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Rollup merge of #107271 - Zeegomo:drop-rmw, r=oli-obk

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Treat Drop as a rmw operation

Previously, a Drop terminator was considered a move in MIR. This commit changes the behavior to only treat Drop as a mutable access to the dropped place.

In order for this change to be correct, we need to guarantee that

1.  A dropped value won't be used again
   2.  Places that appear in a drop won't be used again before a
     subsequent initialization.

We can ensure this to be correct at MIR construction because Drop will only be emitted when a variable goes out of scope, thus having:
*   (1) as there is no way of reaching the old value. drop-elaboration
     will also remove any uninitialized drop.
 * (2) as the place can't be named following the end of the scope.

However, the initialization status, previously tracked by moves, should also be tied to the execution of a Drop, hence the additional logic in the dataflow analyses.

From discussion in [this thread](https://rust-lang.zulipchat.com/#narrow/stream/233931-t-compiler.2Fmajor-changes/topic/.60DROP.60.20to.20.60DROP_IF.60.20compiler-team.23558), originating from https://github.com/rust-lang/compiler-team/issues/558.
See also https://github.com/rust-lang/rust/pull/104488#discussion_r1085556010
This commit is contained in:
Matthias Krüger 2023-02-08 18:32:41 +01:00 • committed by GitHub
commit 05748c66a0
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4 changed files with 46 additions and 9 deletions
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6
compiler/rustc_mir_dataflow/src/value_analysis.rs
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@ -223,13 +223,13 @@ pub trait ValueAnalysis<'tcx> {
self.super_terminator(terminator, state)
}
fn super_terminator(&self, terminator: &Terminator<'tcx>, _state: &mut State<Self::Value>) {
fn super_terminator(&self, terminator: &Terminator<'tcx>, state: &mut State<Self::Value>) {
match &terminator.kind {
TerminatorKind::Call { .. } | TerminatorKind::InlineAsm { .. } => {
// Effect is applied by `handle_call_return`.
}
TerminatorKind::Drop { .. } => {
// We don't track dropped places.
TerminatorKind::Drop { place, .. } => {
state.flood_with(place.as_ref(), self.map(), Self::Value::bottom());
}
TerminatorKind::DropAndReplace { .. } | TerminatorKind::Yield { .. } => {
// They would have an effect, but are not allowed in this phase.