1 //! This query borrow-checks the MIR to (further) ensure it is not broken.
3 #![allow(rustc::potential_query_instability)]
4 #![feature(bool_to_option)]
5 #![feature(box_patterns)]
6 #![feature(crate_visibility_modifier)]
7 #![feature(let_chains)]
9 #![feature(min_specialization)]
10 #![feature(stmt_expr_attributes)]
11 #![feature(trusted_step)]
12 #![feature(try_blocks)]
13 #![recursion_limit = "256"]
16 extern crate rustc_middle;
20 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
21 use rustc_data_structures::graph::dominators::Dominators;
22 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
24 use rustc_hir::def_id::LocalDefId;
26 use rustc_index::bit_set::ChunkedBitSet;
27 use rustc_index::vec::IndexVec;
28 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
29 use rustc_middle::mir::{
30 traversal, Body, ClearCrossCrate, Local, Location, Mutability, Operand, Place, PlaceElem,
31 PlaceRef, VarDebugInfoContents,
33 use rustc_middle::mir::{AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind};
34 use rustc_middle::mir::{Field, ProjectionElem, Promoted, Rvalue, Statement, StatementKind};
35 use rustc_middle::mir::{InlineAsmOperand, Terminator, TerminatorKind};
36 use rustc_middle::ty::query::Providers;
37 use rustc_middle::ty::{self, CapturedPlace, ParamEnv, RegionVid, TyCtxt};
38 use rustc_session::lint::builtin::{MUTABLE_BORROW_RESERVATION_CONFLICT, UNUSED_MUT};
39 use rustc_span::{Span, Symbol, DUMMY_SP};
42 use smallvec::SmallVec;
43 use std::cell::RefCell;
44 use std::collections::BTreeMap;
48 use rustc_mir_dataflow::impls::{
49 EverInitializedPlaces, MaybeInitializedPlaces, MaybeUninitializedPlaces,
51 use rustc_mir_dataflow::move_paths::{InitIndex, MoveOutIndex, MovePathIndex};
52 use rustc_mir_dataflow::move_paths::{InitLocation, LookupResult, MoveData, MoveError};
53 use rustc_mir_dataflow::Analysis;
54 use rustc_mir_dataflow::MoveDataParamEnv;
56 use self::diagnostics::{AccessKind, RegionName};
57 use self::location::LocationTable;
58 use self::prefixes::PrefixSet;
61 use self::path_utils::*;
65 mod constraint_generation;
73 mod member_constraints;
82 mod universal_regions;
85 // A public API provided for the Rust compiler consumers.
88 use borrow_set::{BorrowData, BorrowSet};
89 use dataflow::{BorrowIndex, BorrowckFlowState as Flows, BorrowckResults, Borrows};
90 use nll::{PoloniusOutput, ToRegionVid};
91 use place_ext::PlaceExt;
92 use places_conflict::{places_conflict, PlaceConflictBias};
93 use region_infer::RegionInferenceContext;
95 // FIXME(eddyb) perhaps move this somewhere more centrally.
98 place: CapturedPlace<'tcx>,
100 /// If true, the capture is behind a reference.
104 const DEREF_PROJECTION: &[PlaceElem<'_>; 1] = &[ProjectionElem::Deref];
106 pub fn provide(providers: &mut Providers) {
107 *providers = Providers {
108 mir_borrowck: |tcx, did| {
109 if let Some(def) = ty::WithOptConstParam::try_lookup(did, tcx) {
110 tcx.mir_borrowck_const_arg(def)
112 mir_borrowck(tcx, ty::WithOptConstParam::unknown(did))
115 mir_borrowck_const_arg: |tcx, (did, param_did)| {
116 mir_borrowck(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
122 fn mir_borrowck<'tcx>(
124 def: ty::WithOptConstParam<LocalDefId>,
125 ) -> &'tcx BorrowCheckResult<'tcx> {
126 let (input_body, promoted) = tcx.mir_promoted(def);
127 debug!("run query mir_borrowck: {}", tcx.def_path_str(def.did.to_def_id()));
129 let opt_closure_req = tcx.infer_ctxt().with_opaque_type_inference(def.did).enter(|infcx| {
130 let input_body: &Body<'_> = &input_body.borrow();
131 let promoted: &IndexVec<_, _> = &promoted.borrow();
132 do_mir_borrowck(&infcx, input_body, promoted, false).0
134 debug!("mir_borrowck done");
136 tcx.arena.alloc(opt_closure_req)
139 /// Perform the actual borrow checking.
141 /// If `return_body_with_facts` is true, then return the body with non-erased
142 /// region ids on which the borrow checking was performed together with Polonius
144 #[instrument(skip(infcx, input_body, input_promoted), level = "debug")]
145 fn do_mir_borrowck<'a, 'tcx>(
146 infcx: &InferCtxt<'a, 'tcx>,
147 input_body: &Body<'tcx>,
148 input_promoted: &IndexVec<Promoted, Body<'tcx>>,
149 return_body_with_facts: bool,
150 ) -> (BorrowCheckResult<'tcx>, Option<Box<BodyWithBorrowckFacts<'tcx>>>) {
151 let def = input_body.source.with_opt_param().as_local().unwrap();
156 let param_env = tcx.param_env(def.did);
157 let id = tcx.hir().local_def_id_to_hir_id(def.did);
159 let mut local_names = IndexVec::from_elem(None, &input_body.local_decls);
160 for var_debug_info in &input_body.var_debug_info {
161 if let VarDebugInfoContents::Place(place) = var_debug_info.value {
162 if let Some(local) = place.as_local() {
163 if let Some(prev_name) = local_names[local] && var_debug_info.name != prev_name {
165 var_debug_info.source_info.span,
166 "local {:?} has many names (`{}` vs `{}`)",
172 local_names[local] = Some(var_debug_info.name);
177 let mut errors = error::BorrowckErrors::new();
179 // Gather the upvars of a closure, if any.
180 let tables = tcx.typeck_opt_const_arg(def);
181 if let Some(ErrorGuaranteed { .. }) = tables.tainted_by_errors {
182 infcx.set_tainted_by_errors();
183 errors.set_tainted_by_errors();
185 let upvars: Vec<_> = tables
186 .closure_min_captures_flattened(def.did.to_def_id())
187 .map(|captured_place| {
188 let capture = captured_place.info.capture_kind;
189 let by_ref = match capture {
190 ty::UpvarCapture::ByValue => false,
191 ty::UpvarCapture::ByRef(..) => true,
193 Upvar { place: captured_place.clone(), by_ref }
197 // Replace all regions with fresh inference variables. This
198 // requires first making our own copy of the MIR. This copy will
199 // be modified (in place) to contain non-lexical lifetimes. It
200 // will have a lifetime tied to the inference context.
201 let mut body_owned = input_body.clone();
202 let mut promoted = input_promoted.clone();
204 nll::replace_regions_in_mir(infcx, param_env, &mut body_owned, &mut promoted);
205 let body = &body_owned; // no further changes
207 let location_table_owned = LocationTable::new(body);
208 let location_table = &location_table_owned;
210 let (move_data, move_errors): (MoveData<'tcx>, Vec<(Place<'tcx>, MoveError<'tcx>)>) =
211 match MoveData::gather_moves(&body, tcx, param_env) {
212 Ok(move_data) => (move_data, Vec::new()),
213 Err((move_data, move_errors)) => (move_data, move_errors),
215 let promoted_errors = promoted
217 .map(|(idx, body)| (idx, MoveData::gather_moves(&body, tcx, param_env)));
219 let mdpe = MoveDataParamEnv { move_data, param_env };
221 let mut flow_inits = MaybeInitializedPlaces::new(tcx, &body, &mdpe)
222 .into_engine(tcx, &body)
223 .pass_name("borrowck")
224 .iterate_to_fixpoint()
225 .into_results_cursor(&body);
227 let locals_are_invalidated_at_exit = tcx.hir().body_owner_kind(id).is_fn_or_closure();
229 Rc::new(BorrowSet::build(tcx, body, locals_are_invalidated_at_exit, &mdpe.move_data));
231 let use_polonius = return_body_with_facts || infcx.tcx.sess.opts.debugging_opts.polonius;
233 // Compute non-lexical lifetimes.
241 } = nll::compute_regions(
255 // Dump MIR results into a file, if that is enabled. This let us
256 // write unit-tests, as well as helping with debugging.
257 nll::dump_mir_results(infcx, &body, ®ioncx, &opt_closure_req);
259 // We also have a `#[rustc_regions]` annotation that causes us to dump
261 nll::dump_annotation(
270 // The various `flow_*` structures can be large. We drop `flow_inits` here
271 // so it doesn't overlap with the others below. This reduces peak memory
272 // usage significantly on some benchmarks.
