de7a8305ae
By using SCC for better performance, we also have to take into account SCCs whose representative is an existential region but also contains a placeholder. By only checking the representative, we may miss that the loan escapes the function. This can be fixed by picking a better representative, or removing placeholders from the main path. This is the simplest fix: forgo efficiency and traverse the region graph instead of the SCCs.
638 lines
24 KiB
Rust
638 lines
24 KiB
Rust
#![deny(rustc::untranslatable_diagnostic)]
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#![deny(rustc::diagnostic_outside_of_impl)]
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use rustc_data_structures::fx::FxIndexMap;
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use rustc_data_structures::graph::WithSuccessors;
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use rustc_index::bit_set::BitSet;
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use rustc_middle::mir::{
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self, BasicBlock, Body, CallReturnPlaces, Location, Place, TerminatorEdges,
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};
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use rustc_middle::ty::RegionVid;
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use rustc_middle::ty::TyCtxt;
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use rustc_mir_dataflow::impls::{EverInitializedPlaces, MaybeUninitializedPlaces};
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use rustc_mir_dataflow::ResultsVisitable;
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use rustc_mir_dataflow::{self, fmt::DebugWithContext, GenKill};
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use rustc_mir_dataflow::{Analysis, Direction, Results};
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use std::fmt;
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use crate::{places_conflict, BorrowSet, PlaceConflictBias, PlaceExt, RegionInferenceContext};
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/// A tuple with named fields that can hold either the results or the transient state of the
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/// dataflow analyses used by the borrow checker.
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#[derive(Debug)]
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pub struct BorrowckAnalyses<B, U, E> {
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pub borrows: B,
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pub uninits: U,
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pub ever_inits: E,
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}
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/// The results of the dataflow analyses used by the borrow checker.
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pub type BorrowckResults<'mir, 'tcx> = BorrowckAnalyses<
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Results<'tcx, Borrows<'mir, 'tcx>>,
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Results<'tcx, MaybeUninitializedPlaces<'mir, 'tcx>>,
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Results<'tcx, EverInitializedPlaces<'mir, 'tcx>>,
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>;
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/// The transient state of the dataflow analyses used by the borrow checker.
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pub type BorrowckFlowState<'mir, 'tcx> =
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<BorrowckResults<'mir, 'tcx> as ResultsVisitable<'tcx>>::FlowState;
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macro_rules! impl_visitable {
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( $(
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$T:ident { $( $field:ident : $A:ident ),* $(,)? }
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)* ) => { $(
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impl<'tcx, $($A),*, D: Direction> ResultsVisitable<'tcx> for $T<$( Results<'tcx, $A> ),*>
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where
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$( $A: Analysis<'tcx, Direction = D>, )*
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{
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type Direction = D;
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type FlowState = $T<$( $A::Domain ),*>;
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fn new_flow_state(&self, body: &mir::Body<'tcx>) -> Self::FlowState {
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$T {
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$( $field: self.$field.analysis.bottom_value(body) ),*
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}
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}
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fn reset_to_block_entry(
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&self,
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state: &mut Self::FlowState,
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block: BasicBlock,
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) {
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$( state.$field.clone_from(&self.$field.entry_set_for_block(block)); )*
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}
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fn reconstruct_before_statement_effect(
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&mut self,
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state: &mut Self::FlowState,
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stmt: &mir::Statement<'tcx>,
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loc: Location,
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) {
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$( self.$field.analysis
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.apply_before_statement_effect(&mut state.$field, stmt, loc); )*
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}
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fn reconstruct_statement_effect(
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&mut self,
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state: &mut Self::FlowState,
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stmt: &mir::Statement<'tcx>,
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loc: Location,
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) {
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$( self.$field.analysis
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.apply_statement_effect(&mut state.$field, stmt, loc); )*
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}
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fn reconstruct_before_terminator_effect(
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&mut self,
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state: &mut Self::FlowState,
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term: &mir::Terminator<'tcx>,
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loc: Location,
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) {
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$( self.$field.analysis
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.apply_before_terminator_effect(&mut state.$field, term, loc); )*
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}
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fn reconstruct_terminator_effect(
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&mut self,
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state: &mut Self::FlowState,
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term: &mir::Terminator<'tcx>,
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loc: Location,
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) {
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$( self.$field.analysis
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.apply_terminator_effect(&mut state.$field, term, loc); )*
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}
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}
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)* }
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}
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impl_visitable! {
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BorrowckAnalyses { borrows: B, uninits: U, ever_inits: E }
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}
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rustc_index::newtype_index! {
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#[debug_format = "bw{}"]
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pub struct BorrowIndex {}
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}
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/// `Borrows` stores the data used in the analyses that track the flow
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/// of borrows.
