Don't show const in docs when it's not available

[quodlibetor]

Fixes#31098

AFAICT this is the only place where rustdoc explicitly checks if we are on stable before emitting content, so I can't tell if this is the sane way to handle this, or if anything else should be done to make sure that nobody forgets to remove this check when const is stabilized.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[alexcrichton]

Does this break rendering other parts like *const T? I think the helper here may be used in more than one spot

[quodlibetor]

It might, although I can't find it in render.rs and all the uses of hir::Constness that I could find were in building methods or functions, but I'll double check that some *cont T still shows up in the output.

Originally when I implemented this I added const filtering to the "build function or method" fns, but that required lying about the constness in the struct. If filtering in the render helper doesn't work out do you have an opinion on what I should do? It's not really lying about the constness of an object if it can't be made const in stable, is it?

[alexcrichton]

After checking it looks like this is indeed only used for const fn, but I'd personally feel a little more comfortable at least if this were a bit more of a targeted conditional. Could this conditional be placed directly where const fn is being handled because that's what this is actually trying to apply to?

Also can this use the get_unstable_features_setting function in the compiler instead of reading the environment variable manually?

[quodlibetor]

Sort of like so?

[alexcrichton]

@bors: r+ d59372b

Indeed, thanks!

[quodlibetor]

Awesome! First rust PR :)