src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs

   1 use std::collections::VecDeque;
   2
   3 use crate::borrow_check::borrow_set::BorrowData;
   4 use crate::borrow_check::error_reporting::UseSpans;
   5 use crate::borrow_check::nll::region_infer::{Cause, RegionName};
   6 use crate::borrow_check::nll::ConstraintDescription;
   7 use crate::borrow_check::{MirBorrowckCtxt, WriteKind};
   8 use rustc::mir::{
   9     CastKind, ConstraintCategory, FakeReadCause, Local, Location, Mir, Operand, Place, PlaceBase,
  10     Projection, ProjectionElem, Rvalue, Statement, StatementKind, TerminatorKind,
  11 };
  12 use rustc::ty::{self, TyCtxt};
  13 use rustc::ty::adjustment::{PointerCast};
  14 use rustc_data_structures::fx::FxHashSet;
  15 use rustc_errors::DiagnosticBuilder;
  16 use syntax_pos::Span;
  17
  18 mod find_use;
  19
  20 pub(in crate::borrow_check) enum BorrowExplanation {
  21     UsedLater(LaterUseKind, Span),
  22     UsedLaterInLoop(LaterUseKind, Span),
  23     UsedLaterWhenDropped {
  24         drop_loc: Location,
  25         dropped_local: Local,
  26         should_note_order: bool,
  27     },
  28     MustBeValidFor {
  29         category: ConstraintCategory,
  30         from_closure: bool,
  31         span: Span,
  32         region_name: RegionName,
  33         opt_place_desc: Option<String>,
  34     },
  35     Unexplained,
  36 }
  37
  38 #[derive(Clone, Copy)]
  39 pub(in crate::borrow_check) enum LaterUseKind {
  40     TraitCapture,
  41     ClosureCapture,
  42     Call,
  43     FakeLetRead,
  44     Other,
  45 }
  46
  47 impl BorrowExplanation {
  48     pub(in crate::borrow_check) fn is_explained(&self) -> bool {
  49         match self {
  50             BorrowExplanation::Unexplained => false,
  51             _ => true,
  52         }
  53     }
  54     pub(in crate::borrow_check) fn add_explanation_to_diagnostic<'cx, 'gcx, 'tcx>(
  55         &self,
  56         tcx: TyCtxt<'cx, 'gcx, 'tcx>,
  57         mir: &Mir<'tcx>,
  58         err: &mut DiagnosticBuilder<'_>,
  59         borrow_desc: &str,
  60         borrow_span: Option<Span>,
  61     ) {
  62         match *self {
  63             BorrowExplanation::UsedLater(later_use_kind, var_or_use_span) => {
  64                 let message = match later_use_kind {
  65                     LaterUseKind::TraitCapture => "captured here by trait object",
  66                     LaterUseKind::ClosureCapture => "captured here by closure",
  67                     LaterUseKind::Call => "used by call",
  68                     LaterUseKind::FakeLetRead => "stored here",
  69                     LaterUseKind::Other => "used here",
  70                 };
  71                 if !borrow_span.map(|sp| sp.overlaps(var_or_use_span)).unwrap_or(false) {
  72                     err.span_label(
  73                         var_or_use_span,
  74                         format!("{}borrow later {}", borrow_desc, message),
  75                     );
  76                 }
  77             }
  78             BorrowExplanation::UsedLaterInLoop(later_use_kind, var_or_use_span) => {
  79                 let message = match later_use_kind {
  80                     LaterUseKind::TraitCapture => {
  81                         "borrow captured here by trait object, in later iteration of loop"
  82                     }
  83                     LaterUseKind::ClosureCapture => {
  84                         "borrow captured here by closure, in later iteration of loop"
  85                     }
  86                     LaterUseKind::Call => "borrow used by call, in later iteration of loop",
  87                     LaterUseKind::FakeLetRead => "borrow later stored here",
  88                     LaterUseKind::Other => "borrow used here, in later iteration of loop",
  89                 };
  90                 err.span_label(var_or_use_span, format!("{}{}", borrow_desc, message));
  91             }
  92             BorrowExplanation::UsedLaterWhenDropped {
  93                 drop_loc,
  94                 dropped_local,
  95                 should_note_order,
  96             } => {
  97                 let local_decl = &mir.local_decls[dropped_local];
  98                 let (dtor_desc, type_desc) = match local_decl.ty.sty {
  99                     // If type is an ADT that implements Drop, then
 100                     // simplify output by reporting just the ADT name.
