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use crate::marker::{ConstParamTy_, UnsizedConstParamTy};
/// Marks that `Src` is transmutable into `Self`.
///
/// # Implementation
///
/// This trait cannot be implemented explicitly. It is implemented on-the-fly by
/// the compiler for all types `Src` and `Self` such that, given a set of safety
/// obligations on the programmer (see [`Assume`]), the compiler has proved that
/// the bits of a value of type `Src` can be soundly reinterpreted as a `Self`.
///
/// # Safety
///
/// If `Dst: TransmuteFrom<Src, ASSUMPTIONS>`, the compiler guarantees that
/// `Src` is soundly *union-transmutable* into a value of type `Dst`, provided
/// that the programmer has guaranteed that the given [`ASSUMPTIONS`](Assume)
/// are satisfied.
///
/// A union-transmute is any bit-reinterpretation conversion in the form of:
///
/// ```rust
/// pub unsafe fn transmute_via_union<Src, Dst>(src: Src) -> Dst {
/// use core::mem::ManuallyDrop;
///
/// #[repr(C)]
/// union Transmute<Src, Dst> {
/// src: ManuallyDrop<Src>,
/// dst: ManuallyDrop<Dst>,
/// }
///
/// let transmute = Transmute {
/// src: ManuallyDrop::new(src),
/// };
///
/// let dst = transmute.dst;
///
/// ManuallyDrop::into_inner(dst)
/// }
/// ```
///
/// Note that this construction is more permissive than
/// [`mem::transmute_copy`](super::transmute_copy); union-transmutes permit
/// conversions that extend the bits of `Src` with trailing padding to fill
/// trailing uninitialized bytes of `Self`; e.g.:
///
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
#[cfg_attr(not(bootstrap), doc = "```rust")]
/// #![feature(transmutability)]
///
/// use core::mem::{Assume, TransmuteFrom};
///
/// let src = 42u8; // size = 1
///
/// #[repr(C, align(2))]
/// struct Dst(u8); // size = 2
//
/// let _ = unsafe {
/// <Dst as TransmuteFrom<u8, { Assume::SAFETY }>>::transmute(src)
/// };
/// ```
///
/// # Caveats
///
/// ## Portability
///
/// Implementations of this trait do not provide any guarantee of portability
/// across toolchains, targets or compilations. This trait may be implemented
/// for certain combinations of `Src`, `Self` and `ASSUME` on some toolchains,
/// targets or compilations, but not others. For example, if the layouts of
/// `Src` or `Self` are non-deterministic, the presence or absence of an
/// implementation of this trait may also be non-deterministic. Even if `Src`
/// and `Self` have deterministic layouts (e.g., they are `repr(C)` structs),
/// Rust does not specify the alignments of its primitive integer types, and
/// layouts that involve these types may vary across toolchains, targets or
/// compilations.
///
/// ## Stability
///
/// Implementations of this trait do not provide any guarantee of SemVer
/// stability across the crate versions that define the `Src` and `Self` types.
/// If SemVer stability is crucial to your application, you must consult the
/// documentation of `Src` and `Self`s' defining crates. Note that the presence
/// of `repr(C)`, alone, does not carry a safety invariant of SemVer stability.
/// Furthermore, stability does not imply portability. For example, the size of
/// `usize` is stable, but not portable.
#[unstable(feature = "transmutability", issue = "99571")]
#[lang = "transmute_trait"]
#[rustc_deny_explicit_impl(implement_via_object = false)]
#[rustc_coinductive]
pub unsafe trait TransmuteFrom<Src, const ASSUME: Assume = { Assume::NOTHING }>
where
Src: ?Sized,
{
/// Transmutes a `Src` value into a `Self`.
///
/// # Safety
///
/// The safety obligations of the caller depend on the value of `ASSUME`:
/// - If [`ASSUME.alignment`](Assume::alignment), the caller must guarantee
/// that the addresses of references in the returned `Self` satisfy the
/// alignment requirements of their referent types.
/// - If [`ASSUME.lifetimes`](Assume::lifetimes), the caller must guarantee
/// that references in the returned `Self` will not outlive their
/// referents.
/// - If [`ASSUME.safety`](Assume::safety), the returned value might not
/// satisfy the library safety invariants of `Self`, and the caller must
/// guarantee that undefined behavior does not arise from uses of the
/// returned value.
/// - If [`ASSUME.validity`](Assume::validity), the caller must guarantee
/// that `src` is a bit-valid instance of `Self`.
