more

src/lib.rs

@@ -3,7 +3,7 @@ pub(crate) use core::arch::x86;
 #[cfg(target_arch = "x86_64")]
 pub(crate) use core::arch::x86_64;
 //
-pub(crate) use core::{marker::*, num::*, ptr::*};
+pub(crate) use core::{marker::*, mem::*, num::*, ptr::*};
 
 macro_rules! impl_unsafe_marker_for_array {
   ( $marker:ident , $( $n:expr ),* ) => {
@@ -11,11 +11,226 @@ macro_rules! impl_unsafe_marker_for_array {
   }
 }
 
-macro_rules! impl_unsafe_marker_for_type {
+mod zeroable;
-  ( $marker:ident , $( $t:ty ),* ) => {
+pub use zeroable::*;
-    $(unsafe impl<T> $marker for $t {})*
+
+mod pod;
+pub use pod::*;
+
+/// Re-interprets `&T` as `&[u8]`.
+///
+/// Any ZST becomes an empty slice, and in that case the pointer value of that
+/// empty slice might not match the pointer value of the input reference.
+#[inline]
+pub fn bytes_of<T: Pod>(t: &T) -> &[u8] {
+  try_cast_slice::<T, u8>(core::slice::from_ref(t)).unwrap_or(&[])
+}
+
+/// Re-interprets `&mut T` as `&mut [u8]`.
+///
+/// Any ZST becomes an empty slice, and in that case the pointer value of that
+/// empty slice might not match the pointer value of the input reference.
+#[inline]
+pub fn bytes_of_mut<T: Pod>(t: &mut T) -> &mut [u8] {
+  try_cast_slice_mut::<T, u8>(core::slice::from_mut(t)).unwrap_or(&mut [])
+}
+
+/// The things that can go wrong when casting between [`Pod`] data forms.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PodCastError {
+  /// You tried to cast a slice to an element type with a higher alignment
+  /// requirement but the slice wasn't aligned.
+  TargetAlignmentGreaterAndInputNotAligned,
+  /// You tried to cast between a zero-sized type and a non-zero-sized type.
+  /// Because the output slice resizes based on the input and output types, it's
+  /// fairly nonsensical to throw a ZST into the mix. You can go from a ZST to
+  /// another ZST, if you want.
+  CantConvertBetweenZSTAndNonZST,
+  /// If the element size changes then the output slice changes length
+  /// accordingly. If the output slice wouldn't be a whole number of elements
+  /// then the conversion fails.
+  OutputSliceWouldHaveSlop,
+  /// When casting an individual `T`, `&T`, or `&mut T` value the source size
+  /// and destination size must be an exact match.
+  SizeMismatch,
+}
+
+/// Cast `T` into `U`
+///
+/// ## Panics
+///
+/// This is [`try_cast`] with an unwrap.
+#[inline]
+pub fn cast<A: Pod, B: Pod>(a: A) -> B {
+  try_cast(a).unwrap()
+}
+
+/// Cast `&mut T` into `&mut U`.
+///
+/// ## Panics
+///
+/// This is [`try_cast_mut`] with an unwrap.
+#[inline]
+pub fn cast_mut<A: Pod, B: Pod>(a: &mut A) -> &mut B {
+  try_cast_mut(a).unwrap()
+}
+
+/// Cast `&T` into `&U`.
+///
+/// ## Panics
+///
+/// This is [`try_cast_ref`] with an unwrap.
+#[inline]
+pub fn cast_ref<A: Pod, B: Pod>(a: &A) -> &B {
+  try_cast_ref(a).unwrap()
+}
+
+/// Cast `&[T]` into `&[U]`.
+///
+/// ## Panics
+///
+/// This is [`try_cast_slice`] with an unwrap.
+#[inline]
+pub fn cast_slice<A: Pod, B: Pod>(a: &[A]) -> &[B] {
+  try_cast_slice(a).unwrap()
+}
+
+/// Cast `&mut [T]` into `&mut [U]`.
+///
+/// ## Panics
+///
+/// This is [`try_cast_slice_mut`] with an unwrap.
+#[inline]
+pub fn cast_slice_mut<A: Pod, B: Pod>(a: &mut [A]) -> &mut [B] {
+  try_cast_slice_mut(a).unwrap()
+}
+
+/// As `align_to`, but safe because of the [`Pod`] bound.
+#[inline]
+pub fn pod_align_to<T: Pod, U: Pod>(vals: &[T]) -> (&[T], &[U], &[T]) {
+  unsafe { vals.align_to::<U>() }
+}
+
+/// As `align_to_mut`, but safe because of the [`Pod`] bound.
+#[inline]
+pub fn pod_align_to_mut<T: Pod, U: Pod>(vals: &mut [T]) -> (&mut [T], &mut [U], &mut [T]) {
+  unsafe { vals.align_to_mut::<U>() }
+}
+
+/// Try to cast `T` into `U`.
+///
+/// ## Failure
+///
+/// * If the types don't have the same size this fails.
+#[inline]
+pub fn try_cast<A: Pod, B: Pod>(a: A) -> Result<B, PodCastError> {
+  if size_of::<A>() == size_of::<B>() {
+    let mut b = B::zeroed();
+    // Note(Lokathor): We copy in terms of `u8` because that allows us to bypass
+    // any potential alignment difficulties.
+    let ap = &a as *const A as *const u8;
+    let bp = &mut b as *mut B as *mut u8;
+    unsafe { ap.copy_to_nonoverlapping(bp, size_of::<A>()) };
+    Ok(b)
+  } else {
+    Err(PodCastError::SizeMismatch)
   }
 }
 
