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@ -1,10 +1,88 @@
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use crate::marker::{ConstParamTy_, UnsizedConstParamTy};
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/// Are values of a type transmutable into values of another type?
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/// Marks that `Src` is transmutable into `Self`.
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///
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/// This trait is implemented on-the-fly by the compiler for types `Src` and `Self` when the bits of
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/// any value of type `Self` are safely transmutable into a value of type `Dst`, in a given `Context`,
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/// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
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/// # Implementation
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///
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/// This trait cannot be implemented explicitly. It is implemented on-the-fly by
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/// the compiler for all types `Src` and `Self` such that, given a set of safety
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/// obligations on the programmer (see [`Assume`]), the compiler has proved that
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/// the bits of a value of type `Src` can be soundly reinterpreted as a `Self`.
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///
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/// # Safety
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///
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/// If `Dst: BikeshedIntrinsicFrom<Src, ASSUMPTIONS>`, the compiler guarantees
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/// that `Src` is soundly *union-transmutable* into a value of type `Dst`,
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/// provided that the programmer has guaranteed that the given
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/// [`ASSUMPTIONS`](Assume) are satisfied.
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///
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/// A union-transmute is any bit-reinterpretation conversion in the form of:
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///
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/// ```rust
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/// pub unsafe fn transmute_via_union<Src, Dst>(src: Src) -> Dst {
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/// use core::mem::ManuallyDrop;
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///
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/// #[repr(C)]
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/// union Transmute<Src, Dst> {
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/// src: ManuallyDrop<Src>,
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/// dst: ManuallyDrop<Dst>,
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/// }
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///
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/// let transmute = Transmute {
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/// src: ManuallyDrop::new(src),
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/// };
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///
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/// let dst = transmute.dst;
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///
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/// ManuallyDrop::into_inner(dst)
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/// }
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/// ```
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///
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/// Note that this construction is more permissive than
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/// [`mem::transmute_copy`](super::transmute_copy); union-transmutes permit
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/// conversions that extend the bits of `Src` with trailing padding to fill
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/// trailing uninitialized bytes of `Self`; e.g.:
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///
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#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
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#[cfg_attr(not(bootstrap), doc = "```rust")]
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/// #![feature(transmutability)]
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///
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/// use core::mem::{Assume, BikeshedIntrinsicFrom};
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///
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/// let src = 42u8; // size = 1
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///
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/// #[repr(C, align(2))]
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/// struct Dst(u8); // size = 2
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//
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/// let _ = unsafe {
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/// <Dst as BikeshedIntrinsicFrom<u8, { Assume::SAFETY }>>::transmute(src)
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/// };
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/// ```
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///
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/// # Caveats
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///
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/// ## Portability
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///
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/// Implementations of this trait do not provide any guarantee of portability
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/// across toolchains, targets or compilations. This trait may be implemented
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/// for certain combinations of `Src`, `Self` and `ASSUME` on some toolchains,
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/// targets or compilations, but not others. For example, if the layouts of
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/// `Src` or `Self` are non-deterministic, the presence or absence of an
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/// implementation of this trait may also be non-deterministic. Even if `Src`
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/// and `Self` have deterministic layouts (e.g., they are `repr(C)` structs),
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/// Rust does not specify the alignments of its primitive integer types, and
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/// layouts that involve these types may vary across toolchains, targets or
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/// compilations.
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///
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/// ## Stability
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///
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/// Implementations of this trait do not provide any guarantee of SemVer
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/// stability across the crate versions that define the `Src` and `Self` types.
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/// If SemVer stability is crucial to your application, you must consult the
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/// documentation of `Src` and `Self`s' defining crates. Note that the presence
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/// of `repr(C)`, alone, does not carry a safety invariant of SemVer stability.
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/// Furthermore, stability does not imply portability. For example, the size of
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/// `usize` is stable, but not portable.
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#[unstable(feature = "transmutability", issue = "99571")]
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#[lang = "transmute_trait"]
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#[rustc_deny_explicit_impl(implement_via_object = false)]
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@ -13,28 +91,207 @@ pub unsafe trait BikeshedIntrinsicFrom<Src, const ASSUME: Assume = { Assume::NOT
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where
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Src: ?Sized,
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{
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/// Transmutes a `Src` value into a `Self`.
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///
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/// # Safety
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///
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/// The safety obligations of the caller depend on the value of `ASSUME`:
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/// - If [`ASSUME.alignment`](Assume::alignment), the caller must guarantee
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/// that the addresses of references in the returned `Self` satisfy the
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/// alignment requirements of their referent types.
