBeValidFor {
 795                     category: ConstraintCategory::Assignment,
 796                     from_closure: false,
 797                     region_name: RegionName {
 798                         source: RegionNameSource::AnonRegionFromUpvar(upvar_span, ref upvar_name),
 799                         ..
 800                     },
 801                     span,
 802                     ..
 803                 },
 804             ) => self.report_escaping_data(borrow_span, name, upvar_span, upvar_name, span),
 805             (Some(name), explanation) => self.report_local_value_does_not_live_long_enough(
 806                 location,
 807                 &name,
 808                 &borrow,
 809                 drop_span,
 810                 borrow_spans,
 811                 explanation,
 812             ),
 813             (None, explanation) => self.report_temporary_value_does_not_live_long_enough(
 814                 location,
 815                 &borrow,
 816                 drop_span,
 817                 borrow_spans,
 818                 proper_span,
 819                 explanation,
 820             ),
 821         };
 822
 823         err.buffer(&mut self.errors_buffer);
 824     }
 825
 826     fn report_local_value_does_not_live_long_enough(
 827         &mut self,
 828         location: Location,
 829         name: &str,
 830         borrow: &BorrowData<'tcx>,
 831         drop_span: Span,
 832         borrow_spans: UseSpans,
 833         explanation: BorrowExplanation,
 834     ) -> DiagnosticBuilder<'cx> {
 835         debug!(
 836             "report_local_value_does_not_live_long_enough(\
 837              {:?}, {:?}, {:?}, {:?}, {:?}\
 838              )",
 839             location, name, borrow, drop_span, borrow_spans
 840         );
 841
 842         let borrow_span = borrow_spans.var_or_use();
 843         if let BorrowExplanation::MustBeValidFor {
 844             category,
 845             span,
 846             ref opt_place_desc,
 847             from_closure: false,
 848             ..
 849         } = explanation {
 850             if let Some(diag) = self.try_report_cannot_return_reference_to_local(
 851                 borrow,
 852                 borrow_span,
 853                 span,
 854                 category,
 855                 opt_place_desc.as_ref(),
 856             ) {
 857                 return diag;
 858             }
 859         }
 860
 861         let mut err = self.path_does_not_live_long_enough(
 862             borrow_span,
 863             &format!("`{}`", name),
 864         );
 865
 866         if let Some(annotation) = self.annotate_argument_and_return_for_borrow(borrow) {
 867             let region_name = annotation.emit(self, &mut err);
 868
 869             err.span_label(
 870                 borrow_span,
 871                 format!("`{}` would have to be valid for `{}`...", name, region_name),
 872             );
 873
 874             if let Some(fn_hir_id) = self.infcx.tcx.hir().as_local_hir_id(self.mir_def_id) {
 875                 err.span_label(
 876                     drop_span,
 877                     format!(
 878                         "...but `{}` will be dropped here, when the function `{}` returns",
 879                         name,
 880                         self.infcx.tcx.hir().name(fn_hir_id),
 881                     ),
 882                 );
 883
 884                 err.note(
 885                     "functions cannot return a borrow to data owned within the function's scope, \
 886                      functions can only return borrows to data passed as arguments",
 887                 );
 888                 err.note(
 889                     "to learn more, visit <https://doc.rust-lang.org/book/ch04-02-\
 890                      references-and-borrowing.html#dangling-references>",
 891                 );
 892             } else {
 893                 err.span_label(
 894                     drop_span,
 895                     format!("...but `{}` dropped here while still borrowed", name),
 896                 );
 897             }
 898
 899             if let BorrowExplanation::MustBeValidFor { .. } = explanation {
 900             } else {
 901                 explanation.add_explanation_to_diagnostic(
 902                     self.infcx.tcx,
 903                     self.body,
 904                     &mut err,
 905                     "",
 906                     None,
 907                 );
 908             }
 909         } else {
 910             err.span_label(borrow_span, "borrowed value does not live long enough");
 911             err.span_label(
 912                 drop_span,
 913                 format!("`{}` dropped here while still borrowed", name),
 914             );
 915
 916             let within = if borrow_spans.for_generator() {
 917                 " by generator"
 918             } else {
 919                 ""
 920             };
 921
 922             borrow_spans.args_span_label(
 923                 &mut err,
 924                 format!("value captured here{}", within),
 925             );
 926
 927             explanation.add_explanation_to_diagnostic(
 928                 self.infcx.tcx, self.body, &mut err, "", None);
 929         }
 930
 931         err
 932     }
 933
 934     fn report_borrow_conflicts_with_destructor(
 935         &mut self,
 936         location: Location,
 937         borrow: &BorrowData<'tcx>,
 938         (place, drop_span): (&Place<'tcx>, Span),
 939         kind: Option<WriteKind>,
 940         dropped_ty: Ty<'tcx>,
 941     ) {
 942         debug!(
 943             "report_borrow_conflicts_with_destructor(\
 944              {:?}, {:?}, ({:?}, {:?}), {:?}\
 945              )",
 946             location, borrow, place, drop_span, kind,
 947         );
 948
 949         let borrow_spans = self.retrieve_borrow_spans(borrow);
 950         let borrow_span = borrow_spans.var_or_use();
 951
 952         let mut err = self.cannot_borrow_across_destructor(borrow_span);
 953
 954         let what_was_dropped = match self.describe_place(place.as_ref()) {
 955             Some(name) => format!("`{}`", name.as_str()),
 956             None => String::from("temporary value"),
 957         };
 958
 959         let label = match self.describe_place(borrow.borrowed_place.as_ref()) {
 960             Some(borrowed) => format!(
 961                 "here, drop of {D} needs exclusive access to `{B}`, \
 962                  because the type `{T}` implements the `Drop` trait",
 963                 D = what_was_dropped,
 964                 T = dropped_ty,
 965                 B = borrowed
 966             ),
 967             None => format!(
 968                 "here is drop of {D}; whose type `{T}` implements the `Drop` trait",
 969                 D = what_was_dropped,
 970                 T = dropped_ty
 971             ),
 972         };
 973         err.span_label(drop_span, label);
 974
 975         // Only give this note and suggestion if they could be relevant.
