Take MIR Analysis by &mut

[Jarcho]

Proposal

The MIR Analysis trait currently takes &self on all functions. This makes it awkward to use the results of a previous analysis to inform the current one as ResultsCursor requires &mut self; and to make use of pre-allocated buffers.

The case with results cursor is already in use and currently uses RefCell to work around the issue. This works, but loses the compilers guarantee that it won't crash (very unlikely in this case), and results in a large number of unnecessary borrow_mut calls.

Pre-allocated buffers are currently not used, but would be useful for checking potential state. As an example, tracking the possible local targets for each reference/pointer. When doing so, tracking the results of a dereference can result in multiple possible values. When doing multiple dereferences in a row this requires either a buffer to store intermediary results, or very awkward recursion. Leaking a reference from this analysis also requires a work queue.

Related PR: rust-lang/rust#108293

Mentors or Reviewers

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Process

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

• 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

[pnkfelix]

@rustbot second

[apiraino]

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