feat(diagnostics): split TraitImplIncorrectSafety into distinct rustc errors

[ada4a]

No description provided.

[ada4a]

some questions

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[ada4a]

should I split the file into 5 as well?

[ChayimFriedman2]

I feel like this should remain one error but with an enum to distinguish the kind.

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[ada4a]

If I make it just an enum, then the fields file_id and impl_ would be repeated everywhere, which I find unfortunate. What do you think about doing something like this?

struct ImplIncorrectSafety { // since this actually also handles inherent impls
    file_id: HirFileId,
    impl_: AstPtr<ast::Impl>,
    kind: ImplIncorrectSafetyKind,
}

enum ImplIncorrectSafetyKind {
    UnsafeImplOfSafeTrait { trait_: Trait }
    /* ..other variants */
}

[ada4a]

What do you think about doing something like this?

I've gone ahead and implemented that, turned out pretty well imo. I've left it as a separate commit for now -- if you like the approach, please let me know before merging so that I can squash the commits