275 let regioncx = Rc::new(regioncx);
277 let flow_borrows = Borrows::new(tcx, body, ®ioncx, &borrow_set)
278 .into_engine(tcx, body)
279 .pass_name("borrowck")
280 .iterate_to_fixpoint();
281 let flow_uninits = MaybeUninitializedPlaces::new(tcx, body, &mdpe)
282 .into_engine(tcx, body)
283 .pass_name("borrowck")
284 .iterate_to_fixpoint();
285 let flow_ever_inits = EverInitializedPlaces::new(tcx, body, &mdpe)
286 .into_engine(tcx, body)
287 .pass_name("borrowck")
288 .iterate_to_fixpoint();
290 let movable_generator = !matches!(
292 Node::Expr(&hir::Expr {
293 kind: hir::ExprKind::Closure(.., Some(hir::Movability::Static)),
298 for (idx, move_data_results) in promoted_errors {
299 let promoted_body = &promoted[idx];
301 if let Err((move_data, move_errors)) = move_data_results {
302 let mut promoted_mbcx = MirBorrowckCtxt {
306 move_data: &move_data,
307 location_table, // no need to create a real one for the promoted, it is not used
309 fn_self_span_reported: Default::default(),
310 locals_are_invalidated_at_exit,
311 access_place_error_reported: Default::default(),
312 reservation_error_reported: Default::default(),
313 reservation_warnings: Default::default(),
314 uninitialized_error_reported: Default::default(),
315 regioncx: regioncx.clone(),
316 used_mut: Default::default(),
317 used_mut_upvars: SmallVec::new(),
318 borrow_set: Rc::clone(&borrow_set),
319 dominators: Dominators::dummy(), // not used
321 local_names: IndexVec::from_elem(None, &promoted_body.local_decls),
322 region_names: RefCell::default(),
323 next_region_name: RefCell::new(1),
324 polonius_output: None,
327 promoted_mbcx.report_move_errors(move_errors);
328 errors = promoted_mbcx.errors;
332 let dominators = body.dominators();
334 let mut mbcx = MirBorrowckCtxt {
338 move_data: &mdpe.move_data,
341 locals_are_invalidated_at_exit,
342 fn_self_span_reported: Default::default(),
343 access_place_error_reported: Default::default(),
344 reservation_error_reported: Default::default(),
345 reservation_warnings: Default::default(),
346 uninitialized_error_reported: Default::default(),
347 regioncx: Rc::clone(®ioncx),
348 used_mut: Default::default(),
349 used_mut_upvars: SmallVec::new(),
350 borrow_set: Rc::clone(&borrow_set),
354 region_names: RefCell::default(),
355 next_region_name: RefCell::new(1),
360 // Compute and report region errors, if any.
361 mbcx.report_region_errors(nll_errors);
363 let results = BorrowckResults {
364 ever_inits: flow_ever_inits,
365 uninits: flow_uninits,
366 borrows: flow_borrows,
369 mbcx.report_move_errors(move_errors);
371 rustc_mir_dataflow::visit_results(
373 traversal::reverse_postorder(body).map(|(bb, _)| bb),
378 // Convert any reservation warnings into lints.
379 let reservation_warnings = mem::take(&mut mbcx.reservation_warnings);
380 for (_, (place, span, location, bk, borrow)) in reservation_warnings {
381 let initial_diag = mbcx.report_conflicting_borrow(location, (place, span), bk, &borrow);
383 let scope = mbcx.body.source_info(location).scope;
384 let lint_root = match &mbcx.body.source_scopes[scope].local_data {
385 ClearCrossCrate::Set(data) => data.lint_root,
389 // Span and message don't matter; we overwrite them below anyway
390 mbcx.infcx.tcx.struct_span_lint_hir(
391 MUTABLE_BORROW_RESERVATION_CONFLICT,
395 let mut diag = lint.build("");
397 diag.message = initial_diag.styled_message().clone();
398 diag.span = initial_diag.span.clone();
400 mbcx.buffer_non_error_diag(diag);
403 initial_diag.cancel();
406 // For each non-user used mutable variable, check if it's been assigned from
407 // a user-declared local. If so, then put that local into the used_mut set.
408 // Note that this set is expected to be small - only upvars from closures
409 // would have a chance of erroneously adding non-user-defined mutable vars
411 let temporary_used_locals: FxHashSet<Local> = mbcx
414 .filter(|&local| !mbcx.body.local_decls[*local].is_user_variable())
417 // For the remaining unused locals that are marked as mutable, we avoid linting any that
418 // were never initialized. These locals may have been removed as unreachable code; or will be
419 // linted as unused variables.
420 let unused_mut_locals =
421 mbcx.body.mut_vars_iter().filter(|local| !mbcx.used_mut.contains(local)).collect();
422 mbcx.gather_used_muts(temporary_used_locals, unused_mut_locals);
424 debug!("mbcx.used_mut: {:?}", mbcx.used_mut);
425 let used_mut = std::mem::take(&mut mbcx.used_mut);
426 for local in mbcx.body.mut_vars_and_args_iter().filter(|local| !used_mut.contains(local)) {
427 let local_decl = &mbcx.body.local_decls[local];
428 let lint_root = match &mbcx.body.source_scopes[local_decl.source_info.scope].local_data {
429 ClearCrossCrate::Set(data) => data.lint_root,
433 // Skip over locals that begin with an underscore or have no name
434 match mbcx.local_names[local] {
436 if name.as_str().starts_with('_') {
443 let span = local_decl.source_info.span;
444 if span.desugaring_kind().is_some() {
445 // If the `mut` arises as part of a desugaring, we should ignore it.
449 tcx.struct_span_lint_hir(UNUSED_MUT, lint_root, span, |lint| {
450 let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
451 lint.build("variable does not need to be mutable")
452 .span_suggestion_short(
456 Applicability::MachineApplicable,
462 let tainted_by_errors = mbcx.emit_errors();
464 let result = BorrowCheckResult {
465 concrete_opaque_types: opaque_type_values,
466 closure_requirements: opt_closure_req,
467 used_mut_upvars: mbcx.used_mut_upvars,
471 let body_with_facts = if return_body_with_facts {
472 let output_facts = mbcx.polonius_output.expect("Polonius output was not computed");
473 Some(Box::new(BodyWithBorrowckFacts {
475 input_facts: *polonius_input.expect("Polonius input facts were not generated"),
477 location_table: location_table_owned,
483 debug!("do_mir_borrowck: result = {:#?}", result);
485 (result, body_with_facts)
488 /// A `Body` with information computed by the borrow checker. This struct is
489 /// intended to be consumed by compiler consumers.
491 /// We need to include the MIR body here because the region identifiers must
492 /// match the ones in the Polonius facts.
493 pub struct BodyWithBorrowckFacts<'tcx> {
494 /// A mir body that contains region identifiers.
495 pub body: Body<'tcx>,
496 /// Polonius input facts.
497 pub input_facts: AllFacts,
498 /// Polonius output facts.
499 pub output_facts: Rc<self::nll::PoloniusOutput>,
500 /// The table that maps Polonius points to locations in the table.
501 pub location_table: LocationTable,
504 struct MirBorrowckCtxt<'cx, 'tcx> {
505 infcx: &'cx InferCtxt<'cx, 'tcx>,
506 param_env: ParamEnv<'tcx>,
507 body: &'cx Body<'tcx>,
508 move_data: &'cx MoveData<'tcx>,
510 /// Map from MIR `Location` to `LocationIndex`; created
511 /// when MIR borrowck begins.
512 location_table: &'cx LocationTable,
514 movable_generator: bool,
515 /// This keeps track of whether local variables are free-ed when the function
516 /// exits even without a `StorageDead`, which appears to be the case for
519 /// I'm not sure this is the right approach - @eddyb could you try and
521 locals_are_invalidated_at_exit: bool,
522 /// This field keeps track of when borrow errors are reported in the access_place function
523 /// so that there is no duplicate reporting. This field cannot also be used for the conflicting
524 /// borrow errors that is handled by the `reservation_error_reported` field as the inclusion
525 /// of the `Span` type (while required to mute some errors) stops the muting of the reservation
527 access_place_error_reported: FxHashSet<(Place<'tcx>, Span)>,
528 /// This field keeps track of when borrow conflict errors are reported
529 /// for reservations, so that we don't report seemingly duplicate
530 /// errors for corresponding activations.
532 // FIXME: ideally this would be a set of `BorrowIndex`, not `Place`s,
533 // but it is currently inconvenient to track down the `BorrowIndex`
534 // at the time we detect and report a reservation error.
535 reservation_error_reported: FxHashSet<Place<'tcx>>,
536 /// This fields keeps track of the `Span`s that we have
537 /// used to report extra information for `FnSelfUse`, to avoid
538 /// unnecessarily verbose errors.
539 fn_self_span_reported: FxHashSet<Span>,
540 /// Migration warnings to be reported for #56254. We delay reporting these
541 /// so that we can suppress the warning if there's a corresponding error
542 /// for the activation of the borrow.
543 reservation_warnings:
544 FxHashMap<BorrowIndex, (Place<'tcx>, Span, Location, BorrowKind, BorrowData<'tcx>)>,
545 /// This field keeps track of errors reported in the checking of uninitialized variables,
546 /// so that we don't report seemingly duplicate errors.
547 uninitialized_error_reported: FxHashSet<PlaceRef<'tcx>>,
548 /// This field keeps track of all the local variables that are declared mut and are mutated.
549 /// Used for the warning issued by an unused mutable local variable.
550 used_mut: FxHashSet<Local>,
551 /// If the function we're checking is a closure, then we'll need to report back the list of
552 /// mutable upvars that have been used. This field keeps track of them.
553 used_mut_upvars: SmallVec<[Field; 8]>,
554 /// Region inference context. This contains the results from region inference and lets us e.g.
555 /// find out which CFG points are contained in each borrow region.
556 regioncx: Rc<RegionInferenceContext<'tcx>>,
558 /// The set of borrows extracted from the MIR
559 borrow_set: Rc<BorrowSet<'tcx>>,
561 /// Dominators for MIR
562 dominators: Dominators<BasicBlock>,
564 /// Information about upvars not necessarily preserved in types or MIR
565 upvars: Vec<Upvar<'tcx>>,
567 /// Names of local (user) variables (extracted from `var_debug_info`).
