Add transmute_ref! macro

This macro is like the existing `transmute!`, but it transmutes
immutable references rather than values.

Issue #159

Author: joshlf

Diff for: src/lib.rs

@@ -1463,6 +1463,96 @@ macro_rules! transmute {
     }}
 }
 
+/// A type whose size is equal to `mem::align_of::<T>()`.
+///
+/// This type is used internally in `transmute_ref!`, and so needs to be
+/// exported. It is not intended for direct use by users of zerocopy, and so is
+/// `#[doc(hidden)]`.
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+#[repr(C)]
+pub struct SizeIsAlign<T> {
+    // This field ensures that:
+    // - The size is always at least 1 (the minimum possible alignment).
+    // - If the alignment is greater than 1, Rust has to round up to the next
+    //   multiple of it in order to make sure that `SizeIsAlign`'s size is a
+    //   multiple of that alignment. Without this field, its size could be 0,
+    //   which is a valid multiple of any alignment.
+    _u: u8,
+    _a: [T; 0],
+}
+
+impl<T> SizeIsAlign<T> {
+    #[doc(hidden)]
+    pub fn new(_t: T) -> SizeIsAlign<T> {
+        SizeIsAlign { _u: 0, _a: [] }
+    }
+
+    #[doc(hidden)]
+    pub fn into_t(self) -> T {
+        unreachable!()
+    }
+}
+
+/// Safely transmutes an immutable reference of one type to an immutable
+/// reference of another type of the same size and alignment.
+///
+/// The expression `$e` must have a concrete type, `&T`, where `T: Sized +
+/// AsBytes`. The `transmute_ref!` expression must also have a concrete type,
+/// `&U` (`U` is inferred from the calling context), where `U: Sized +
+/// FromBytes`.
+///
+/// The lifetime of the input type, `&T`, must be the same as or outlive the
+/// lifetime of the output type, `&U`.
+#[macro_export]
+macro_rules! transmute_ref {
+    ($e:expr) => {{
+        // NOTE: This must be a macro (rather than a function with trait bounds)
+        // because there's no way, in a generic context, to enforce that two
+        // types have the same size or alignment. `core::mem::transmute` uses
+        // compiler magic to enforce size equality so long as the types are
+        // concrete. We use `SizeIsAlign` to create a type whose size is equal
+        // to the alignment of another type so that we can use `transmute` to
+        // check alignment as well.
+
+        let e = $e;
+        #[allow(unused, clippy::diverging_sub_expression)]
+        if false {
+            // This branch, though never taken, ensures that the type of `e` is
+            // `&T` where `T: 't + Sized + AsBytes`, that the type of this macro
+            // expression is `&U` where `U: 'u + Sized + FromBytes`, and that
+            // `'t` outlives `'u`.
+            const fn transmute<'u, 't: 'u, T: 't + Sized + $crate::AsBytes, U: 'u + Sized + $crate::FromBytes>(_t: &'t T) -> &'u U {
+                unreachable!()
+            }
+            transmute(e)
+        } else if false {
+            // This branch, though never taken, ensures that the alignment of
+            // `T` is equal to the alignment of `U`.
+            let target = unreachable!();
+            e = &target;
+
+            // SAFETY: This code is never executed.
+            let ret: $crate::SizeIsAlign<_> = unsafe { $crate::__real_transmute($crate::SizeIsAlign::new(target)) };
+            &ret.into_t()
+        } else {
+            // SAFETY: `core::mem::transmute` ensures that the type of `e` and
+            // the type of this macro invocation expression have the same size.
+            // We know this transmute is safe thanks to the `AsBytes` and
+            // `FromBytes` bounds enforced by the `false` branch.
+            //
+            // We use `$crate::__real_transmute` because we know it will always
+            // be available for crates which are using the 2015 edition of Rust.
+            // By contrast, if we were to use `std::mem::transmute`, this macro
+            // would not work for such crates in `no_std` contexts, and if we
+            // were to use `core::mem::transmute`, this macro would not work in
+            // `std` contexts in which `core` was not manually imported. This is
+            // not a problem for 2018 edition crates.
+            unsafe { $crate::__real_transmute(e) }
+        }
+    }}
+}
+
 /// A length- and alignment-checked reference to a byte slice which can safely
 /// be reinterpreted as another type.
 ///
@@ -3195,6 +3285,43 @@ mod tests {
         assert_eq!(X, ARRAY_OF_ARRAYS);
     }
 
+    #[test]
+    fn test_size_is_align() {
+        macro_rules! test {
+            ($ty:ty) => {
+                assert_eq!(mem::size_of::<SizeIsAlign<$ty>>(), mem::align_of::<$ty>());
+            };
+        }
+
+        test!(());
+        test!(u8);
+        test!(AU64);
+        test!([AU64; 2]);
+    }
+
+    #[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]
     fn test_address() {
         // Test that the `Deref` and `DerefMut` implementations return a

Diff for: tests/ui-msrv/transmute-illegal-lifetime.rs

@@ -0,0 +1 @@
+../ui/transmute-illegal-lifetime.rs

Diff for: tests/ui-msrv/transmute-illegal-lifetime.stderr

@@ -0,0 +1,9 @@
+error[E0597]: `x` does not live long enough
+  --> tests/ui-msrv/transmute-illegal-lifetime.rs:12:52
+   |
+12 |     let _: &'static u64 = zerocopy::transmute_ref!(&x);
+   |            ------------                            ^^ borrowed value does not live long enough
+   |            |
+   |            type annotation requires that `x` is borrowed for `'static`
+13 | }
+   | - `x` dropped here while still borrowed