Make PlaceElem::Index take a Place.

[cjgillot]

Fixes #106141

This makes the handling of Places much more general. I'm not completely sure this is a good idea.

[rustbot]

r? @petrochenkov

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[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

[rustbot]

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[bjorn3]

I believe we actually moved away from arbitrary nesting of places in the past.

[RalfJung]

Yeah, I think this is un-doing a simplification that happened a few years back.

Why are general places in index projections needed to fix #106141?

[JakobDegen]

Yeah, I also feel pretty strongly that we should not do this. For example, this opens up a whole can of worms around evaluation order that I would greatly prefer we don't make any harder on ourselves

[oli-obk]

I would prefer to actually remove the index in favor of having an rvalue for it, similar to the ongoing work to removing the deref projection. Everything else has already been said. I'm going to go ahead and close this, as afaik no on wg-mir-opt wants to move in this direction.