diff --git a/src/allocation.rs b/src/allocation.rs index 735f6db..888d3c1 100644 --- a/src/allocation.rs +++ b/src/allocation.rs @@ -339,6 +339,30 @@ pub trait TransparentWrapperAlloc: TransparentWrapper { } } + /// Convert a box to the inner type into a box to the wrapper + /// type. + #[inline] + fn wrap_box(s: Box) -> Box { + assert!(size_of::<*mut Inner>() == size_of::<*mut Self>()); + + unsafe { + // A pointer cast doesn't work here because rustc can't tell that + // the vtables match (because of the `?Sized` restriction relaxation). + // A `transmute` doesn't work because the sizes are unspecified. + // + // SAFETY: + // * The unsafe contract requires that pointers to Inner and Self + // have identical representations + // * Box is guaranteed to have representation identical to a + // (non-null) pointer + // * The pointer comes from a box (and thus satisfies all safety + // requirements of Box) + let inner_ptr: *mut Inner = Box::into_raw(s); + let wrapper_ptr: *mut Self = transmute!(inner_ptr); + Box::from_raw(wrapper_ptr) + } + } + /// Convert a vec of the wrapper type into a vec of the inner type. fn peel_vec(s: Vec) -> Vec where @@ -363,5 +387,29 @@ pub trait TransparentWrapperAlloc: TransparentWrapper { ) } } + + /// Convert a box to the wrapper type into a box to the inner + /// type. + #[inline] + fn peel_box(s: Box) -> Box { + assert!(size_of::<*mut Inner>() == size_of::<*mut Self>()); + + unsafe { + // A pointer cast doesn't work here because rustc can't tell that + // the vtables match (because of the `?Sized` restriction relaxation). + // A `transmute` doesn't work because the sizes are unspecified. + // + // SAFETY: + // * The unsafe contract requires that pointers to Inner and Self + // have identical representations + // * Box is guaranteed to have representation identical to a + // (non-null) pointer + // * The pointer comes from a box (and thus satisfies all safety + // requirements of Box) + let wrapper_ptr: *mut Self = Box::into_raw(s); + let inner_ptr: *mut Inner = transmute!(wrapper_ptr); + Box::from_raw(inner_ptr) + } + } } impl> TransparentWrapperAlloc for T {} diff --git a/tests/transparent.rs b/tests/transparent.rs index fc4ec1b..ad3435f 100644 --- a/tests/transparent.rs +++ b/tests/transparent.rs @@ -5,12 +5,12 @@ #[test] fn test_transparent_wrapper() { // An external type defined in a different crate. - #[derive(Copy, Clone, Default)] + #[derive(Debug, Copy, Clone, Default)] struct Foreign(u8); use bytemuck::TransparentWrapper; - #[derive(Copy, Clone)] + #[derive(Debug, Copy, Clone)] #[repr(transparent)] struct Wrapper(Foreign); @@ -20,6 +20,14 @@ fn test_transparent_wrapper() { unsafe impl bytemuck::Zeroable for Wrapper {} unsafe impl bytemuck::Pod for Wrapper {} + impl PartialEq for Foreign { + fn eq(&self, &other: &u8) -> bool { self.0 == other } + } + + impl PartialEq for Wrapper { + fn eq(&self, &other: &u8) -> bool { self.0 == other } + } + let _: u8 = bytemuck::cast(Wrapper::wrap(Foreign::default())); let _: Foreign = Wrapper::peel(bytemuck::cast(u8::default())); @@ -69,13 +77,16 @@ fn test_transparent_wrapper() { let a: Vec = vec![Foreign::default(); 2]; let b: Vec = Wrapper::wrap_vec(a); - assert_eq!( - bytemuck::cast_slice::(b.as_slice()), &[0u8, 0] - ); + assert_eq!(b, [0, 0]); let c: Vec = Wrapper::peel_vec(b); - assert_eq!( - bytemuck::cast_slice::(Wrapper::wrap_slice(c.as_slice())), &[0, 0] - ); + assert_eq!(c, [0, 0]); + + let d: Box = Box::new(Foreign::default()); + + let e: Box = Wrapper::wrap_box(d); + assert_eq!(&*e, &0); + let f: Box = Wrapper::peel_box(e); + assert_eq!(&*f, &0); } }