 101                     ty::Adt(adt, _substs) if adt.has_dtor(tcx) && !adt.is_box() => (
 102                         "`Drop` code",
 103                         format!("type `{}`", tcx.def_path_str(adt.did)),
 104                     ),
 105
 106                     // Otherwise, just report the whole type (and use
 107                     // the intentionally fuzzy phrase "destructor")
 108                     ty::Closure(..) => ("destructor", "closure".to_owned()),
 109                     ty::Generator(..) => ("destructor", "generator".to_owned()),
 110
 111                     _ => ("destructor", format!("type `{}`", local_decl.ty)),
 112                 };
 113
 114                 match local_decl.name {
 115                     Some(local_name) if !local_decl.from_compiler_desugaring() => {
 116                         let message = format!(
 117                             "{B}borrow might be used here, when `{LOC}` is dropped \
 118                              and runs the {DTOR} for {TYPE}",
 119                             B = borrow_desc,
 120                             LOC = local_name,
 121                             TYPE = type_desc,
 122                             DTOR = dtor_desc
 123                         );
 124                         err.span_label(mir.source_info(drop_loc).span, message);
 125
 126                         if should_note_order {
 127                             err.note(
 128                                 "values in a scope are dropped \
 129                                  in the opposite order they are defined",
 130                             );
 131                         }
 132                     }
 133                     _ => {
 134                         err.span_label(
 135                             local_decl.source_info.span,
 136                             format!(
 137                                 "a temporary with access to the {B}borrow \
 138                                  is created here ...",
 139                                 B = borrow_desc
 140                             ),
 141                         );
 142                         let message = format!(
 143                             "... and the {B}borrow might be used here, \
 144                              when that temporary is dropped \
 145                              and runs the {DTOR} for {TYPE}",
 146                             B = borrow_desc,
 147                             TYPE = type_desc,
 148                             DTOR = dtor_desc
 149                         );
 150                         err.span_label(mir.source_info(drop_loc).span, message);
 151
 152                         if let Some(info) = &local_decl.is_block_tail {
 153                             // FIXME: use span_suggestion instead, highlighting the
 154                             // whole block tail expression.
 155                             let msg = if info.tail_result_is_ignored {
 156                                 "The temporary is part of an expression at the end of a block. \
 157                                  Consider adding semicolon after the expression so its temporaries \
 158                                  are dropped sooner, before the local variables declared by the \
 159                                  block are dropped."
 160                             } else {
 161                                 "The temporary is part of an expression at the end of a block. \
 162                                  Consider forcing this temporary to be dropped sooner, before \
 163                                  the block's local variables are dropped. \
 164                                  For example, you could save the expression's value in a new \
 165                                  local variable `x` and then make `x` be the expression \
 166                                  at the end of the block."
 167                             };
 168
 169                             err.note(msg);
 170                         }
 171                     }
 172                 }
 173             }
 174             BorrowExplanation::MustBeValidFor {
 175                 category,
 176                 span,
 177                 ref region_name,
 178                 ref opt_place_desc,
 179                 from_closure: _,
 180             } => {
 181                 region_name.highlight_region_name(err);
 182
 183                 if let Some(desc) = opt_place_desc {
 184                     err.span_label(
 185                         span,
 186                         format!(
 187                             "{}requires that `{}` is borrowed for `{}`",
 188                             category.description(),
 189                             desc,
 190                             region_name,
 191                         ),
 192                     );
 193                 } else {
 194                     err.span_label(
 195                         span,
 196                         format!(
 197                             "{}requires that {}borrow lasts for `{}`",
 198                             category.description(),
 199                             borrow_desc,
 200                             region_name,
 201                         ),
 202                     );
 203                 };
 204             }
 205             _ => {}
 206         }
 207     }
 208 }
 209
 210 impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
 211     /// Returns structured explanation for *why* the borrow contains the
 212     /// point from `location`. This is key for the "3-point errors"
 213     /// [described in the NLL RFC][d].