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///
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/// It uniquely identifies every borrow (`Rvalue::Ref`) by a
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/// `BorrowIndex`, and maps each such index to a `BorrowData`
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/// describing the borrow. These indexes are used for representing the
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/// borrows in compact bitvectors.
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pub struct Borrows<'a, 'tcx> {
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tcx: TyCtxt<'tcx>,
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body: &'a Body<'tcx>,
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borrow_set: &'a BorrowSet<'tcx>,
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borrows_out_of_scope_at_location: FxIndexMap<Location, Vec<BorrowIndex>>,
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}
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struct OutOfScopePrecomputer<'a, 'tcx> {
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visited: BitSet<mir::BasicBlock>,
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visit_stack: Vec<mir::BasicBlock>,
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body: &'a Body<'tcx>,
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regioncx: &'a RegionInferenceContext<'tcx>,
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borrows_out_of_scope_at_location: FxIndexMap<Location, Vec<BorrowIndex>>,
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}
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impl<'a, 'tcx> OutOfScopePrecomputer<'a, 'tcx> {
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fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
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OutOfScopePrecomputer {
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visited: BitSet::new_empty(body.basic_blocks.len()),
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visit_stack: vec![],
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body,
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regioncx,
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borrows_out_of_scope_at_location: FxIndexMap::default(),
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}
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}
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}
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impl<'tcx> OutOfScopePrecomputer<'_, 'tcx> {
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fn precompute_borrows_out_of_scope(
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&mut self,
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borrow_index: BorrowIndex,
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borrow_region: RegionVid,
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first_location: Location,
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) {
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let first_block = first_location.block;
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let first_bb_data = &self.body.basic_blocks[first_block];
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// This is the first block, we only want to visit it from the creation of the borrow at
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// `first_location`.
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let first_lo = first_location.statement_index;
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let first_hi = first_bb_data.statements.len();
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if let Some(kill_stmt) = self.regioncx.first_non_contained_inclusive(
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borrow_region,
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first_block,
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first_lo,
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first_hi,
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) {
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let kill_location = Location { block: first_block, statement_index: kill_stmt };
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// If region does not contain a point at the location, then add to list and skip
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// successor locations.
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debug!("borrow {:?} gets killed at {:?}", borrow_index, kill_location);
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self.borrows_out_of_scope_at_location
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.entry(kill_location)
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.or_default()
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.push(borrow_index);
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// The borrow is already dead, there is no need to visit other blocks.
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return;
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}
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// The borrow is not dead. Add successor BBs to the work list, if necessary.
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for succ_bb in first_bb_data.terminator().successors() {
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if self.visited.insert(succ_bb) {
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self.visit_stack.push(succ_bb);
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}
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}
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// We may end up visiting `first_block` again. This is not an issue: we know at this point
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// that it does not kill the borrow in the `first_lo..=first_hi` range, so checking the
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// `0..first_lo` range and the `0..first_hi` range give the same result.
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while let Some(block) = self.visit_stack.pop() {
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let bb_data = &self.body[block];
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let num_stmts = bb_data.statements.len();
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if let Some(kill_stmt) =
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self.regioncx.first_non_contained_inclusive(borrow_region, block, 0, num_stmts)
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{
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let kill_location = Location { block, statement_index: kill_stmt };
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// If region does not contain a point at the location, then add to list and skip
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// successor locations.
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debug!("borrow {:?} gets killed at {:?}", borrow_index, kill_location);
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self.borrows_out_of_scope_at_location
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.entry(kill_location)
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.or_default()
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.push(borrow_index);
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// We killed the borrow, so we do not visit this block's successors.
