Add try_transmute!, try_transmute_{ref,mut}!

TODO: Commit message body

TODO:
- Update try_transmute! docs to mention the size equality constraint?
  Add an example of this failing?
- In `try_transmute!`, should the argument be dropped or forgotten (ie,
  `mem::forget`) when the transmute fails?

Makes progress on #5

src/lib.rs

@@ -1939,14 +1939,16 @@ mod simd {
     simd_arch_mod!(arm, int8x4_t, uint8x4_t);
 }
 
-// Used in `transmute!` below.
+// Used in macros below.
 #[doc(hidden)]
 pub use core::mem::transmute as __real_transmute;
+#[doc(hidden)]
+pub use core::mem::ManuallyDrop as __RealManuallyDrop;
 
 /// Safely transmutes a value of one type to a value of another type of the same
 /// size.
 ///
-/// The expression `$e` must have a concrete type, `T`, which implements
+/// The expression, `$e`, must have a concrete type, `T`, which implements
 /// `AsBytes`. The `transmute!` expression must also have a concrete type, `U`
 /// (`U` is inferred from the calling context), and `U` must implement
 /// `FromBytes`.
@@ -1989,6 +1991,168 @@ macro_rules! transmute {
     }}
 }
 
+/// Safely attempts to transmute a value of one type to a value of another type
+/// of the same size, failing if the transmute would be unsound.
+///
+/// The expression, `$e`, must have a concrete type, `T`, which implements
+/// `AsBytes`. The `try_transmute!` expression must also have a concrete type,
+/// `Option<U>` (`U` is inferred from the calling context), and `U` must
+/// implement `TryFromBytes`.
+///
+/// [`TryFromBytes::try_read_from`] is used to attempt to convert `$e` to the
+/// output type `U`. This will fail if the bytes of `$e` do not correspond to a
+/// valid instance of `U`.
+///
+/// Note that the `T` produced by the expression `$e` will *not* be dropped.
+/// Semantically, its bits will be copied into a new value of type `U`, the
+/// original `T` will be forgotten, and the value of type `U` will be returned.
+///
+/// # Examples
+///
+/// ```rust
+/// # use zerocopy::try_transmute;
+/// assert_eq!(try_transmute!(1u8), Some(true));
+/// assert_eq!(try_transmute!(2u8), None::<bool>);
+///
+/// assert_eq!(try_transmute!(108u32), Some('l'));
+/// assert_eq!(try_transmute!(0xD800u32), None::<char>);
+/// ```
+#[macro_export]
+macro_rules! try_transmute {
+    ($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. `core::mem::transmute` uses compiler magic
+        // to enforce this so long as the types are concrete.
+
+        let e = $e;
+        if false {
+            // This branch, though never taken, ensures that the type of `e` is
+            // `AsBytes` and that the type of this macro invocation expression
+            // is `TryFromBytes`.
+            const fn transmute<T: $crate::AsBytes, U: $crate::TryFromBytes>(_t: T) -> U {
+                unreachable!()
+            }
+            Some(transmute(e))
+        } else if false {
+            // Though never executed, this ensures that the source and
+            // destination types have the same size. This isn't strictly
+            // necessary for soundness, but it turns what would otherwise be
+            // runtime errors into compile-time errors.
+            //
+            // SAFETY: This branch never executes.
+            Some(unsafe { $crate::__real_transmute(e) })
+        } else {
+            // TODO: What's the correct drop behavior on `None`? Does this just
+            // behave like `mem::forget` in that case?
+            let m = $crate::__RealManuallyDrop::new(e);
+            $crate::TryFromBytes::try_read_from($crate::AsBytes::as_bytes(&m))
+        }
+    }}
+}
+
+/// Safely attempts to transmute a reference of one type to a reference of
+/// another type, failing if the transmute would be unsound.
+///
+/// The expression, `$e`, must have a concrete type, `&T`, where `T: AsBytes`.
+/// The `try_transmute_ref!` expression must also have a concrete type,
+/// `Option<&U>` (`U` is inferred from the calling context), and `U` must
+/// implement `TryFromBytes`.
+///
+/// [`TryFromBytes::try_from_ref`] is used to attempt to convert `$e` to the
