google · jswrenn · Mar 15, 2024
diff --git a/src/lib.rs b/src/lib.rs
@@ -4550,9 +4550,10 @@ macro_rules! transmute_ref {
 /// another type of the same size.
 ///
 /// The expression `$e` must have a concrete type, `&mut T`, where `T: Sized +
-/// IntoBytes`. The `transmute_mut!` expression must also have a concrete type,
-/// `&mut U` (`U` is inferred from the calling context), where `U: Sized +
-/// FromBytes`. It must be the case that `align_of::<T>() >= align_of::<U>()`.
+/// FromBytes + IntoBytes`. The `transmute_mut!` expression must also have a
+/// concrete type, `&mut U` (`U` is inferred from the calling context), where
+/// `U: Sized + FromBytes + IntoBytes`. It must be the case that
+/// `align_of::<T>() >= align_of::<U>()`.
 ///
 /// The lifetime of the input type, `&mut T`, must be the same as or outlive the
 /// lifetime of the output type, `&mut U`.
@@ -4617,9 +4618,9 @@ macro_rules! transmute_mut {
         #[allow(unused, clippy::diverging_sub_expression)]
         if false {
             // This branch, though never taken, ensures that the type of `e` is
-            // `&mut T` where `T: 't + Sized + FromBytes + IntoBytes + NoCell`
-            // and that the type of this macro expression is `&mut U` where `U:
-            // 'u + Sized + FromBytes + IntoBytes + NoCell`.
+            // `&mut T` where `T: 't + Sized + FromBytes + IntoBytes` and that
+            // the type of this macro expression is `&mut U` where `U: 'u +
+            // Sized + FromBytes + IntoBytes`.
 
             // We use immutable references here rather than mutable so that, if
             // this macro is used in a const context (in which, as of this
@@ -4629,20 +4630,16 @@ macro_rules! transmute_mut {
             struct AssertSrcIsSized<'a, T: ::core::marker::Sized>(&'a T);
             struct AssertSrcIsFromBytes<'a, T: ?::core::marker::Sized + $crate::FromBytes>(&'a T);
             struct AssertSrcIsIntoBytes<'a, T: ?::core::marker::Sized + $crate::IntoBytes>(&'a T);
-            struct AssertSrcIsNoCell<'a, T: ?::core::marker::Sized + $crate::NoCell>(&'a T);
             struct AssertDstIsSized<'a, T: ::core::marker::Sized>(&'a T);
             struct AssertDstIsFromBytes<'a, T: ?::core::marker::Sized + $crate::FromBytes>(&'a T);
             struct AssertDstIsIntoBytes<'a, T: ?::core::marker::Sized + $crate::IntoBytes>(&'a T);
-            struct AssertDstIsNoCell<'a, T: ?::core::marker::Sized + $crate::NoCell>(&'a T);
 
             if true {
                 let _ = AssertSrcIsSized(&*e);
             } else if true {
                 let _ = AssertSrcIsFromBytes(&*e);
-            } else if true {
-                let _ = AssertSrcIsIntoBytes(&*e);
             } else {
-                let _ = AssertSrcIsNoCell(&*e);
+                let _ = AssertSrcIsIntoBytes(&*e);
             }
 
