Document `compile_fail,E1234` syntax for marking compile_fail tests as failing with a particular error

[Manishearth]

It's possible to mark tests as failing with a particular error. This largely seems to be used within rustc itself.

/// ```compile_fail,E0425
///  let x = x;
/// ```

Ought we document this in the rustdoc book somewhere? It currently does not appear to be.

[ehuss]

I believe it is documented here: https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html#error-numbers-for-compile-fail-doctests

I don't think it is in the doctests chapter because it is unstable.

(As an aside, I'm curious if ya'll want to stabilize it.)

[Manishearth]

Seems somewhat useful. But there's also the danger of relying on it too much, Rust code can start failing under different errors over time.

[jieyouxu]

I think we shuffle error numbers around quite liberally (i.e. we can change diagnostics to emit one error code over another even in the same major version). As mentioned, it seems like not a good idea outside rustc/alt compiler to depend on the specific error code, otherwise the test can even break over minor versions.

[hkBst]

Since this seems to already be documented as an unstable feature and appetite for stabling it seems low, is there anything left to do here?