rustdoc: Header color has regressed

[camelid]

They used to be blue and are now black. I don't think this change was intentional, although we have been thinking about migrating more headers to be black recently. I think this regressed very recently, since I probably would have noticed it otherwise.

[camelid]

I wonder if this was caused by #91062 or #91223? Those were both merged recently, though the second one doesn't seem like it could have changed colors.

cc @jsha @GuillaumeGomez

[jsha]

This regressed in #91179, which added .section-header a as one of the rules for an anchor to be styled black.

The goal with that change was to make the section headings on module pages black (e.g. https://doc.rust-lang.org/nightly/std/index.html#macros). Those have class="section-header". However, headings generated from Markdown in a docblock also have class="section-header".

I think the right short-term fix is:

• Section headings on module pages get class="small-section-header", like their peers on other pages.
• Remove the CSS rule for .section-header a to be black.

It's not really clear to me why Markdown gets section-header, and Rustdoc-generated headings get small-section-header, particularly given that the Rustdoc headings are almost always bigger. Probably we should rename these to docblock-prose-heading and rustdoc-prose-heading.

[camelid]

@jsha Since you figured out a way to fix this regression, do you want to assign yourself and open a PR with the fix?

[jsha]

Will do.