Teach transmute_{ref,mut}! to handle slice DSTs

TODO:
- Figure out how to make this backwards-compatible (since current macros
  work with `T: Sized`, but `T: Sized` does not imply `T: KnownLayout`)
- Make it so that `Sized` bounds are still enforced on older toolchains
  that don't support slice DST transmutes
- Write safety comments
- Get rid of now-unused macros and functions in util::macro_util

Makes progress on #1817

gherrit-pr-id: Ib4bc62202e0b3b09d155333b525087f7aa8f02c2

Author: joshlf

src/macros.rs

@@ -182,22 +182,15 @@ macro_rules! transmute_ref {
             // this macro expression is `&U` where `U: 'u + Sized + FromBytes +
             // Immutable`, and that `'t` outlives `'u`.
 
-            struct AssertSrcIsSized<'a, T: ::core::marker::Sized>(&'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, U: ?::core::marker::Sized + $crate::FromBytes>(&'a U);
             struct AssertDstIsImmutable<'a, T: ?::core::marker::Sized + $crate::Immutable>(&'a T);
 
-            let _ = AssertSrcIsSized(e);
             let _ = AssertSrcIsIntoBytes(e);
             let _ = AssertSrcIsImmutable(e);
 
             if true {
-                #[allow(unused, unreachable_code)]
-                let u = AssertDstIsSized(loop {});
-                u.0
-            } else if true {
                 #[allow(unused, unreachable_code)]
                 let u = AssertDstIsFromBytes(loop {});
                 u.0
@@ -206,36 +199,13 @@ macro_rules! transmute_ref {
                 let u = AssertDstIsImmutable(loop {});
                 u.0
             }
-        } else if false {
-            // This branch, though never taken, ensures that `size_of::<T>() ==
-            // size_of::<U>()` and that that `align_of::<T>() >=
-            // align_of::<U>()`.
-
-            // `t` is inferred to have type `T` because it's assigned to `e` (of
-            // type `&T`) as `&t`.
-            let mut t = loop {};
-            e = &t;
-
-            // `u` is inferred to have type `U` because it's used as `&u` as the
-            // value returned from this branch.
-            let u;
-
-            $crate::assert_size_eq!(t, u);
-            $crate::assert_align_gt_eq!(t, u);
-
-            &u
         } else {
-            // SAFETY: For source type `Src` and destination type `Dst`:
-            // - We know that `Src: IntoBytes + Immutable` and `Dst: FromBytes +
-            //   Immutable` thanks to the uses of `AssertSrcIsIntoBytes`,
-            //   `AssertSrcIsImmutable`, `AssertDstIsFromBytes`, 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
-            //   the use of `assert_align_gt_eq!` above.
-            let u = unsafe { $crate::util::macro_util::transmute_ref(e) };
-            $crate::util::macro_util::must_use(u)
+            use $crate::util::macro_util::TransmuteRefDst;
+            let t = $crate::util::macro_util::Wrap(e);
+            // SAFETY: todo
+            unsafe {
+                t.transmute_ref()
+            }
         }
     }}
 }
@@ -334,26 +304,18 @@ macro_rules! transmute_mut {
             // writing, mutable references are banned), the error message
             // appears to originate in the user's code rather than in the
             // internals of this macro.
-            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 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);
 
             if true {
-                let _ = AssertSrcIsSized(&*e);
-            } else if true {
                 let _ = AssertSrcIsFromBytes(&*e);
             } else {
                 let _ = AssertSrcIsIntoBytes(&*e);
             }
 
