Remove Immutable where it's no longer needed

There are some APIs which still spuriously require `Immutable`, but
which require more work to update. These will be fixed in a separate
commit.

Makes progress on #251

Co-authored-by: Jack Wrenn <jswrenn@amazon.com>

src/lib.rs

@@ -1427,7 +1427,7 @@ pub unsafe trait TryFromBytes {
     #[inline]
     fn try_mut_from(bytes: &mut [u8]) -> Result<&mut Self, TryCastError<&mut [u8], Self>>
     where
-        Self: KnownLayout + Immutable, // TODO(#251): Remove the `Immutable` bound.
+        Self: KnownLayout,
     {
         util::assert_dst_is_not_zst::<Self>();
         match Ptr::from_mut(bytes).try_cast_into_no_leftover::<Self, BecauseExclusive>() {
@@ -1532,7 +1532,7 @@ pub unsafe trait TryFromBytes {
         candidate: &mut [u8],
     ) -> Result<(&mut Self, &mut [u8]), TryCastError<&mut [u8], Self>>
     where
-        Self: KnownLayout + Immutable,
+        Self: KnownLayout,
     {
         util::assert_dst_is_not_zst::<Self>();
         try_mut_from_prefix_suffix(candidate, CastType::Prefix)
@@ -1619,7 +1619,7 @@ pub unsafe trait TryFromBytes {
         candidate: &mut [u8],
     ) -> Result<(&mut [u8], &mut Self), TryCastError<&mut [u8], Self>>
     where
-        Self: KnownLayout + Immutable,
+        Self: KnownLayout,
     {
         util::assert_dst_is_not_zst::<Self>();
         try_mut_from_prefix_suffix(candidate, CastType::Suffix).map(swap)
@@ -2535,7 +2535,7 @@ pub unsafe trait FromBytes: FromZeros {
     #[inline]
     fn mut_from(bytes: &mut [u8]) -> Result<&mut Self, CastError<&mut [u8], Self>>
     where
-        Self: IntoBytes + KnownLayout + Immutable,
+        Self: IntoBytes + KnownLayout,
     {
         util::assert_dst_is_not_zst::<Self>();
         match Ptr::from_mut(bytes).try_cast_into_no_leftover::<_, BecauseExclusive>() {
@@ -2610,7 +2610,7 @@ pub unsafe trait FromBytes: FromZeros {
         bytes: &mut [u8],
     ) -> Result<(&mut Self, &mut [u8]), CastError<&mut [u8], Self>>
     where
-        Self: IntoBytes + KnownLayout + Immutable,
+        Self: IntoBytes + KnownLayout,
     {
         util::assert_dst_is_not_zst::<Self>();
         let (slf, suffix) = Ptr::from_mut(bytes)
@@ -2679,7 +2679,7 @@ pub unsafe trait FromBytes: FromZeros {
         bytes: &mut [u8],
     ) -> Result<(&mut [u8], &mut Self), CastError<&mut [u8], Self>>
     where
-        Self: IntoBytes + KnownLayout + Immutable,
+        Self: IntoBytes + KnownLayout,
     {
         util::assert_dst_is_not_zst::<Self>();
         let (slf, prefix) = Ptr::from_mut(bytes)
@@ -3483,7 +3483,7 @@ pub unsafe trait IntoBytes {
     #[inline(always)]
     fn as_mut_bytes(&mut self) -> &mut [u8]
     where
-        Self: FromBytes + Immutable,
+        Self: FromBytes,
     {
         // Note that this method does not have a `Self: Sized` bound;
         // `size_of_val` works for unsized values too.
@@ -3718,7 +3718,7 @@ pub unsafe trait IntoBytes {
     #[inline]
     fn as_bytes_mut(&mut self) -> &mut [u8]
     where
-        Self: FromBytes + Immutable,
+        Self: FromBytes,
     {
         self.as_mut_bytes()
     }
@@ -5076,20 +5076,16 @@ 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 AssertSrcIsImmutable<'a, T: ?::core::marker::Sized + $crate::Immutable>(&'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 AssertDstIsImmutable<'a, T: ?::core::marker::Sized + $crate::Immutable>(&'a T);
 
             if true {
                 let _ = AssertSrcIsSized(&*e);
             } else if true {
                 let _ = AssertSrcIsFromBytes(&*e);
-            } else if true {
-                let _ = AssertSrcIsIntoBytes(&*e);
             } else {
-                let _ = AssertSrcIsImmutable(&*e);
+                let _ = AssertSrcIsIntoBytes(&*e);
             }
 
             if true {
@@ -5100,13 +5096,9 @@ 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 = AssertDstIsImmutable(loop {});
+                let u = AssertDstIsIntoBytes(loop {});
                 &mut *u.0
             }
         } else if false {
@@ -5129,11 +5121,6 @@ macro_rules! transmute_mut {
             &mut u
         } else {
             // SAFETY: For source type `Src` and destination type `Dst`:
-            // - We know that `Src: FromBytes + IntoBytes + Immutable` and `Dst:
-            //   FromBytes + IntoBytes + Immutable` thanks to the uses of
-            //   `AssertSrcIsFromBytes`, `AssertSrcIsIntoBytes`,
-            //   `AssertSrcIsImmutable`, `AssertDstIsFromBytes`,
-            //   `AssertDstIsIntoBytes`, and `AssertDstIsImmutable` 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

src/macro_util.rs

@@ -386,8 +386,8 @@ pub const unsafe fn transmute_ref<'dst, 'src: 'dst, Src: 'src, Dst: 'dst>(
 /// # Safety
 ///
 /// The caller must guarantee that:
-/// - `Src: FromBytes + IntoBytes + Immutable`
-/// - `Dst: FromBytes + IntoBytes + Immutable`
+/// - `Src: FromBytes + IntoBytes`
+/// - `Dst: FromBytes + IntoBytes`
 /// - `size_of::<Src>() == size_of::<Dst>()`
 /// - `align_of::<Src>() >= align_of::<Dst>()`
 // TODO(#686): Consider removing the `Immutable` requirement.
@@ -403,9 +403,6 @@ 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: Immutable`
-    //   and `Dst: Immutable`.
     // - 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.

src/ref.rs

@@ -677,7 +677,7 @@ where
 impl<'a, B, T> Ref<B, T>
 where
     B: 'a + IntoByteSliceMut<'a>,
-    T: FromBytes + IntoBytes + KnownLayout + Immutable + ?Sized,
+    T: FromBytes + IntoBytes + KnownLayout + ?Sized,
 {
     /// Converts this `Ref` into a mutable reference.
     ///
@@ -802,6 +802,9 @@ where
 impl<B, T> DerefMut for Ref<B, T>
 where
     B: ByteSliceMut,
+    // TODO(#251): We can't remove `Immutable` here because it's required by
+    // the impl of `Deref`, which is a super-trait of `DerefMut`. Maybe we can
+    // add a separate inherent method for this?
     T: FromBytes + IntoBytes + KnownLayout + Immutable + ?Sized,
 {
     #[inline]

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)

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
    |

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
    |