[Crater experiment] Adjust `PlaceMention` borrow checking for MIR move elimination semantics by Jules-Bertholet · Pull Request #165046 · rust-lang/rust

Jules-Bertholet · 2026-10-27T16:38:16Z

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#164812 (comment)

I didn't bother adjusting all the UI tests, so they will fail.

@Amanieu can you confirm that tests/ui/borrowck/placemention-mir-move-elim.rs corresponds to the behavior you want?

Co-authored-by: dianne <diannes.gm@gmail.com>

theemathas · 2026-10-27T16:43:35Z

@bors try

…=<try> [Crater experiment] Adjust `PlaceMention` borrow checking for MIR move elimination semantics

rust-log-analyzer · 2026-10-27T17:14:47Z

The job test-aarch64-gnu-llvm-22-1 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/binding/issue-53114-borrow-checks.rs stdout ----

error: test compilation failed although it shouldn't!
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/binding/issue-53114-borrow-checks.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/binding/issue-53114-borrow-checks" "-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" "--edition=2015"
stdout: none
--- stderr -------------------------------
error[E0382]: use of moved value: `m`
##[error]  --> /checkout/tests/ui/binding/issue-53114-borrow-checks.rs:12:13
   |
LL |     let m = M;
   |         - move occurs because `m` has type `M`, which does not implement the `Copy` trait
LL |     drop(m);
   |          - value moved here
LL |     let _ = m; // accepted, and want it to continue to be
   |             ^ value used here after move
   |
note: if `M` implemented `Clone`, you could clone the value
  --> /checkout/tests/ui/binding/issue-53114-borrow-checks.rs:7:1
   |
LL | struct M;
   | ^^^^^^^^ consider implementing `Clone` for this type
...
LL |     drop(m);
   |          - you could clone this value

error[E0382]: use of moved value: `m`
##[error]  --> /checkout/tests/ui/binding/issue-53114-borrow-checks.rs:23:11
   |
LL |     let m = M;
   |         - move occurs because `m` has type `M`, which does not implement the `Copy` trait
LL |     drop(m);
   |          - value moved here
LL |     match m { _ => { } } // #53114: accepted too
   |           ^ value used here after move
   |
note: if `M` implemented `Clone`, you could clone the value
  --> /checkout/tests/ui/binding/issue-53114-borrow-checks.rs:7:1
   |
LL | struct M;
   | ^^^^^^^^ consider implementing `Clone` for this type
...
LL |     drop(m);
   |          - you could clone this value

error[E0382]: use of moved value: `m`
##[error]  --> /checkout/tests/ui/binding/issue-53114-borrow-checks.rs:34:16
   |
LL |     let m = M;
   |         - move occurs because `m` has type `M`, which does not implement the `Copy` trait
LL |     drop(m);
   |          - value moved here
LL |     if let _ = m { } // #53114: accepted too
   |                ^ value used here after move
   |
note: if `M` implemented `Clone`, you could clone the value
  --> /checkout/tests/ui/binding/issue-53114-borrow-checks.rs:7:1
   |
LL | struct M;
   | ^^^^^^^^ consider implementing `Clone` for this type
...
---
---- [ui] tests/ui/reachable/type_ascribe_never_field.rs stdout ----

error: test compilation failed although it shouldn't!
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/reachable/type_ascribe_never_field.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/reachable/type_ascribe_never_field" "-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" "--edition=2015"
stdout: none
--- stderr -------------------------------
error[E0381]: used binding `x` isn't initialized
##[error]  --> /checkout/tests/ui/reachable/type_ascribe_never_field.rs:13:12
   |
LL |    let x: (!,);
   |        - binding declared here but left uninitialized
LL |    let _ = type_ascribe!(x.0, _);
   |            ^^^^^^^^^^^^^^^^^^^^^ `x` used here but it isn't initialized

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0381`.
------------------------------------------