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/// - If [`ASSUME.lifetimes`](Assume::lifetimes), the caller must guarantee
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/// that references in the returned `Self` will not outlive their
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/// referents.
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/// - If [`ASSUME.safety`](Assume::safety), the returned value might not
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/// satisfy the library safety invariants of `Self`, and the caller must
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/// guarantee that undefined behavior does not arise from uses of the
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/// returned value.
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/// - If [`ASSUME.validity`](Assume::validity), the caller must guarantee
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/// that `src` is a bit-valid instance of `Self`.
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///
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/// When satisfying the above obligations (if any), the caller must *not*
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/// assume that this trait provides any inherent guarantee of layout
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/// [portability](#portability) or [stability](#stability).
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unsafe fn transmute(src: Src) -> Self
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where
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Src: Sized,
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Self: Sized,
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{
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use super::ManuallyDrop;
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#[repr(C)]
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union Transmute<Src, Dst> {
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src: ManuallyDrop<Src>,
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dst: ManuallyDrop<Dst>,
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}
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let transmute = Transmute { src: ManuallyDrop::new(src) };
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// SAFETY: It is safe to reinterpret the bits of `src` as a value of
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// type `Self`, because, by combination of invariant on this trait and
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// contract on the caller, `src` has been proven to satisfy both the
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// language and library invariants of `Self`. For all invariants not
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// `ASSUME`'d by the caller, the safety obligation is supplied by the
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// compiler. Conversely, for all invariants `ASSUME`'d by the caller,
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// the safety obligation is supplied by contract on the caller.
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let dst = unsafe { transmute.dst };
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ManuallyDrop::into_inner(dst)
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}
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}
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/// What transmutation safety conditions shall the compiler assume that *you* are checking?
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/// Configurable proof assumptions of [`BikeshedIntrinsicFrom`].
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///
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/// When `false`, the respective proof obligation belongs to the compiler. When
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/// `true`, the onus of the safety proof belongs to the programmer.
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/// [`BikeshedIntrinsicFrom`].
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#[unstable(feature = "transmutability", issue = "99571")]
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#[lang = "transmute_opts"]
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#[derive(PartialEq, Eq, Clone, Copy, Debug)]
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pub struct Assume {
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/// When `true`, the compiler assumes that *you* are ensuring (either dynamically or statically) that
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/// destination referents do not have stricter alignment requirements than source referents.
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/// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
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/// transmutations that might violate the the alignment requirements of
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/// references; e.g.:
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///
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#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
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#[cfg_attr(not(bootstrap), doc = "```compile_fail,E0277")]
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/// #![feature(transmutability)]
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/// use core::mem::{align_of, BikeshedIntrinsicFrom};
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///
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/// assert_eq!(align_of::<[u8; 2]>(), 1);
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/// assert_eq!(align_of::<u16>(), 2);
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///
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/// let src: &[u8; 2] = &[0xFF, 0xFF];
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///
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/// // SAFETY: No safety obligations.
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/// let dst: &u16 = unsafe {
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/// <_ as BikeshedIntrinsicFrom<_>>::transmute(src)
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/// };
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/// ```
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///
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/// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
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/// that references in the transmuted value satisfy the alignment
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/// requirements of their referent types; e.g.:
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///
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#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
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#[cfg_attr(not(bootstrap), doc = "```rust")]
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/// #![feature(pointer_is_aligned_to, transmutability)]
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/// use core::mem::{align_of, Assume, BikeshedIntrinsicFrom};
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///
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/// let src: &[u8; 2] = &[0xFF, 0xFF];
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///
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/// let maybe_dst: Option<&u16> = if <*const _>::is_aligned_to(src, align_of::<u16>()) {
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/// // SAFETY: We have checked above that the address of `src` satisfies the
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/// // alignment requirements of `u16`.
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/// Some(unsafe {
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/// <_ as BikeshedIntrinsicFrom<_, { Assume::ALIGNMENT }>>::transmute(src)
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/// })
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/// } else {
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/// None
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/// };
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///
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/// assert!(matches!(maybe_dst, Some(&u16::MAX) | None));
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/// ```
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pub alignment: bool,
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/// When `true`, the compiler assume that *you* are ensuring that lifetimes are not extended in a manner
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/// that violates Rust's memory model.
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/// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
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/// transmutations that extend the lifetimes of references.
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///
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/// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
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/// that references in the transmuted value do not outlive their referents.
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pub lifetimes: bool,
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/// When `true`, the compiler assumes that *you* have ensured that no
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/// unsoundness will arise from violating the safety invariants of the
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/// destination type (and sometimes of the source type, too).
