prevent `Freeze` and `UnsafeUnpin` from being implemented outside of `core` - #161811

joboet · 2026-08-26T12:06:12Z

impl(crate) only prevents explicit implementations, but not the automatically derived ones (playground), so we can use it to the same effect as #[rustc_deny_explicit_impl] while still having the fundamental implementations of these traits in core.

`core`

rustbot · 2026-08-26T12:06:18Z

r? @clarfonthey

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joboet · 2026-08-26T12:07:05Z

@@ -85,7 +85,3 @@ impl<T: PointeeSized, U: PointeeSized> CoerceUnsized<pattern_type!(*const U is !
 impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<pattern_type!(U is !null)> for pattern_type!(T is !null) {}

 impl<T: PointeeSized> Unpin for pattern_type!(*const T is !null) {}
-
-unsafe impl<T: PointeeSized> Freeze for pattern_type!(*const T is !null) {}


The UnsafeUnpin implementation already lives in marker, so I've moved this too for consistency.

View changes since the review

bjorn3 · 2026-08-26T12:35:18Z

 #[lang = "freeze"]
 #[unstable(feature = "freeze", issue = "121675")]
-pub unsafe auto trait Freeze {}
+pub impl(crate) unsafe auto trait Freeze {}


Can these be impl(self)?

View changes since the review

There is a manual Freeze implementation for NonZero that's used to provide nicer error messages. Since it's in core, I don't think it matters too much, but I can change it if you want.

I figured impl(self) would make it less likely that someone adds a manual impl outside this module not knowing that it is probably a bad idea. But a manual impl for better error messages makes sense I guess.

On second thought, it's very unlikely for this to show up anyway: there's no way to require the absence of a trait, and NonZero is Freeze for all types that can implement ZeroablePrimitive. So I think you could never run into a situation where the error is "NonZero doesn't implement Freeze because <T as ZeroablePrimitive>::NonZeroInner doesn't implement it". I'll constrain it to impl(self).

Wait, it's about the documentation, so this is still relevant. Nevermind!

rust-log-analyzer · 2026-08-26T12:40:47Z

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [ui] tests/ui/feature-gates/feature-gate-freeze-impls.rs stdout ----
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/feature-gates/feature-gate-freeze-impls/feature-gate-freeze-impls.stderr`
diff of stderr:

8    = help: add `#![feature(freeze_impls)]` to the crate attributes to enable
9    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
10 
+ error: trait cannot be implemented outside `core`
+   --> $DIR/feature-gate-freeze-impls.rs:7:1
+    |
+ LL | unsafe impl Freeze for Foo {}
---
11 error[E0658]: explicit impls for the `Freeze` trait are not permitted
12   --> $DIR/feature-gate-freeze-impls.rs:12:1
13    |

18    = help: add `#![feature(freeze_impls)]` to the crate attributes to enable
19    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
20 
- error: aborting due to 2 previous errors
+ error: trait cannot be implemented outside `core`
+   --> $DIR/feature-gate-freeze-impls.rs:12:1
+    |
+ LL | impl !Freeze for Bar {}
+    | ^^^^^^^^^^^^^^^^^^^^
+    |
+ note: trait restricted here
+   --> $SRC_DIR/core/src/marker.rs:LL:COL
+ 
---
+ 
+ error: trait cannot be implemented outside `core`
+   --> $DIR/feature-gate-freeze-impls.rs:12:1
+    |
+ LL | impl !Freeze for Bar {}
+    | ^^^^^^^^^^^^^^^^^^^^
+    |
+ note: trait restricted here
+   --> $SRC_DIR/core/src/marker.rs:LL:COL
+ 
---
To only update this specific test, also pass `--test-args feature-gates/feature-gate-freeze-impls.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/feature-gates/feature-gate-freeze-impls.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/feature-gates/feature-gate-freeze-impls" "-Znext-solver=coherence" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0658]: explicit impls for the `Freeze` trait are not permitted
##[error]  --> /checkout/tests/ui/feature-gates/feature-gate-freeze-impls.rs:7:1
   |
LL | unsafe impl Freeze for Foo {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ impl of `Freeze` not allowed
   |
   = note: see issue #121675 <https://github.com/rust-lang/rust/issues/121675> for more information
   = help: add `#![feature(freeze_impls)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: trait cannot be implemented outside `core`
##[error]  --> /checkout/tests/ui/feature-gates/feature-gate-freeze-impls.rs:7:1
   |
LL | unsafe impl Freeze for Foo {}
---

error[E0658]: explicit impls for the `Freeze` trait are not permitted
##[error]  --> /checkout/tests/ui/feature-gates/feature-gate-freeze-impls.rs:12:1
   |
LL | impl !Freeze for Bar {}
   | ^^^^^^^^^^^^^^^^^^^^ impl of `Freeze` not allowed
   |
   = note: see issue #121675 <https://github.com/rust-lang/rust/issues/121675> for more information
   = help: add `#![feature(freeze_impls)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: trait cannot be implemented outside `core`
##[error]  --> /checkout/tests/ui/feature-gates/feature-gate-freeze-impls.rs:12:1
   |
LL | impl !Freeze for Bar {}
   | ^^^^^^^^^^^^^^^^^^^^
   |
note: trait restricted here
  --> /rustc/FAKE_PREFIX/library/core/src/marker.rs:904:4

ChayimFriedman2 · 2026-08-26T15:05:18Z

Wait, does that mean we can get rid of rustc_deny_explicit_impl now (edit: except maybe for better error messages)?

clarfonthey · 2026-08-27T00:04:56Z

Hmm, is the behaviour on derived impls something explicitly documented anywhere? The code looks fine otherwise, but I would like at least some assurance that this isn't going to change down the line.

clarfonthey · 2026-08-27T00:24:31Z

Wait, never mind, I misinterpreted "auto-derived" to mean… ones that came from derive, not ones that came from auto, and now I understand. (No, we don't derive these. Obviously.)

Yes, this makes a lot more sense. We should be able to merge this once tests are good.

joboet · 2026-08-27T13:40:19Z

I think this means we could also get rid of the freeze_impls feature that previously restricted manual impl Freezes, CC @oli-obk since you added this in 7849230. Should I go ahead and do that here?

jhpratt · 2026-08-28T05:42:43Z

For behavior on auto-impls (i.e. from an auto trait), this was never explicitly documented anywhere (nor discussed). I can't imagine any situation where that behavior changes.

mejrs · 2026-08-28T18:12:13Z

Wait, does that mean we can get rid of rustc_deny_explicit_impl now (edit: except maybe for better error messages)?

No, these traits still need to be implemented, it's just done by the compiler.

clarfonthey · 2026-08-31T02:41:58Z

@rustbot author

prevent Freeze and UnsafeUnpin from being implemented outside of

961f8d4

`core`

rustbot assigned clarfonthey Aug 26, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Aug 26, 2026

joboet commented Aug 26, 2026

View reviewed changes

bjorn3 reviewed Aug 26, 2026

View reviewed changes

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 31, 2026

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