Rustdoc renders associated constants poorly

[sgrif]

We recently changed Diesel to use associated constants in a few places, and noticed that our docs look a lot worse as a result.

It stands out far too much, and looks nothing like any other section of the docs related to traits. It'd be great if the definition could be inlined and look the same as it does for associated types.

[killercup]

FTR, original implementation was in #39944 and current design suggested in #39944 (comment)

[GuillaumeGomez]

Seems like it has been fixed by #39944 so I close.

[sgrif]

@GuillaumeGomez That was the pull request that introduced this bug, not the one that fixed it. Please re-open this issue.

[nox]

Why are definitions of constants even printed? This leaks implementation details about how opaque wrappers etc are defined. Could we instead display them only if --document-private-items is on?

[ollie27]

I think we can consider this fixed by #53409.