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/// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
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/// transmutations that might violate the library safety invariants of the
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/// destination type; e.g.:
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///
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#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
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#[cfg_attr(not(bootstrap), doc = "```compile_fail,E0277")]
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/// #![feature(transmutability)]
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/// use core::mem::BikeshedIntrinsicFrom;
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///
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/// let src: u8 = 3;
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///
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/// struct EvenU8 {
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/// // SAFETY: `val` must be an even number.
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/// val: u8,
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/// }
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///
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/// // SAFETY: No safety obligations.
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/// let dst: EvenU8 = unsafe {
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/// <_ as BikeshedIntrinsicFrom<_>>::transmute(src)
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/// };
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/// ```
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///
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/// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
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/// that undefined behavior does not arise from using the transmuted value;
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/// e.g.:
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///
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#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
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#[cfg_attr(not(bootstrap), doc = "```rust")]
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/// #![feature(transmutability)]
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/// use core::mem::{Assume, BikeshedIntrinsicFrom};
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///
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/// let src: u8 = 42;
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///
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/// struct EvenU8 {
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/// // SAFETY: `val` must be an even number.
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/// val: u8,
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/// }
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///
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/// let maybe_dst: Option<EvenU8> = if src % 2 == 0 {
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/// // SAFETY: We have checked above that the value of `src` is even.
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/// Some(unsafe {
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/// <_ as BikeshedIntrinsicFrom<_, { Assume::SAFETY }>>::transmute(src)
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/// })
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/// } else {
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/// None
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/// };
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///
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/// assert!(matches!(maybe_dst, Some(EvenU8 { val: 42 })));
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/// ```
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pub safety: bool,
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/// When `true`, the compiler assumes that *you* are ensuring that the source type is actually a valid
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/// instance of the destination type.
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/// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
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/// transmutations that might violate the language-level bit-validity
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/// invariant of the destination type; e.g.:
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///
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#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
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#[cfg_attr(not(bootstrap), doc = "```compile_fail,E0277")]
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/// #![feature(transmutability)]
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/// use core::mem::BikeshedIntrinsicFrom;
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///
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/// let src: u8 = 3;
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///
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/// // SAFETY: No safety obligations.
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/// let dst: bool = unsafe {
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/// <_ as BikeshedIntrinsicFrom<_>>::transmute(src)
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/// };
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/// ```
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///
|
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/// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
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/// that the value being transmuted is a bit-valid instance of the
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/// transmuted value; e.g.:
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///
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#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
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#[cfg_attr(not(bootstrap), doc = "```rust")]
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/// #![feature(transmutability)]
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/// use core::mem::{Assume, BikeshedIntrinsicFrom};
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///
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/// let src: u8 = 1;
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///
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/// let maybe_dst: Option<bool> = if src == 0 || src == 1 {
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/// // SAFETY: We have checked above that the value of `src` is a bit-valid
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/// // instance of `bool`.
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/// Some(unsafe {
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/// <_ as BikeshedIntrinsicFrom<_, { Assume::VALIDITY }>>::transmute(src)
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/// })
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/// } else {
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/// None
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/// };
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///
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/// assert_eq!(maybe_dst, Some(true));
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/// ```
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pub validity: bool,
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}
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@ -44,29 +301,87 @@ impl ConstParamTy_ for Assume {}
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impl UnsizedConstParamTy for Assume {}
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impl Assume {
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/// Do not assume that *you* have ensured any safety properties are met.
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/// With this, [`BikeshedIntrinsicFrom`] does not assume you have ensured
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/// any safety obligations are met, and relies only upon its own analysis to
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/// (dis)prove transmutability.
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#[unstable(feature = "transmutability", issue = "99571")]
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pub const NOTHING: Self =
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Self { alignment: false, lifetimes: false, safety: false, validity: false };
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/// Assume only that alignment conditions are met.
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/// With this, [`BikeshedIntrinsicFrom`] assumes only that you have ensured
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/// that references in the transmuted value satisfy the alignment
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/// requirements of their referent types. See [`Assume::alignment`] for
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/// examples.
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#[unstable(feature = "transmutability", issue = "99571")]
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pub const ALIGNMENT: Self = Self { alignment: true, ..Self::NOTHING };
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/// Assume only that lifetime conditions are met.
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/// With this, [`BikeshedIntrinsicFrom`] assumes only that you have ensured
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/// that references in the transmuted value do not outlive their referents.
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/// See [`Assume::lifetimes`] for examples.
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#[unstable(feature = "transmutability", issue = "99571")]
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pub const LIFETIMES: Self = Self { lifetimes: true, ..Self::NOTHING };
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/// Assume only that safety conditions are met.
