Implement Write for Cursor<W: AsMut<[u8]>>

[bushrat011899]

Change WriteThroughCursor into a specialization which allows Vec and &mut Vec to extend their allocation for writing. This makes discoverability of the Write implementation better.

ACP: rust-lang/libs-team#853
Tracking Issue: #154046
Follow Up To: #160952 & #158537

Description

Currently, Write is implemented for Cursor<W>, where W is one of:

• &mut [u8]
• for<const N: usize> [u8; N]
• for<A: Allocator> Box<[u8], A>
• for<A: Allocator> Vec<u8, A>
• for<A: Allocator> &mut Vec<u8, A>

When moving Write into core::io, it was noted here that the documentation of Cursor would be degraded due to the introduction of WriteThroughCursor, an indirection trait to allow Cursor and Write in core, while Box and Vec exist in alloc.

To resolve this documentation regression, and add additional functionality as well, I'm proposing we instead implement Write for Cursor<W> where W: AsMut<[u8]>. All 5 of the original types above implement AsMut<[u8]>, so we'll correctly document the breadth of support.

However, for Vec and &mut Vec, I use specialization to allow their implementation to extend, rather than only writing to the existing slice. While this still uses an indirection trait (SpecCursorWrite), but now this is purely for specialization, and therefore doesn't need to be documented publicly.

I'm opening a PR directly as a reference point, but I suspect this will need an ACP since this changes the public API.

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Notes

• No AI tooling of any kind was used during the creation of this PR.

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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.