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continue;
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}
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// Add successor BBs to the work list, if necessary.
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for succ_bb in bb_data.terminator().successors() {
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if self.visited.insert(succ_bb) {
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self.visit_stack.push(succ_bb);
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}
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}
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}
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self.visited.clear();
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}
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}
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// This is `pub` because it's used by unstable external borrowck data users, see `consumers.rs`.
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pub fn calculate_borrows_out_of_scope_at_location<'tcx>(
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body: &Body<'tcx>,
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regioncx: &RegionInferenceContext<'tcx>,
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borrow_set: &BorrowSet<'tcx>,
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) -> FxIndexMap<Location, Vec<BorrowIndex>> {
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let mut prec = OutOfScopePrecomputer::new(body, regioncx);
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for (borrow_index, borrow_data) in borrow_set.iter_enumerated() {
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let borrow_region = borrow_data.region;
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let location = borrow_data.reserve_location;
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prec.precompute_borrows_out_of_scope(borrow_index, borrow_region, location);
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}
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prec.borrows_out_of_scope_at_location
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}
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struct PoloniusOutOfScopePrecomputer<'a, 'tcx> {
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visited: BitSet<mir::BasicBlock>,
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visit_stack: Vec<mir::BasicBlock>,
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body: &'a Body<'tcx>,
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regioncx: &'a RegionInferenceContext<'tcx>,
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loans_out_of_scope_at_location: FxIndexMap<Location, Vec<BorrowIndex>>,
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}
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impl<'a, 'tcx> PoloniusOutOfScopePrecomputer<'a, 'tcx> {
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fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
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Self {
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visited: BitSet::new_empty(body.basic_blocks.len()),
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visit_stack: vec![],
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body,
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regioncx,
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loans_out_of_scope_at_location: FxIndexMap::default(),
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}
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}
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}
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impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
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/// Loans are in scope while they are live: whether they are contained within any live region.
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/// In the location-insensitive analysis, a loan will be contained in a region if the issuing
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/// region can reach it in the subset graph. So this is a reachability problem.
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fn precompute_loans_out_of_scope(
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&mut self,
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loan_idx: BorrowIndex,
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issuing_region: RegionVid,
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loan_issued_at: Location,
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) {
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let sccs = self.regioncx.constraint_sccs();
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let universal_regions = self.regioncx.universal_regions();
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// We first handle the cases where the loan doesn't go out of scope, depending on the issuing
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// region's successors.
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for successor in self.regioncx.region_graph().depth_first_search(issuing_region) {
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// 1. Via applied member constraints
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//
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// The issuing region can flow into the choice regions, and they are either:
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// - placeholders or free regions themselves,
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// - or also transitively outlive a free region.
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//
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// That is to say, if there are applied member constraints here, the loan escapes the
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// function and cannot go out of scope. We could early return here.
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//
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// For additional insurance via fuzzing and crater, we verify that the constraint's min
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// choice indeed escapes the function. In the future, we could e.g. turn this check into
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// a debug assert and early return as an optimization.
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let scc = sccs.scc(successor);
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for constraint in self.regioncx.applied_member_constraints(scc) {
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if universal_regions.is_universal_region(constraint.min_choice) {
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return;
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}
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}
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// 2. Via regions that are live at all points: placeholders and free regions.
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//
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// If the issuing region outlives such a region, its loan escapes the function and
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// cannot go out of scope. We can early return.
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if self.regioncx.is_region_live_at_all_points(successor) {
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return;
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}
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}
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let first_block = loan_issued_at.block;
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let first_bb_data = &self.body.basic_blocks[first_block];
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// The first block we visit is the one where the loan is issued, starting from the statement
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// where the loan is issued: at `loan_issued_at`.
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let first_lo = loan_issued_at.statement_index;
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let first_hi = first_bb_data.statements.len();
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if let Some(kill_location) =
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self.loan_kill_location(loan_idx, loan_issued_at, first_block, first_lo, first_hi)
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{
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debug!("loan {:?} gets killed at {:?}", loan_idx, kill_location);
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self.loans_out_of_scope_at_location.entry(kill_location).or_default().push(loan_idx);
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// The loan dies within the first block, we're done and can early return.