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/// With this, [`BikeshedIntrinsicFrom`] assumes only that you have ensured
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/// that undefined behavior does not arise from using the transmuted value.
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/// See [`Assume::safety`] for examples.
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#[unstable(feature = "transmutability", issue = "99571")]
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pub const SAFETY: Self = Self { safety: true, ..Self::NOTHING };
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/// Assume only that dynamically-satisfiable validity conditions are met.
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/// With this, [`BikeshedIntrinsicFrom`] assumes only that you have ensured
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/// that the value being transmuted is a bit-valid instance of the
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/// transmuted value. See [`Assume::validity`] for examples.
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#[unstable(feature = "transmutability", issue = "99571")]
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pub const VALIDITY: Self = Self { validity: true, ..Self::NOTHING };
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/// Assume both `self` and `other_assumptions`.
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/// Combine the assumptions of `self` and `other_assumptions`.
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///
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/// This is especially useful for extending [`Assume`] in generic contexts;
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/// e.g.:
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///
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#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
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#[cfg_attr(not(bootstrap), doc = "```rust")]
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#[unstable(feature = "transmutability", issue = "99571")]
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/// #![feature(
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/// adt_const_params,
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/// generic_const_exprs,
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/// pointer_is_aligned_to,
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/// transmutability,
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/// )]
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/// #![allow(incomplete_features)]
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/// use core::mem::{align_of, Assume, BikeshedIntrinsicFrom};
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///
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/// /// Attempts to transmute `src` to `&Dst`.
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/// ///
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/// /// Returns `None` if `src` violates the alignment requirements of `&Dst`.
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/// ///
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/// /// # Safety
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/// ///
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/// /// The caller guarantees that the obligations required by `ASSUME`, except
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/// /// alignment, are satisfied.
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/// unsafe fn try_transmute_ref<'a, Src, Dst, const ASSUME: Assume>(src: &'a Src) -> Option<&'a Dst>
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/// where
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/// &'a Dst: BikeshedIntrinsicFrom<&'a Src, { ASSUME.and(Assume::ALIGNMENT) }>,
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/// {
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/// if <*const _>::is_aligned_to(src, align_of::<Dst>()) {
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/// // SAFETY: By the above dynamic check, we have ensured that the address
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/// // of `src` satisfies the alignment requirements of `&Dst`. By contract
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/// // on the caller, the safety obligations required by `ASSUME` have also
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/// // been satisfied.
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/// Some(unsafe {
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/// <_ as BikeshedIntrinsicFrom<_, { ASSUME.and(Assume::ALIGNMENT) }>>::transmute(src)
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/// })
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/// } else {
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/// None
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/// }
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/// }
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///
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/// let src: &[u8; 2] = &[0xFF, 0xFF];
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///
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/// // SAFETY: No safety obligations.
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/// let maybe_dst: Option<&u16> = unsafe {
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/// try_transmute_ref::<_, _, { Assume::NOTHING }>(src)
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/// };
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///```
|
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|
|
pub const fn and(self, other_assumptions: Self) -> Self {
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|
Self {
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|
alignment: self.alignment || other_assumptions.alignment,
|
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|
|
@ -76,7 +391,21 @@ pub const fn and(self, other_assumptions: Self) -> Self {
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}
|
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|
}
|
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|
/// Assume `self`, excepting `other_assumptions`.
|
|
|
|
|
/// Remove `other_assumptions` the obligations of `self`; e.g.:
|
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|
|
///
|
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|
|
|
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
|
|
|
|
|
#[cfg_attr(not(bootstrap), doc = "```rust")]
|
|
|
|
|
/// #![feature(transmutability)]
|
|
|
|
|
/// use core::mem::Assume;
|
|
|
|
|
///
|
|
|
|
|
/// let assumptions = Assume::ALIGNMENT.and(Assume::SAFETY);
|
|
|
|
|
/// let to_be_removed = Assume::SAFETY.and(Assume::VALIDITY);
|
|
|
|
|
///
|
|
|
|
|
/// assert_eq!(
|
|
|
|
|
/// assumptions.but_not(to_be_removed),
|
|
|
|
|
/// Assume::ALIGNMENT,
|
|
|
|
|
/// );
|
|
|
|
|
/// ```
|
|
|
|
|
#[unstable(feature = "transmutability", issue = "99571")]
|
|
|
|
|
pub const fn but_not(self, other_assumptions: Self) -> Self {
|
|
|
|
|
Self {
|
|
|
|
|
|
Jack Wrenn