Deref is not a projection

[ouz-a]

Proposal

Motivation

The MIR uses Place to refer to memory locations. A Place consists of a base, either a static or a local, and a series of projections starting from that base. With one exception, these projections are merely refinements of the base: they specify a field, an array element, or the portion of memory corresponding to a given enum variant. However, the Deref projection is unique in that the target of the Place refers to a completely different region of memory than the base. As a result, several analyses must iterate over each place to see whether that place contains a Deref.

Solution

Convert

let x = (*a.b).c

into

let tmp = a.b;
let x = (*tmp).c

Plan

• Create a new mir-opt(derefer) that does the above-mentioned conversion
• Pull derefer, earlier in the optimization phases
• Create new Rvalue for derefer
• everything above has already been done
• Pull derefer before borrowck
• Make it so MIR Body is built in a way that the transformation becomes unnecessary
• Remove ProjectionElem::Deref entirely.
  • add a deref: bool field to Place, which indicates that the place must be dereferenced before its projections are applied.

Read more about it here

Mentors or Reviewers

@oli-obk

Process

The main points of the Major Change Process are as follows:

• [x] File an issue describing the proposal.
• [ ] A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• [ ] Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

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.

[rustbot]

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.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[eddyb]

@rustbot second _{(hope I'm not overstepping here - this has been one of my major gripes with MIR over the years)}

[safinaskar]

I hope this major change doesn't conflict with deref patterns ( https://github.com/rust-lang/lang-team/issues/77 )

[apiraino]

@rustbot label -final-comment-period +major-change-accepted