+/// output reference type `&U`. This will fail if `$e` is not the right size, is
+/// not properly aligned, or if the bytes of `$e` do not correspond to a valid
+/// instance of `U`.
+///
+/// Note that, if `U` is an unsized type, there will be multiple sizes for `$e`
+/// which correspond to valid values of `U`.
+///
+/// # Examples
+///
+/// ```rust
+/// # use zerocopy::try_transmute_ref;
+/// # use zerocopy::AsBytes as _;
+/// let s: Option<&str> = try_transmute_ref!(&[104u8, 101, 108, 108, 111]);
+/// assert_eq!(s, Some("hello"));
+///
+/// // Invalid UTF-8
+/// assert_eq!(try_transmute_ref!(&0xFFFFFFFFu32), None::<&str>);
+///
+/// // Not enough bytes for a `u8`
+/// assert_eq!(try_transmute_ref!(&()), None::<&u8>);
+///
+/// // Valid `&[[u8; 2]]` slices could be 2 or 4 bytes long,
+/// // but not 3.
+/// assert_eq!(try_transmute_ref!(&[0u8, 1, 2]), None::<&[[u8; 2]]>);
+///
+/// // Guaranteed to be invalidly-aligned so long as
+/// // `align_of::<u16>() == 2` and `align_of::<u32>() >= 2`
+/// // (this is true on most targets, but it isn't guaranteed).
+/// assert_eq!(try_transmute_ref!(&0u32.as_bytes()[1..]), None::<&u16>);
+/// ```
+#[macro_export]
+macro_rules! try_transmute_ref {
+    ($e:expr) => {
+        $crate::TryFromBytes::try_from_ref($crate::AsBytes::as_bytes($e))
+    };
+}
+
+/// Safely attempts to transmute a mutable reference of one type to a mutable
+/// reference of another type, failing if the transmute would be unsound.
+///
+/// The expression, `$e`, must have a concrete type, `&mut T`, where `T:
+/// FromBytes + AsBytes`. The `try_transmute_ref!` expression must also have a
+/// concrete type, `Option<&mut U>` (`U` is inferred from the calling context),
+/// and `U` must implement `TryFromBytes`.
+///
+/// [`TryFromBytes::try_from_mut`] is used to attempt to convert `$e` to the
+/// output reference type, `&mut U`. This will fail if `$e` is not the right
+/// size, is not properly aligned, or if the bytes of `$e` do not correspond to
+/// a valid instance of `U`.
+///
+/// Note that, if `U` is an unsized type, there will be multiple sizes for `$e`
+/// which correspond to valid values of `U`.
+///
+/// # Examples
+///
+/// ```rust
+/// # use zerocopy::try_transmute_mut;
+/// # use zerocopy::AsBytes as _;
+/// let bytes = &mut [104u8, 101, 108, 108, 111];
+/// let mut s = try_transmute_mut!(bytes);
+/// assert_eq!(s, Some(String::from("hello").as_mut_str()));
+///
+/// // Mutations to the transmuted reference are reflected
+/// // in the original reference.
+/// s.as_mut().unwrap().make_ascii_uppercase();
+/// assert_eq!(bytes, &[72, 69, 76, 76, 79]);
+///
+/// // Invalid UTF-8
+/// let mut u = 0xFFFFFFFFu32;
+/// assert_eq!(try_transmute_mut!(&mut u), None::<&mut str>);
+///
+/// // Not enough bytes for a `u8`
+/// let mut tuple = ();
+/// assert_eq!(try_transmute_mut!(&mut tuple), None::<&mut u8>);
+///
+/// // Valid `&mut [[u8; 2]]` slices could be 2 or 4 bytes
+/// // long, but not 3.
+/// let bytes = &mut [0u8, 1, 2];
+/// assert_eq!(try_transmute_mut!(bytes), None::<&mut [[u8; 2]]>);
+///
+/// // Guaranteed to be invalidly-aligned so long as
+/// // `align_of::<u16>() == 2` and `align_of::<u32>() >= 2`
+/// // (this is true on most targets, but it isn't guaranteed).
+/// let mut u = 0u32;
+/// assert_eq!(try_transmute_mut!(&mut u.as_bytes_mut()[1..]), None::<&mut u16>);
+/// ```
+#[macro_export]
+macro_rules! try_transmute_mut {
+    ($e:expr) => {
+        $crate::TryFromBytes::try_from_mut($crate::AsBytes::as_bytes_mut($e))
+    };
+}
+
 /// A typed reference derived from a byte slice.
 ///
 /// A `Ref<B, T>` is a reference to a `T` which is stored in a byte slice, `B`.
@@ -3599,10 +3763,16 @@ mod tests {
         // 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!(array_of_u8s);
         assert_eq!(x, array_of_arrays);
+        let x: Option<[[u8; 2]; 4]> = try_transmute!(array_of_u8s);
+        assert_eq!(x, Some(array_of_arrays));
+
         let x: [u8; 8] = transmute!(array_of_arrays);
         assert_eq!(x, array_of_u8s);
+        let x: Option<[u8; 8]> = try_transmute!(array_of_arrays);
+        assert_eq!(x, Some(array_of_u8s));
 
