Edition 2024: Make ! fall back to !

[WaffleLapkin]

This PR changes never type fallback to be ! (the never type itself) in the next, 2024, edition.

This makes the never type's behavior more intuitive (in 2024 edition) and is the first step of the path to stabilize it.

r? @compiler-errors

[WaffleLapkin]

Nominating for T-lang discussion. This PR changes never type fallback in the 2024 edition. I'm asking for approval, given that this is part of the following plan:

1. Prior to the stabilization of Rust 2024:
   • Change fallback to ! on the nightly 2024 edition (this PR).
   • Add a lint against fallback affecting a generic that is passed to an unsafe function.
     • Perhaps make this lint deny-by-default or a hard error in Rust 2024.
   • Add a future incompat lint for some/most of the code that will be broken by fallback to ! (i.e. check trait obligations on diverging type variables which are affected by fallback).
   • Add documentation/migration tools for people adopting 2024 edition.
2. Stabilize Rust 2024.
3. At a later date when 2024 edition adoption is widespread:
   • Make the breaking change to fall back to ! always everywhere.
   • Change Infallible = !.
   • Stabilize ! (!).

[crlf0710]

Might worth add #66173 and #66757 as tests.

fn test66173() {
     let x = match true {
          true => Default::default(),
          false => panic!("..."),
     };
     // print `x` type
}

fn test66757() {
     struct E;

     impl From<!> for E {
          fn from(_: !) -> E {
               E
          }
     }

     #[allow(unreachable_code)]
     fn foo(never: !) {
          <E as From<!>>::from(never);  // Ok
          <E as From<_>>::from(never);  // Should the inference fail?
     }
}

[nikomatsakis]

@rfcbot merge

This PR changes fallback to ! in Rust 2024. This has raised concerns about two particular kinds of problems, and we have plans to add two lints to catch common errors

• Add a lint against fallback affecting a generic that is passed to an unsafe function.
• Add a future incompat lint for some/most of the code that will be broken by fallback to ! (i.e. check trait obligations on diverging type variables which are affected by fallback).

These lints are valuable in all editions because they both address "suspicious" / error-prone code. They also catch the cases that could change behavior. Ideally we'll have them before we get to the new edition, if not, we'll want to revisit whether to proceed.

[rfcbot]

Team member @nikomatsakis has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[WaffleLapkin]

@crlf0710 do those tests look good to you?

[tmandry]

I'm happy with doing this, contingent on having mitigations to avoid unsafe code changing behavior.

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[WaffleLapkin]

@tmandry fyi the unsafe mitigation lint is being implemented in #123939. Needs a bit more polish, but it is generally working

c:

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.