568 local_names: IndexVec<Local, Option<Symbol>>,
570 /// Record the region names generated for each region in the given
571 /// MIR def so that we can reuse them later in help/error messages.
572 region_names: RefCell<FxHashMap<RegionVid, RegionName>>,
574 /// The counter for generating new region names.
575 next_region_name: RefCell<usize>,
577 /// Results of Polonius analysis.
578 polonius_output: Option<Rc<PoloniusOutput>>,
580 errors: error::BorrowckErrors<'tcx>,
584 // 1. assignments are always made to mutable locations (FIXME: does that still really go here?)
585 // 2. loans made in overlapping scopes do not conflict
586 // 3. assignments do not affect things loaned out as immutable
587 // 4. moves do not affect things loaned out in any way
588 impl<'cx, 'tcx> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtxt<'cx, 'tcx> {
589 type FlowState = Flows<'cx, 'tcx>;
591 fn visit_statement_before_primary_effect(
593 flow_state: &Flows<'cx, 'tcx>,
594 stmt: &'cx Statement<'tcx>,
597 debug!("MirBorrowckCtxt::process_statement({:?}, {:?}): {:?}", location, stmt, flow_state);
598 let span = stmt.source_info.span;
600 self.check_activations(location, span, flow_state);
603 StatementKind::Assign(box (lhs, ref rhs)) => {
604 self.consume_rvalue(location, (rhs, span), flow_state);
606 self.mutate_place(location, (*lhs, span), Shallow(None), flow_state);
608 StatementKind::FakeRead(box (_, ref place)) => {
609 // Read for match doesn't access any memory and is used to
610 // assert that a place is safe and live. So we don't have to
611 // do any checks here.
613 // FIXME: Remove check that the place is initialized. This is
614 // needed for now because matches don't have never patterns yet.
615 // So this is the only place we prevent
619 self.check_if_path_or_subpath_is_moved(
621 InitializationRequiringAction::Use,
622 (place.as_ref(), span),
626 StatementKind::SetDiscriminant { place, variant_index: _ } => {
627 self.mutate_place(location, (**place, span), Shallow(None), flow_state);
629 StatementKind::CopyNonOverlapping(box rustc_middle::mir::CopyNonOverlapping {
634 "Unexpected CopyNonOverlapping, should only appear after lower_intrinsics",
638 | StatementKind::Coverage(..)
639 | StatementKind::AscribeUserType(..)
640 | StatementKind::Retag { .. }
641 | StatementKind::StorageLive(..) => {
642 // `Nop`, `AscribeUserType`, `Retag`, and `StorageLive` are irrelevant
645 StatementKind::StorageDead(local) => {
648 (Place::from(*local), span),
649 (Shallow(None), Write(WriteKind::StorageDeadOrDrop)),
650 LocalMutationIsAllowed::Yes,
657 fn visit_terminator_before_primary_effect(
659 flow_state: &Flows<'cx, 'tcx>,
660 term: &'cx Terminator<'tcx>,
663 debug!("MirBorrowckCtxt::process_terminator({:?}, {:?}): {:?}", loc, term, flow_state);
664 let span = term.source_info.span;
666 self.check_activations(loc, span, flow_state);
669 TerminatorKind::SwitchInt { ref discr, switch_ty: _, targets: _ } => {
670 self.consume_operand(loc, (discr, span), flow_state);
672 TerminatorKind::Drop { place, target: _, unwind: _ } => {
674 "visit_terminator_drop \
675 loc: {:?} term: {:?} place: {:?} span: {:?}",
676 loc, term, place, span
682 (AccessDepth::Drop, Write(WriteKind::StorageDeadOrDrop)),
683 LocalMutationIsAllowed::Yes,
687 TerminatorKind::DropAndReplace {
689 value: ref new_value,
693 self.mutate_place(loc, (drop_place, span), Deep, flow_state);
694 self.consume_operand(loc, (new_value, span), flow_state);
696 TerminatorKind::Call {
704 self.consume_operand(loc, (func, span), flow_state);
706 self.consume_operand(loc, (arg, span), flow_state);
708 if let Some((dest, _ /*bb*/)) = *destination {
709 self.mutate_place(loc, (dest, span), Deep, flow_state);
712 TerminatorKind::Assert { ref cond, expected: _, ref msg, target: _, cleanup: _ } => {
713 self.consume_operand(loc, (cond, span), flow_state);
714 use rustc_middle::mir::AssertKind;
715 if let AssertKind::BoundsCheck { ref len, ref index } = *msg {
716 self.consume_operand(loc, (len, span), flow_state);
717 self.consume_operand(loc, (index, span), flow_state);
721 TerminatorKind::Yield { ref value, resume: _, resume_arg, drop: _ } => {
722 self.consume_operand(loc, (value, span), flow_state);
723 self.mutate_place(loc, (resume_arg, span), Deep, flow_state);
726 TerminatorKind::InlineAsm {
736 InlineAsmOperand::In { reg: _, ref value } => {
737 self.consume_operand(loc, (value, span), flow_state);
739 InlineAsmOperand::Out { reg: _, late: _, place, .. } => {
740 if let Some(place) = place {
741 self.mutate_place(loc, (place, span), Shallow(None), flow_state);
744 InlineAsmOperand::InOut { reg: _, late: _, ref in_value, out_place } => {
745 self.consume_operand(loc, (in_value, span), flow_state);
746 if let Some(out_place) = out_place {
755 InlineAsmOperand::Const { value: _ }
756 | InlineAsmOperand::SymFn { value: _ }
757 | InlineAsmOperand::SymStatic { def_id: _ } => {}
762 TerminatorKind::Goto { target: _ }
763 | TerminatorKind::Abort
764 | TerminatorKind::Unreachable
765 | TerminatorKind::Resume
766 | TerminatorKind::Return
767 | TerminatorKind::GeneratorDrop
768 | TerminatorKind::FalseEdge { real_target: _, imaginary_target: _ }
769 | TerminatorKind::FalseUnwind { real_target: _, unwind: _ } => {
770 // no data used, thus irrelevant to borrowck
775 fn visit_terminator_after_primary_effect(
777 flow_state: &Flows<'cx, 'tcx>,
778 term: &'cx Terminator<'tcx>,
781 let span = term.source_info.span;
784 TerminatorKind::Yield { value: _, resume: _, resume_arg: _, drop: _ } => {
785 if self.movable_generator {
786 // Look for any active borrows to locals
787 let borrow_set = self.borrow_set.clone();
788 for i in flow_state.borrows.iter() {
789 let borrow = &borrow_set[i];
790 self.check_for_local_borrow(borrow, span);
795 TerminatorKind::Resume | TerminatorKind::Return | TerminatorKind::GeneratorDrop => {
796 // Returning from the function implicitly kills storage for all locals and statics.
797 // Often, the storage will already have been killed by an explicit
798 // StorageDead, but we don't always emit those (notably on unwind paths),
799 // so this "extra check" serves as a kind of backup.
800 let borrow_set = self.borrow_set.clone();
801 for i in flow_state.borrows.iter() {
802 let borrow = &borrow_set[i];
803 self.check_for_invalidation_at_exit(loc, borrow, span);
807 TerminatorKind::Abort
808 | TerminatorKind::Assert { .. }
809 | TerminatorKind::Call { .. }
810 | TerminatorKind::Drop { .. }
811 | TerminatorKind::DropAndReplace { .. }
812 | TerminatorKind::FalseEdge { real_target: _, imaginary_target: _ }
813 | TerminatorKind::FalseUnwind { real_target: _, unwind: _ }
814 | TerminatorKind::Goto { .. }
815 | TerminatorKind::SwitchInt { .. }
816 | TerminatorKind::Unreachable
817 | TerminatorKind::InlineAsm { .. } => {}
822 use self::AccessDepth::{Deep, Shallow};
823 use self::ReadOrWrite::{Activation, Read, Reservation, Write};
825 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
826 enum ArtificialField {
831 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
833 /// From the RFC: "A *shallow* access means that the immediate
834 /// fields reached at P are accessed, but references or pointers
835 /// found within are not dereferenced. Right now, the only access
836 /// that is shallow is an assignment like `x = ...;`, which would
837 /// be a *shallow write* of `x`."
838 Shallow(Option<ArtificialField>),
840 /// From the RFC: "A *deep* access means that all data reachable
841 /// through the given place may be invalidated or accesses by
845 /// Access is Deep only when there is a Drop implementation that
846 /// can reach the data behind the reference.
850 /// Kind of access to a value: read or write
851 /// (For informational purposes only)
852 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
854 /// From the RFC: "A *read* means that the existing data may be
855 /// read, but will not be changed."
858 /// From the RFC: "A *write* means that the data may be mutated to
859 /// new values or otherwise invalidated (for example, it could be
860 /// de-initialized, as in a move operation).
863 /// For two-phase borrows, we distinguish a reservation (which is treated
864 /// like a Read) from an activation (which is treated like a write), and
865 /// each of those is furthermore distinguished from Reads/Writes above.
866 Reservation(WriteKind),
867 Activation(WriteKind, BorrowIndex),
870 /// Kind of read access to a value
871 /// (For informational purposes only)
872 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
878 /// Kind of write access to a value
879 /// (For informational purposes only)
880 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
883 MutableBorrow(BorrowKind),
888 /// When checking permissions for a place access, this flag is used to indicate that an immutable
889 /// local place can be mutated.
