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