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Showing content from https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/trait.Diagnostic.html below:

Diagnostic in rustc_errors - Rust

pub trait Diagnostic<'a, G: EmissionGuarantee = ErrorGuaranteed> {
    // Required method
    fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, G>;
}

Trait implemented by error types. This is rarely implemented manually. Instead, use #[derive(Diagnostic)] – see rustc_macros::Diagnostic.

When implemented manually, it should be generic over the emission guarantee, i.e.:

impl<'a, G: EmissionGuarantee> Diagnostic<'a, G> for Foo { ... }

rather than being specific:

impl<'a> Diagnostic<'a> for Bar { ... }  impl<'a> Diagnostic<'a, ()> for Baz { ... }

There are two reasons for this.

• A diagnostic like Foo could be emitted at any level – level is passed in to into_diag from outside. Even if in practice it is always emitted at a single level, we let the diagnostic creation/emission site determine the level (by using create_err, emit_warn, etc.) rather than the Diagnostic impl.
• Derived impls are always generic, and it’s good for the hand-written impls to be consistent with them.

Write out as a diagnostic out of DiagCtxt.

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