891 // FIXME: @nikomatsakis suggested that this flag could be removed with the following modifications:
892 // - Merge `check_access_permissions()` and `check_if_reassignment_to_immutable_state()`.
893 // - Split `is_mutable()` into `is_assignable()` (can be directly assigned) and
894 // `is_declared_mutable()`.
895 // - Take flow state into consideration in `is_assignable()` for local variables.
896 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
897 enum LocalMutationIsAllowed {
899 /// We want use of immutable upvars to cause a "write to immutable upvar"
900 /// error, not an "reassignment" error.
905 #[derive(Copy, Clone, Debug)]
906 enum InitializationRequiringAction {
914 struct RootPlace<'tcx> {
916 place_projection: &'tcx [PlaceElem<'tcx>],
917 is_local_mutation_allowed: LocalMutationIsAllowed,
920 impl InitializationRequiringAction {
921 fn as_noun(self) -> &'static str {
923 InitializationRequiringAction::Borrow => "borrow",
924 InitializationRequiringAction::MatchOn => "use", // no good noun
925 InitializationRequiringAction::Use => "use",
926 InitializationRequiringAction::Assignment => "assign",
927 InitializationRequiringAction::PartialAssignment => "assign to part",
931 fn as_verb_in_past_tense(self) -> &'static str {
933 InitializationRequiringAction::Borrow => "borrowed",
934 InitializationRequiringAction::MatchOn => "matched on",
935 InitializationRequiringAction::Use => "used",
936 InitializationRequiringAction::Assignment => "assigned",
937 InitializationRequiringAction::PartialAssignment => "partially assigned",
942 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
943 fn body(&self) -> &'cx Body<'tcx> {
947 /// Checks an access to the given place to see if it is allowed. Examines the set of borrows
948 /// that are in scope, as well as which paths have been initialized, to ensure that (a) the
949 /// place is initialized and (b) it is not borrowed in some way that would prevent this
952 /// Returns `true` if an error is reported.
956 place_span: (Place<'tcx>, Span),
957 kind: (AccessDepth, ReadOrWrite),
958 is_local_mutation_allowed: LocalMutationIsAllowed,
959 flow_state: &Flows<'cx, 'tcx>,
963 if let Activation(_, borrow_index) = rw {
964 if self.reservation_error_reported.contains(&place_span.0) {
966 "skipping access_place for activation of invalid reservation \
967 place: {:?} borrow_index: {:?}",
968 place_span.0, borrow_index
974 // Check is_empty() first because it's the common case, and doing that
975 // way we avoid the clone() call.
976 if !self.access_place_error_reported.is_empty()
977 && self.access_place_error_reported.contains(&(place_span.0, place_span.1))
980 "access_place: suppressing error place_span=`{:?}` kind=`{:?}`",
986 let mutability_error = self.check_access_permissions(
989 is_local_mutation_allowed,
994 self.check_access_for_conflict(location, place_span, sd, rw, flow_state);
996 if let (Activation(_, borrow_idx), true) = (kind.1, conflict_error) {
997 // Suppress this warning when there's an error being emitted for the
998 // same borrow: fixing the error is likely to fix the warning.
999 self.reservation_warnings.remove(&borrow_idx);
1002 if conflict_error || mutability_error {
1003 debug!("access_place: logging error place_span=`{:?}` kind=`{:?}`", place_span, kind);
1004 self.access_place_error_reported.insert((place_span.0, place_span.1));
1008 fn check_access_for_conflict(
1011 place_span: (Place<'tcx>, Span),
1014 flow_state: &Flows<'cx, 'tcx>,
1017 "check_access_for_conflict(location={:?}, place_span={:?}, sd={:?}, rw={:?})",
1018 location, place_span, sd, rw,
1021 let mut error_reported = false;
1022 let tcx = self.infcx.tcx;
1023 let body = self.body;
1024 let borrow_set = self.borrow_set.clone();
1026 // Use polonius output if it has been enabled.
1027 let polonius_output = self.polonius_output.clone();
1028 let borrows_in_scope = if let Some(polonius) = &polonius_output {
1029 let location = self.location_table.start_index(location);
1030 Either::Left(polonius.errors_at(location).iter().copied())
1032 Either::Right(flow_state.borrows.iter())
1035 each_borrow_involving_path(
1043 |this, borrow_index, borrow| match (rw, borrow.kind) {
1044 // Obviously an activation is compatible with its own
1045 // reservation (or even prior activating uses of same
1046 // borrow); so don't check if they interfere.
1048 // NOTE: *reservations* do conflict with themselves;
1049 // thus aren't injecting unsoundenss w/ this check.)
1050 (Activation(_, activating), _) if activating == borrow_index => {
1052 "check_access_for_conflict place_span: {:?} sd: {:?} rw: {:?} \
1053 skipping {:?} b/c activation of same borrow_index",
1057 (borrow_index, borrow),
1062 (Read(_), BorrowKind::Shared | BorrowKind::Shallow)
1064 Read(ReadKind::Borrow(BorrowKind::Shallow)),
1065 BorrowKind::Unique | BorrowKind::Mut { .. },
1066 ) => Control::Continue,
1068 (Write(WriteKind::Move), BorrowKind::Shallow) => {
1069 // Handled by initialization checks.
1073 (Read(kind), BorrowKind::Unique | BorrowKind::Mut { .. }) => {
1074 // Reading from mere reservations of mutable-borrows is OK.
1075 if !is_active(&this.dominators, borrow, location) {
1076 assert!(allow_two_phase_borrow(borrow.kind));
1077 return Control::Continue;
1080 error_reported = true;
1084 .report_use_while_mutably_borrowed(location, place_span, borrow);
1085 this.buffer_error(err);
1087 ReadKind::Borrow(bk) => {
1089 this.report_conflicting_borrow(location, place_span, bk, borrow);
1090 this.buffer_error(err);
1097 Reservation(WriteKind::MutableBorrow(bk)),
1098 BorrowKind::Shallow | BorrowKind::Shared,
1099 ) if { tcx.migrate_borrowck() && this.borrow_set.contains(&location) } => {
1100 let bi = this.borrow_set.get_index_of(&location).unwrap();
1102 "recording invalid reservation of place: {:?} with \
1103 borrow index {:?} as warning",
1106 // rust-lang/rust#56254 - This was previously permitted on
1107 // the 2018 edition so we emit it as a warning. We buffer
1108 // these sepately so that we only emit a warning if borrow
1109 // checking was otherwise successful.
1110 this.reservation_warnings
1111 .insert(bi, (place_span.0, place_span.1, location, bk, borrow.clone()));
1113 // Don't suppress actual errors.
1117 (Reservation(kind) | Activation(kind, _) | Write(kind), _) => {
1119 Reservation(..) => {
1121 "recording invalid reservation of \
1125 this.reservation_error_reported.insert(place_span.0);
1127 Activation(_, activating) => {
1129 "observing check_place for activation of \
1130 borrow_index: {:?}",
1134 Read(..) | Write(..) => {}
1137 error_reported = true;
1139 WriteKind::MutableBorrow(bk) => {
1141 this.report_conflicting_borrow(location, place_span, bk, borrow);
1142 this.buffer_error(err);
1144 WriteKind::StorageDeadOrDrop => this
1145 .report_borrowed_value_does_not_live_long_enough(
1151 WriteKind::Mutate => {
1152 this.report_illegal_mutation_of_borrowed(location, place_span, borrow)
1154 WriteKind::Move => {
1155 this.report_move_out_while_borrowed(location, place_span, borrow)
1169 place_span: (Place<'tcx>, Span),
1171 flow_state: &Flows<'cx, 'tcx>,
1173 // Write of P[i] or *P requires P init'd.
1174 self.check_if_assigned_path_is_moved(location, place_span, flow_state);
1176 // Special case: you can assign an immutable local variable
1177 // (e.g., `x = ...`) so long as it has never been initialized
1178 // before (at this point in the flow).
1179 if let Some(local) = place_span.0.as_local() {
1180 if let Mutability::Not = self.body.local_decls[local].mutability {
1181 // check for reassignments to immutable local variables
1182 self.check_if_reassignment_to_immutable_state(
1183 location, local, place_span, flow_state,
1189 // Otherwise, use the normal access permission rules.