 976         let explanation =
 977             self.explain_why_borrow_contains_point(location, borrow, kind.map(|k| (k, place)));
 978         match explanation {
 979             BorrowExplanation::UsedLater { .. }
 980             | BorrowExplanation::UsedLaterWhenDropped { .. } => {
 981                 err.note("consider using a `let` binding to create a longer lived value");
 982             }
 983             _ => {}
 984         }
 985
 986         explanation.add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
 987
 988         err.buffer(&mut self.errors_buffer);
 989     }
 990
 991     fn report_thread_local_value_does_not_live_long_enough(
 992         &mut self,
 993         drop_span: Span,
 994         borrow_span: Span,
 995     ) -> DiagnosticBuilder<'cx> {
 996         debug!(
 997             "report_thread_local_value_does_not_live_long_enough(\
 998              {:?}, {:?}\
 999              )",
1000             drop_span, borrow_span
1001         );
1002
1003         let mut err = self.thread_local_value_does_not_live_long_enough(borrow_span);
1004
1005         err.span_label(
1006             borrow_span,
1007             "thread-local variables cannot be borrowed beyond the end of the function",
1008         );
1009         err.span_label(drop_span, "end of enclosing function is here");
1010
1011         err
1012     }
1013
1014     fn report_temporary_value_does_not_live_long_enough(
1015         &mut self,
1016         location: Location,
1017         borrow: &BorrowData<'tcx>,
1018         drop_span: Span,
1019         borrow_spans: UseSpans,
1020         proper_span: Span,
1021         explanation: BorrowExplanation,
1022     ) -> DiagnosticBuilder<'cx> {
1023         debug!(
1024             "report_temporary_value_does_not_live_long_enough(\
1025              {:?}, {:?}, {:?}, {:?}\
1026              )",
1027             location, borrow, drop_span, proper_span
1028         );
1029
1030         if let BorrowExplanation::MustBeValidFor {
1031             category,
1032             span,
1033             from_closure: false,
1034             ..
1035         } = explanation {
1036             if let Some(diag) = self.try_report_cannot_return_reference_to_local(
1037                 borrow,
1038                 proper_span,
1039                 span,
1040                 category,
1041                 None,
1042             ) {
1043                 return diag;
1044             }
1045         }
1046
1047         let mut err = self.temporary_value_borrowed_for_too_long(proper_span);
1048         err.span_label(
1049             proper_span,
1050             "creates a temporary which is freed while still in use",
1051         );
1052         err.span_label(
1053             drop_span,
1054             "temporary value is freed at the end of this statement",
1055         );
1056
1057         match explanation {
1058             BorrowExplanation::UsedLater(..)
1059             | BorrowExplanation::UsedLaterInLoop(..)
1060             | BorrowExplanation::UsedLaterWhenDropped { .. } => {
1061                 // Only give this note and suggestion if it could be relevant.