 214     ///
 215     /// # Parameters
 216     ///
 217     /// - `borrow`: the borrow in question
 218     /// - `location`: where the borrow occurs
 219     /// - `kind_place`: if Some, this describes the statement that triggered the error.
 220     ///   - first half is the kind of write, if any, being performed
 221     ///   - second half is the place being accessed
 222     ///
 223     /// [d]: https://rust-lang.github.io/rfcs/2094-nll.html#leveraging-intuition-framing-errors-in-terms-of-points
 224     pub(in crate::borrow_check) fn explain_why_borrow_contains_point(
 225         &self,
 226         location: Location,
 227         borrow: &BorrowData<'tcx>,
 228         kind_place: Option<(WriteKind, &Place<'tcx>)>,
 229     ) -> BorrowExplanation {
 230         debug!(
 231             "explain_why_borrow_contains_point(location={:?}, borrow={:?}, kind_place={:?})",
 232             location, borrow, kind_place
 233         );
 234
 235         let regioncx = &self.nonlexical_regioncx;
 236         let mir = self.mir;
 237         let tcx = self.infcx.tcx;
 238
 239         let borrow_region_vid = borrow.region;
 240         debug!(
 241             "explain_why_borrow_contains_point: borrow_region_vid={:?}",
 242             borrow_region_vid
 243         );
 244
 245         let region_sub = regioncx.find_sub_region_live_at(borrow_region_vid, location);
 246         debug!(
 247             "explain_why_borrow_contains_point: region_sub={:?}",
 248             region_sub
 249         );
 250
 251         match find_use::find(mir, regioncx, tcx, region_sub, location) {
 252             Some(Cause::LiveVar(local, location)) => {
 253                 let span = mir.source_info(location).span;
 254                 let spans = self
 255                     .move_spans(&Place::Base(PlaceBase::Local(local)), location)
 256                     .or_else(|| self.borrow_spans(span, location));
 257
 258                 let borrow_location = location;
 259                 if self.is_use_in_later_iteration_of_loop(borrow_location, location) {
 260                     let later_use = self.later_use_kind(borrow, spans, location);
 261                     BorrowExplanation::UsedLaterInLoop(later_use.0, later_use.1)
 262                 } else {
 263                     // Check if the location represents a `FakeRead`, and adapt the error
 264                     // message to the `FakeReadCause` it is from: in particular,
 265                     // the ones inserted in optimized `let var = <expr>` patterns.
 266                     let later_use = self.later_use_kind(borrow, spans, location);
 267                     BorrowExplanation::UsedLater(later_use.0, later_use.1)
 268                 }
 269             }
 270
 271             Some(Cause::DropVar(local, location)) => {
 272                 let mut should_note_order = false;
 273                 if mir.local_decls[local].name.is_some() {
 274                     if let Some((WriteKind::StorageDeadOrDrop, place)) = kind_place {
 275                         if let Place::Base(PlaceBase::Local(borrowed_local)) = place {
 276                              if mir.local_decls[*borrowed_local].name.is_some()
 277                                 && local != *borrowed_local
 278                             {
 279                                 should_note_order = true;
 280                             }
 281                         }
 282                     }
 283                 }
 284
 285                 BorrowExplanation::UsedLaterWhenDropped {
 286                     drop_loc: location,
 287                     dropped_local: local,
 288                     should_note_order,
 289                 }
 290             }
 291
 292             None => {
 293                 if let Some(region) = regioncx.to_error_region_vid(borrow_region_vid) {
 294                     let (category, from_closure, span, region_name) =
 295                         self.nonlexical_regioncx.free_region_constraint_info(
 296                             self.mir,
 297                         &self.upvars,
 298                             self.mir_def_id,
 299                             self.infcx,
 300                             borrow_region_vid,
 301                             region,
 302                         );
 303                     if let Some(region_name) = region_name {
 304                         let opt_place_desc = self.describe_place(&borrow.borrowed_place);
 305                         BorrowExplanation::MustBeValidFor {
 306                             category,
 307                             from_closure,
 308                             span,
 309                             region_name,
 310                             opt_place_desc,
 311                         }
 312                     } else {
 313                         debug!("explain_why_borrow_contains_point: \
 314                                 Could not generate a region name");
 315                         BorrowExplanation::Unexplained
 316                     }
 317                 } else {
 318                     debug!("explain_why_borrow_contains_point: \
 319                             Could not generate an error region vid");
 320                     BorrowExplanation::Unexplained
 321                 }
 322             }
 323         }
 324     }
 325