1193 (kind, Write(WriteKind::Mutate)),
1194 LocalMutationIsAllowed::No,
1202 (rvalue, span): (&'cx Rvalue<'tcx>, Span),
1203 flow_state: &Flows<'cx, 'tcx>,
1206 Rvalue::Ref(_ /*rgn*/, bk, place) => {
1207 let access_kind = match bk {
1208 BorrowKind::Shallow => {
1209 (Shallow(Some(ArtificialField::ShallowBorrow)), Read(ReadKind::Borrow(bk)))
1211 BorrowKind::Shared => (Deep, Read(ReadKind::Borrow(bk))),
1212 BorrowKind::Unique | BorrowKind::Mut { .. } => {
1213 let wk = WriteKind::MutableBorrow(bk);
1214 if allow_two_phase_borrow(bk) {
1215 (Deep, Reservation(wk))
1226 LocalMutationIsAllowed::No,
1230 let action = if bk == BorrowKind::Shallow {
1231 InitializationRequiringAction::MatchOn
1233 InitializationRequiringAction::Borrow
1236 self.check_if_path_or_subpath_is_moved(
1239 (place.as_ref(), span),
1244 Rvalue::AddressOf(mutability, place) => {
1245 let access_kind = match mutability {
1246 Mutability::Mut => (
1248 Write(WriteKind::MutableBorrow(BorrowKind::Mut {
1249 allow_two_phase_borrow: false,
1252 Mutability::Not => (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
1259 LocalMutationIsAllowed::No,
1263 self.check_if_path_or_subpath_is_moved(
1265 InitializationRequiringAction::Borrow,
1266 (place.as_ref(), span),
1271 Rvalue::ThreadLocalRef(_) => {}
1273 Rvalue::Use(ref operand)
1274 | Rvalue::Repeat(ref operand, _)
1275 | Rvalue::UnaryOp(_ /*un_op*/, ref operand)
1276 | Rvalue::Cast(_ /*cast_kind*/, ref operand, _ /*ty*/)
1277 | Rvalue::ShallowInitBox(ref operand, _ /*ty*/) => {
1278 self.consume_operand(location, (operand, span), flow_state)
1281 Rvalue::Len(place) | Rvalue::Discriminant(place) => {
1282 let af = match *rvalue {
1283 Rvalue::Len(..) => Some(ArtificialField::ArrayLength),
1284 Rvalue::Discriminant(..) => None,
1285 _ => unreachable!(),
1290 (Shallow(af), Read(ReadKind::Copy)),
1291 LocalMutationIsAllowed::No,
1294 self.check_if_path_or_subpath_is_moved(
1296 InitializationRequiringAction::Use,
1297 (place.as_ref(), span),
1302 Rvalue::BinaryOp(_bin_op, box (ref operand1, ref operand2))
1303 | Rvalue::CheckedBinaryOp(_bin_op, box (ref operand1, ref operand2)) => {
1304 self.consume_operand(location, (operand1, span), flow_state);
1305 self.consume_operand(location, (operand2, span), flow_state);
1308 Rvalue::NullaryOp(_op, _ty) => {
1309 // nullary ops take no dynamic input; no borrowck effect.
1312 Rvalue::Aggregate(ref aggregate_kind, ref operands) => {
1313 // We need to report back the list of mutable upvars that were
1314 // moved into the closure and subsequently used by the closure,
1315 // in order to populate our used_mut set.
1316 match **aggregate_kind {
1317 AggregateKind::Closure(def_id, _) | AggregateKind::Generator(def_id, _, _) => {
1318 let BorrowCheckResult { used_mut_upvars, .. } =
1319 self.infcx.tcx.mir_borrowck(def_id.expect_local());
1320 debug!("{:?} used_mut_upvars={:?}", def_id, used_mut_upvars);
1321 for field in used_mut_upvars {
1322 self.propagate_closure_used_mut_upvar(&operands[field.index()]);
1325 AggregateKind::Adt(..)
1326 | AggregateKind::Array(..)
1327 | AggregateKind::Tuple { .. } => (),
1330 for operand in operands {
1331 self.consume_operand(location, (operand, span), flow_state);
1337 fn propagate_closure_used_mut_upvar(&mut self, operand: &Operand<'tcx>) {
1338 let propagate_closure_used_mut_place = |this: &mut Self, place: Place<'tcx>| {
1339 // We have three possibilities here:
1340 // a. We are modifying something through a mut-ref
1341 // b. We are modifying something that is local to our parent
1342 // c. Current body is a nested closure, and we are modifying path starting from
1343 // a Place captured by our parent closure.
1345 // Handle (c), the path being modified is exactly the path captured by our parent
1346 if let Some(field) = this.is_upvar_field_projection(place.as_ref()) {
1347 this.used_mut_upvars.push(field);
1351 for (place_ref, proj) in place.iter_projections().rev() {
1353 if proj == ProjectionElem::Deref {
1354 match place_ref.ty(this.body(), this.infcx.tcx).ty.kind() {
1355 // We aren't modifying a variable directly
1356 ty::Ref(_, _, hir::Mutability::Mut) => return,
1363 if let Some(field) = this.is_upvar_field_projection(place_ref) {
1364 this.used_mut_upvars.push(field);
1370 this.used_mut.insert(place.local);
1373 // This relies on the current way that by-value
1374 // captures of a closure are copied/moved directly
1375 // when generating MIR.
1377 Operand::Move(place) | Operand::Copy(place) => {
1378 match place.as_local() {
1379 Some(local) if !self.body.local_decls[local].is_user_variable() => {
1380 if self.body.local_decls[local].ty.is_mutable_ptr() {
1381 // The variable will be marked as mutable by the borrow.
1384 // This is an edge case where we have a `move` closure
1385 // inside a non-move closure, and the inner closure
1386 // contains a mutation:
1389 // || { move || { i += 1; }; };
1391 // In this case our usual strategy of assuming that the
1392 // variable will be captured by mutable reference is
1393 // wrong, since `i` can be copied into the inner
1394 // closure from a shared reference.
1396 // As such we have to search for the local that this
1397 // capture comes from and mark it as being used as mut.
1399 let temp_mpi = self.move_data.rev_lookup.find_local(local);
1400 let init = if let [init_index] = *self.move_data.init_path_map[temp_mpi] {
1401 &self.move_data.inits[init_index]
1403 bug!("temporary should be initialized exactly once")
1406 let InitLocation::Statement(loc) = init.location else {
1407 bug!("temporary initialized in arguments")
1410 let body = self.body;
1411 let bbd = &body[loc.block];
1412 let stmt = &bbd.statements[loc.statement_index];
1413 debug!("temporary assigned in: stmt={:?}", stmt);
1415 if let StatementKind::Assign(box (_, Rvalue::Ref(_, _, source))) = stmt.kind
1417 propagate_closure_used_mut_place(self, source);
1420 "closures should only capture user variables \
1421 or references to user variables"
1425 _ => propagate_closure_used_mut_place(self, place),
1428 Operand::Constant(..) => {}
1435 (operand, span): (&'cx Operand<'tcx>, Span),
1436 flow_state: &Flows<'cx, 'tcx>,
1439 Operand::Copy(place) => {
1440 // copy of place: check if this is "copy of frozen path"
1441 // (FIXME: see check_loans.rs)
1445 (Deep, Read(ReadKind::Copy)),
1446 LocalMutationIsAllowed::No,
1450 // Finally, check if path was already moved.
1451 self.check_if_path_or_subpath_is_moved(
1453 InitializationRequiringAction::Use,
1454 (place.as_ref(), span),
1458 Operand::Move(place) => {
1459 // move of place: check if this is move of already borrowed path
1463 (Deep, Write(WriteKind::Move)),
1464 LocalMutationIsAllowed::Yes,
1468 // Finally, check if path was already moved.
1469 self.check_if_path_or_subpath_is_moved(
1471 InitializationRequiringAction::Use,
1472 (place.as_ref(), span),
1476 Operand::Constant(_) => {}
1480 /// Checks whether a borrow of this place is invalidated when the function
1482 fn check_for_invalidation_at_exit(
1485 borrow: &BorrowData<'tcx>,
1488 debug!("check_for_invalidation_at_exit({:?})", borrow);
1489 let place = borrow.borrowed_place;
1490 let mut root_place = PlaceRef { local: place.local, projection: &[] };
1492 // FIXME(nll-rfc#40): do more precise destructor tracking here. For now
1493 // we just know that all locals are dropped at function exit (otherwise
1494 // we'll have a memory leak) and assume that all statics have a destructor.
1496 // FIXME: allow thread-locals to borrow other thread locals?
1498 let (might_be_alive, will_be_dropped) =
1499 if self.body.local_decls[root_place.local].is_ref_to_thread_local() {
1500 // Thread-locals might be dropped after the function exits
1501 // We have to dereference the outer reference because
1502 // borrows don't conflict behind shared references.
1503 root_place.projection = DEREF_PROJECTION;
1506 (false, self.locals_are_invalidated_at_exit)
1509 if !will_be_dropped {
1510 debug!("place_is_invalidated_at_exit({:?}) - won't be dropped", place);
1514 let sd = if might_be_alive { Deep } else { Shallow(None) };
1516 if places_conflict::borrow_conflicts_with_place(
1523 places_conflict::PlaceConflictBias::Overlap,
1525 debug!("check_for_invalidation_at_exit({:?}): INVALID", place);
1526 // FIXME: should be talking about the region lifetime instead
1527 // of just a span here.
1528 let span = self.infcx.tcx.sess.source_map().end_point(span);
1529 self.report_borrowed_value_does_not_live_long_enough(
1538 /// Reports an error if this is a borrow of local data.
1539 /// This is called for all Yield expressions on movable generators
1540 fn check_for_local_borrow(&mut self, borrow: &BorrowData<'tcx>, yield_span: Span) {
1541 debug!("check_for_local_borrow({:?})", borrow);
1543 if borrow_of_local_data(borrow.borrowed_place) {
1544 let err = self.cannot_borrow_across_generator_yield(
1545 self.retrieve_borrow_spans(borrow).var_or_use(),
1549 self.buffer_error(err);
1553 fn check_activations(&mut self, location: Location, span: Span, flow_state: &Flows<'cx, 'tcx>) {
1554 // Two-phase borrow support: For each activation that is newly
1555 // generated at this statement, check if it interferes with
1557 let borrow_set = self.borrow_set.clone();
1558 for &borrow_index in borrow_set.activations_at_location(location) {
1559 let borrow = &borrow_set[borrow_index];
1561 // only mutable borrows should be 2-phase
1562 assert!(match borrow.kind {
1563 BorrowKind::Shared | BorrowKind::Shallow => false,
1564 BorrowKind::Unique | BorrowKind::Mut { .. } => true,
1569 (borrow.borrowed_place, span),
1570 (Deep, Activation(WriteKind::MutableBorrow(borrow.kind), borrow_index)),
1571 LocalMutationIsAllowed::No,
1574 // We do not need to call `check_if_path_or_subpath_is_moved`
1575 // again, as we already called it when we made the
1576 // initial reservation.