1062                 err.note("consider using a `let` binding to create a longer lived value");
1063             }
1064             _ => {}
1065         }
1066         explanation.add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
1067
1068         let within = if borrow_spans.for_generator() {
1069             " by generator"
1070         } else {
1071             ""
1072         };
1073
1074         borrow_spans.args_span_label(
1075             &mut err,
1076             format!("value captured here{}", within),
1077         );
1078
1079         err
1080     }
1081
1082     fn try_report_cannot_return_reference_to_local(
1083         &self,
1084         borrow: &BorrowData<'tcx>,
1085         borrow_span: Span,
1086         return_span: Span,
1087         category: ConstraintCategory,
1088         opt_place_desc: Option<&String>,
1089     ) -> Option<DiagnosticBuilder<'cx>> {
1090         let return_kind = match category {
1091             ConstraintCategory::Return => "return",
1092             ConstraintCategory::Yield => "yield",
1093             _ => return None,
1094         };
1095
1096         // FIXME use a better heuristic than Spans
1097         let reference_desc = if return_span == self.body.source_info(borrow.reserve_location).span {
1098             "reference to"
1099         } else {
1100             "value referencing"
1101         };
1102
1103         let (place_desc, note) = if let Some(place_desc) = opt_place_desc {
1104             let local_kind = match borrow.borrowed_place {
1105                 Place {
1106                     base: PlaceBase::Local(local),
1107                     projection: None,
1108                 } => {
1109                     match self.body.local_kind(local) {
1110                         LocalKind::ReturnPointer
1111                         | LocalKind::Temp => bug!("temporary or return pointer with a name"),
1112                         LocalKind::Var => "local variable ",
1113                         LocalKind::Arg
1114                         if !self.upvars.is_empty()
1115                             && local == Local::new(1) => {
1116                             "variable captured by `move` "
1117                         }
1118                         LocalKind::Arg => {
1119                             "function parameter "
1120                         }
1121                     }
1122                 }
1123                 _ => "local data ",
1124             };
1125             (
1126                 format!("{}`{}`", local_kind, place_desc),
1127                 format!("`{}` is borrowed here", place_desc),
1128             )
1129         } else {
1130             let root_place = self.prefixes(borrow.borrowed_place.as_ref(),
1131                                            PrefixSet::All)
1132                 .last()
1133                 .unwrap();
1134             let local = if let PlaceRef {
1135                 base: PlaceBase::Local(local),
1136                 projection: None,
1137             } = root_place {
1138                 local
1139             } else {
1140                 bug!("try_report_cannot_return_reference_to_local: not a local")
1141             };
1142             match self.body.local_kind(*local) {
1143                 LocalKind::ReturnPointer | LocalKind::Temp => {
1144                     (
1145                         "temporary value".to_string(),
1146                         "temporary value created here".to_string(),
1147                     )
1148                 }
1149                 LocalKind::Arg => {
1150                     (
1151                         "function parameter".to_string(),
1152                         "function parameter borrowed here".to_string(),
1153                     )
1154                 },
1155                 LocalKind::Var => bug!("local variable without a name"),
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(string) => format!("move {}", string),
1192             Err(_) => "move |<args>| <body>".to_string()
1193         };
1194
1195         err.span_suggestion(
1196             args_span,
1197             &format!("to force the closure to take ownership of {} (and any \
1198                       other referenced variables), use the `move` keyword",
1199                       captured_var),
1200             suggestion,
1201             Applicability::MachineApplicable,
1202         );
1203
1204         match category {
1205             ConstraintCategory::Return => {
1206                 err.span_note(constraint_span, "closure is returned here");
1207             }
1208             ConstraintCategory::CallArgument => {
1209                 fr_name.highlight_region_name(&mut err);
1210                 err.span_note(
1211                     constraint_span,
1212                     &format!("function requires argument type to outlive `{}`", fr_name),
1213                 );
1214             }
1215             _ => bug!("report_escaping_closure_capture called with unexpected constraint \
1216                        category: `{:?}`", category),
1217         }
1218         err
1219     }
1220
1221     fn report_escaping_data(
1222         &mut self,
1223         borrow_span: Span,
1224         name: &Option<String>,
1225         upvar_span: Span,
1226         upvar_name: &str,
1227         escape_span: Span,
1228     ) -> DiagnosticBuilder<'cx> {
1229         let tcx = self.infcx.tcx;
1230
1231         let escapes_from = if tcx.is_closure(self.mir_def_id) {
1232             let tables = tcx.typeck_tables_of(self.mir_def_id);
1233             let mir_hir_id = tcx.hir().def_index_to_hir_id(self.mir_def_id.index);
1234             match tables.node_type(mir_hir_id).sty {
1235                 ty::Closure(..) => "closure",
1236                 ty::Generator(..) => "generator",
1237                 _ => bug!("Closure body doesn't have a closure or generator type"),
1238             }
1239         } else {
1240             "function"
1241         };
1242
1243         let mut err = borrowck_errors::borrowed_data_escapes_closure(
1244             tcx,
1245             escape_span,
1246             escapes_from,
1247         );
1248
1249         err.span_label(
1250             upvar_span,
1251             format!(
1252                 "`{}` is declared here, outside of the {} body",
1253                 upvar_name, escapes_from
1254             ),
1255         );
1256
1257         err.span_label(
1258             borrow_span,
1259             format!(
1260                 "borrow is only valid in the {} body",
1261                 escapes_from
1262             ),
1263         );
1264
1265         if let Some(name) = name {
1266             err.span_label(
1267                 escape_span,
1268                 format!("reference to `{}` escapes the {} body here", name, escapes_from),
1269             );
1270         } else {
1271             err.span_label(
1272                 escape_span,
1273                 format!("reference escapes the {} body here", escapes_from),
1274             );
1275         }
1276
1277         err
1278     }
1279
1280     fn get_moved_indexes(&mut self, location: Location, mpi: MovePathIndex) -> Vec<MoveSite> {
1281         let body = self.body;
1282
1283         let mut stack = Vec::new();
1284         stack.extend(body.predecessor_locations(location).map(|predecessor| {
1285             let is_back_edge = location.dominates(predecessor, &self.dominators);
1286             (predecessor, is_back_edge)
1287         }));
1288
1289         let mut visited = FxHashSet::default();
1290         let mut result = vec![];
1291
1292         'dfs: while let Some((location, is_back_edge)) = stack.pop() {
1293             debug!(
1294                 "report_use_of_moved_or_uninitialized: (current_location={:?}, back_edge={})",
1295                 location, is_back_edge
1296             );
1297
1298             if !visited.insert(location) {
1299                 continue;
1300             }
1301
1302             // check for moves
1303             let stmt_kind = body[location.block]
1304                 .statements
1305                 .get(location.statement_index)
1306                 .map(|s| &s.kind);
1307             if let Some(StatementKind::StorageDead(..)) = stmt_kind {
1308                 // this analysis only tries to find moves explicitly
1309                 // written by the user, so we ignore the move-outs
1310                 // created by `StorageDead` and at the beginning
1311                 // of a function.
