rustdoc: show attributes on enum variants

[lolbinarycat]

mostly for #[non_exhaustive]

unsure if there's any attributes we should take care to not include, it could use render_code_attribute and is_non_exhaustive instead, if that is a concern.

fixes #142599

[rustbot]

r? @GuillaumeGomez

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[GuillaumeGomez]

Looks good to me, thanks! Since @fmease opened the issue, I'll let them take a look before r+ing.

[fmease]

Attribute-related changes look fine to me.

It might be worth noting that for non-exhaustive structs, below the "introductory code snippet" we say Fields (Non-exhaustive) and have a <details> section explaining what that implies whereas for enum variants, we just say Fields but we do have that same <details> section. Not sure if y'all want to address that in this PR.

(Does this show up in Aliased Type sections? Just curious. I know that we only allowlist repr for aliased enums and structs and we should include #[non_exhaustive] at some point but that's a another story for another day).

[GuillaumeGomez]

Normally it should since #140863 which unified alias rendering with the other ADTs. However I suppose adding a test to ensure it is rendering as expected would be a good idea.

Can you add a type alias on the enum in your rustdoc test @lolbinarycat please?

[lolbinarycat]

Yes it shows up in the aliased type. Added a test for that (unfortunately I couldn't make it super robust due to not having access to all of XPath)