1580 fn check_if_reassignment_to_immutable_state(
1584 place_span: (Place<'tcx>, Span),
1585 flow_state: &Flows<'cx, 'tcx>,
1587 debug!("check_if_reassignment_to_immutable_state({:?})", local);
1589 // Check if any of the initializiations of `local` have happened yet:
1590 if let Some(init_index) = self.is_local_ever_initialized(local, flow_state) {
1591 // And, if so, report an error.
1592 let init = &self.move_data.inits[init_index];
1593 let span = init.span(&self.body);
1594 self.report_illegal_reassignment(location, place_span, span, place_span.0);
1598 fn check_if_full_path_is_moved(
1601 desired_action: InitializationRequiringAction,
1602 place_span: (PlaceRef<'tcx>, Span),
1603 flow_state: &Flows<'cx, 'tcx>,
1605 let maybe_uninits = &flow_state.uninits;
1609 // 1. Move of `a.b.c`, use of `a.b.c`
1610 // 2. Move of `a.b.c`, use of `a.b.c.d` (without first reinitializing `a.b.c.d`)
1611 // 3. Uninitialized `(a.b.c: &_)`, use of `*a.b.c`; note that with
1612 // partial initialization support, one might have `a.x`
1613 // initialized but not `a.b`.
1617 // 4. Move of `a.b.c`, use of `a.b.d`
1618 // 5. Uninitialized `a.x`, initialized `a.b`, use of `a.b`
1619 // 6. Copied `(a.b: &_)`, use of `*(a.b).c`; note that `a.b`
1620 // must have been initialized for the use to be sound.
1621 // 7. Move of `a.b.c` then reinit of `a.b.c.d`, use of `a.b.c.d`
1623 // The dataflow tracks shallow prefixes distinctly (that is,
1624 // field-accesses on P distinctly from P itself), in order to
1625 // track substructure initialization separately from the whole
1628 // E.g., when looking at (*a.b.c).d, if the closest prefix for
1629 // which we have a MovePath is `a.b`, then that means that the
1630 // initialization state of `a.b` is all we need to inspect to
1631 // know if `a.b.c` is valid (and from that we infer that the
1632 // dereference and `.d` access is also valid, since we assume
1633 // `a.b.c` is assigned a reference to an initialized and
1634 // well-formed record structure.)
1636 // Therefore, if we seek out the *closest* prefix for which we
1637 // have a MovePath, that should capture the initialization
1638 // state for the place scenario.
1640 // This code covers scenarios 1, 2, and 3.
1642 debug!("check_if_full_path_is_moved place: {:?}", place_span.0);
1643 let (prefix, mpi) = self.move_path_closest_to(place_span.0);
1644 if maybe_uninits.contains(mpi) {
1645 self.report_use_of_moved_or_uninitialized(
1648 (prefix, place_span.0, place_span.1),
1651 } // Only query longest prefix with a MovePath, not further
1652 // ancestors; dataflow recurs on children when parents
1653 // move (to support partial (re)inits).
1655 // (I.e., querying parents breaks scenario 7; but may want
1656 // to do such a query based on partial-init feature-gate.)
1659 /// Subslices correspond to multiple move paths, so we iterate through the
1660 /// elements of the base array. For each element we check
1662 /// * Does this element overlap with our slice.
1663 /// * Is any part of it uninitialized.
1664 fn check_if_subslice_element_is_moved(
1667 desired_action: InitializationRequiringAction,
1668 place_span: (PlaceRef<'tcx>, Span),
1669 maybe_uninits: &ChunkedBitSet<MovePathIndex>,
1673 if let Some(mpi) = self.move_path_for_place(place_span.0) {
1674 let move_paths = &self.move_data.move_paths;
1676 let root_path = &move_paths[mpi];
1677 for (child_mpi, child_move_path) in root_path.children(move_paths) {
1678 let last_proj = child_move_path.place.projection.last().unwrap();
1679 if let ProjectionElem::ConstantIndex { offset, from_end, .. } = last_proj {
1680 debug_assert!(!from_end, "Array constant indexing shouldn't be `from_end`.");
1682 if (from..to).contains(offset) {
1684 self.move_data.find_in_move_path_or_its_descendants(child_mpi, |mpi| {
1685 maybe_uninits.contains(mpi)
1688 if let Some(uninit_child) = uninit_child {
1689 self.report_use_of_moved_or_uninitialized(
1692 (place_span.0, place_span.0, place_span.1),
1695 return; // don't bother finding other problems.
1703 fn check_if_path_or_subpath_is_moved(
1706 desired_action: InitializationRequiringAction,
1707 place_span: (PlaceRef<'tcx>, Span),
1708 flow_state: &Flows<'cx, 'tcx>,
1710 let maybe_uninits = &flow_state.uninits;
1714 // 1. Move of `a.b.c`, use of `a` or `a.b`
1715 // partial initialization support, one might have `a.x`
1716 // initialized but not `a.b`.
1717 // 2. All bad scenarios from `check_if_full_path_is_moved`
1721 // 3. Move of `a.b.c`, use of `a.b.d`
1722 // 4. Uninitialized `a.x`, initialized `a.b`, use of `a.b`
1723 // 5. Copied `(a.b: &_)`, use of `*(a.b).c`; note that `a.b`
1724 // must have been initialized for the use to be sound.
1725 // 6. Move of `a.b.c` then reinit of `a.b.c.d`, use of `a.b.c.d`
1727 self.check_if_full_path_is_moved(location, desired_action, place_span, flow_state);
1729 if let Some((place_base, ProjectionElem::Subslice { from, to, from_end: false })) =
1730 place_span.0.last_projection()
1732 let place_ty = place_base.ty(self.body(), self.infcx.tcx);
1733 if let ty::Array(..) = place_ty.ty.kind() {
1734 self.check_if_subslice_element_is_moved(
1737 (place_base, place_span.1),
1746 // A move of any shallow suffix of `place` also interferes
1747 // with an attempt to use `place`. This is scenario 3 above.
1749 // (Distinct from handling of scenarios 1+2+4 above because
1750 // `place` does not interfere with suffixes of its prefixes,
1751 // e.g., `a.b.c` does not interfere with `a.b.d`)
1753 // This code covers scenario 1.
1755 debug!("check_if_path_or_subpath_is_moved place: {:?}", place_span.0);
1756 if let Some(mpi) = self.move_path_for_place(place_span.0) {
1757 let uninit_mpi = self
1759 .find_in_move_path_or_its_descendants(mpi, |mpi| maybe_uninits.contains(mpi));
1761 if let Some(uninit_mpi) = uninit_mpi {
1762 self.report_use_of_moved_or_uninitialized(
1765 (place_span.0, place_span.0, place_span.1),
1768 return; // don't bother finding other problems.
1773 /// Currently MoveData does not store entries for all places in
1774 /// the input MIR. For example it will currently filter out
1775 /// places that are Copy; thus we do not track places of shared
1776 /// reference type. This routine will walk up a place along its
1777 /// prefixes, searching for a foundational place that *is*
1778 /// tracked in the MoveData.
1780 /// An Err result includes a tag indicated why the search failed.
1781 /// Currently this can only occur if the place is built off of a
1782 /// static variable, as we do not track those in the MoveData.
1783 fn move_path_closest_to(&mut self, place: PlaceRef<'tcx>) -> (PlaceRef<'tcx>, MovePathIndex) {
1784 match self.move_data.rev_lookup.find(place) {
1785 LookupResult::Parent(Some(mpi)) | LookupResult::Exact(mpi) => {
1786 (self.move_data.move_paths[mpi].place.as_ref(), mpi)
1788 LookupResult::Parent(None) => panic!("should have move path for every Local"),
1792 fn move_path_for_place(&mut self, place: PlaceRef<'tcx>) -> Option<MovePathIndex> {
1793 // If returns None, then there is no move path corresponding
1794 // to a direct owner of `place` (which means there is nothing
1795 // that borrowck tracks for its analysis).
1797 match self.move_data.rev_lookup.find(place) {
1798 LookupResult::Parent(_) => None,
1799 LookupResult::Exact(mpi) => Some(mpi),
1803 fn check_if_assigned_path_is_moved(
1806 (place, span): (Place<'tcx>, Span),
1807 flow_state: &Flows<'cx, 'tcx>,
1809 debug!("check_if_assigned_path_is_moved place: {:?}", place);
1811 // None case => assigning to `x` does not require `x` be initialized.
1812 for (place_base, elem) in place.iter_projections().rev() {
1814 ProjectionElem::Index(_/*operand*/) |
1815 ProjectionElem::ConstantIndex { .. } |
1816 // assigning to P[i] requires P to be valid.
1817 ProjectionElem::Downcast(_/*adt_def*/, _/*variant_idx*/) =>
1818 // assigning to (P->variant) is okay if assigning to `P` is okay
1820 // FIXME: is this true even if P is an adt with a dtor?
