Rustdoc shows a _DERIVE_..._FOR_... const when HashStable is derived

[bjorn3]

Eg https://doc.rust-lang.org/nightly/nightly-rustc/rustc/mir/constant._DERIVE_rustc_data_structures_stable_hasher_HashStable_rustc_ich_StableHashingContext_ctx_FOR_CastKind.html

List of all of these items in rustc::mir:

[euclio]

I'm working on this.

[euclio]

So, this is trickier than I thought. The _DERIVE_ constants are an implementation detail of synstructure, which are used to ensure hygiene for the generated impls. They appear because we pass --document-private-items while generating the compiler docs.

I thought that I would be able to add #[doc(hidden)] to synstructure, but unfortunately the --document-private-items flag also removes the strip-hidden pass.

I suppose we have two options:

1. Modify synstructure to add #[doc(hidden)] to the generated constants, and modify rustbuild to keep the strip-hidden pass when generating compiler docs.
2. Wait until synstructure can achieve the same hygiene effect through a feature like underscore constants.