Add missing checks for #[doc(alias = "")]

[GuillaumeGomez]

Now that #72780 has been merged, only a few checks remain to be added in order to stabilize this feature.

First, we need to actually perform those checks in the compiler directly instead of rustdoc.

You mentioned other checks too @ollie27 ?

This issue is part of #50146.

[ollie27]

You mentioned other checks too @ollie27 ?

We should reject #[doc(alias = "")] on items that don't actually appear in the search index such as extern blocks and impls.

[GuillaumeGomez]

Ok, makes sense! Last question: I remember you telling that you'd prefer this check in rustc directly instead of rustdoc. I can move things there, just wanted to check one last time before doing it.

[ollie27]

I remember you telling that you'd prefer this check in rustc directly instead of rustdoc. I can move things there, just wanted to check one last time before doing it.

Yeah, rustc validates some attributes in https://github.com/rust-lang/rust/blob/master/src/librustc_passes/check_attr.rs and it might make sense to validate the #[doc] attribute there as well.

[GuillaumeGomez]

Since #74148 has been merged, we can close this issue.

[ollie27]

We're still not rejecting #[doc(alias)] on associated consts in trait impls like:

pub struct Foo;
pub trait Bar {
    const BAZ: u8;
}
impl Bar for Foo {
    #[doc(alias = "thing")]
    const BAZ: u8 = 0;
}