1312             } else {
1313                 // If we are found a use of a.b.c which was in error, then we want to look for
1314                 // moves not only of a.b.c but also a.b and a.
1315                 //
1316                 // Note that the moves data already includes "parent" paths, so we don't have to
1317                 // worry about the other case: that is, if there is a move of a.b.c, it is already
1318                 // marked as a move of a.b and a as well, so we will generate the correct errors
1319                 // there.
1320                 let mut mpis = vec![mpi];
1321                 let move_paths = &self.move_data.move_paths;
1322                 mpis.extend(move_paths[mpi].parents(move_paths));
1323
1324                 for moi in &self.move_data.loc_map[location] {
1325                     debug!("report_use_of_moved_or_uninitialized: moi={:?}", moi);
1326                     if mpis.contains(&self.move_data.moves[*moi].path) {
1327                         debug!("report_use_of_moved_or_uninitialized: found");
1328                         result.push(MoveSite {
1329                             moi: *moi,
1330                             traversed_back_edge: is_back_edge,
1331                         });
1332
1333                         // Strictly speaking, we could continue our DFS here. There may be
1334                         // other moves that can reach the point of error. But it is kind of
1335                         // confusing to highlight them.
1336                         //
1337                         // Example:
1338                         //
1339                         // ```
1340                         // let a = vec![];
1341                         // let b = a;
1342                         // let c = a;
1343                         // drop(a); // <-- current point of error
1344                         // ```
1345                         //
1346                         // Because we stop the DFS here, we only highlight `let c = a`,
1347                         // and not `let b = a`. We will of course also report an error at
1348                         // `let c = a` which highlights `let b = a` as the move.
1349                         continue 'dfs;
1350                     }
1351                 }
1352             }
1353
1354             // check for inits
1355             let mut any_match = false;
1356             drop_flag_effects::for_location_inits(
1357                 self.infcx.tcx,
1358                 self.body,
1359                 self.move_data,
1360                 location,
1361                 |m| {
1362                     if m == mpi {
1363                         any_match = true;
1364                     }
1365                 },
1366             );
1367             if any_match {
1368                 continue 'dfs;
1369             }
1370
1371             stack.extend(body.predecessor_locations(location).map(|predecessor| {
1372                 let back_edge = location.dominates(predecessor, &self.dominators);
1373                 (predecessor, is_back_edge || back_edge)
1374             }));
1375         }
1376
1377         result
1378     }
1379
1380     pub(super) fn report_illegal_mutation_of_borrowed(
1381         &mut self,
1382         location: Location,
1383         (place, span): (&Place<'tcx>, Span),
1384         loan: &BorrowData<'tcx>,
1385     ) {
1386         let loan_spans = self.retrieve_borrow_spans(loan);
1387         let loan_span = loan_spans.args_or_use();
1388
1389         if loan.kind == BorrowKind::Shallow {
1390             let mut err = self.cannot_mutate_in_match_guard(
1391                 span,
1392                 loan_span,
1393                 &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
1394                 "assign",
1395             );
1396             loan_spans.var_span_label(
1397                 &mut err,
1398                 format!("borrow occurs due to use{}", loan_spans.describe()),
1399             );
1400
1401             err.buffer(&mut self.errors_buffer);
1402
1403             return;
1404         }
1405
1406         let mut err = self.cannot_assign_to_borrowed(
1407             span,
1408             loan_span,
1409             &self.describe_place(place.as_ref()).unwrap_or_else(|| "_".to_owned()),
1410         );
1411
1412         loan_spans.var_span_label(
1413             &mut err,
1414             format!("borrow occurs due to use{}", loan_spans.describe()),
1415         );
1416
1417         self.explain_why_borrow_contains_point(location, loan, None)
1418             .add_explanation_to_diagnostic(self.infcx.tcx, self.body, &mut err, "", None);
1419
1420         err.buffer(&mut self.errors_buffer);
1421     }
1422
1423     /// Reports an illegal reassignment; for example, an assignment to
1424     /// (part of) a non-`mut` local that occurs potentially after that
1425     /// local has already been initialized. `place` is the path being
1426     /// assigned; `err_place` is a place providing a reason why
1427     /// `place` is not mutable (e.g., the non-`mut` local `x` in an
1428     /// assignment to `x.f`).
1429     pub(super) fn report_illegal_reassignment(
1430         &mut self,
1431         _location: Location,
1432         (place, span): (&Place<'tcx>, Span),
1433         assigned_span: Span,
1434         err_place: &Place<'tcx>,
1435     ) {
1436         let (from_arg, local_decl) = if let Place {
1437             base: PlaceBase::Local(local),
1438             projection: None,
1439         } = *err_place {
1440             if let LocalKind::Arg = self.body.local_kind(local) {
1441                 (true, Some(&self.body.local_decls[local]))
1442             } else {
1443                 (false, Some(&self.body.local_decls[local]))
1444             }
1445         } else {
1446             (false, None)
1447         };
1448
1449         // If root local is initialized immediately (everything apart from let
1450         // PATTERN;) then make the error refer to that local, rather than the
1451         // place being assigned later.
