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Rollup merge of #121840 - oli-obk:freeze, r=dtolnay

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Expose the Freeze trait again (unstably) and forbid implementing it manually

non-emoji version of https://github.com/rust-lang/rust/pull/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: https://github.com/google/zerocopy/issues/941

cc ```@RalfJung```

T-lang signed off on reexposing this unstably: https://github.com/rust-lang/rust/pull/121501#issuecomment-1969827742
This commit is contained in:
Jacob Pratt 2024-03-11 03:47:19 -04:00 • committed by GitHub
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2
compiler/rustc_feature/src/unstable.rs
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@ -471,6 +471,8 @@ declare_features! (
(unstable, fn_align, "1.53.0", Some(82232)),
/// Support delegating implementation of functions to other already implemented functions.
(incomplete, fn_delegation, "1.76.0", Some(118212)),
/// Allows impls for the Freeze trait.
(internal, freeze_impls, "CURRENT_RUSTC_VERSION", Some(121675)),
/// Allows defining gen blocks and `gen fn`.
(unstable, gen_blocks, "1.75.0", Some(117078)),
/// Infer generic args for both consts and types.