         // Test that the source expression's value is forgotten rather than
         // dropped.
@@ -3615,12 +3785,37 @@ mod tests {
             }
         }
         let _: () = transmute!(PanicOnDrop(()));
+        let _: Option<()> = try_transmute!(PanicOnDrop(()));
 
         // Test that `transmute!` 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]];
         const X: [[u8; 2]; 4] = transmute!(ARRAY_OF_U8S);
         assert_eq!(X, ARRAY_OF_ARRAYS);
+
+        // Test fallible transmutations with `try_transmute!`.
+        let mut b: Option<bool> = try_transmute!(0u8);
+        assert_eq!(b, Some(false));
+        b = try_transmute!(1u8);
+        assert_eq!(b, Some(true));
+        b = try_transmute!(2u8);
+        assert_eq!(b, None);
+    }
+
+    #[test]
+    fn test_try_transmute_ref_mut() {
+        // These macros are dead-simple thin wrappers which delegate to other
+        // traits. We only have this test to ensure that the macros are uesd
+        // somewhere so our tests will break if the paths to various items
+        // break.
+        let x: Option<&[u8; 2]> = try_transmute_ref!(&0xFFFFu16);
+        assert_eq!(x, Some(&[255, 255]));
+
+        let mut u = 0xFFFFu16;
+        let x: Option<&mut [u8; 2]> = try_transmute_mut!(&mut u);
+        assert_eq!(x, Some(&mut [255, 255]));
+        *x.unwrap() = [0, 0];
+        assert_eq!(u, 0);
     }
 
     #[test]

tests/ui-nightly/transmute-illegal.rs

@@ -8,3 +8,9 @@ fn main() {}
 
 // It is unsound to inspect the usize value of a pointer during const eval.
 const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
+
+// `transmute!` and `try_transmute!` enforce size equality.
+const TOO_LARGE: u64 = zerocopy::transmute!(0u8);
+const TRY_TOO_LARGE: Option<u64> = zerocopy::try_transmute!(0u8);
+const TOO_SMALL: u8 = zerocopy::transmute!(0u64);
+const TRY_TOO_SMALL: Option<u8> = zerocopy::try_transmute!(0u64);

tests/ui-nightly/transmute-illegal.stderr

@@ -14,3 +14,43 @@ note: required by a bound in `POINTER_VALUE::transmute`
 10 | const POINTER_VALUE: usize = zerocopy::transmute!(&0usize as *const usize);
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `transmute`
    = note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> tests/ui-nightly/transmute-illegal.rs:13:24
+   |
+13 | const TOO_LARGE: u64 = zerocopy::transmute!(0u8);
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `u8` (8 bits)
+   = note: target type: `u64` (64 bits)
+   = note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> tests/ui-nightly/transmute-illegal.rs:14:36
+   |
+14 | const TRY_TOO_LARGE: Option<u64> = zerocopy::try_transmute!(0u8);
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `u8` (8 bits)
+   = note: target type: `u64` (64 bits)
+   = note: this error originates in the macro `zerocopy::try_transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> tests/ui-nightly/transmute-illegal.rs:15:23
+   |
+15 | const TOO_SMALL: u8 = zerocopy::transmute!(0u64);
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `u64` (64 bits)
+   = note: target type: `u8` (8 bits)
+   = note: this error originates in the macro `zerocopy::transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> tests/ui-nightly/transmute-illegal.rs:16:35
+   |
+16 | const TRY_TOO_SMALL: Option<u8> = zerocopy::try_transmute!(0u64);
+   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `u64` (64 bits)
+   = note: target type: `u8` (8 bits)
+   = note: this error originates in the macro `zerocopy::try_transmute` (in Nightly builds, run with -Z macro-backtrace for more info)