Handle index projections in call destinations in DSE

[Amanieu]

Since call destinations are evaluated after call arguments, we can't turn copy arguments into moves if the same local is later used as an index projection in the call destination.

DSE call arg optimization: #113758

r? @cjgillot
cc @RalfJung

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[cjgillot]

@bors r+ rollup

[rust-bors]

📌 Commit b15544d has been approved by cjgillot

It is now in the queue for this repository.

[RalfJung]

call destinations are evaluated after call arguments

Are they? Hm I guess in Miri they really are. That seems like a mistake to me. For assignments we evaluate the destination first, and we generally use left-to-right evaluation order.

Do you know what codegen does / whether it even matters there?

[RalfJung]

we can't turn copy arguments into moves if the same local is later used as an index projection in the call destination.

This is a Miri-only problem, right? Is there an example/issue somewhere that this is fixing, or how did you even notice this problem?

[Amanieu]

I don't think we ever generate MIR that triggers this, I only noticed this as a possibility while reviewing existing MIR optimization in preparation for implementing the MIR move optimization.

[RalfJung]

I don't see any reason why a[b] = passthrough_usize(Move(b)) would or should be problematic under our current MIR semantics. So adjusting the optimization here seems a bit premature to me.