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return;
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}
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// The loan is not dead. Add successor BBs to the work list, if necessary.
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for succ_bb in first_bb_data.terminator().successors() {
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if self.visited.insert(succ_bb) {
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self.visit_stack.push(succ_bb);
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}
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}
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// We may end up visiting `first_block` again. This is not an issue: we know at this point
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// that the loan is not killed in the `first_lo..=first_hi` range, so checking the
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// `0..first_lo` range and the `0..first_hi` range gives the same result.
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while let Some(block) = self.visit_stack.pop() {
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let bb_data = &self.body[block];
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let num_stmts = bb_data.statements.len();
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if let Some(kill_location) =
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self.loan_kill_location(loan_idx, loan_issued_at, block, 0, num_stmts)
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{
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debug!("loan {:?} gets killed at {:?}", loan_idx, kill_location);
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self.loans_out_of_scope_at_location
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.entry(kill_location)
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.or_default()
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.push(loan_idx);
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// The loan dies within this block, so we don't need to visit its successors.
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continue;
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}
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// Add successor BBs to the work list, if necessary.
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for succ_bb in bb_data.terminator().successors() {
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if self.visited.insert(succ_bb) {
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self.visit_stack.push(succ_bb);
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}
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}
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}
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self.visited.clear();
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assert!(self.visit_stack.is_empty(), "visit stack should be empty");
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}
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/// Returns the lowest statement in `start..=end`, where the loan goes out of scope, if any.
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/// This is the statement where the issuing region can't reach any of the regions that are live
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/// at this point.
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fn loan_kill_location(
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&self,
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loan_idx: BorrowIndex,
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loan_issued_at: Location,
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block: BasicBlock,
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start: usize,
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end: usize,
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) -> Option<Location> {
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for statement_index in start..=end {
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let location = Location { block, statement_index };
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// Check whether the issuing region can reach local regions that are live at this point:
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// - a loan is always live at its issuing location because it can reach the issuing
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// region, which is always live at this location.
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if location == loan_issued_at {
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continue;
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}
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// - the loan goes out of scope at `location` if it's not contained within any regions
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// live at this point.
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//
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// FIXME: if the issuing region `i` can reach a live region `r` at point `p`, and `r` is
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// live at point `q`, then it's guaranteed that `i` would reach `r` at point `q`.
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// Reachability is location-insensitive, and we could take advantage of that, by jumping
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// to a further point than just the next statement: we can jump to the furthest point
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// within the block where `r` is live.
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if self.regioncx.is_loan_live_at(loan_idx, location) {
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continue;
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}
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// No live region is reachable from the issuing region: the loan is killed at this
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// point.
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return Some(location);
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}
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None
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}
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}
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|
|
impl<'a, 'tcx> Borrows<'a, 'tcx> {
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pub fn new(
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tcx: TyCtxt<'tcx>,
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body: &'a Body<'tcx>,
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regioncx: &'a RegionInferenceContext<'tcx>,
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borrow_set: &'a BorrowSet<'tcx>,
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) -> Self {
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let mut borrows_out_of_scope_at_location =
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calculate_borrows_out_of_scope_at_location(body, regioncx, borrow_set);
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|
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// The in-tree polonius analysis computes loans going out of scope using the set-of-loans
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// model, and makes sure they're identical to the existing computation of the set-of-points
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// model.
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if tcx.sess.opts.unstable_opts.polonius.is_next_enabled() {
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let mut polonius_prec = PoloniusOutOfScopePrecomputer::new(body, regioncx);
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for (loan_idx, loan_data) in borrow_set.iter_enumerated() {
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let issuing_region = loan_data.region;
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let loan_issued_at = loan_data.reserve_location;
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polonius_prec.precompute_loans_out_of_scope(
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loan_idx,
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issuing_region,
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loan_issued_at,
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);
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}
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assert_eq!(
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borrows_out_of_scope_at_location, polonius_prec.loans_out_of_scope_at_location,
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"the loans out of scope must be the same as the borrows out of scope"
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);
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borrows_out_of_scope_at_location = polonius_prec.loans_out_of_scope_at_location;
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}
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Borrows { tcx, body, borrow_set, borrows_out_of_scope_at_location }
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}
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pub fn location(&self, idx: BorrowIndex) -> &Location {
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&self.borrow_set[idx].reserve_location
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}
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/// Add all borrows to the kill set, if those borrows are out of scope at `location`.