1452         let (place_description, assigned_span) = match local_decl {
1453             Some(LocalDecl {
1454                 is_user_variable: Some(ClearCrossCrate::Clear),
1455                 ..
1456             })
1457             | Some(LocalDecl {
1458                 is_user_variable:
1459                     Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm {
1460                         opt_match_place: None,
1461                         ..
1462                     }))),
1463                 ..
1464             })
1465             | Some(LocalDecl {
1466                 is_user_variable: None,
1467                 ..
1468             })
1469             | None => (self.describe_place(place.as_ref()), assigned_span),
1470             Some(decl) => (self.describe_place(err_place.as_ref()), decl.source_info.span),
1471         };
1472
1473         let mut err = self.cannot_reassign_immutable(
1474             span,
1475             place_description.as_ref().map(AsRef::as_ref).unwrap_or("_"),
1476             from_arg,
1477         );
1478         let msg = if from_arg {
1479             "cannot assign to immutable argument"
1480         } else {
1481             "cannot assign twice to immutable variable"
1482         };
1483         if span != assigned_span {
1484             if !from_arg {
1485                 let value_msg = match place_description {
1486                     Some(name) => format!("`{}`", name),
1487                     None => "value".to_owned(),
1488                 };
1489                 err.span_label(assigned_span, format!("first assignment to {}", value_msg));
1490             }
1491         }
1492         if let Some(decl) = local_decl {
1493             if let Some(name) = decl.name {
1494                 if decl.can_be_made_mutable() {
1495                     err.span_suggestion(
1496                         decl.source_info.span,
1497                         "make this binding mutable",
1498                         format!("mut {}", name),
1499                         Applicability::MachineApplicable,
1500                     );
1501                 }
1502             }
1503         }
1504         err.span_label(span, msg);
1505         err.buffer(&mut self.errors_buffer);
1506     }
1507
1508     fn classify_drop_access_kind(&self, place: PlaceRef<'cx, 'tcx>) -> StorageDeadOrDrop<'tcx> {
1509         let tcx = self.infcx.tcx;
1510         match place.projection {
1511             None => {
1512                 StorageDeadOrDrop::LocalStorageDead
1513             }
1514             Some(box Projection { ref base, ref elem }) => {
1515                 let base_access = self.classify_drop_access_kind(PlaceRef {
1516                     base: place.base,
1517                     projection: base,
1518                 });
1519                 match elem {
1520                     ProjectionElem::Deref => match base_access {
1521                         StorageDeadOrDrop::LocalStorageDead
1522                         | StorageDeadOrDrop::BoxedStorageDead => {
1523                             assert!(
1524                                 Place::ty_from(&place.base, base, self.body, tcx).ty.is_box(),
1525                                 "Drop of value behind a reference or raw pointer"
1526                             );
1527                             StorageDeadOrDrop::BoxedStorageDead
1528                         }
1529                         StorageDeadOrDrop::Destructor(_) => base_access,
1530                     },
1531                     ProjectionElem::Field(..) | ProjectionElem::Downcast(..) => {
1532                         let base_ty = Place::ty_from(&place.base, base, self.body, tcx).ty;
1533                         match base_ty.sty {
1534                             ty::Adt(def, _) if def.has_dtor(tcx) => {
1535                                 // Report the outermost adt with a destructor
1536                                 match base_access {
1537                                     StorageDeadOrDrop::Destructor(_) => base_access,
1538                                     StorageDeadOrDrop::LocalStorageDead
1539                                     | StorageDeadOrDrop::BoxedStorageDead => {
1540                                         StorageDeadOrDrop::Destructor(base_ty)
1541                                     }
1542                                 }
1543                             }
1544                             _ => base_access,
1545                         }
1546                     }
1547
1548                     ProjectionElem::ConstantIndex { .. }
1549                     | ProjectionElem::Subslice { .. }
1550                     | ProjectionElem::Index(_) => base_access,
1551                 }
1552             }
1553         }
1554     }
1555
1556     /// Annotate argument and return type of function and closure with (synthesized) lifetime for
1557     /// borrow of local value that does not live long enough.
1558     fn annotate_argument_and_return_for_borrow(
1559         &self,
1560         borrow: &BorrowData<'tcx>,
1561     ) -> Option<AnnotatedBorrowFnSignature<'tcx>> {
1562         // Define a fallback for when we can't match a closure.
1563         let fallback = || {
1564             let is_closure = self.infcx.tcx.is_closure(self.mir_def_id);
1565             if is_closure {
1566                 None
1567             } else {
1568                 let ty = self.infcx.tcx.type_of(self.mir_def_id);
1569                 match ty.sty {
1570                     ty::FnDef(_, _) | ty::FnPtr(_) => self.annotate_fn_sig(
1571                         self.mir_def_id,
1572                         self.infcx.tcx.fn_sig(self.mir_def_id),
1573                     ),
1574                     _ => None,
1575                 }
1576             }
1577         };
1578
1579         // In order to determine whether we need to annotate, we need to check whether the reserve
1580         // place was an assignment into a temporary.
1581         //
1582         // If it was, we check whether or not that temporary is eventually assigned into the return
1583         // place. If it was, we can add annotations about the function's return type and arguments
1584         // and it'll make sense.