---- [ui] tests/ui/reachable/type_ascribe_never_field.rs stdout end ----
---- [ui] tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs#current stdout ----

error in revision `current`: test compilation failed although it shouldn't!
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/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.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" "--cfg" "current" "--check-cfg" "cfg(test,FALSE,current,next)" "--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/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.current" "-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" "--edition=2015"
stdout: none
--- stderr -------------------------------
error[E0382]: use of moved value: `obj`
##[error]  --> /checkout/tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs:31:40
   |
LL |     let (obj, t) = foo();
   |          --- move occurs because `obj` has type `Inv<dyn Object<&(), &()>>`, which does not implement the `Copy` trait
LL |     impls_trait(obj, t);
   |                 --- value moved here
LL |     let _: Inv<dyn Object<&(), &()>> = obj;
   |                                        ^^^ value used here after move
   |
note: consider changing this parameter type in function `impls_trait` to borrow instead if owning the value isn't necessary
  --> /checkout/tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs:26:35
   |
LL | fn impls_trait<T: Trait<U>, U>(_: Inv<T>, _: Inv<U>) {}
   |    ----------- in this function   ^^^^^^ this parameter takes ownership of the value
note: if all bounds were met, you could clone the value
  --> /checkout/tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs:22:1
   |
LL | #[derive(Clone, Copy)]
   |          ----- derived `Clone` adds implicit bounds on type parameters
LL | struct Inv<T>(*mut T);
   | ^^^^^^^^^^^-^
   |            |
   |            introduces an implicit `T: Clone` bound
...
LL |     impls_trait(obj, t);
   |                 --- you could clone this value
   = help: consider manually implementing `Clone` to avoid undesired bounds

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0382`.
------------------------------------------

---- [ui] tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs#current stdout end ----
---- [ui] tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs#next stdout ----

error in revision `next`: test compilation failed although it shouldn't!
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/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.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" "--cfg" "next" "--check-cfg" "cfg(test,FALSE,current,next)" "--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/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.next" "-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" "--edition=2015" "-Znext-solver"
stdout: none
--- stderr -------------------------------
error[E0382]: use of moved value: `obj`
##[error]  --> /checkout/tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs:31:40
   |
LL |     let (obj, t) = foo();
   |          --- move occurs because `obj` has type `Inv<dyn Object<&(), &()>>`, which does not implement the `Copy` trait
LL |     impls_trait(obj, t);
   |                 --- value moved here
LL |     let _: Inv<dyn Object<&(), &()>> = obj;
   |                                        ^^^ value used here after move
   |
note: consider changing this parameter type in function `impls_trait` to borrow instead if owning the value isn't necessary
  --> /checkout/tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs:26:35
   |
LL | fn impls_trait<T: Trait<U>, U>(_: Inv<T>, _: Inv<U>) {}
   |    ----------- in this function   ^^^^^^ this parameter takes ownership of the value
note: if all bounds were met, you could clone the value
  --> /checkout/tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs:22:1
   |
LL | #[derive(Clone, Copy)]
   |          ----- derived `Clone` adds implicit bounds on type parameters
LL | struct Inv<T>(*mut T);
   | ^^^^^^^^^^^-^
   |            |
   |            introduces an implicit `T: Clone` bound
...
LL |     impls_trait(obj, t);
   |                 --- you could clone this value
   = help: consider manually implementing `Clone` to avoid undesired bounds

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0382`.
------------------------------------------

---- [ui] tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-3.rs#next stdout end ----
---- [ui] tests/ui/traits/next-solver/generalize/subtype-obligations-bivariant-args.rs#next stdout ----

error in revision `next`: test compilation failed although it shouldn't!
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/traits/next-solver/generalize/subtype-obligations-bivariant-args.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" "--cfg" "next" "--check-cfg" "cfg(test,FALSE,old,next)" "--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/traits/next-solver/generalize/subtype-obligations-bivariant-args.next" "-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" "--edition=2015" "-Znext-solver"
stdout: none
--- stderr -------------------------------
error[E0382]: use of moved value: `x`
##[error]  --> /checkout/tests/ui/traits/next-solver/generalize/subtype-obligations-bivariant-args.rs:50:25
   |
LL |     let x = None.unwrap();
   |         - move occurs because `x` has type `Foo<u32, u32>`, which does not implement the `Copy` trait
LL |     let y = x;
   |             - value moved here
LL |     let _: Foo<_,  _> = x;
   |                         ^ value used here after move
   |
note: if `Foo<u32, u32>` implemented `Clone`, you could clone the value
  --> /checkout/tests/ui/traits/next-solver/generalize/subtype-obligations-bivariant-args.rs:45:1
   |
LL | struct Foo<T: Trait<Assoc = U>, U>(T);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ consider implementing `Clone` for this type
...
LL |     let y = x;
   |             - you could clone this value