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/// That means they went out of a nonlexical scope
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|
fn kill_loans_out_of_scope_at_location(
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&self,
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trans: &mut impl GenKill<BorrowIndex>,
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location: Location,
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) {
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// NOTE: The state associated with a given `location`
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// reflects the dataflow on entry to the statement.
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// Iterate over each of the borrows that we've precomputed
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// to have went out of scope at this location and kill them.
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//
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// We are careful always to call this function *before* we
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// set up the gen-bits for the statement or
|
|
// terminator. That way, if the effect of the statement or
|
|
// terminator *does* introduce a new loan of the same
|
|
// region, then setting that gen-bit will override any
|
|
// potential kill introduced here.
|
|
if let Some(indices) = self.borrows_out_of_scope_at_location.get(&location) {
|
|
trans.kill_all(indices.iter().copied());
|
|
}
|
|
}
|
|
|
|
/// Kill any borrows that conflict with `place`.
|
|
fn kill_borrows_on_place(&self, trans: &mut impl GenKill<BorrowIndex>, place: Place<'tcx>) {
|
|
debug!("kill_borrows_on_place: place={:?}", place);
|
|
|
|
let other_borrows_of_local = self
|
|
.borrow_set
|
|
.local_map
|
|
.get(&place.local)
|
|
.into_iter()
|
|
.flat_map(|bs| bs.iter())
|
|
.copied();
|
|
|
|
// If the borrowed place is a local with no projections, all other borrows of this
|
|
// local must conflict. This is purely an optimization so we don't have to call
|
|
// `places_conflict` for every borrow.
|
|
if place.projection.is_empty() {
|
|
if !self.body.local_decls[place.local].is_ref_to_static() {
|
|
trans.kill_all(other_borrows_of_local);
|
|
}
|
|
return;
|
|
}
|
|
|
|
// By passing `PlaceConflictBias::NoOverlap`, we conservatively assume that any given
|
|
// pair of array indices are not equal, so that when `places_conflict` returns true, we
|
|
// will be assured that two places being compared definitely denotes the same sets of
|
|
// locations.
|
|
let definitely_conflicting_borrows = other_borrows_of_local.filter(|&i| {
|
|
places_conflict(
|
|
self.tcx,
|
|
self.body,
|
|
self.borrow_set[i].borrowed_place,
|
|
place,
|
|
PlaceConflictBias::NoOverlap,
|
|
)
|
|
});
|
|
|
|
trans.kill_all(definitely_conflicting_borrows);
|
|
}
|
|
}
|
|
|
|
impl<'tcx> rustc_mir_dataflow::AnalysisDomain<'tcx> for Borrows<'_, 'tcx> {
|
|
type Domain = BitSet<BorrowIndex>;
|
|
|
|
const NAME: &'static str = "borrows";
|
|
|
|
fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
|
|
// bottom = nothing is reserved or activated yet;
|
|
BitSet::new_empty(self.borrow_set.len())
|
|
}
|
|
|
|
fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
|
|
// no borrows of code region_scopes have been taken prior to
|
|
// function execution, so this method has no effect.
|
|
}
|
|
}
|
|
|
|
/// Forward dataflow computation of the set of borrows that are in scope at a particular location.
|
|
/// - we gen the introduced loans
|
|
/// - we kill loans on locals going out of (regular) scope
|
|
/// - we kill the loans going out of their region's NLL scope: in NLL terms, the frontier where a
|
|
/// region stops containing the CFG points reachable from the issuing location.
|
|
/// - we also kill loans of conflicting places when overwriting a shared path: e.g. borrows of
|
|
/// `a.b.c` when `a` is overwritten.
