move leak-check to during coherence, candidate eval

[nikomatsakis]

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

• MCP filed. Automatically, as a result of filing this issue:
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  • A Zulip topic in the stream #t-compiler/major changes will be created for this issue.
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• Final comment period (FCP). Once the MCP is approved, the FCP begins and lasts for 10 days. This is a time for other members to review and raise concerns -- concerns that should block acceptance should be noted as comments on the thread, ideally with a link to Zulip for further discussion.
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  • At this point, the major-change-accepted label is added and the issue is closed. You can link to it for future reference.

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.

TL;DR

• Alter the leak-check to be more precise and to work by examining all placeholders created during a snapshot
• Remove the leak check so it occurs only during candidate evaluation + coherence

Links and Details

• Full write-up available in this gist

Mentors or Reviewers

• Reviewer probably @matthewjasper

[matthewjasper]

@rustbot second

[nikomatsakis]

Closing, this is accepted.