-mod zeroable;
+/// Try to convert a `&T` into `&U`.
-pub use zeroable::*;
+///
+/// ## Failure
+///
+/// * If the reference isn't aligned in the new type
+/// * If the source type and target type aren't the same size.
+#[inline]
+pub fn try_cast_ref<A: Pod, B: Pod>(a: &A) -> Result<&B, PodCastError> {
+  // Note(Lokathor): everything with `align_of` and `size_of` will optimize away
+  // after monomorphization.
+  if align_of::<B>() > align_of::<A>() && (a as *const A as usize) % align_of::<B>() != 0 {
+    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
+  } else if size_of::<B>() == size_of::<A>() {
+    Ok(unsafe {
+      (a as *const A as *const B)
+        .as_ref()
+        .unwrap_or_else(|| core::hint::unreachable_unchecked())
+    })
+  } else {
+    Err(PodCastError::SizeMismatch)
+  }
+}
+
+/// Try to convert a `&mut T` into `&mut U`.
+///
+/// As [`try_cast_ref`], but `mut`.
+#[inline]
+pub fn try_cast_mut<A: Pod, B: Pod>(a: &mut A) -> Result<&mut B, PodCastError> {
+  // Note(Lokathor): everything with `align_of` and `size_of` will optimize away
+  // after monomorphization.
+  if align_of::<B>() > align_of::<A>() && (a as *mut A as usize) % align_of::<B>() != 0 {
+    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
+  } else if size_of::<B>() == size_of::<A>() {
+    Ok(unsafe {
+      (a as *mut A as *mut B)
+        .as_mut()
+        .unwrap_or_else(|| core::hint::unreachable_unchecked())
+    })
+  } else {
+    Err(PodCastError::SizeMismatch)
+  }
+}
+
+/// Try to convert `&[T]` into `&[U]` (possibly with a change in length).
+///
+/// * `input.as_ptr() as usize == output.as_ptr() as usize`
+/// * `input.len() * size_of::<A>() == output.len() * size_of::<B>()`
+///
+/// ## Failure
+///
+/// * If the target type has a greater alignment requirement and the input slice
+///   isn't aligned.
+/// * If the target element type is a different size from the current element
+///   type, and the output slice wouldn't be a whole number of elements when
+///   accounting for the size change (eg: three `u16` values is 1.5 `u32`
+///   values, so that's a failure).
+/// * Similarly, you can't convert between a
+///   [ZST](https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts)
+///   and a non-ZST.
+#[inline]
+pub fn try_cast_slice<A: Pod, B: Pod>(a: &[A]) -> Result<&[B], PodCastError> {
+  // Note(Lokathor): everything with `align_of` and `size_of` will optimize away
+  // after monomorphization.
+  if align_of::<B>() > align_of::<A>() && (a.as_ptr() as usize) % align_of::<B>() != 0 {
+    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
+  } else if size_of::<B>() == size_of::<A>() {
+    Ok(unsafe { core::slice::from_raw_parts(a.as_ptr() as *const B, a.len()) })
+  } else if size_of::<A>() == 0 || size_of::<B>() == 0 {
+    Err(PodCastError::CantConvertBetweenZSTAndNonZST)
+  } else if core::mem::size_of_val(a) % size_of::<B>() == 0 {
+    let new_len = core::mem::size_of_val(a) / size_of::<B>();
+    Ok(unsafe { core::slice::from_raw_parts(a.as_ptr() as *const B, new_len) })
+  } else {
+    Err(PodCastError::OutputSliceWouldHaveSlop)
+  }
+}
+
+/// Try to convert `&mut [T]` into `mut [U]` (possibly with a change in length).
+///
+/// As [`try_cast_slice`], but `mut`.
+#[inline]
+pub fn try_cast_slice_mut<A: Pod, B: Pod>(a: &mut [A]) -> Result<&mut [B], PodCastError> {
+  // Note(Lokathor): everything with `align_of` and `size_of` will optimize away
+  // after monomorphization.
+  if align_of::<B>() > align_of::<A>() && (a.as_ptr() as usize) % align_of::<B>() != 0 {
+    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
+  } else if size_of::<B>() == size_of::<A>() {
+    Ok(unsafe { core::slice::from_raw_parts_mut(a.as_ptr() as *mut B, a.len()) })
+  } else if size_of::<A>() == 0 || size_of::<B>() == 0 {
+    Err(PodCastError::CantConvertBetweenZSTAndNonZST)
+  } else if core::mem::size_of_val(a) % size_of::<B>() == 0 {
+    let new_len = core::mem::size_of_val(a) / size_of::<B>();
+    Ok(unsafe { core::slice::from_raw_parts_mut(a.as_ptr() as *mut B, new_len) })
+  } else {
+    Err(PodCastError::OutputSliceWouldHaveSlop)
+  }
+}