|
|
impl<'tcx> rustc_mir_dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
|
|
type Idx = BorrowIndex;
|
|
|
|
fn domain_size(&self, _: &mir::Body<'tcx>) -> usize {
|
|
self.borrow_set.len()
|
|
}
|
|
|
|
fn before_statement_effect(
|
|
&mut self,
|
|
trans: &mut impl GenKill<Self::Idx>,
|
|
_statement: &mir::Statement<'tcx>,
|
|
location: Location,
|
|
) {
|
|
self.kill_loans_out_of_scope_at_location(trans, location);
|
|
}
|
|
|
|
fn statement_effect(
|
|
&mut self,
|
|
trans: &mut impl GenKill<Self::Idx>,
|
|
stmt: &mir::Statement<'tcx>,
|
|
location: Location,
|
|
) {
|
|
match &stmt.kind {
|
|
mir::StatementKind::Assign(box (lhs, rhs)) => {
|
|
if let mir::Rvalue::Ref(_, _, place) = rhs {
|
|
if place.ignore_borrow(
|
|
self.tcx,
|
|
self.body,
|
|
&self.borrow_set.locals_state_at_exit,
|
|
) {
|
|
return;
|
|
}
|
|
let index = self.borrow_set.get_index_of(&location).unwrap_or_else(|| {
|
|
panic!("could not find BorrowIndex for location {location:?}");
|
|
});
|
|
|
|
trans.gen(index);
|
|
}
|
|
|
|
// Make sure there are no remaining borrows for variables
|
|
// that are assigned over.
|
|
self.kill_borrows_on_place(trans, *lhs);
|
|
}
|
|
|
|
mir::StatementKind::StorageDead(local) => {
|
|
// Make sure there are no remaining borrows for locals that
|
|
// are gone out of scope.
|
|
self.kill_borrows_on_place(trans, Place::from(*local));
|
|
}
|
|
|
|
mir::StatementKind::FakeRead(..)
|
|
| mir::StatementKind::SetDiscriminant { .. }
|
|
| mir::StatementKind::Deinit(..)
|
|
| mir::StatementKind::StorageLive(..)
|
|
| mir::StatementKind::Retag { .. }
|
|
| mir::StatementKind::PlaceMention(..)
|
|
| mir::StatementKind::AscribeUserType(..)
|
|
| mir::StatementKind::Coverage(..)
|
|
| mir::StatementKind::Intrinsic(..)
|
|
| mir::StatementKind::ConstEvalCounter
|
|
| mir::StatementKind::Nop => {}
|
|
}
|
|
}
|
|
|
|
fn before_terminator_effect(
|
|
&mut self,
|
|
trans: &mut impl GenKill<Self::Idx>,
|
|
_terminator: &mir::Terminator<'tcx>,
|
|
location: Location,
|
|
) {
|
|
self.kill_loans_out_of_scope_at_location(trans, location);
|
|
}
|
|
|
|
fn terminator_effect<'mir>(
|
|
&mut self,
|
|
trans: &mut Self::Domain,
|
|
terminator: &'mir mir::Terminator<'tcx>,
|
|
_location: Location,
|
|
) -> TerminatorEdges<'mir, 'tcx> {
|
|
if let mir::TerminatorKind::InlineAsm { operands, .. } = &terminator.kind {
|
|
for op in operands {
|
|
if let mir::InlineAsmOperand::Out { place: Some(place), .. }
|
|
| mir::InlineAsmOperand::InOut { out_place: Some(place), .. } = *op
|
|
{
|
|
self.kill_borrows_on_place(trans, place);
|
|
}
|
|
}
|
|
}
|
|
terminator.edges()
|
|
}
|
|
|
|
fn call_return_effect(
|
|
&mut self,
|
|
_trans: &mut impl GenKill<Self::Idx>,
|
|
_block: mir::BasicBlock,
|
|
_return_places: CallReturnPlaces<'_, 'tcx>,
|
|
) {
|
|
}
|
|
}
|
|
|
|
impl DebugWithContext<Borrows<'_, '_>> for BorrowIndex {
|
|
fn fmt_with(&self, ctxt: &Borrows<'_, '_>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
|
|
write!(f, "{:?}", ctxt.location(*self))
|
|
}
|
|
}
|