1585         let location = borrow.reserve_location;
1586         debug!(
1587             "annotate_argument_and_return_for_borrow: location={:?}",
1588             location
1589         );
1590         if let Some(&Statement { kind: StatementKind::Assign(ref reservation, _), ..})
1591              = &self.body[location.block].statements.get(location.statement_index)
1592         {
1593             debug!(
1594                 "annotate_argument_and_return_for_borrow: reservation={:?}",
1595                 reservation
1596             );
1597             // Check that the initial assignment of the reserve location is into a temporary.
1598             let mut target = *match reservation {
1599                 Place {
1600                     base: PlaceBase::Local(local),
1601                     projection: None,
1602                 } if self.body.local_kind(*local) == LocalKind::Temp => local,
1603                 _ => return None,
1604             };
1605
1606             // Next, look through the rest of the block, checking if we are assigning the
1607             // `target` (that is, the place that contains our borrow) to anything.
1608             let mut annotated_closure = None;
1609             for stmt in &self.body[location.block].statements[location.statement_index + 1..] {
1610                 debug!(
1611                     "annotate_argument_and_return_for_borrow: target={:?} stmt={:?}",
1612                     target, stmt
1613                 );
1614                 if let StatementKind::Assign(
1615                     Place {
1616                         base: PlaceBase::Local(assigned_to),
1617                         projection: None,
1618                     },
1619                     box rvalue
1620                 ) = &stmt.kind {
1621                     debug!(
1622                         "annotate_argument_and_return_for_borrow: assigned_to={:?} \
1623                          rvalue={:?}",
1624                         assigned_to, rvalue
1625                     );
1626                     // Check if our `target` was captured by a closure.
1627                     if let Rvalue::Aggregate(
1628                         box AggregateKind::Closure(def_id, substs),
1629                         operands,
1630                     ) = rvalue
1631                     {
1632                         for operand in operands {
1633                             let assigned_from = match operand {
1634                                 Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1635                                     assigned_from
1636                                 }
1637                                 _ => continue,
1638                             };
1639                             debug!(
1640                                 "annotate_argument_and_return_for_borrow: assigned_from={:?}",
1641                                 assigned_from
1642                             );
1643
1644                             // Find the local from the operand.
1645                             let assigned_from_local = match assigned_from.local_or_deref_local() {
1646                                 Some(local) => local,
1647                                 None => continue,
1648                             };
1649
1650                             if assigned_from_local != target {
1651                                 continue;
1652                             }
1653
1654                             // If a closure captured our `target` and then assigned
1655                             // into a place then we should annotate the closure in
1656                             // case it ends up being assigned into the return place.
1657                             annotated_closure = self.annotate_fn_sig(
1658                                 *def_id,
1659                                 self.infcx.closure_sig(*def_id, *substs),
1660                             );
1661                             debug!(
1662                                 "annotate_argument_and_return_for_borrow: \
1663                                  annotated_closure={:?} assigned_from_local={:?} \
1664                                  assigned_to={:?}",
1665                                 annotated_closure, assigned_from_local, assigned_to
1666                             );
1667
1668                             if *assigned_to == mir::RETURN_PLACE {
1669                                 // If it was assigned directly into the return place, then
1670                                 // return now.
1671                                 return annotated_closure;
1672                             } else {
1673                                 // Otherwise, update the target.
1674                                 target = *assigned_to;
1675                             }
1676                         }
1677
1678                         // If none of our closure's operands matched, then skip to the next
1679                         // statement.
1680                         continue;
1681                     }
1682
1683                     // Otherwise, look at other types of assignment.
1684                     let assigned_from = match rvalue {
1685                         Rvalue::Ref(_, _, assigned_from) => assigned_from,
1686                         Rvalue::Use(operand) => match operand {
1687                             Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1688                                 assigned_from
1689                             }
1690                             _ => continue,
1691                         },
1692                         _ => continue,
1693                     };
1694                     debug!(
1695                         "annotate_argument_and_return_for_borrow: \
1696                          assigned_from={:?}",
1697                         assigned_from,
1698                     );
1699
1700                     // Find the local from the rvalue.
1701                     let assigned_from_local = match assigned_from.local_or_deref_local() {
1702                         Some(local) => local,
1703                         None => continue,
1704                     };
1705                     debug!(
1706                         "annotate_argument_and_return_for_borrow: \
1707                          assigned_from_local={:?}",
1708                         assigned_from_local,
1709                     );
1710
1711                     // Check if our local matches the target - if so, we've assigned our
1712                     // borrow to a new place.
1713                     if assigned_from_local != target {
1714                         continue;
1715                     }
1716
1717                     // If we assigned our `target` into a new place, then we should
1718                     // check if it was the return place.
1719                     debug!(
1720                         "annotate_argument_and_return_for_borrow: \
1721                          assigned_from_local={:?} assigned_to={:?}",
1722                         assigned_from_local, assigned_to
1723                     );
1724                     if *assigned_to == mir::RETURN_PLACE {
1725                         // If it was then return the annotated closure if there was one,
1726                         // else, annotate this function.
1727                         return annotated_closure.or_else(fallback);
1728                     }
1729
1730                     // If we didn't assign into the return place, then we just update
1731                     // the target.
1732                     target = *assigned_to;
1733                 }
1734             }
1735
1736             // Check the terminator if we didn't find anything in the statements.