src/pod.rs

@@ -0,0 +1,90 @@
+use super::*;
+
+/// Marker trait for "plain old data".
+///
+/// The point of this trait is that once something is marked "plain old data"
+/// you can really go to town with the bit fiddling and bit casting. Therefore,
+/// it's a relatively strong claim to make about a type. Do not add this to your
+/// type casually.
+///
+/// **Reminder:** The results of casting around bytes between data types are
+/// _endian dependant_. Little-endian machines are the most common, but
+/// big-endian machines do exist (and big-endian is also used for "network
+/// order" bytes).
+///
+/// ## Safety
+///
+/// * The type must be inhabited (eg: no
+///   [Infallible](core::convert::Infallible)).
+/// * The type must allow any bit pattern (eg: no `bool` or `char`).
+/// * The type must not contain any padding bytes (eg: no `(u8, u16)`).
+/// * A struct needs to be `repr(C)`, or a `repr(transparent)` wrapper around a
+///   `Pod` type.
+pub unsafe trait Pod: Zeroable + Copy + 'static {}
+
+unsafe impl Pod for () {}
+unsafe impl Pod for u8 {}
+unsafe impl Pod for i8 {}
+unsafe impl Pod for u16 {}
+unsafe impl Pod for i16 {}
+unsafe impl Pod for u32 {}
+unsafe impl Pod for i32 {}
+unsafe impl Pod for u64 {}
+unsafe impl Pod for i64 {}
+unsafe impl Pod for usize {}
+unsafe impl Pod for isize {}
+unsafe impl Pod for u128 {}
+unsafe impl Pod for i128 {}
+unsafe impl Pod for f32 {}
+unsafe impl Pod for f64 {}
+unsafe impl<T: Pod> Pod for Wrapping<T> {}
+
+unsafe impl Pod for Option<NonZeroI8> {}
+unsafe impl Pod for Option<NonZeroI16> {}
+unsafe impl Pod for Option<NonZeroI32> {}
+unsafe impl Pod for Option<NonZeroI64> {}
+unsafe impl Pod for Option<NonZeroI128> {}
+unsafe impl Pod for Option<NonZeroIsize> {}
+unsafe impl Pod for Option<NonZeroU8> {}
+unsafe impl Pod for Option<NonZeroU16> {}
+unsafe impl Pod for Option<NonZeroU32> {}
+unsafe impl Pod for Option<NonZeroU64> {}
+unsafe impl Pod for Option<NonZeroU128> {}
+unsafe impl Pod for Option<NonZeroUsize> {}
+
+unsafe impl<T: 'static> Pod for *mut T {}
+unsafe impl<T: 'static> Pod for *const T {}
+unsafe impl<T: 'static> Pod for Option<NonNull<T>> {}
+unsafe impl<T: Pod> Pod for PhantomData<T> {}
+unsafe impl<T: Pod> Pod for ManuallyDrop<T> {}
+
+impl_unsafe_marker_for_array!(
+  Pod, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+  25, 26, 27, 28, 29, 30, 31, 32, 48, 64, 96, 128, 256, 512, 1024
+);
+
+#[cfg(target_arch = "x86")]
+unsafe impl Pod for x86::__m128i {}
+#[cfg(target_arch = "x86")]
+unsafe impl Pod for x86::__m128 {}
+#[cfg(target_arch = "x86")]
+unsafe impl Pod for x86::__m128d {}
+#[cfg(target_arch = "x86")]
+unsafe impl Pod for x86::__m256i {}
+#[cfg(target_arch = "x86")]
+unsafe impl Pod for x86::__m256 {}
+#[cfg(target_arch = "x86")]
+unsafe impl Pod for x86::__m256d {}
+
+#[cfg(target_arch = "x86_64")]
+unsafe impl Pod for x86_64::__m128i {}
+#[cfg(target_arch = "x86_64")]
+unsafe impl Pod for x86_64::__m128 {}
+#[cfg(target_arch = "x86_64")]
+unsafe impl Pod for x86_64::__m128d {}
+#[cfg(target_arch = "x86_64")]
+unsafe impl Pod for x86_64::__m256i {}
+#[cfg(target_arch = "x86_64")]
+unsafe impl Pod for x86_64::__m256 {}
+#[cfg(target_arch = "x86_64")]
+unsafe impl Pod for x86_64::__m256d {}

