replace `box_patterns` in the compiler with `deref_patterns`

[cyrgani]

Proposal

A lot of types in the compiler are wrapped in Boxes. To make matching on them more ergonomic, the box_patterns (tracking issue) feature is used at the moment. The current plan for this feature is that it will not be stabilized; instead, deref_patterns (tracking issue) supersedes it.

I propose to replace the usage of box_patterns in the compiler with deref_patterns. This has several advantages:

• It makes it possible to remove box_patterns entirely in the future
• It removes the need for using the weird box keyword in the compiler, making the code slightly easier to read
• We might find possible bugs in the deref_patterns implementation by using it in practice

The feature was previously moved from incomplete to unstable in rust-lang/rust#153053.

Mentors or Reviewers

I am willing to create the PRs that update the affected compiler crates to use the newer feature.

Process

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

• File an issue describing the proposal.
• A compiler team member who is knowledgeable in the area can second by writing @rustbot second or kickoff a team FCP with @rfcbot fcp $RESOLUTION.
  • Refer to Proposals, Approvals and Stabilization docs for when a second is sufficient, or when a full team FCP is required.
• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
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cc @rust-lang/compiler

[fmease]

@rustbot second

[rustbot]

The final comment period is now complete, this major change is now accepted. 🎉

As the automated representative, I want to express gratitude to the author for their work and everyone else who contributed to this major change proposal.

If you think this major change shouldn't have been accepted, feel free to remove the major-change-accepted label and reopen this issue.

Note

Further progress is now tracked over at rust-lang/rust#156110.