Expose the Freeze trait again (unstably) and forbid implementing it manually by oli-obk · Pull Request #121840 · rust-lang/rust (original) (raw)

non-emoji version of #121501

cc #60715

This trait is useful for generic constants (associated consts of generic traits). See the test (tests/ui/associated-consts/freeze.rs) added in this PR for a usage example. The builtin Freeze trait is the only way to do it, users cannot work around this issue.

It's also a useful trait for building some very specific abstrations, as shown by the usage by the zerocopy crate: google/zerocopy#941

cc @RalfJung

T-lang signed off on reexposing this unstably: #121501 (comment)