Preserve PlaceContext through projections

[ecstatic-morse]

TL;DR

Remove the Projection variants of PlaceContext, instead propagating the original context of the use (e.g., Borrow, Store, etc.) or a newly added Deref variant if that place is dereferenced.

Links and Details

The visit_place, visit_local and similar methods on MIR visitors have access to a PlaceContext so the visitor knows how that Place or Local was being used without having to match on StatementKind or TerminatorKind themselves. However, PlaceContext basically becomes useless as soon as projections are involved. For example, the following uses of x.y are all given MutatingUseContext::Projection, despite being very different in meaning. I want to instead preserve the original use context or a newly added Deref variant.

let b = &mut x.y; // `MutatingUseContext::Borrow`
x.y = 42;         // `MutatingUseContext::Store`
*x.y = 42;        // `MutatingUseContext::Store` for the place `*x.y`, `MutatingUseContext::Deref` for the place `x.y` and the local `x`.

This will allow more code to make use of PlaceContext, which is easier (and thus less prone to breakage) than manually inspecting statement or terminator kinds and/or place projections. Because Projection is used for all projections, the dataflow liveness analysis for locals (which uses PlaceContext) is not as precise as it could be.

Mentors or Reviewers

???

(Not sure who "owns" PlaceContext)

What is this issue?

This is a major change proposal, which means a proposal to make a notable change to the compiler -- one that either alters the architecture of some component, affects a lot of people, or makes a small but noticeable public change (e.g., adding a compiler flag). You can read more about the MCP process on https://forge.rust-lang.org/.

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

MCP Checklist

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A note on stability. If your change is proposing a new stable feature, such as a -C flag, then a full team checkoff will be required before the feature can be landed. Often it is better to start with an unstable flag, like a -Z flag, and then move to stabilize as a secondary step.

[oli-obk]

@rustbot seconded