stabilize smart pointer map functions

[Qelxiros]

Tracking issue: #144419
@rustbot modify labels: +T-libs-api

[rustbot]

r? @Darksonn

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Darksonn]

Sorry I'm at capacity for the next two weeks.

@rustbot reroll

[dtolnay]

This stabilizes a subset of #144419 — just the map functions and not try_map.

• https://doc.rust-lang.org/nightly/std/boxed/struct.Box.html#method.map
• https://doc.rust-lang.org/nightly/std/rc/struct.UniqueRc.html#method.map
• https://doc.rust-lang.org/nightly/std/sync/struct.UniqueArc.html#method.map
• https://doc.rust-lang.org/nightly/std/rc/struct.Rc.html#method.map
• https://doc.rust-lang.org/nightly/std/sync/struct.Arc.html#method.map

@rfcbot fcp merge libs-api

[rust-rfcbot]

@dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

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