1737             let terminator = &self.body[location.block].terminator();
1738             debug!(
1739                 "annotate_argument_and_return_for_borrow: target={:?} terminator={:?}",
1740                 target, terminator
1741             );
1742             if let TerminatorKind::Call {
1743                 destination: Some((Place {
1744                     base: PlaceBase::Local(assigned_to),
1745                     projection: None,
1746                 }, _)),
1747                 args,
1748                 ..
1749             } = &terminator.kind
1750             {
1751                 debug!(
1752                     "annotate_argument_and_return_for_borrow: assigned_to={:?} args={:?}",
1753                     assigned_to, args
1754                 );
1755                 for operand in args {
1756                     let assigned_from = match operand {
1757                         Operand::Copy(assigned_from) | Operand::Move(assigned_from) => {
1758                             assigned_from
1759                         }
1760                         _ => continue,
1761                     };
1762                     debug!(
1763                         "annotate_argument_and_return_for_borrow: assigned_from={:?}",
1764                         assigned_from,
1765                     );
1766
1767                     if let Some(assigned_from_local) = assigned_from.local_or_deref_local() {
1768                         debug!(
1769                             "annotate_argument_and_return_for_borrow: assigned_from_local={:?}",
1770                             assigned_from_local,
1771                         );
1772
1773                         if *assigned_to == mir::RETURN_PLACE && assigned_from_local == target {
1774                             return annotated_closure.or_else(fallback);
1775                         }
1776                     }
1777                 }
1778             }
1779         }
1780
1781         // If we haven't found an assignment into the return place, then we need not add
1782         // any annotations.
1783         debug!("annotate_argument_and_return_for_borrow: none found");
1784         None
1785     }
1786
1787     /// Annotate the first argument and return type of a function signature if they are
1788     /// references.
1789     fn annotate_fn_sig(
1790         &self,
1791         did: DefId,
1792         sig: ty::PolyFnSig<'tcx>,
1793     ) -> Option<AnnotatedBorrowFnSignature<'tcx>> {
1794         debug!("annotate_fn_sig: did={:?} sig={:?}", did, sig);
1795         let is_closure = self.infcx.tcx.is_closure(did);
1796         let fn_hir_id = self.infcx.tcx.hir().as_local_hir_id(did)?;
1797         let fn_decl = self.infcx.tcx.hir().fn_decl_by_hir_id(fn_hir_id)?;
1798
1799         // We need to work out which arguments to highlight. We do this by looking
1800         // at the return type, where there are three cases:
1801         //
1802         // 1. If there are named arguments, then we should highlight the return type and
1803         //    highlight any of the arguments that are also references with that lifetime.
1804         //    If there are no arguments that have the same lifetime as the return type,
1805         //    then don't highlight anything.
1806         // 2. The return type is a reference with an anonymous lifetime. If this is
1807         //    the case, then we can take advantage of (and teach) the lifetime elision
1808         //    rules.
1809         //
1810         //    We know that an error is being reported. So the arguments and return type
1811         //    must satisfy the elision rules. Therefore, if there is a single argument
1812         //    then that means the return type and first (and only) argument have the same
1813         //    lifetime and the borrow isn't meeting that, we can highlight the argument
1814         //    and return type.
1815         //
1816         //    If there are multiple arguments then the first argument must be self (else
1817         //    it would not satisfy the elision rules), so we can highlight self and the
1818         //    return type.
1819         // 3. The return type is not a reference. In this case, we don't highlight
1820         //    anything.
1821         let return_ty = sig.output();
1822         match return_ty.skip_binder().sty {
1823             ty::Ref(return_region, _, _) if return_region.has_name() && !is_closure => {
1824                 // This is case 1 from above, return type is a named reference so we need to
1825                 // search for relevant arguments.
1826                 let mut arguments = Vec::new();
1827                 for (index, argument) in sig.inputs().skip_binder().iter().enumerate() {
1828                     if let ty::Ref(argument_region, _, _) = argument.sty {
1829                         if argument_region == return_region {
1830                             // Need to use the `rustc::ty` types to compare against the
1831                             // `return_region`. Then use the `rustc::hir` type to get only
1832                             // the lifetime span.
1833                             if let hir::TyKind::Rptr(lifetime, _) = &fn_decl.inputs[index].node {
1834                                 // With access to the lifetime, we can get
1835                                 // the span of it.
1836                                 arguments.push((*argument, lifetime.span));
1837                             } else {
1838                                 bug!("ty type is a ref but hir type is not");
1839                             }
1840                         }
1841                     }
1842                 }
1843
1844                 // We need to have arguments. This shouldn't happen, but it's worth checking.
1845                 if arguments.is_empty() {
1846                     return None;
1847                 }
1848
1849                 // We use a mix of the HIR and the Ty types to get information
1850                 // as the HIR doesn't have full types for closure arguments.
1851                 let return_ty = *sig.output().skip_binder();
1852                 let mut return_span = fn_decl.output.span();
1853                 if let hir::FunctionRetTy::Return(ty) = &fn_decl.output {
1854                     if let hir::TyKind::Rptr(lifetime, _) = ty.node {
1855                         return_span = lifetime.span;
1856                     }
1857                 }
1858
1859                 Some(AnnotatedBorrowFnSignature::NamedFunction {
1860                     arguments,
1861                     return_ty,
1862                     return_span,
1863                 })
1864             }
1865             ty::Ref(_, _, _) if is_closure => {
1866                 // This is case 2 from above but only for closures, return type is anonymous
1867                 // reference so we select
1868                 // the first argument.