1823 // assigning to (*P) requires P to be initialized
1824 ProjectionElem::Deref => {
1825 self.check_if_full_path_is_moved(
1826 location, InitializationRequiringAction::Use,
1827 (place_base, span), flow_state);
1828 // (base initialized; no need to
1833 ProjectionElem::Subslice { .. } => {
1834 panic!("we don't allow assignments to subslices, location: {:?}",
1838 ProjectionElem::Field(..) => {
1839 // if type of `P` has a dtor, then
1840 // assigning to `P.f` requires `P` itself
1841 // be already initialized
1842 let tcx = self.infcx.tcx;
1843 let base_ty = place_base.ty(self.body(), tcx).ty;
1844 match base_ty.kind() {
1845 ty::Adt(def, _) if def.has_dtor(tcx) => {
1846 self.check_if_path_or_subpath_is_moved(
1847 location, InitializationRequiringAction::Assignment,
1848 (place_base, span), flow_state);
1850 // (base initialized; no need to
1855 // Once `let s; s.x = V; read(s.x);`,
1856 // is allowed, remove this match arm.
1857 ty::Adt(..) | ty::Tuple(..) => {
1858 check_parent_of_field(self, location, place_base, span, flow_state);
1860 // rust-lang/rust#21232, #54499, #54986: during period where we reject
1861 // partial initialization, do not complain about unnecessary `mut` on
1862 // an attempt to do a partial initialization.
1863 self.used_mut.insert(place.local);
1872 fn check_parent_of_field<'cx, 'tcx>(
1873 this: &mut MirBorrowckCtxt<'cx, 'tcx>,
1875 base: PlaceRef<'tcx>,
1877 flow_state: &Flows<'cx, 'tcx>,
1879 // rust-lang/rust#21232: Until Rust allows reads from the
1880 // initialized parts of partially initialized structs, we
1881 // will, starting with the 2018 edition, reject attempts
1882 // to write to structs that are not fully initialized.
1884 // In other words, *until* we allow this:
1886 // 1. `let mut s; s.x = Val; read(s.x);`
1888 // we will for now disallow this:
1890 // 2. `let mut s; s.x = Val;`
1894 // 3. `let mut s = ...; drop(s); s.x=Val;`
1896 // This does not use check_if_path_or_subpath_is_moved,
1897 // because we want to *allow* reinitializations of fields:
1898 // e.g., want to allow
1900 // `let mut s = ...; drop(s.x); s.x=Val;`
1902 // This does not use check_if_full_path_is_moved on
1903 // `base`, because that would report an error about the
1904 // `base` as a whole, but in this scenario we *really*
1905 // want to report an error about the actual thing that was
1906 // moved, which may be some prefix of `base`.
1908 // Shallow so that we'll stop at any dereference; we'll
1909 // report errors about issues with such bases elsewhere.
1910 let maybe_uninits = &flow_state.uninits;
1912 // Find the shortest uninitialized prefix you can reach
1913 // without going over a Deref.
1914 let mut shortest_uninit_seen = None;
1915 for prefix in this.prefixes(base, PrefixSet::Shallow) {
1916 let Some(mpi) = this.move_path_for_place(prefix) else { continue };
1918 if maybe_uninits.contains(mpi) {
1920 "check_parent_of_field updating shortest_uninit_seen from {:?} to {:?}",
1921 shortest_uninit_seen,
1924 shortest_uninit_seen = Some((prefix, mpi));
1926 debug!("check_parent_of_field {:?} is definitely initialized", (prefix, mpi));
1930 if let Some((prefix, mpi)) = shortest_uninit_seen {
1931 // Check for a reassignment into an uninitialized field of a union (for example,
1932 // after a move out). In this case, do not report an error here. There is an
1933 // exception, if this is the first assignment into the union (that is, there is
1934 // no move out from an earlier location) then this is an attempt at initialization
1935 // of the union - we should error in that case.
1936 let tcx = this.infcx.tcx;
1937 if base.ty(this.body(), tcx).ty.is_union() {
1938 if this.move_data.path_map[mpi].iter().any(|moi| {
1939 this.move_data.moves[*moi].source.is_predecessor_of(location, this.body)
1945 this.report_use_of_moved_or_uninitialized(
1947 InitializationRequiringAction::PartialAssignment,
1948 (prefix, base, span),
1955 /// Checks the permissions for the given place and read or write kind
1957 /// Returns `true` if an error is reported.
1958 fn check_access_permissions(
1960 (place, span): (Place<'tcx>, Span),
1962 is_local_mutation_allowed: LocalMutationIsAllowed,
1963 flow_state: &Flows<'cx, 'tcx>,
1967 "check_access_permissions({:?}, {:?}, is_local_mutation_allowed: {:?})",
1968 place, kind, is_local_mutation_allowed
1975 Reservation(WriteKind::MutableBorrow(
1976 borrow_kind @ (BorrowKind::Unique | BorrowKind::Mut { .. }),
1978 | Write(WriteKind::MutableBorrow(
1979 borrow_kind @ (BorrowKind::Unique | BorrowKind::Mut { .. }),
1981 let is_local_mutation_allowed = match borrow_kind {
1982 BorrowKind::Unique => LocalMutationIsAllowed::Yes,
1983 BorrowKind::Mut { .. } => is_local_mutation_allowed,
1984 BorrowKind::Shared | BorrowKind::Shallow => unreachable!(),
1986 match self.is_mutable(place.as_ref(), is_local_mutation_allowed) {
1988 self.add_used_mut(root_place, flow_state);
1992 error_access = AccessKind::MutableBorrow;
1993 the_place_err = place_err;
1997 Reservation(WriteKind::Mutate) | Write(WriteKind::Mutate) => {
1998 match self.is_mutable(place.as_ref(), is_local_mutation_allowed) {
2000 self.add_used_mut(root_place, flow_state);
2004 error_access = AccessKind::Mutate;
2005 the_place_err = place_err;
2012 | WriteKind::StorageDeadOrDrop
2013 | WriteKind::MutableBorrow(BorrowKind::Shared)
2014 | WriteKind::MutableBorrow(BorrowKind::Shallow),
2018 | WriteKind::StorageDeadOrDrop
2019 | WriteKind::MutableBorrow(BorrowKind::Shared)
2020 | WriteKind::MutableBorrow(BorrowKind::Shallow),
2022 if self.is_mutable(place.as_ref(), is_local_mutation_allowed).is_err()
2023 && !self.has_buffered_errors()
2025 // rust-lang/rust#46908: In pure NLL mode this code path should be
2026 // unreachable, but we use `delay_span_bug` because we can hit this when
2027 // dereferencing a non-Copy raw pointer *and* have `-Ztreat-err-as-bug`
2028 // enabled. We don't want to ICE for that case, as other errors will have
2029 // been emitted (#52262).
2030 self.infcx.tcx.sess.delay_span_bug(
2033 "Accessing `{:?}` with the kind `{:?}` shouldn't be possible",
2041 // permission checks are done at Reservation point.
2047 | BorrowKind::Mut { .. }
2048 | BorrowKind::Shared
2049 | BorrowKind::Shallow,
2053 // Access authorized
2058 // rust-lang/rust#21232, #54986: during period where we reject
2059 // partial initialization, do not complain about mutability
2060 // errors except for actual mutation (as opposed to an attempt
2061 // to do a partial initialization).
2062 let previously_initialized =
2063 self.is_local_ever_initialized(place.local, flow_state).is_some();
2065 // at this point, we have set up the error reporting state.
2066 if previously_initialized {
2067 self.report_mutability_error(place, span, the_place_err, error_access, location);
2074 fn is_local_ever_initialized(
2077 flow_state: &Flows<'cx, 'tcx>,
2078 ) -> Option<InitIndex> {
2079 let mpi = self.move_data.rev_lookup.find_local(local);
2080 let ii = &self.move_data.init_path_map[mpi];
2082 if flow_state.ever_inits.contains(index) {
2089 /// Adds the place into the used mutable variables set
2090 fn add_used_mut(&mut self, root_place: RootPlace<'tcx>, flow_state: &Flows<'cx, 'tcx>) {
2092 RootPlace { place_local: local, place_projection: [], is_local_mutation_allowed } => {
2093 // If the local may have been initialized, and it is now currently being
2094 // mutated, then it is justified to be annotated with the `mut`
2095 // keyword, since the mutation may be a possible reassignment.
2096 if is_local_mutation_allowed != LocalMutationIsAllowed::Yes
2097 && self.is_local_ever_initialized(local, flow_state).is_some()
2099 self.used_mut.insert(local);
2104 place_projection: _,
2105 is_local_mutation_allowed: LocalMutationIsAllowed::Yes,
2109 place_projection: place_projection @ [.., _],
2110 is_local_mutation_allowed: _,
2112 if let Some(field) = self.is_upvar_field_projection(PlaceRef {
2114 projection: place_projection,
2116 self.used_mut_upvars.push(field);
2122 /// Whether this value can be written or borrowed mutably.
2123 /// Returns the root place if the place passed in is a projection.