             if true {
@@ -4653,13 +4650,9 @@ macro_rules! transmute_mut {
                 #[allow(unused, unreachable_code)]
                 let u = AssertDstIsFromBytes(loop {});
                 &mut *u.0
-            } else if true {
-                #[allow(unused, unreachable_code)]
-                let u = AssertDstIsIntoBytes(loop {});
-                &mut *u.0
             } else {
                 #[allow(unused, unreachable_code)]
-                let u = AssertDstIsNoCell(loop {});
+                let u = AssertDstIsIntoBytes(loop {});
                 &mut *u.0
             }
         } else if false {
@@ -4682,11 +4675,10 @@ macro_rules! transmute_mut {
             &mut u
         } else {
             // SAFETY: For source type `Src` and destination type `Dst`:
-            // - We know that `Src: FromBytes + IntoBytes + NoCell` and `Dst:
-            //   FromBytes + IntoBytes + NoCell` thanks to the uses of
-            //   `AssertSrcIsFromBytes`, `AssertSrcIsIntoBytes`,
-            //   `AssertSrcIsNoCell`, `AssertDstIsFromBytes`,
-            //   `AssertDstIsIntoBytes`, and `AssertDstIsNoCell` above.
+            // - We know that `Src: FromBytes + IntoBytes` and `Dst: FromBytes +
+            //   IntoBytes` thanks to the uses of `AssertSrcIsFromBytes`,
+            //   `AssertSrcIsIntoBytes`, `AssertDstIsFromBytes`, and
+            //   `AssertDstIsIntoBytes` above.
             // - We know that `size_of::<Src>() == size_of::<Dst>()` thanks to
             //   the use of `assert_size_eq!` above.
             // - We know that `align_of::<Src>() >= align_of::<Dst>()` thanks to
@@ -7418,6 +7410,14 @@ mod tests {
         #[allow(clippy::useless_transmute)]
         let y: &u8 = transmute_mut!(&mut x);
         assert_eq!(*y, 0);
+
+        // Test that the referents can contain `UnsafeCell`s.
+        let mut src = AtomicU8::new(42);
+        {
+            let dst: &mut AtomicI8 = transmute_mut!(&mut src);
+            *dst.get_mut() += 1;
+        }
+        assert_eq!(src.into_inner(), 43);
     }
 
     #[test]

diff --git a/src/macro_util.rs b/src/macro_util.rs
@@ -378,8 +378,8 @@ pub const unsafe fn transmute_ref<'dst, 'src: 'dst, Src: 'src, Dst: 'dst>(
 /// # Safety
 ///
 /// The caller must guarantee that:
-/// - `Src: FromBytes + IntoBytes + NoCell`
-/// - `Dst: FromBytes + IntoBytes + NoCell`
+/// - `Src: FromBytes + IntoBytes`
+/// - `Dst: FromBytes + IntoBytes`
 /// - `size_of::<Src>() == size_of::<Dst>()`
 /// - `align_of::<Src>() >= align_of::<Dst>()`
 // TODO(#686): Consider removing the `NoCell` requirement.
@@ -395,9 +395,8 @@ pub unsafe fn transmute_mut<'dst, 'src: 'dst, Src: 'src, Dst: 'dst>(
     //   vice-versa because the caller has guaranteed that `Src: FromBytes +
     //   IntoBytes`, `Dst: FromBytes + IntoBytes`, and `size_of::<Src>() ==
     //   size_of::<Dst>()`.
-    // - We know that there are no `UnsafeCell`s, and thus we don't have to
-    //   worry about `UnsafeCell` overlap, because `Src: NoCell`
-    //   and `Dst: NoCell`.
+    // - We know that `src` is exclusively aliased, so there are no other active
+    //   references that could be used to create a data race with `&mut dst`.
     // - The caller has guaranteed that alignment is not increased.
     // - We know that the returned lifetime will not outlive the input lifetime
     //   thanks to the lifetime bounds on this function.