src/zeroable.rs

@@ -35,6 +35,7 @@ unsafe impl Zeroable for u128 {}
 unsafe impl Zeroable for i128 {}
 unsafe impl Zeroable for f32 {}
 unsafe impl Zeroable for f64 {}
+unsafe impl<T: Zeroable> Zeroable for Wrapping<T> {}
 
 unsafe impl Zeroable for Option<NonZeroI8> {}
 unsafe impl Zeroable for Option<NonZeroI16> {}
@@ -52,62 +53,26 @@ unsafe impl Zeroable for Option<NonZeroUsize> {}
 unsafe impl<T> Zeroable for *mut T {}
 unsafe impl<T> Zeroable for *const T {}
 unsafe impl<T> Zeroable for Option<NonNull<T>> {}
-unsafe impl<T> Zeroable for PhantomData<T> where T: Zeroable {}
+unsafe impl<T: Zeroable> Zeroable for PhantomData<T> {}
+unsafe impl<T: Zeroable> Zeroable for ManuallyDrop<T> {}
 
-unsafe impl<A> Zeroable for (A,) where A: Zeroable {}
+unsafe impl<A: Zeroable> Zeroable for (A,) {}
-unsafe impl<A, B> Zeroable for (A, B)
+unsafe impl<A: Zeroable, B: Zeroable> Zeroable for (A, B) {}
-where
+unsafe impl<A: Zeroable, B: Zeroable, C: Zeroable> Zeroable for (A, B, C) {}
-  A: Zeroable,
+unsafe impl<A: Zeroable, B: Zeroable, C: Zeroable, D: Zeroable> Zeroable for (A, B, C, D) {}
-  B: Zeroable,
+unsafe impl<A: Zeroable, B: Zeroable, C: Zeroable, D: Zeroable, E: Zeroable> Zeroable
+  for (A, B, C, D, E)
 {
 }
-unsafe impl<A, B, C> Zeroable for (A, B, C)
+unsafe impl<A: Zeroable, B: Zeroable, C: Zeroable, D: Zeroable, E: Zeroable, F: Zeroable> Zeroable
-where
+  for (A, B, C, D, E, F)
-  A: Zeroable,
-  B: Zeroable,
-  C: Zeroable,
 {
 }
-unsafe impl<A, B, C, D> Zeroable for (A, B, C, D)
+unsafe impl<A: Zeroable, B: Zeroable, C: Zeroable, D: Zeroable, E: Zeroable, F: Zeroable, G: Zeroable>
-where
+  Zeroable for (A, B, C, D, E, F, G)
-  A: Zeroable,
-  B: Zeroable,
-  C: Zeroable,
-  D: Zeroable,
 {
 }
-unsafe impl<A, B, C, D, E> Zeroable for (A, B, C, D, E)
+unsafe impl<
-where
-  A: Zeroable,
-  B: Zeroable,
-  C: Zeroable,
-  D: Zeroable,
-  E: Zeroable,
-{
-}
-unsafe impl<A, B, C, D, E, F> Zeroable for (A, B, C, D, E, F)
-where
-  A: Zeroable,
-  B: Zeroable,
-  C: Zeroable,
-  D: Zeroable,
-  E: Zeroable,
-  F: Zeroable,
-{
-}
-unsafe impl<A, B, C, D, E, F, G> Zeroable for (A, B, C, D, E, F, G)
-where
-  A: Zeroable,
-  B: Zeroable,
-  C: Zeroable,
-  D: Zeroable,
-  E: Zeroable,
-  F: Zeroable,
-  G: Zeroable,
-{
-}
-unsafe impl<A, B, C, D, E, F, G, H> Zeroable for (A, B, C, D, E, F, G, H)
-where
     A: Zeroable,
     B: Zeroable,
     C: Zeroable,
@@ -116,6 +81,7 @@ where
     F: Zeroable,
     G: Zeroable,
     H: Zeroable,
+  > Zeroable for (A, B, C, D, E, F, G, H)
 {
 }