Tracking issue for `on_unimplemented` feature

[aturon]

The on_unimplemented feature provides the #[rustc_on_unimplemented] attribute, which allows trait definitions to add specialized notes to error messages when an implementation was expected but not found.

WARNING: This attribute, as all other rustc_ attributes, is an internal implementation detail of rustc and is not meant to become stable.

[seanmonstar]

I think this would be 'private' to rustc.

[aturon]

@seanmonstar You may well be right, although it may be a useful feature for tailoring error messages around complicated user-space trait setups as well.

[sgrif]

As someone creating a library with complicated user-space trait setups, this would be an enormously useful feature. However I did notice an issue while using it, due to blanket impls.

When adding this line, I would expect this compile fail test to contain "collections::string::String cannot be used in an expression of type yaqb::types::Serial". However, it doesn't even mention the annotated trait, since due to this blanket impl, it doesn't even mention the annotated trait (and therefore doesn't include my note). Only the one that would satisfy the constraints of that blanket impl. (This is likely another manifestation of #28894).

Also worth noting that it's impossible to test these w/ compile-fail at the moment, but that's tangential to this feature.

[SimonSapin]

Is it possible to improve the default diagnostics, without an opt-in feature?

[steveklabnik]

Triage: no real change. apparently servo uses this.

[SimonSapin]

Update: I’m moving usage of nice to have but not really required (optimizations, compiler diagnostics like this, …) unstable features in Servo behind an optional Cargo feature flag.

[Centril]

I would like to see an RFC for getting custom errors a la https://ghc.haskell.org/trac/ghc/wiki/Proposal/CustomTypeErrors in Rust.