             if true {
-                #[allow(unused, unreachable_code)]
-                let u = AssertDstIsSized(loop {});
-                &mut *u.0
-            } else if true {
                 #[allow(unused, unreachable_code)]
                 let u = AssertDstIsFromBytes(loop {});
                 &mut *u.0
@@ -362,32 +324,13 @@ macro_rules! transmute_mut {
                 let u = AssertDstIsIntoBytes(loop {});
                 &mut *u.0
             }
-        } else if false {
-            // This branch, though never taken, ensures that `size_of::<T>() ==
-            // size_of::<U>()` and that that `align_of::<T>() >=
-            // align_of::<U>()`.
-
-            // `t` is inferred to have type `T` because it's assigned to `e` (of
-            // type `&mut T`) as `&mut t`.
-            let mut t = loop {};
-            e = &mut t;
-
-            // `u` is inferred to have type `U` because it's used as `&mut u` as
-            // the value returned from this branch.
-            let u;
-
-            $crate::assert_size_eq!(t, u);
-            $crate::assert_align_gt_eq!(t, u);
-
-            &mut u
         } else {
-            // SAFETY: For source type `Src` and destination type `Dst`:
-            // - 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
-            //   the use of `assert_align_gt_eq!` above.
-            let u = unsafe { $crate::util::macro_util::transmute_mut(e) };
-            $crate::util::macro_util::must_use(u)
+            use $crate::util::macro_util::TransmuteMutDst;
+            let t = $crate::util::macro_util::Wrap(e);
+            // SAFETY: todo
+            unsafe {
+                t.transmute_mut()
+            }
         }
     }}
 }
@@ -1038,40 +981,71 @@ mod tests {
         assert_eq!(x.into_inner(), 1);
     }
 
-    #[test]
-    fn test_transmute_ref() {
-        // Test that memory is transmuted as expected.
-        let array_of_u8s = [0u8, 1, 2, 3, 4, 5, 6, 7];
-        let array_of_arrays = [[0, 1], [2, 3], [4, 5], [6, 7]];
-        let x: &[[u8; 2]; 4] = transmute_ref!(&array_of_u8s);
-        assert_eq!(*x, array_of_arrays);
-        let x: &[u8; 8] = transmute_ref!(&array_of_arrays);
-        assert_eq!(*x, array_of_u8s);
-
-        // Test that `transmute_ref!` is legal in a const context.
-        const ARRAY_OF_U8S: [u8; 8] = [0u8, 1, 2, 3, 4, 5, 6, 7];
-        const ARRAY_OF_ARRAYS: [[u8; 2]; 4] = [[0, 1], [2, 3], [4, 5], [6, 7]];
-        #[allow(clippy::redundant_static_lifetimes)]
-        const X: &'static [[u8; 2]; 4] = transmute_ref!(&ARRAY_OF_U8S);
-        assert_eq!(*X, ARRAY_OF_ARRAYS);
-
-        // Test that it's legal to transmute a reference while shrinking the
-        // lifetime (note that `X` has the lifetime `'static`).
-        let x: &[u8; 8] = transmute_ref!(X);
-        assert_eq!(*x, ARRAY_OF_U8S);
-
-        // Test that `transmute_ref!` supports decreasing alignment.
-        let u = AU64(0);
-        let array = [0, 0, 0, 0, 0, 0, 0, 0];
-        let x: &[u8; 8] = transmute_ref!(&u);
-        assert_eq!(*x, array);
-
-        // Test that a mutable reference can be turned into an immutable one.
-        let mut x = 0u8;
-        #[allow(clippy::useless_transmute)]
-        let y: &u8 = transmute_ref!(&mut x);
-        assert_eq!(*y, 0);
-    }
+    // #[test]
+    // fn test_transmute_ref() {
+    //     // Test that memory is transmuted as expected.
+    //     let array_of_u8s = [0u8, 1, 2, 3, 4, 5, 6, 7];
+    //     let array_of_arrays = [[0, 1], [2, 3], [4, 5], [6, 7]];
+    //     let x: &[[u8; 2]; 4] = transmute_ref!(&array_of_u8s);
+    //     assert_eq!(*x, array_of_arrays);
+    //     let x: &[u8; 8] = transmute_ref!(&array_of_arrays);
+    //     assert_eq!(*x, array_of_u8s);
+
+    //     // Test that `transmute_ref!` is legal in a const context.
+    //     const ARRAY_OF_U8S: [u8; 8] = [0u8, 1, 2, 3, 4, 5, 6, 7];
+    //     const ARRAY_OF_ARRAYS: [[u8; 2]; 4] = [[0, 1], [2, 3], [4, 5], [6, 7]];
+    //     #[allow(clippy::redundant_static_lifetimes)]
+    //     const X: &'static [[u8; 2]; 4] = transmute_ref!(&ARRAY_OF_U8S);
+    //     assert_eq!(*X, ARRAY_OF_ARRAYS);
+
+    //     // Test that `transmute_ref!` works on slice DSTs in and that memory is
+    //     // transmuted as expected.
+    //     #[derive(KnownLayout, Immutable, FromBytes, IntoBytes, PartialEq, Debug)]
+    //     #[repr(C)]
+    //     struct SliceDst<T> {
+    //         a: u8,
+    //         b: [T],
+    //     }
+
+    //     use crate::byteorder::native_endian::U16;
+    //     let slice_dst_of_u8s =
+    //         SliceDst::<[u8; 2]>::ref_from_bytes(&[0, 1, 2, 3, 4, 5, 6][..]).unwrap();
+    //     let slice_dst_of_u16s =
+    //         SliceDst::<U16>::ref_from_bytes(&[0, 1, 2, 3, 4, 5, 6][..]).unwrap();
+    //     let x: &SliceDst<U16> = transmute_ref!(slice_dst_of_u8s);
+    //     assert_eq!(x, slice_dst_of_u16s);
+
+    //     // Test that `transmute_ref!` works on slices in a const context and
+    //     // that memory is transmuted as expected.
+    //     const SLICE_OF_U16S: &[u16] = &[0u16, 1, 2];
+    //     const SLICE_OF_I16S: &[i16] = &[0i16, 1, 2];
+    //     let y: &[i16] = transmute_ref!(SLICE_OF_U16S);
+    //     assert_eq!(Y, SLICE_OF_I16S);
+
+    //     // Test that `transmute_ref!` works on slice DSTs in a const context. We
+    //     // have no way of synthesizing slice DST values in a const context, so
+    //     // this just tests that it compiles.
+    //     const fn _transmute_slice_dst_ref(from: &SliceDst<[u8; 2]>) -> &SliceDst<U16> {
+    //         transmute_ref!(from)
+    //     }
+
+    //     // Test that it's legal to transmute a reference while shrinking the
+    //     // lifetime (note that `X` has the lifetime `'static`).
+    //     let x: &[u8; 8] = transmute_ref!(X);
+    //     assert_eq!(*x, ARRAY_OF_U8S);
+
+    //     // Test that `transmute_ref!` supports decreasing alignment.
+    //     let u = AU64(0);
+    //     let array = [0, 0, 0, 0, 0, 0, 0, 0];
+    //     let x: &[u8; 8] = transmute_ref!(&u);
+    //     assert_eq!(*x, array);
+
+    //     // Test that a mutable reference can be turned into an immutable one.
+    //     let mut x = 0u8;
+    //     #[allow(clippy::useless_transmute)]
+    //     let y: &u8 = transmute_ref!(&mut x);
+    //     assert_eq!(*y, 0);
+    // }
 
