rustdoc: highlight default enum variant

[H2WO4]

Problem

It would be convenient and help readability if enum variants marked with #[default] in the code would be also marked as such in the documentation.

While it is certainly possible to just use a doc comment, it would be better to have a standardized automatic solution.

Proposed Solution

Displaying (default) next to the default variant would be more than sufficient.

Quick mockup of what it could look like:

Additionally, pre- or postfixing the variant with a symbol could also be a viable solution.

Notes

A similar issue was already created in the docs.rs repository but closed as it should have been opened here.

This would only apply to variant using the #[default] attribute; manual Default impls would clearly be out of scope (due to the insanity of parsing the behavior of such a function).

[weihanglo]

The rustdoc binary controls over how the HTML output looks like so this should be submitted to rust-lang/rust where rustdoc resides.

@rustbot transfer rust

[fmease]

This feature request was FCP-closed in #115575. Please see that PR and the linked issue for the rationale.