Don't warn on never to any as casts as unreachable

[WaffleLapkin]

I'm doing this while being sleep deprived on a night train, let's hope this is coherent.

Fixes #67227

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[compiler-errors]

This seems reasonable to me.

I don't think it's really particularly important to warn that the as cast is unreachable, even if it was expected to have operational side effects (e.g. an unsizing operation). A more important part is to warn that, e.g., a method call is unreachable, since it's expected that the operation have meaningful side effects.

[compiler-errors]

Not sure if this should be T-lang nominated but if it is, it should be an easy decision. Thoughts?

[BoxyUwU]

on the one hand it technically has lang approval already, on the other hand that was almost 6 years ago and also not an FCP :clueless:

[WaffleLapkin]

I guess just to be safe, let's nominate this for T-lang.

T-lang nomination

This PR makes it so we don't warn on as casts from the never type to any type as unreachable code. This is nice because sometimes those casts are required to appeal the type checker, for example if you want to specify a type for an RPIT:

fn blah() -> impl Iterator<Item = u32> {
    todo!() as std::iter::Empty<_> // just using `todo!()` would't work since `!` does not implement `Iterator`
}

Non-cast expressions are still warned against if they have the cast inside:

fn f() {
    Some(todo!() as u8);
    //~^ warn: unreachable call
}

This was considered a desirable change by T-lang 6 years ago (see #67227), but just to doable check, are you still on board?

[traviscross]

Thanks for the PR and nomination. Seems reasonable to me.

@rfcbot fcp merge

[rfcbot]

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

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

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