Move unreachable code lint from HIR type check to a proper lint

[WaffleLapkin]

Proposal

Currently we emit the unreachable code lint inside of rustc_hir_typeck using the same machinery as we use for making diverging blocks !. This has two problems:

• The lint doesn't have full information, as inference hasn't happened yet. See for example unreachable code is not detected if it depends on never type fallback rust#146085
• Changing the lint behavior changes code semantics, because we use the same reachability information for type checking

I propose to fix this by moving the lint code from rustc_hir_typeck to working on THIR (as suggested by @oli-obk when I talked to them) or to a normal late lint.

Mentors or Reviewers

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.
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.

Concerns or objections can formally be registered here by adding a comment.

@rustbot concern reason-for-concern
<description of the concern>

Concerns can be lifted with:

@rustbot resolve reason-for-concern

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler

[Urgau]

@rustbot second