 326     /// true if `borrow_location` can reach `use_location` by going through a loop and
 327     /// `use_location` is also inside of that loop
 328     fn is_use_in_later_iteration_of_loop(
 329         &self,
 330         borrow_location: Location,
 331         use_location: Location,
 332     ) -> bool {
 333         let back_edge = self.reach_through_backedge(borrow_location, use_location);
 334         back_edge.map_or(false, |back_edge| {
 335             self.can_reach_head_of_loop(use_location, back_edge)
 336         })
 337     }
 338
 339     /// Returns the outmost back edge if `from` location can reach `to` location passing through
 340     /// that back edge
 341     fn reach_through_backedge(&self, from: Location, to: Location) -> Option<Location> {
 342         let mut visited_locations = FxHashSet::default();
 343         let mut pending_locations = VecDeque::new();
 344         visited_locations.insert(from);
 345         pending_locations.push_back(from);
 346         debug!("reach_through_backedge: from={:?} to={:?}", from, to,);
 347
 348         let mut outmost_back_edge = None;
 349         while let Some(location) = pending_locations.pop_front() {
 350             debug!(
 351                 "reach_through_backedge: location={:?} outmost_back_edge={:?}
 352                    pending_locations={:?} visited_locations={:?}",
 353                 location, outmost_back_edge, pending_locations, visited_locations
 354             );
 355
 356             if location == to && outmost_back_edge.is_some() {
 357                 // We've managed to reach the use location
 358                 debug!("reach_through_backedge: found!");
 359                 return outmost_back_edge;
 360             }
 361
 362             let block = &self.mir.basic_blocks()[location.block];
 363
 364             if location.statement_index < block.statements.len() {
 365                 let successor = location.successor_within_block();
 366                 if visited_locations.insert(successor) {
 367                     pending_locations.push_back(successor);
 368                 }
 369             } else {
 370                 pending_locations.extend(
 371                     block
 372                         .terminator()
 373                         .successors()
 374                         .map(|bb| Location {
 375                             statement_index: 0,
 376                             block: *bb,
 377                         })
 378                         .filter(|s| visited_locations.insert(*s))
 379                         .map(|s| {
 380                             if self.is_back_edge(location, s) {
 381                                 match outmost_back_edge {
 382                                     None => {
 383                                         outmost_back_edge = Some(location);
 384                                     }
 385
 386                                     Some(back_edge)
 387                                         if location.dominates(back_edge, &self.dominators) =>
 388                                     {
 389                                         outmost_back_edge = Some(location);
 390                                     }
 391
 392                                     Some(_) => {}
 393                                 }
 394                             }
 395
 396                             s
 397                         }),
 398                 );
 399             }
 400         }
 401
 402         None
 403     }
 404
 405     /// true if `from` location can reach `loop_head` location and `loop_head` dominates all the
 406     /// intermediate nodes
 407     fn can_reach_head_of_loop(&self, from: Location, loop_head: Location) -> bool {
 408         self.find_loop_head_dfs(from, loop_head, &mut FxHashSet::default())
 409     }
 410
 411     fn find_loop_head_dfs(
 412         &self,
 413         from: Location,
 414         loop_head: Location,
 415         visited_locations: &mut FxHashSet<Location>,
 416     ) -> bool {
 417         visited_locations.insert(from);
 418
 419         if from == loop_head {
 420             return true;
 421         }
 422
 423         if loop_head.dominates(from, &self.dominators) {
 424             let block = &self.mir.basic_blocks()[from.block];
 425
 426             if from.statement_index < block.statements.len() {
 427                 let successor = from.successor_within_block();
 428
 429                 if !visited_locations.contains(&successor)
 430                     && self.find_loop_head_dfs(successor, loop_head, visited_locations)
 431                 {
 432                     return true;
 433                 }
 434             } else {
 435                 for bb in block.terminator().successors() {
 436                     let successor = Location {
 437                         statement_index: 0,
 438                         block: *bb,
 439                     };
 440
 441                     if !visited_locations.contains(&successor)
 442                         && self.find_loop_head_dfs(successor, loop_head, visited_locations)
 443                     {
 444                         return true;
 445                     }
 446                 }
 447             }
 448         }
 449
 450         false
 451     }
 452
 453     /// True if an edge `source -> target` is a backedge -- in other words, if the target
 454     /// dominates the source.