---
---- [ui] tests/ui/traits/next-solver/generalize/subtype-obligations-bivariant-args.rs#old stdout ----

error in revision `old`: test compilation failed although it shouldn't!
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/traits/next-solver/generalize/subtype-obligations-bivariant-args.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" "--cfg" "old" "--check-cfg" "cfg(test,FALSE,old,next)" "--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/traits/next-solver/generalize/subtype-obligations-bivariant-args.old" "-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" "--edition=2015"
stdout: none
--- stderr -------------------------------
error[E0382]: use of moved value: `x`
##[error]  --> /checkout/tests/ui/traits/next-solver/generalize/subtype-obligations-bivariant-args.rs:50:25
   |
LL |     let x = None.unwrap();
   |         - move occurs because `x` has type `Foo<u32, u32>`, which does not implement the `Copy` trait
LL |     let y = x;
   |             - value moved here
LL |     let _: Foo<_,  _> = x;
   |                         ^ value used here after move
   |
note: if `Foo<u32, u32>` implemented `Clone`, you could clone the value
  --> /checkout/tests/ui/traits/next-solver/generalize/subtype-obligations-bivariant-args.rs:45:1
   |
LL | struct Foo<T: Trait<Assoc = U>, U>(T);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ consider implementing `Clone` for this type
...
LL |     let y = x;
   |             - you could clone this value

---
---- [ui] tests/ui/uninhabited/diverging-guard.rs stdout ----

error: test compilation failed although it shouldn't!
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/uninhabited/diverging-guard.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/uninhabited/diverging-guard" "-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" "--edition=2015"
stdout: none
--- stderr -------------------------------
error[E0381]: used binding `x` isn't initialized
##[error]  --> /checkout/tests/ui/uninhabited/diverging-guard.rs:7:11
   |
LL |     let x: Void;
   |         - binding declared here but left uninitialized
LL |     match x {
   |           ^ `x` used here but it isn't initialized
   |
help: consider assigning a value
   |
LL |     let x: Void = /* value */;
   |                 +++++++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0381`.

rust-bors · 2026-10-27T17:45:55Z

☀️ Try build successful (CI)
Build commit: 636e83d (636e83d88d52bef087e37ddcb9b224b2284c758e)
Base parent: 16d1ffa (16d1ffac17ad8b739e32483ac2e65fe7e1804977)

theemathas · 2026-10-27T17:49:12Z

@craterbot check

craterbot · 2026-10-27T17:50:03Z

👌 Experiment pr-165046 created and queued.
🤖 Automatically detected try build 636e83d
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-11-03T02:39:42Z

🚧 Experiment pr-165046 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-11-05T16:55:13Z

🎉 Experiment pr-165046 is completed!
📊 264 regressed and 2 fixed (1217468 total)
📊 5862 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-165046/retry-regressed-list.txt ↩

Amanieu · 2026-11-05T17:05:13Z

Based on a quick skim of the crater results I don't think this is going to fly. let _ = foo seems to be widely used to suppress the dead code warning, even when foo is not initialized.

We might have to simply define PlaceMention to be a no-op if the corresponding local isn't initialized.

RalfJung · 2026-11-05T18:22:15Z

I don't see how a semantics like that could be coherent. let _ = ***x switches between place and value expressions multiple times, there's no way to even evaluate most of that expression if x is not initialized. I don't think we should fundamentally alter the structure of place computation itself to account for this.

We could say that PlaceMention is just always a NOP. That's still a bad semantics IMO since let _ = foo now behaves very differently depending on whether foo is a value expression or a place expression. The desugaring from surface Rust into a sane explicit core calculus will be a messy nightmare.

Or... I've seen worse than 270 regressions. They come from fairly few roots. Even most of the "root results" are not root results, they come from a single macro in "blobby".

Jules-Bertholet · 2026-11-06T05:47:13Z

Even most of the "root results" are not root results, they come from a single macro in "blobby".

And in the non-root results, there are 115 failures from an obsolete alpha version of zerocopy.

RalfJung · 2026-11-06T07:09:04Z

115 failures from an alpha version of zerocopy.

Cc @joshlf

joshlf · 2026-11-06T16:49:32Z

Feel free to disregard the zerocopy failures. They come from this function which did the following on to guard against an accidental type-alias change:

        let c = candidate.into_exclusive_or_post_monomorphization_error();

        // ...

