rustdoc: make #[doc(hidden)] render the same as other attrs

[karolzwolak]

Makes #[doc(hidden)] render the same as other attrs, instead of being rendered it the same line like visibility specifiers (pub etc.).
This only has effect when unstable flag document-hidden-items is on, as otherwise items with this attr wouldn't be rendered at all.
Also makes #[doc(hidden)] render for macro_rules!.

Fixes #132304.
r? @GuillaumeGomez

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[GuillaumeGomez]

Normally we have a specific display for this attribute, so we don't want to display it with the other attributes.

[karolzwolak]

So you're saying that we don't actually want this attribute to look the same as other attributes?
How do you propose we differentiate it then?
Maybe we could add 👻 emoji next to the attribute like we do in other places?

[GuillaumeGomez]

It's the same for private items: it's only displayed at the module levels for types. We can add a sentence under the type declaration mentioning (with the ghost for the hidden items) that it's a hidden item.

[fmease]

Since I created the linked issue, my 2¢ are that users rarely want to use --document-hidden-items (regardless of whether it will ever be stabilized or not) and if they do, they likely only want to double-check things I'm assuming. Therefore, rendering doc(hidden) like a normal attribute – which it is – is more than sufficient and in my opinion we needn't spend any extra effort designing and maintaining a special display for it.

[GuillaumeGomez]

I agree with you. However, doc attributes are always rendered differently and not supposed to be displayed as is in the documentation as they convey extra info that needs to be displayed differently. Displayed attributes are about how they "modify" the item they're applied to, whereas doc attributes only impact how they're displayed in the docs.

So as such, I think doc(hidden) should be handled differently, like I suggested above for example.

[bors]

☔ The latest upstream changes (presumably #146879) made this pull request unmergeable. Please resolve the merge conflicts.