     #[test]
     fn test_try_transmute() {
@@ -1209,6 +1183,30 @@ mod tests {
         #[allow(clippy::useless_transmute)]
         let y: &u8 = transmute_mut!(&mut x);
         assert_eq!(*y, 0);
+
+        // Test that `transmute_mut!` works on slice DSTs in and that memory is
+        // transmuted as expected.
+        #[derive(KnownLayout, Immutable, FromBytes, IntoBytes, PartialEq, Debug)]
+        #[repr(C)]
+        struct SliceDst<T> {
+            a: u8,
+            b: [T],
+        }
+
+        use crate::byteorder::native_endian::U16;
+        let mut bytes = [0, 1, 2, 3, 4, 5, 6];
+        let slice_dst_of_u8s = SliceDst::<[u8; 2]>::mut_from_bytes(&mut bytes[..]).unwrap();
+        let mut bytes = [0, 1, 2, 3, 4, 5, 6];
+        let slice_dst_of_u16s = SliceDst::<U16>::mut_from_bytes(&mut bytes[..]).unwrap();
+        let x: &mut SliceDst<U16> = transmute_mut!(slice_dst_of_u8s);
+        assert_eq!(x, slice_dst_of_u16s);
+
+        // Test that `transmute_mut!` works on slices that memory is transmuted
+        // as expected.
+        let array_of_u16s: &mut [u16] = &mut [0u16, 1, 2];
+        let array_of_i16s: &mut [i16] = &mut [0i16, 1, 2];
+        let x: &mut [i16] = transmute_mut!(array_of_u16s);
+        assert_eq!(x, array_of_i16s);
     }
 
     #[test]