diff --git a/tests/ui-msrv/transmute-mut-dst-not-a-reference.stderr b/tests/ui-msrv/transmute-mut-dst-not-a-reference.stderr
@@ -47,13 +47,3 @@ error[E0308]: mismatched types
    = note:           expected type `usize`
            found mutable reference `&mut _`
    = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0308]: mismatched types
-  --> tests/ui-msrv/transmute-mut-dst-not-a-reference.rs:17:36
-   |
-17 | const DST_NOT_A_REFERENCE: usize = transmute_mut!(&mut 0u8);
-   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^ expected `usize`, found `&mut _`
-   |
-   = note:           expected type `usize`
-           found mutable reference `&mut _`
-   = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/tests/ui-msrv/transmute-mut-dst-not-nocell.stderr b/tests/ui-msrv/transmute-mut-dst-not-nocell.stderr
@@ -1,12 +1,25 @@
-error[E0277]: the trait bound `Dst: NoCell` is not satisfied
-  --> tests/ui-msrv/transmute-mut-dst-not-nocell.rs:24:35
+error[E0658]: mutable references are not allowed in constants
+  --> tests/ui-msrv/transmute-mut-dst-not-nocell.rs:24:50
    |
 24 | const DST_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NoCell` is not implemented for `Dst`
+   |                                                  ^^^^^^^^
    |
-note: required by `AssertDstIsNoCell`
+   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
+
+error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
   --> tests/ui-msrv/transmute-mut-dst-not-nocell.rs:24:35
    |
 24 | const DST_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
    = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0716]: temporary value dropped while borrowed
+  --> tests/ui-msrv/transmute-mut-dst-not-nocell.rs:24:55
+   |
+24 | const DST_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
+   |                                   --------------------^^^-
+   |                                   |                   |
+   |                                   |                   creates a temporary which is freed while still in use
+   |                                   temporary value is freed at the end of this statement
+   |                                   using this value as a constant requires that borrow lasts for `'static`
diff --git a/tests/ui-msrv/transmute-mut-src-not-nocell.stderr b/tests/ui-msrv/transmute-mut-src-not-nocell.stderr
@@ -1,25 +1,25 @@
-error[E0277]: the trait bound `Src: NoCell` is not satisfied
-  --> tests/ui-msrv/transmute-mut-src-not-nocell.rs:24:35
+error[E0658]: mutable references are not allowed in constants
+  --> tests/ui-msrv/transmute-mut-src-not-nocell.rs:24:50
    |
 24 | const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NoCell` is not implemented for `Src`
+   |                                                  ^^^^^^^^
    |
-note: required by `AssertSrcIsNoCell`
+   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
+
+error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
   --> tests/ui-msrv/transmute-mut-src-not-nocell.rs:24:35
    |
 24 | const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
    = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0277]: the trait bound `Src: NoCell` is not satisfied
-  --> tests/ui-msrv/transmute-mut-src-not-nocell.rs:24:35
+error[E0716]: temporary value dropped while borrowed
+  --> tests/ui-msrv/transmute-mut-src-not-nocell.rs:24:55
    |
 24 | const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NoCell` is not implemented for `Src`
-   |
-note: required by a bound in `AssertSrcIsNoCell`
-  --> tests/ui-msrv/transmute-mut-src-not-nocell.rs:24:35
-   |
-24 | const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `AssertSrcIsNoCell`
-   = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
+   |                                   --------------------^^^-
+   |                                   |                   |
+   |                                   |                   creates a temporary which is freed while still in use
+   |                                   temporary value is freed at the end of this statement
+   |                                   using this value as a constant requires that borrow lasts for `'static`
diff --git a/tests/ui-nightly/transmute-mut-dst-not-a-reference.stderr b/tests/ui-nightly/transmute-mut-dst-not-a-reference.stderr
@@ -38,16 +38,6 @@ error[E0308]: mismatched types
            found mutable reference `&mut _`
    = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0308]: mismatched types
-  --> tests/ui-nightly/transmute-mut-dst-not-a-reference.rs:17:36
-   |
-17 | const DST_NOT_A_REFERENCE: usize = transmute_mut!(&mut 0u8);
-   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^ expected `usize`, found `&mut _`
-   |
-   = note:           expected type `usize`
-           found mutable reference `&mut _`
-   = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
-
 error[E0308]: mismatched types
   --> tests/ui-nightly/transmute-mut-dst-not-a-reference.rs:17:36
    |

diff --git a/tests/ui-nightly/transmute-mut-dst-not-nocell.rs b/tests/ui-nightly/transmute-mut-dst-not-nocell.rs
@@ -12,13 +12,13 @@ use zerocopy::transmute_mut;
 
 fn main() {}
 
-#[derive(zerocopy::FromBytes, zerocopy::IntoBytes, zerocopy::NoCell)]
+#[derive(zerocopy::FromBytes, zerocopy::IntoBytes)]
 #[repr(C)]
 struct Src;
 
 #[derive(zerocopy::FromBytes, zerocopy::IntoBytes)]
 #[repr(C)]
 struct Dst;
 
