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authorValentin Popov <valentin@popov.link>2024-07-19 15:37:58 +0300
committerValentin Popov <valentin@popov.link>2024-07-19 15:37:58 +0300
commita990de90fe41456a23e58bd087d2f107d321f3a1 (patch)
tree15afc392522a9e85dc3332235e311b7d39352ea9 /vendor/bytemuck/src/transparent.rs
parent3d48cd3f81164bbfc1a755dc1d4a9a02f98c8ddd (diff)
downloadfparkan-a990de90fe41456a23e58bd087d2f107d321f3a1.tar.xz
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-use super::*;
-
-/// A trait which indicates that a type is a `#[repr(transparent)]` wrapper
-/// around the `Inner` value.
-///
-/// This allows safely copy transmuting between the `Inner` type and the
-/// `TransparentWrapper` type. Functions like `wrap_{}` convert from the inner
-/// type to the wrapper type and `peel_{}` functions do the inverse conversion
-/// from the wrapper type to the inner type. We deliberately do not call the
-/// wrapper-removing methods "unwrap" because at this point that word is too
-/// strongly tied to the Option/ Result methods.
-///
-/// # Safety
-///
-/// The safety contract of `TransparentWrapper` is relatively simple:
-///
-/// For a given `Wrapper` which implements `TransparentWrapper<Inner>`:
-///
-/// 1. `Wrapper` must be a wrapper around `Inner` with an identical data
-/// representations. This either means that it must be a
-/// `#[repr(transparent)]` struct which contains a either a field of type
-/// `Inner` (or a field of some other transparent wrapper for `Inner`) as
-/// the only non-ZST field.
-///
-/// 2. Any fields *other* than the `Inner` field must be trivially constructable
-/// ZSTs, for example `PhantomData`, `PhantomPinned`, etc. (When deriving
-/// `TransparentWrapper` on a type with ZST fields, the ZST fields must be
-/// [`Zeroable`]).
-///
-/// 3. The `Wrapper` may not impose additional alignment requirements over
-/// `Inner`.
-/// - Note: this is currently guaranteed by `repr(transparent)`, but there
-/// have been discussions of lifting it, so it's stated here explicitly.
-///
-/// 4. All functions on `TransparentWrapper` **may not** be overridden.
-///
-/// ## Caveats
-///
-/// If the wrapper imposes additional constraints upon the inner type which are
-/// required for safety, it's responsible for ensuring those still hold -- this
-/// generally requires preventing access to instances of the inner type, as
-/// implementing `TransparentWrapper<U> for T` means anybody can call
-/// `T::cast_ref(any_instance_of_u)`.
-///
-/// For example, it would be invalid to implement TransparentWrapper for `str`
-/// to implement `TransparentWrapper` around `[u8]` because of this.
-///
-/// # Examples
-///
-/// ## Basic
-///
-/// ```
-/// use bytemuck::TransparentWrapper;
-/// # #[derive(Default)]
-/// # struct SomeStruct(u32);
-///
-/// #[repr(transparent)]
-/// struct MyWrapper(SomeStruct);
-///
-/// unsafe impl TransparentWrapper<SomeStruct> for MyWrapper {}
-///
-/// // interpret a reference to &SomeStruct as a &MyWrapper
-/// let thing = SomeStruct::default();
-/// let inner_ref: &MyWrapper = MyWrapper::wrap_ref(&thing);
-///
-/// // Works with &mut too.
-/// let mut mut_thing = SomeStruct::default();
-/// let inner_mut: &mut MyWrapper = MyWrapper::wrap_mut(&mut mut_thing);
-///
-/// # let _ = (inner_ref, inner_mut); // silence warnings
-/// ```
-///
-/// ## Use with dynamically sized types
-///
-/// ```
-/// use bytemuck::TransparentWrapper;
-///
-/// #[repr(transparent)]
-/// struct Slice<T>([T]);
-///
-/// unsafe impl<T> TransparentWrapper<[T]> for Slice<T> {}
-///
-/// let s = Slice::wrap_ref(&[1u32, 2, 3]);
-/// assert_eq!(&s.0, &[1, 2, 3]);
-///
-/// let mut buf = [1, 2, 3u8];
-/// let sm = Slice::wrap_mut(&mut buf);
-/// ```
-///
-/// ## Deriving
-///
-/// When deriving, the non-wrapped fields must uphold all the normal requirements,
-/// and must also be `Zeroable`.
-///
-#[cfg_attr(feature = "derive", doc = "```")]
-#[cfg_attr(
- not(feature = "derive"),
- doc = "```ignore
-// This example requires the `derive` feature."
-)]
-/// use bytemuck::TransparentWrapper;
-/// use std::marker::PhantomData;
-///
-/// #[derive(TransparentWrapper)]
-/// #[repr(transparent)]
-/// #[transparent(usize)]
-/// struct Wrapper<T: ?Sized>(usize, PhantomData<T>); // PhantomData<T> implements Zeroable for all T
-/// ```
-///
-/// Here, an error will occur, because `MyZst` does not implement `Zeroable`.
-///
-#[cfg_attr(feature = "derive", doc = "```compile_fail")]
-#[cfg_attr(
- not(feature = "derive"),
- doc = "```ignore
-// This example requires the `derive` feature."
