[RFC 2011] Library code

[c410-f3r]

CC #96496

Based on https://github.com/dtolnay/case-studies/tree/master/autoref-specialization.

Basically creates two traits with the same method name. One trait is generic over any T and the other is specialized to any T: Printable.

The compiler will then call the corresponding trait method through auto reference.

fn main() {
    let mut a = Capture::new();
    let mut b = Capture::new();

    (&Wrapper(&1i32)).try_capture(&mut a); // `try_capture` from `TryCapturePrintable`
    (&Wrapper(&vec![1i32])).try_capture(&mut b); // `try_capture` from `TryCaptureGeneric`

    assert_eq!(format!("{:?}", a), "1");
    assert_eq!(format!("{:?}", b), "N/A");
}

r? @scottmcm

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[bors]

☔ The latest upstream changes (presumably #94119) made this pull request unmergeable. Please resolve the merge conflicts.

[scottmcm]

Thanks for splitting this out! Looks reasonable to me, and since it's internal we can always tweak it later.

@bors r+

[bors]

📌 Commit 664e8a9 has been approved by scottmcm

[c410-f3r]

Thank you @scottmcm !