2126 place: PlaceRef<'tcx>,
2127 is_local_mutation_allowed: LocalMutationIsAllowed,
2128 ) -> Result<RootPlace<'tcx>, PlaceRef<'tcx>> {
2129 debug!("is_mutable: place={:?}, is_local...={:?}", place, is_local_mutation_allowed);
2130 match place.last_projection() {
2132 let local = &self.body.local_decls[place.local];
2133 match local.mutability {
2134 Mutability::Not => match is_local_mutation_allowed {
2135 LocalMutationIsAllowed::Yes => Ok(RootPlace {
2136 place_local: place.local,
2137 place_projection: place.projection,
2138 is_local_mutation_allowed: LocalMutationIsAllowed::Yes,
2140 LocalMutationIsAllowed::ExceptUpvars => Ok(RootPlace {
2141 place_local: place.local,
2142 place_projection: place.projection,
2143 is_local_mutation_allowed: LocalMutationIsAllowed::ExceptUpvars,
2145 LocalMutationIsAllowed::No => Err(place),
2147 Mutability::Mut => Ok(RootPlace {
2148 place_local: place.local,
2149 place_projection: place.projection,
2150 is_local_mutation_allowed,
2154 Some((place_base, elem)) => {
2156 ProjectionElem::Deref => {
2157 let base_ty = place_base.ty(self.body(), self.infcx.tcx).ty;
2159 // Check the kind of deref to decide
2160 match base_ty.kind() {
2161 ty::Ref(_, _, mutbl) => {
2163 // Shared borrowed data is never mutable
2164 hir::Mutability::Not => Err(place),
2165 // Mutably borrowed data is mutable, but only if we have a
2166 // unique path to the `&mut`
2167 hir::Mutability::Mut => {
2168 let mode = match self.is_upvar_field_projection(place) {
2169 Some(field) if self.upvars[field.index()].by_ref => {
2170 is_local_mutation_allowed
2172 _ => LocalMutationIsAllowed::Yes,
2175 self.is_mutable(place_base, mode)
2179 ty::RawPtr(tnm) => {
2181 // `*const` raw pointers are not mutable
2182 hir::Mutability::Not => Err(place),
2183 // `*mut` raw pointers are always mutable, regardless of
2184 // context. The users have to check by themselves.
2185 hir::Mutability::Mut => Ok(RootPlace {
2186 place_local: place.local,
2187 place_projection: place.projection,
2188 is_local_mutation_allowed,
2192 // `Box<T>` owns its content, so mutable if its location is mutable
2193 _ if base_ty.is_box() => {
2194 self.is_mutable(place_base, is_local_mutation_allowed)
2196 // Deref should only be for reference, pointers or boxes
2197 _ => bug!("Deref of unexpected type: {:?}", base_ty),
2200 // All other projections are owned by their base path, so mutable if
2201 // base path is mutable
2202 ProjectionElem::Field(..)
2203 | ProjectionElem::Index(..)
2204 | ProjectionElem::ConstantIndex { .. }
2205 | ProjectionElem::Subslice { .. }
2206 | ProjectionElem::Downcast(..) => {
2207 let upvar_field_projection = self.is_upvar_field_projection(place);
2208 if let Some(field) = upvar_field_projection {
2209 let upvar = &self.upvars[field.index()];
2211 "is_mutable: upvar.mutability={:?} local_mutation_is_allowed={:?} \
2212 place={:?}, place_base={:?}",
2213 upvar, is_local_mutation_allowed, place, place_base
2215 match (upvar.place.mutability, is_local_mutation_allowed) {
2218 LocalMutationIsAllowed::No
2219 | LocalMutationIsAllowed::ExceptUpvars,
2221 (Mutability::Not, LocalMutationIsAllowed::Yes)
2222 | (Mutability::Mut, _) => {
2223 // Subtle: this is an upvar
2224 // reference, so it looks like
2225 // `self.foo` -- we want to double
2226 // check that the location `*self`
2227 // is mutable (i.e., this is not a
2228 // `Fn` closure). But if that
2229 // check succeeds, we want to
2230 // *blame* the mutability on
2231 // `place` (that is,
2232 // `self.foo`). This is used to
2233 // propagate the info about
2234 // whether mutability declarations
2235 // are used outwards, so that we register
2236 // the outer variable as mutable. Otherwise a
2237 // test like this fails to record the `mut`
2241 // fn foo<F: FnOnce()>(_f: F) { }
2243 // let var = Vec::new();
2250 self.is_mutable(place_base, is_local_mutation_allowed)?;
2252 place_local: place.local,
2253 place_projection: place.projection,
2254 is_local_mutation_allowed,
2259 self.is_mutable(place_base, is_local_mutation_allowed)
2267 /// If `place` is a field projection, and the field is being projected from a closure type,
2268 /// then returns the index of the field being projected. Note that this closure will always
2269 /// be `self` in the current MIR, because that is the only time we directly access the fields
2270 /// of a closure type.
2271 fn is_upvar_field_projection(&self, place_ref: PlaceRef<'tcx>) -> Option<Field> {
2272 path_utils::is_upvar_field_projection(self.infcx.tcx, &self.upvars, place_ref, self.body())
2277 use rustc_errors::ErrorGuaranteed;
2281 pub struct BorrowckErrors<'tcx> {
2282 /// This field keeps track of move errors that are to be reported for given move indices.
2284 /// There are situations where many errors can be reported for a single move out (see #53807)
2285 /// and we want only the best of those errors.
2287 /// The `report_use_of_moved_or_uninitialized` function checks this map and replaces the
2288 /// diagnostic (if there is one) if the `Place` of the error being reported is a prefix of the
2289 /// `Place` of the previous most diagnostic. This happens instead of buffering the error. Once
2290 /// all move errors have been reported, any diagnostics in this map are added to the buffer
2293 /// `BTreeMap` is used to preserve the order of insertions when iterating. This is necessary
2294 /// when errors in the map are being re-added to the error buffer so that errors with the
2295 /// same primary span come out in a consistent order.
2296 buffered_move_errors:
2297 BTreeMap<Vec<MoveOutIndex>, (PlaceRef<'tcx>, DiagnosticBuilder<'tcx, ErrorGuaranteed>)>,
2298 /// Diagnostics to be reported buffer.
2299 buffered: Vec<Diagnostic>,
2300 /// Set to Some if we emit an error during borrowck
2301 tainted_by_errors: Option<ErrorGuaranteed>,
2304 impl BorrowckErrors<'_> {
2305 pub fn new() -> Self {
2307 buffered_move_errors: BTreeMap::new(),
2308 buffered: Default::default(),
2309 tainted_by_errors: None,
2313 // FIXME(eddyb) this is a suboptimal API because `tainted_by_errors` is
2314 // set before any emission actually happens (weakening the guarantee).
2315 pub fn buffer_error(&mut self, t: DiagnosticBuilder<'_, ErrorGuaranteed>) {
2316 self.tainted_by_errors = Some(ErrorGuaranteed::unchecked_claim_error_was_emitted());
2317 t.buffer(&mut self.buffered);
2320 pub fn buffer_non_error_diag(&mut self, t: DiagnosticBuilder<'_, ()>) {
2321 t.buffer(&mut self.buffered);
2324 pub fn set_tainted_by_errors(&mut self) {
2325 self.tainted_by_errors = Some(ErrorGuaranteed::unchecked_claim_error_was_emitted());
2329 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
2330 pub fn buffer_error(&mut self, t: DiagnosticBuilder<'_, ErrorGuaranteed>) {
2331 self.errors.buffer_error(t);
2334 pub fn buffer_non_error_diag(&mut self, t: DiagnosticBuilder<'_, ()>) {
2335 self.errors.buffer_non_error_diag(t);
2338 pub fn buffer_move_error(
2340 move_out_indices: Vec<MoveOutIndex>,
2341 place_and_err: (PlaceRef<'tcx>, DiagnosticBuilder<'tcx, ErrorGuaranteed>),
2343 if let Some((_, diag)) =
2344 self.errors.buffered_move_errors.insert(move_out_indices, place_and_err)
2346 // Cancel the old diagnostic so we don't ICE
2354 pub fn emit_errors(&mut self) -> Option<ErrorGuaranteed> {
2355 // Buffer any move errors that we collected and de-duplicated.
2356 for (_, (_, diag)) in std::mem::take(&mut self.errors.buffered_move_errors) {
2357 // We have already set tainted for this error, so just buffer it.
2358 diag.buffer(&mut self.errors.buffered);
2361 if !self.errors.buffered.is_empty() {
2362 self.errors.buffered.sort_by_key(|diag| diag.sort_span);
2364 for diag in self.errors.buffered.drain(..) {
2365 self.infcx.tcx.sess.diagnostic().emit_diagnostic(&diag);
2369 self.errors.tainted_by_errors
2372 pub fn has_buffered_errors(&self) -> bool {
2373 self.errors.buffered.is_empty()
2376 pub fn has_move_error(
2378 move_out_indices: &[MoveOutIndex],
2379 ) -> Option<&(PlaceRef<'tcx>, DiagnosticBuilder<'cx, ErrorGuaranteed>)> {
2380 self.errors.buffered_move_errors.get(move_out_indices)
2385 /// The degree of overlap between 2 places for borrow-checking.
2387 /// The places might partially overlap - in this case, we give
2388 /// up and say that they might conflict. This occurs when
2389 /// different fields of a union are borrowed. For example,
2390 /// if `u` is a union, we have no way of telling how disjoint
2391 /// `u.a.x` and `a.b.y` are.
2393 /// The places have the same type, and are either completely disjoint
2394 /// or equal - i.e., they can't "partially" overlap as can occur with
2395 /// unions. This is the "base case" on which we recur for extensions
2398 /// The places are disjoint, so we know all extensions of them
2399 /// will also be disjoint.