-)]
-/// use bytemuck::TransparentWrapper;
-/// struct MyZst;
-///
-/// #[derive(TransparentWrapper)]
-/// #[repr(transparent)]
-/// #[transparent(usize)]
-/// struct Wrapper(usize, MyZst); // MyZst does not implement Zeroable
-/// ```
-pub unsafe trait TransparentWrapper<Inner: ?Sized> {
- /// Convert the inner type into the wrapper type.
- #[inline]
- fn wrap(s: Inner) -> Self
- where
- Self: Sized,
- Inner: Sized,
- {
- // SAFETY: The unsafe contract requires that `Self` and `Inner` have
- // identical representations.
- unsafe { transmute!(s) }
- }
-
- /// Convert a reference to the inner type into a reference to the wrapper
- /// type.
- #[inline]
- fn wrap_ref(s: &Inner) -> &Self {
- unsafe {
- assert!(size_of::<*const Inner>() == size_of::<*const Self>());
- // 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 these two have
- // identical representations.
- let inner_ptr = s as *const Inner;
- let wrapper_ptr: *const Self = transmute!(inner_ptr);
- &*wrapper_ptr
- }
- }
-
- /// Convert a mutable reference to the inner type into a mutable reference to
- /// the wrapper type.
- #[inline]
- fn wrap_mut(s: &mut Inner) -> &mut Self {
- unsafe {
- assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
- // 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 these two have
- // identical representations.
- let inner_ptr = s as *mut Inner;
- let wrapper_ptr: *mut Self = transmute!(inner_ptr);
- &mut *wrapper_ptr
- }
- }
-
- /// Convert a slice to the inner type into a slice to the wrapper type.
- #[inline]
- fn wrap_slice(s: &[Inner]) -> &[Self]
- where
- Self: Sized,
- Inner: Sized,
- {
- unsafe {
- assert!(size_of::<*const Inner>() == size_of::<*const Self>());
- assert!(align_of::<*const Inner>() == align_of::<*const Self>());
- // SAFETY: The unsafe contract requires that these two have
- // identical representations (size and alignment).
- core::slice::from_raw_parts(s.as_ptr() as *const Self, s.len())
- }
- }
-
- /// Convert a mutable slice to the inner type into a mutable slice to the
- /// wrapper type.
- #[inline]
- fn wrap_slice_mut(s: &mut [Inner]) -> &mut [Self]
- where
- Self: Sized,
- Inner: Sized,
- {
- unsafe {
- assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
- assert!(align_of::<*mut Inner>() == align_of::<*mut Self>());
- // SAFETY: The unsafe contract requires that these two have
- // identical representations (size and alignment).
- core::slice::from_raw_parts_mut(s.as_mut_ptr() as *mut Self, s.len())
- }
- }
-
- /// Convert the wrapper type into the inner type.
- #[inline]
- fn peel(s: Self) -> Inner
- where
- Self: Sized,
- Inner: Sized,
- {
- unsafe { transmute!(s) }
- }
-
- /// Convert a reference to the wrapper type into a reference to the inner
- /// type.
- #[inline]
- fn peel_ref(s: &Self) -> &Inner {
- unsafe {
- assert!(size_of::<*const Inner>() == size_of::<*const Self>());
- // 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 these two have
- // identical representations.
- let wrapper_ptr = s as *const Self;
- let inner_ptr: *const Inner = transmute!(wrapper_ptr);
- &*inner_ptr
- }
- }
-
- /// Convert a mutable reference to the wrapper type into a mutable reference
- /// to the inner type.
- #[inline]
- fn peel_mut(s: &mut Self) -> &mut Inner {
- unsafe {
- assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
- // 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 these two have
- // identical representations.
- let wrapper_ptr = s as *mut Self;
- let inner_ptr: *mut Inner = transmute!(wrapper_ptr);
- &mut *inner_ptr
- }
- }
-
- /// Convert a slice to the wrapped type into a slice to the inner type.
- #[inline]
- fn peel_slice(s: &[Self]) -> &[Inner]
- where
- Self: Sized,
- Inner: Sized,
- {
- unsafe {
- assert!(size_of::<*const Inner>() == size_of::<*const Self>());
- assert!(align_of::<*const Inner>() == align_of::<*const Self>());
- // SAFETY: The unsafe contract requires that these two have
- // identical representations (size and alignment).
- core::slice::from_raw_parts(s.as_ptr() as *const Inner, s.len())
- }
- }
-
- /// Convert a mutable slice to the wrapped type into a mutable slice to the
- /// inner type.
- #[inline]
- fn peel_slice_mut(s: &mut [Self]) -> &mut [Inner]
- where
- Self: Sized,
- Inner: Sized,
- {
- unsafe {
- assert!(size_of::<*mut Inner>() == size_of::<*mut Self>());
- assert!(align_of::<*mut Inner>() == align_of::<*mut Self>());
- // SAFETY: The unsafe contract requires that these two have
- // identical representations (size and alignment).
- core::slice::from_raw_parts_mut(s.as_mut_ptr() as *mut Inner, s.len())
- }
- }
-}
-
-unsafe impl<T> TransparentWrapper<T> for core::num::Wrapping<T> {}