Vendor things

third-party/vendor/bytemuck/src/no_uninit.rs

@@ -0,0 +1,80 @@
+use crate::Pod;
+use core::num::{
+  NonZeroI128, NonZeroI16, NonZeroI32, NonZeroI64, NonZeroI8, NonZeroIsize,
+  NonZeroU128, NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize,
+};
+
+/// Marker trait for "plain old data" types with no uninit (or padding) bytes.
+///
+/// The requirements for this is very similar to [`Pod`],
+/// except that it doesn't require that all bit patterns of the type are valid,
+/// i.e. it does not require the type to be [`Zeroable`][crate::Zeroable].
+/// This limits what you can do with a type of this kind, but also broadens the
+/// included types to things like C-style enums. Notably, you can only cast from
+/// *immutable* references to a [`NoUninit`] type into *immutable* references of
+/// any other type, no casting of mutable references or mutable references to
+/// slices etc.
+///
+/// [`Pod`] is a subset of [`NoUninit`], meaning that any `T: Pod` is also
+/// [`NoUninit`] but any `T: NoUninit` is not necessarily [`Pod`]. If possible,
+/// prefer implementing [`Pod`] directly. To get more [`Pod`]-like functionality
+/// for a type that is only [`NoUninit`], consider also implementing
+/// [`CheckedBitPattern`][crate::CheckedBitPattern].
+///
+/// # Derive
+///
+/// A `#[derive(NoUninit)]` macro is provided under the `derive` feature flag
+/// which will automatically validate the requirements of this trait and
+/// implement the trait for you for both enums and structs. This is the
+/// recommended method for implementing the trait, however it's also possible to
+/// do manually. If you implement it manually, you *must* carefully follow the
+/// below safety rules.
+///
+/// # Safety
+///
+/// The same as [`Pod`] except we disregard the rule about it must
+/// allow any bit pattern (i.e. it does not need to be
+/// [`Zeroable`][crate::Zeroable]). Still, this is a quite strong guarantee
+/// about a type, so *be careful* whem implementing it manually.
+///
+/// * The type must be inhabited (eg: no
+///   [Infallible](core::convert::Infallible)).
+/// * The type must not contain any uninit (or padding) bytes, either in the
+///   middle or on the end (eg: no `#[repr(C)] struct Foo(u8, u16)`, which has
+///   padding in the middle, and also no `#[repr(C)] struct Foo(u16, u8)`, which
+///   has padding on the end).
+/// * Structs need to have all fields also be `NoUninit`.
+/// * Structs need to be `repr(C)` or `repr(transparent)`. In the case of
+///   `repr(C)`, the `packed` and `align` repr modifiers can be used as long as
+///   all other rules end up being followed.
+/// * Enums need to have an explicit `#[repr(Int)]`
+/// * Enums must have only fieldless variants
+/// * It is disallowed for types to contain pointer types, `Cell`, `UnsafeCell`,
+///   atomics, and any other forms of interior mutability.
+/// * More precisely: A shared reference to the type must allow reads, and
+///   *only* reads. RustBelt's separation logic is based on the notion that a
+///   type is allowed to define a sharing predicate, its own invariant that must
+///   hold for shared references, and this predicate is the reasoning that allow
+///   it to deal with atomic and cells etc. We require the sharing predicate to
+///   be trivial and permit only read-only access.
+/// * There's probably more, don't mess it up (I mean it).
+pub unsafe trait NoUninit: Sized + Copy + 'static {}
+
+unsafe impl<T: Pod> NoUninit for T {}
+
+unsafe impl NoUninit for char {}
+
+unsafe impl NoUninit for bool {}
+
+unsafe impl NoUninit for NonZeroU8 {}
+unsafe impl NoUninit for NonZeroI8 {}
+unsafe impl NoUninit for NonZeroU16 {}
+unsafe impl NoUninit for NonZeroI16 {}
+unsafe impl NoUninit for NonZeroU32 {}
+unsafe impl NoUninit for NonZeroI32 {}
+unsafe impl NoUninit for NonZeroU64 {}
+unsafe impl NoUninit for NonZeroI64 {}
+unsafe impl NoUninit for NonZeroU128 {}
+unsafe impl NoUninit for NonZeroI128 {}
+unsafe impl NoUninit for NonZeroUsize {}
+unsafe impl NoUninit for NonZeroIsize {}