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pub fn cast_b_to_a(b: B) { match b.cast::<A>() { Ok(a) if a.a == 1 => println!("worked"), _ => {} } } pub fn cast_refb_to_refa(b: &B) { match b.cast::<A>() { Ok(a) if a.a == 1 => println!("worked"), _ => {} } } // Note that `A` is not ToBytes since it contains padding. It // can therefore not be cast from. pub fn transmute_refb_to_refa(b: &B) { if (b as *const B).align_offset(std::mem::align_of::<A>()) != 0 { return; } let a = unsafe { std::mem::transmute::<&B, &A>(b) }; if a.a == 1 { println!("worked") } } #[repr(C)] pub struct A { a: u16, b: C } #[repr(u8)] pub enum C { A = 3, B = 4, C = 5 } #[repr(C)] pub struct B { a: u8, b: u8, c: D, // This becomes the padding byte for `A`, remove it // and casting will fail (though the compiler can staticaly) // see this d: u8 } #[repr(u8)] pub enum D { A = 3, B = 4, C = 5 } enum FromBytesError { InsufficientAlignment, InsufficientBytes, InvalidValue, } trait FromBytes { fn from_bytes(bytes: &[u8]) -> Result<&Self, FromBytesError>; } trait FromAnyBytes {} impl<T: FromAnyBytes> FromBytes for T { // Inline to allow optimizing away the length and alignment checks. #[inline] fn from_bytes(bytes: &[u8]) -> Result<&Self, FromBytesError> { if bytes.len() < std::mem::size_of::<Self>() { return Err(FromBytesError::InsufficientBytes); } if bytes.as_ptr().align_offset(std::mem::align_of::<Self>()) != 0 { return Err(FromBytesError::InsufficientAlignment); } Ok(unsafe { std::mem::transmute::<*const u8, &Self>(bytes.as_ptr()) }) } } impl FromAnyBytes for u8 {} impl FromAnyBytes for u16 {} impl FromBytes for A { #[inline] fn from_bytes(bytes: &[u8]) -> Result<&Self, FromBytesError> { if bytes.len() < std::mem::size_of::<Self>() { return Err(FromBytesError::InsufficientBytes); } if bytes.as_ptr().align_offset(std::mem::align_of::<Self>()) != 0 { return Err(FromBytesError::InsufficientAlignment); } match &bytes[2] { b if *b == 3 || *b == 4 || *b == 5 => { Ok(unsafe { std::mem::transmute::<*const u8, &Self>(bytes.as_ptr()) }) } _ => Err(FromBytesError::InvalidValue), } } } impl FromBytes for B { #[inline] fn from_bytes(bytes: &[u8]) -> Result<&Self, FromBytesError> { if bytes.len() < std::mem::size_of::<Self>() { return Err(FromBytesError::InsufficientBytes); } if bytes.as_ptr().align_offset(std::mem::align_of::<Self>()) != 0 { return Err(FromBytesError::InsufficientAlignment); } match &bytes[2] { b if *b == 3 || *b == 4 || *b == 5 => { Ok(unsafe { std::mem::transmute::<*const u8, &Self>(bytes.as_ptr()) }) } _ => Err(FromBytesError::InvalidValue), } } } trait ToBytes: Sized { #[inline] fn to_bytes(&self) -> &[u8] { let pointer = self as *const Self as *const u8; unsafe { std::slice::from_raw_parts(pointer, std::mem::size_of::<Self>()) } } /// Safely cast this type in-place to another type, returning a reference /// to the same memory. fn cast<T: FromBytes>(&self) -> Result<&T, FromBytesError> { T::from_bytes(self.to_bytes()) } } impl ToBytes for bool {} impl ToBytes for B {}
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