 455     fn is_back_edge(&self, source: Location, target: Location) -> bool {
 456         target.dominates(source, &self.mir.dominators())
 457     }
 458
 459     /// Determine how the borrow was later used.
 460     fn later_use_kind(
 461         &self,
 462         borrow: &BorrowData<'tcx>,
 463         use_spans: UseSpans,
 464         location: Location,
 465     ) -> (LaterUseKind, Span) {
 466         match use_spans {
 467             UseSpans::ClosureUse { var_span, .. } => {
 468                 // Used in a closure.
 469                 (LaterUseKind::ClosureCapture, var_span)
 470             }
 471             UseSpans::OtherUse(span) => {
 472                 let block = &self.mir.basic_blocks()[location.block];
 473
 474                 let kind = if let Some(&Statement {
 475                     kind: StatementKind::FakeRead(FakeReadCause::ForLet, _),
 476                     ..
 477                 }) = block.statements.get(location.statement_index)
 478                 {
 479                     LaterUseKind::FakeLetRead
 480                 } else if self.was_captured_by_trait_object(borrow) {
 481                     LaterUseKind::TraitCapture
 482                 } else if location.statement_index == block.statements.len() {
 483                     if let TerminatorKind::Call {
 484                         ref func,
 485                         from_hir_call: true,
 486                         ..
 487                     } = block.terminator().kind
 488                     {
 489                         // Just point to the function, to reduce the chance of overlapping spans.
 490                         let function_span = match func {
 491                             Operand::Constant(c) => c.span,
 492                             Operand::Copy(Place::Base(PlaceBase::Local(l))) |
 493                             Operand::Move(Place::Base(PlaceBase::Local(l))) => {
 494                                 let local_decl = &self.mir.local_decls[*l];
 495                                 if local_decl.name.is_none() {
 496                                     local_decl.source_info.span
 497                                 } else {
 498                                     span
 499                                 }
 500                             }
 501                             _ => span,
 502                         };
 503                         return (LaterUseKind::Call, function_span);
 504                     } else {
 505                         LaterUseKind::Other
 506                     }
 507                 } else {
 508                     LaterUseKind::Other
 509                 };
 510
 511                 (kind, span)
 512             }
 513         }
 514     }
 515
 516     /// Checks if a borrowed value was captured by a trait object. We do this by
 517     /// looking forward in the MIR from the reserve location and checking if we see
 518     /// a unsized cast to a trait object on our data.
 519     fn was_captured_by_trait_object(&self, borrow: &BorrowData<'tcx>) -> bool {
 520         // Start at the reserve location, find the place that we want to see cast to a trait object.
 521         let location = borrow.reserve_location;
 522         let block = &self.mir[location.block];
 523         let stmt = block.statements.get(location.statement_index);
 524         debug!(
 525             "was_captured_by_trait_object: location={:?} stmt={:?}",
 526             location, stmt
 527         );
 528
 529         // We make a `queue` vector that has the locations we want to visit. As of writing, this
 530         // will only ever have one item at any given time, but by using a vector, we can pop from
 531         // it which simplifies the termination logic.
 532         let mut queue = vec![location];
 533         let mut target = if let Some(&Statement {
 534             kind: StatementKind::Assign(Place::Base(PlaceBase::Local(local)), _),
 535             ..