        // Confirm that `Maybe` is a type alias for `Ptr` with the validity
        // invariant `Initialized`. Our safety proof depends upon this
        // invariant, and it might change at some point. If that happens, we
        // want this function to stop compiling.
        let _: Ptr<'_, UnsafeCell<T>, (_, _, invariant::Initialized)> = candidate;

This wasn't control flow load-bearing; it was just meant as a sort of compile-time type assertion. AFAICT, it was never published outside of alpha releases (it was added in 0.8.0-alpha.6 in google/zerocopy#912 and then removed in 0.8.0-alpha.19 in google/zerocopy#1619).

…cts` (#1588) The Rust project is considering a change to the language that would break `let $field: (); let _ = $field;`: rust-lang/rust#165046

newpavlov · 2026-11-07T11:22:01Z

I released blobby v0.4.1 with the fix.

tmiasko · 2026-11-09T09:44:54Z

If it were necessary to keep accepting the same programs (as in #164812), there is also an alternative semantics where PlaceMention takes the role of StorageAlloc (when place has no deref projections).

RalfJung · 2026-11-09T09:54:53Z

We should not have a semantics with ad-hoc special-cases like "do something different if there are no deref projections".

tmiasko · 2026-11-09T10:14:20Z

We should not have a semantics with ad-hoc special-cases like "do something different if there are no deref projections".

This behavior would be consistent with MIR move elimination semantics, which indeed does make such a distinction.

RalfJung · 2026-11-09T10:21:25Z

The behavior of MIR move semantics can be described by carrying a "mode" alongside place evaluation, indicating whether locals should be initialized. That's a compositional way to give rise to the desired semantics without ad-hoc syntactic checks.

I don't think the same is possible here. Place evaluation must either return a place or raise UB. We should IMO not alter the return type of place evaluation to deal with this quirk in the borrow checker.

tmiasko · 2026-11-09T10:38:51Z

I am perplexed by your comments. In MIR move elimination semantics are two modes of place evaluation. It is entirely trivial observation that one could use the other mode in PlaceMention. I don't understand why that wouldn't be possible.

RalfJung · 2026-11-09T10:45:50Z

Please spell out a compositional model of place evaluation that achieves "PlaceMention takes the role of StorageAlloc (when place has no deref projections)". Maybe I am missing the obvious.

tmiasko · 2026-11-09T10:50:12Z

In context of #163340, this would mean PlaceMention using eval_place_for_write as opposed to eval_place.

Amanieu · 2026-11-09T13:15:41Z

Note that this makes RemovePlaceMention as it currently is unsound: you would have to replace these with StorageAlloc instead of removing them.

Amanieu · 2026-11-09T13:20:36Z

What I had in mind instead was to explicitly skip place evaluation if the base local isn't allocated and there's no deref. So evaluation of a PlaceMention becomes:

if !place.has_deref() && local_state(place.local) == LiveUnallocated {
    // nop
} else {
    eval_place(place)
}

It is a hack, but a necessary one to maintain backwards-compatibility. We could maybe phase it out in the long term with a FCW.

Jules-Bertholet and others added 4 commits October 25, 2026 22:50

Don't allow dereferencing dangling refs inside PlaceMention

7c1c254

Avoid extra allocations

58d3745

Co-authored-by: dianne <diannes.gm@gmail.com>

Add tests for partial move out of Box

2766312

Forbid PlaceMentions of fully moved/uninit locals

29ba0e5

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 27, 2026

Jules-Bertholet mentioned this pull request Oct 27, 2026

Don't allow dereferencing uninitialized references inside PlaceMention #164812

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rust-bors Bot pushed a commit that referenced this pull request Oct 27, 2026

Auto merge of #165046 - Jules-Bertholet:placemention-mir-move-elim, r…

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…=<try> [Crater experiment] Adjust `PlaceMention` borrow checking for MIR move elimination semantics

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Jules-Bertholet mentioned this pull request Nov 5, 2026

blobby: don't PlaceMention an uninitialized local in parse_into_structs RustCrypto/utils#1588

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Jules-Bertholet mentioned this pull request Nov 6, 2026

vm: Don't let _ = a moved-out local gluon-lang/gluon#983

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