-// `transmute_mut` requires that the destination type implements `NoCell`
+// `transmute_mut` des not require that the destination type implements `NoCell`
 const DST_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
diff --git a/tests/ui-nightly/transmute-mut-dst-not-nocell.stderr b/tests/ui-nightly/transmute-mut-dst-not-nocell.stderr
@@ -1,25 +1,17 @@
-error[E0277]: the trait bound `Dst: NoCell` is not satisfied
-  --> tests/ui-nightly/transmute-mut-dst-not-nocell.rs:24:35
+error[E0658]: mutable references are not allowed in constants
+  --> tests/ui-nightly/transmute-mut-dst-not-nocell.rs:24:50
    |
 24 | const DST_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^
-   |                                   |
-   |                                   the trait `NoCell` is not implemented for `Dst`
-   |                                   required by a bound introduced by this call
+   |                                                  ^^^^^^^^
    |
-   = help: the following other types implement trait `NoCell`:
-             bool
-             char
-             isize
-             i8
-             i16
-             i32
-             i64
-             i128
-           and $N others
-note: required by a bound in `AssertDstIsNoCell`
+   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+
+error[E0015]: cannot call non-const fn `transmute_mut::<'_, '_, Src, Dst>` in constants
   --> tests/ui-nightly/transmute-mut-dst-not-nocell.rs:24:35
    |
 24 | const DST_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `AssertDstIsNoCell`
+   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: calls in constants are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/tests/ui-nightly/transmute-mut-src-not-nocell.rs b/tests/ui-nightly/transmute-mut-src-not-nocell.rs
@@ -16,9 +16,9 @@ fn main() {}
 #[repr(C)]
 struct Src;
 
-#[derive(zerocopy::FromBytes, zerocopy::IntoBytes, zerocopy::NoCell)]
+#[derive(zerocopy::FromBytes, zerocopy::IntoBytes)]
 #[repr(C)]
 struct Dst;
 
-// `transmute_mut` requires that the source type implements `NoCell`
+// `transmute_mut` does not require that the source type implements `NoCell`
 const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
diff --git a/tests/ui-nightly/transmute-mut-src-not-nocell.stderr b/tests/ui-nightly/transmute-mut-src-not-nocell.stderr
@@ -1,48 +1,17 @@
-error[E0277]: the trait bound `Src: NoCell` is not satisfied
-  --> tests/ui-nightly/transmute-mut-src-not-nocell.rs:24:35
+error[E0658]: mutable references are not allowed in constants
+  --> tests/ui-nightly/transmute-mut-src-not-nocell.rs:24:50
    |
 24 | const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^
-   |                                   |
-   |                                   the trait `NoCell` is not implemented for `Src`
-   |                                   required by a bound introduced by this call
+   |                                                  ^^^^^^^^
    |
-   = help: the following other types implement trait `NoCell`:
-             bool
-             char
-             isize
-             i8
-             i16
-             i32
-             i64
-             i128
-           and $N others
-note: required by a bound in `AssertSrcIsNoCell`
-  --> tests/ui-nightly/transmute-mut-src-not-nocell.rs:24:35
-   |
-24 | const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `AssertSrcIsNoCell`
-   = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
+   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0277]: the trait bound `Src: NoCell` is not satisfied
+error[E0015]: cannot call non-const fn `transmute_mut::<'_, '_, Src, Dst>` in constants
   --> tests/ui-nightly/transmute-mut-src-not-nocell.rs:24:35
    |
 24 | const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NoCell` is not implemented for `Src`
-   |
-   = help: the following other types implement trait `NoCell`:
-             bool
-             char
-             isize
-             i8
-             i16
-             i32
-             i64
-             i128
-           and $N others
-note: required by a bound in `AssertSrcIsNoCell`
-  --> tests/ui-nightly/transmute-mut-src-not-nocell.rs:24:35
+   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^
    |
-24 | const SRC_NOT_NO_CELL: &mut Dst = transmute_mut!(&mut Src);
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `AssertSrcIsNoCell`
+   = note: calls in constants are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/tests/ui-stable/transmute-mut-dst-not-a-reference.stderr b/tests/ui-stable/transmute-mut-dst-not-a-reference.stderr
@@ -38,16 +38,6 @@ error[E0308]: mismatched types
            found mutable reference `&mut _`
    = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0308]: mismatched types
-  --> tests/ui-stable/transmute-mut-dst-not-a-reference.rs:17:36
-   |
-17 | const DST_NOT_A_REFERENCE: usize = transmute_mut!(&mut 0u8);
-   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^ expected `usize`, found `&mut _`
-   |
-   = note:           expected type `usize`
-           found mutable reference `&mut _`
-   = note: this error originates in the macro `transmute_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
-
 error[E0308]: mismatched types
   --> tests/ui-stable/transmute-mut-dst-not-a-reference.rs:17:36
    |