remove support for `typeof`

[lcnr]

Proposal

We currently have support for the typeof keyword, resulting in the following error message:

error[E0516]: `typeof` is a reserved keyword but unimplemented
 --> src/lib.rs:1:10
  |
1 | const F: typeof(1) = 1;
  |          ^^^^^^^^^ reserved keyword
  |
help: consider replacing `typeof(...)` with an actual type
  |
1 - const F: typeof(1) = 1;
1 + const F: i32 = 1;
  |

This does provide the benefit of suggesting the proper type, so it may be useful to users, even if it's always a hard error. The same can be achieved via _ as the type of a constant or a return type

error[E0121]: the placeholder `_` is not allowed within types on item signatures for constants
 --> src/lib.rs:1:10
  |
1 | const F: _ = 1;
  |          ^ not allowed in type signatures
  |
help: replace this with a fully-specified type
  |
1 - const F: _ = 1;
1 + const F: i32 = 1;
  |

Supporting this does add a small maintenance cost however:

• we now need to support unnameable types in part of the type system where they otherwise can't exist, making it harder to reason about this code
• we have an additional place where constants can exist in the type system

Given this small cost and the little benefit together with from what I can tell no path towards stabilization, I am in favor of ripping this out. I've implemented this in rust-lang/rust#148256.

Mentors or Reviewers

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Process

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• File an issue describing the proposal.
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• Once an MCP is seconded, the Final Comment Period begins.
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  • 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

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cc @rust-lang/compiler

[oli-obk]

@rustbot second