Support borrowed locals in DestinationPropagation.

[cjgillot]

Follow-up to #142915

The current implementation of DestinationPropagation refuses to consider locals that are borrowed at any point in the MIR.

This PR attempts to relax this requirement without trying to perform alias analysis.

For each local, we consider they are borrowed from the first Rvalue::Ref or Rvalue::RawPtr statement until they are marked StorageDead.

The liveness analysis is modified to consider that:

• an indirect write may write to any currently borrowed local;
• an indirect read may read to any currently borrowed local;
• a call/tailcall/drop/yield/inlineasm terminator may read and write to any currently borrowed local.

Future extensions may consider a move to discard borrows, but this requires work on MIR semantics beforehand.

r? @Amanieu

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[tmiasko]

Those conditions are insufficient since they make it possible to observe that storage of unified locals overlaps:

$ cat a.rs 
fn main() {
    let x;
    let y;
    {
        let a = 0;
        x = &a as *const _;
        let b = a;
        y = &b as *const _;
    }
    assert!(x != y);
}
$ rustc +stage1 -O a.rs
$ ./a
thread 'main' (34419) panicked at a.rs:10:5:
assertion failed: x != y
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[Amanieu]

Right, the correct way to calculate the live range of a borrowed local is to start from the assignment that initializes it (at the late position) and end at StorageDead. This needs to be done using a separate forward dataflow analysis, but then the produced live range interval should be compatible with those from the backwards analysis for non-borrowed locals.

[bors]

☔ The latest upstream changes (presumably #142771) made this pull request unmergeable. Please resolve the merge conflicts.