rustdoc: the required const feature gate is not shown

[RalfJung]

For regular unstable functions, rustdoc shows something like this

However, for const-unstable functions, all we get is

There's no indication of what the feature gate is. Worse, if the function is unstable and const-unstable, but under different feature gates, then there's not even the "const: unstable" that links to the issue -- there's no way to figure out what the const feature gate is other than looking at the source.

Cc @rust-lang/rustdoc

[RalfJung]

Turns out the "const: unstable" has a tooltip indicating the const feature gate. But that can't be used for copy-paste, and it is quite hard to notice.

[lolbinarycat]

The biggest problem I see is how to handle this on mobile devices.

Also, function and method pages are rendered quite differently.

[RalfJung]

The biggest problem I see is how to handle this on mobile devices.

Well today we use a tooltip which doesn't work well at all here...

Also, if we can fix this for desktop users only, that's still better than the status quo. Most of us don't program on a mobile device, I assume, and so having the docs work well on large formfactors is quite relevant.

[lolbinarycat]

yeah, i was thinking we could just tweak the current rendering to (const: unstable, feature_flag_name), and then simply hide the feature flag name on mobile devices using css.

[GuillaumeGomez]

Shouldn't be a tooltip but full-fledged text thing like feature-gate items.