Stabilize {slice,array}::from_ref

[WaffleLapkin]

This PR stabilizes the following APIs as const functions in Rust 1.63:

// core::array
pub const fn from_ref<T>(s: &T) -> &[T; 1];

// core::slice
pub const fn from_ref<T>(s: &T) -> &[T];

Note that the mut versions are not stabilized as unique references (&mut _) are unstable in const context.

FCP: #90206 (comment)

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs

[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

[WaffleLapkin]

@rustbot label -T-libs

[dtolnay]

@bors r+

[bors]

📌 Commit 7a09b8a has been approved by dtolnay