Stability annotations on generic parameters (take 2.5)

[exrook]

Rebase of #72314 + more tests

Implements rust-lang/wg-allocators#2.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @lcnr (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[varkor]

Thanks for taking this across the finish line!

@bors r+

[bors]

📌 Commit 98eab09 has been approved by varkor

[varkor]

Thanks to @Avi-D-coder for all their hard work implementing this feature!

[bors]

⌛ Testing commit 98eab09 with merge d902752...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: varkor
Pushing d902752 to master...

[TimDiekmann]

When adding an unstable generic parameter to String, this results in the following error:

error: This stability annotation is useless
   --> library/alloc/src/string.rs:280:75
    |
280 | pub struct String<#[unstable(feature = "allocator_api", issue = "32838")] A: AllocRef = Global> {
    |                                                                           ^

error: This stability annotation is useless
   --> library/alloc/src/string.rs:319:82
    |
319 | pub struct FromUtf8Error<#[unstable(feature = "allocator_api", issue = "32838")] A: AllocRef = Global>  {
    |             

Is this a bug in this implementation or is this a false positive of the compiler when searching for stability annotations?