vec_recycle: implementation

[GrigorenkoPV]

Tracking issue: #148227

Going with the TransmuteFrom approach suggested in rust-lang/libs-team#674 (comment), but a bit simplified.

Currently does not work in some places where it should due to the limitations of the current implementation of the transmutability analysis: #148227 (comment)

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[jswrenn]

Is the explicit transmutability feature flag needed for this example?

[GrigorenkoPV]

Yes, the feature flag seems to propagate from the bound on the impl

[jswrenn]

Hm, I wonder how Try avoids that.

[GrigorenkoPV]

The initial TransmuteFrom trait is marked with unstable_feature_bound, while Try is not

[jswrenn]

Oh, interesting! I didn't add that, and I'm curious why it was added (it's usually used to create unstable impls of stable traits). Hey @tiif, can you chime in as to why you added #[unstable_feature_bound(transmutability)] to TransmuteFrom here?

For context: We're using TransmuteFrom as a private implementation detail of Vec::recycle, but requiring users of recycle to have #![feature(transmutability)] even when they don't mention TransmuteFrom leaks that implementation detail.

[tiif]

There is a #[unstable_feature_bound(transmutability)] for impl ConstParamTy_ for Assume {}. Since TransmuteFrom uses const ASSUME: Assume = { Assume::NOTHING } here, it needs to add #[unstable_feature_bound(transmutability)] too.

Unfortunately we haven't made it possible to stop propagating #[unstable_feature_bound] this way for now :(

[bors]

☔ The latest upstream changes (presumably #135634) made this pull request unmergeable. Please resolve the merge conflicts.

[GrigorenkoPV]

@rustbot ready

[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.