 536         }) = stmt
 537         {
 538             local
 539         } else {
 540             return false;
 541         };
 542
 543         debug!(
 544             "was_captured_by_trait: target={:?} queue={:?}",
 545             target, queue
 546         );
 547         while let Some(current_location) = queue.pop() {
 548             debug!("was_captured_by_trait: target={:?}", target);
 549             let block = &self.mir[current_location.block];
 550             // We need to check the current location to find out if it is a terminator.
 551             let is_terminator = current_location.statement_index == block.statements.len();
 552             if !is_terminator {
 553                 let stmt = &block.statements[current_location.statement_index];
 554                 debug!("was_captured_by_trait_object: stmt={:?}", stmt);
 555
 556                 // The only kind of statement that we care about is assignments...
 557                 if let StatementKind::Assign(place, box rvalue) = &stmt.kind {
 558                     let into = match place {
 559                         Place::Base(PlaceBase::Local(into)) => into,
 560                         Place::Projection(box Projection {
 561                             base: Place::Base(PlaceBase::Local(into)),
 562                             elem: ProjectionElem::Deref,
 563                         }) => into,
 564                         _ => {
 565                             // Continue at the next location.
 566                             queue.push(current_location.successor_within_block());
 567                             continue;
 568                         }
 569                     };
 570
 571                     match rvalue {
 572                         // If we see a use, we should check whether it is our data, and if so
 573                         // update the place that we're looking for to that new place.
 574                         Rvalue::Use(operand) => match operand {
 575                             Operand::Copy(Place::Base(PlaceBase::Local(from)))
 576                             | Operand::Move(Place::Base(PlaceBase::Local(from)))
 577                                 if *from == target =>
 578                             {
 579                                 target = *into;
 580                             }
 581                             _ => {}
 582                         },
 583                         // If we see a unsized cast, then if it is our data we should check
 584                         // whether it is being cast to a trait object.
 585                         Rvalue::Cast(
 586                             CastKind::Pointer(PointerCast::Unsize), operand, ty
 587                         ) => match operand {
 588                             Operand::Copy(Place::Base(PlaceBase::Local(from)))
 589                             | Operand::Move(Place::Base(PlaceBase::Local(from)))
 590                                 if *from == target =>
 591                             {
 592                                 debug!("was_captured_by_trait_object: ty={:?}", ty);
 593                                 // Check the type for a trait object.
 594                                 return match ty.sty {
 595                                     // `&dyn Trait`
 596                                     ty::Ref(_, ty, _) if ty.is_trait() => true,
 597                                     // `Box<dyn Trait>`
 598                                     _ if ty.is_box() && ty.boxed_ty().is_trait() => true,
 599                                     // `dyn Trait`
 600                                     _ if ty.is_trait() => true,
 601                                     // Anything else.
 602                                     _ => false,
 603                                 };
 604                             }
 605                             _ => return false,
 606                         },
 607                         _ => {}
 608                     }
 609                 }
 610
 611                 // Continue at the next location.
 612                 queue.push(current_location.successor_within_block());
 613             } else {
 614                 // The only thing we need to do for terminators is progress to the next block.
 615                 let terminator = block.terminator();
 616                 debug!("was_captured_by_trait_object: terminator={:?}", terminator);
 617
 618                 if let TerminatorKind::Call {
 619                     destination: Some((Place::Base(PlaceBase::Local(dest)), block)),
 620                     args,
 621                     ..
 622                 } = &terminator.kind
 623                 {
 624                     debug!(
 625                         "was_captured_by_trait_object: target={:?} dest={:?} args={:?}",
 626                         target, dest, args
 627                     );
 628                     // Check if one of the arguments to this function is the target place.
 629                     let found_target = args.iter().any(|arg| {
 630                         if let Operand::Move(Place::Base(PlaceBase::Local(potential))) = arg {
 631                             *potential == target
 632                         } else {
 633                             false
 634                         }
 635                     });
 636
 637                     // If it is, follow this to the next block and update the target.
 638                     if found_target {
 639                         target = *dest;
 640                         queue.push(block.start_location());
 641                     }
 642                 }
 643             }
 644
 645             debug!("was_captured_by_trait: queue={:?}", queue);
 646         }
 647
 648         // We didn't find anything and ran out of locations to check.
 649         false
 650     }
 651 }