1869                 let argument_span = fn_decl.inputs.first()?.span;
1870                 let argument_ty = sig.inputs().skip_binder().first()?;
1871
1872                 // Closure arguments are wrapped in a tuple, so we need to get the first
1873                 // from that.
1874                 if let ty::Tuple(elems) = argument_ty.sty {
1875                     let argument_ty = elems.first()?.expect_ty();
1876                     if let ty::Ref(_, _, _) = argument_ty.sty {
1877                         return Some(AnnotatedBorrowFnSignature::Closure {
1878                             argument_ty,
1879                             argument_span,
1880                         });
1881                     }
1882                 }
1883
1884                 None
1885             }
1886             ty::Ref(_, _, _) => {
1887                 // This is also case 2 from above but for functions, return type is still an
1888                 // anonymous reference so we select the first argument.
1889                 let argument_span = fn_decl.inputs.first()?.span;
1890                 let argument_ty = sig.inputs().skip_binder().first()?;
1891
1892                 let return_span = fn_decl.output.span();
1893                 let return_ty = *sig.output().skip_binder();
1894
1895                 // We expect the first argument to be a reference.
1896                 match argument_ty.sty {
1897                     ty::Ref(_, _, _) => {}
1898                     _ => return None,
1899                 }
1900
1901                 Some(AnnotatedBorrowFnSignature::AnonymousFunction {
1902                     argument_ty,
1903                     argument_span,
1904                     return_ty,
1905                     return_span,
1906                 })
1907             }
1908             _ => {
1909                 // This is case 3 from above, return type is not a reference so don't highlight
1910                 // anything.
1911                 None
1912             }
1913         }
1914     }
1915 }
1916
1917 #[derive(Debug)]
1918 enum AnnotatedBorrowFnSignature<'tcx> {
1919     NamedFunction {
1920         arguments: Vec<(Ty<'tcx>, Span)>,
1921         return_ty: Ty<'tcx>,
1922         return_span: Span,
1923     },
1924     AnonymousFunction {
1925         argument_ty: Ty<'tcx>,
1926         argument_span: Span,
1927         return_ty: Ty<'tcx>,
1928         return_span: Span,
1929     },
1930     Closure {
1931         argument_ty: Ty<'tcx>,
1932         argument_span: Span,
1933     },
1934 }
1935
1936 impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
1937     /// Annotate the provided diagnostic with information about borrow from the fn signature that
1938     /// helps explain.
1939     pub(super) fn emit(
1940         &self,
1941         cx: &mut MirBorrowckCtxt<'_, 'tcx>,
1942         diag: &mut DiagnosticBuilder<'_>,
1943     ) -> String {
1944         match self {
1945             AnnotatedBorrowFnSignature::Closure {
1946                 argument_ty,
1947                 argument_span,
1948             } => {
1949                 diag.span_label(
1950                     *argument_span,
1951                     format!("has type `{}`", cx.get_name_for_ty(argument_ty, 0)),
1952                 );
1953
1954                 cx.get_region_name_for_ty(argument_ty, 0)
1955             }
1956             AnnotatedBorrowFnSignature::AnonymousFunction {
1957                 argument_ty,
1958                 argument_span,
1959                 return_ty,
1960                 return_span,
1961             } => {
1962                 let argument_ty_name = cx.get_name_for_ty(argument_ty, 0);
1963                 diag.span_label(*argument_span, format!("has type `{}`", argument_ty_name));
1964
1965                 let return_ty_name = cx.get_name_for_ty(return_ty, 0);
1966                 let types_equal = return_ty_name == argument_ty_name;
1967                 diag.span_label(
1968                     *return_span,
1969                     format!(
1970                         "{}has type `{}`",
1971                         if types_equal { "also " } else { "" },
1972                         return_ty_name,
1973                     ),
1974                 );
1975
1976                 diag.note(
1977                     "argument and return type have the same lifetime due to lifetime elision rules",
1978                 );
1979                 diag.note(
1980                     "to learn more, visit <https://doc.rust-lang.org/book/ch10-03-\
1981                      lifetime-syntax.html#lifetime-elision>",
1982                 );
1983
1984                 cx.get_region_name_for_ty(return_ty, 0)
1985             }
1986             AnnotatedBorrowFnSignature::NamedFunction {
1987                 arguments,
1988                 return_ty,
1989                 return_span,
1990             } => {
1991                 // Region of return type and arguments checked to be the same earlier.
1992                 let region_name = cx.get_region_name_for_ty(return_ty, 0);
1993                 for (_, argument_span) in arguments {
1994                     diag.span_label(*argument_span, format!("has lifetime `{}`", region_name));
1995                 }
1996
1997                 diag.span_label(
1998                     *return_span,
1999                     format!("also has lifetime `{}`", region_name,),
2000                 );
2001
2002                 diag.help(&format!(
2003                     "use data from the highlighted arguments which match the `{}` lifetime of \
2004                      the return type",
2005                     region_name,
2006                 ));
2007
2008                 region_name
2009             }
2010         }
2011     }
2012 }