Improve rustdoc const bounds

[fee1-dead]

• Rustdoc no longer displays ~const in trait bounds, because it currently means nothing for stable users, and because we still haven't decided on the final syntax yet.
• Rustdoc will hide trait bounds where the trait is Drop AND it is ~const, i.e. ~const Drop bounds because it has no effect on stable users as well.
• Because of additional logic that hides the whole where statement where it consists of ~const Drop bounds (so it doesn't display struct Foo<T>() where ; like that), bounds that have no trait e.g. where [T; N+1]: ; are also hidden.

Cherry-picked from #92433.

[rust-highfive]

r? @GuillaumeGomez

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[GuillaumeGomez]

Looks good to me overall, thanks! Your PR seems to done a bit more than what the test you updated seems to check. Can you add more checks please?

Also cc @camelid in case you want to take a second look. :)

[camelid]

Please also add some tests of the expected behavior, e.g. for skipping the ~const part or excluding ~const Drop.

[fee1-dead]

I have addressed the comments and added a test for the display.

[GuillaumeGomez]

Looks good to me, thanks! Waiting for @camelid now. ;)

[camelid]

Sorry, I don't have time to review the code. Could you summarize the current version of the changes in this PR?

[fee1-dead]

• Hide ~const in rustdoc, it is provisional syntax.
• Hide ~const Drop bounds completely because they do not mean anything to stable users.

[fee1-dead]

It also hides const generic wf bounds: [T; SIZE]: (no trait) will be hidden

[camelid]

Sounds reasonable. Can you add something to the tracking issue for ~const about reconsidering the rustdoc display when stabilizing? One more thing: are ~const Trait bounds no-ops for stable users or are they interpreted as regular Trait bounds?

[fee1-dead]

Sure, I'll update the issue when I have access to a computer.

As for : ~const Trait they behave as : Trait to stable users. Only : ~const Drop is a no-op to stable users.

[fee1-dead]

@camelid: Did you want to review this PR when you have time again? Or do you think it is fine as is?

[camelid]

The changes sound good to me. I haven't reviewed the code, but as long as Guillaume has, I don't think I need to. It'd be good to update the PR description with your extended description of the changes, but other than that I think this should be good.

[GuillaumeGomez]

Please ping me once the PR description has been updated so I can approve it.

[fee1-dead]

@GuillaumeGomez: I have updated the PR description.

[GuillaumeGomez]

Thanks!

@bors: r=

[bors]

📌 Commit 484936b has been approved by ``

[GuillaumeGomez]

Second time will work better...

@bors r+

[bors]

📌 Commit 484936b has been approved by GuillaumeGomez