///
/// When satisfying the above obligations (if any), the caller must *not*
/// assume that this trait provides any inherent guarantee of layout
/// [portability](#portability) or [stability](#stability).
unsafe fn transmute(src: Src) -> Self
where
Src: Sized,
Self: Sized,
{
use super::ManuallyDrop;
#[repr(C)]
union Transmute<Src, Dst> {
src: ManuallyDrop<Src>,
dst: ManuallyDrop<Dst>,
}
let transmute = Transmute { src: ManuallyDrop::new(src) };
// SAFETY: It is safe to reinterpret the bits of `src` as a value of
// type `Self`, because, by combination of invariant on this trait and
// contract on the caller, `src` has been proven to satisfy both the
// language and library invariants of `Self`. For all invariants not
// `ASSUME`'d by the caller, the safety obligation is supplied by the
// compiler. Conversely, for all invariants `ASSUME`'d by the caller,
// the safety obligation is supplied by contract on the caller.
let dst = unsafe { transmute.dst };
ManuallyDrop::into_inner(dst)
}
}
/// Configurable proof assumptions of [`TransmuteFrom`].
///
/// When `false`, the respective proof obligation belongs to the compiler. When
/// `true`, the onus of the safety proof belongs to the programmer.
#[unstable(feature = "transmutability", issue = "99571")]
#[lang = "transmute_opts"]
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
pub struct Assume {
/// When `false`, [`TransmuteFrom`] is not implemented for transmutations
/// that might violate the alignment requirements of references; e.g.:
///
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
#[cfg_attr(not(bootstrap), doc = "```compile_fail,E0277")]
/// #![feature(transmutability)]
/// use core::mem::{align_of, TransmuteFrom};
///
/// assert_eq!(align_of::<[u8; 2]>(), 1);
/// assert_eq!(align_of::<u16>(), 2);
///
/// let src: &[u8; 2] = &[0xFF, 0xFF];
///
/// // SAFETY: No safety obligations.
/// let dst: &u16 = unsafe {
/// <_ as TransmuteFrom<_>>::transmute(src)
/// };
/// ```
///
/// When `true`, [`TransmuteFrom`] assumes that *you* have ensured
/// that references in the transmuted value satisfy the alignment
/// requirements of their referent types; e.g.:
///
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
#[cfg_attr(not(bootstrap), doc = "```rust")]
/// #![feature(pointer_is_aligned_to, transmutability)]
/// use core::mem::{align_of, Assume, TransmuteFrom};
///
/// let src: &[u8; 2] = &[0xFF, 0xFF];
///
/// let maybe_dst: Option<&u16> = if <*const _>::is_aligned_to(src, align_of::<u16>()) {
/// // SAFETY: We have checked above that the address of `src` satisfies the
/// // alignment requirements of `u16`.
/// Some(unsafe {
/// <_ as TransmuteFrom<_, { Assume::ALIGNMENT }>>::transmute(src)
/// })
/// } else {
/// None
/// };
///
/// assert!(matches!(maybe_dst, Some(&u16::MAX) | None));
/// ```
pub alignment: bool,
/// When `false`, [`TransmuteFrom`] is not implemented for transmutations
/// that extend the lifetimes of references.
///
/// When `true`, [`TransmuteFrom`] assumes that *you* have ensured that
/// references in the transmuted value do not outlive their referents.
pub lifetimes: bool,
/// When `false`, [`TransmuteFrom`] is not implemented for transmutations
/// that might violate the library safety invariants of the destination
/// type; e.g.:
///
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
#[cfg_attr(not(bootstrap), doc = "```compile_fail,E0277")]
/// #![feature(transmutability)]
/// use core::mem::TransmuteFrom;
///
/// let src: u8 = 3;
///
/// struct EvenU8 {
/// // SAFETY: `val` must be an even number.
/// val: u8,
/// }
///
/// // SAFETY: No safety obligations.
/// let dst: EvenU8 = unsafe {
/// <_ as TransmuteFrom<_>>::transmute(src)
/// };
/// ```
///
/// When `true`, [`TransmuteFrom`] assumes that *you* have ensured
/// that undefined behavior does not arise from using the transmuted value;
/// e.g.:
///
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
#[cfg_attr(not(bootstrap), doc = "```rust")]
/// #![feature(transmutability)]
/// use core::mem::{Assume, TransmuteFrom};
///
/// let src: u8 = 42;
///
/// struct EvenU8 {
/// // SAFETY: `val` must be an even number.
/// val: u8,
/// }
///
/// let maybe_dst: Option<EvenU8> = if src % 2 == 0 {
/// // SAFETY: We have checked above that the value of `src` is even.
/// Some(unsafe {
/// <_ as TransmuteFrom<_, { Assume::SAFETY }>>::transmute(src)
/// })
/// } else {
/// None
/// };
///
/// assert!(matches!(maybe_dst, Some(EvenU8 { val: 42 })));
/// ```
pub safety: bool,
/// When `false`, [`TransmuteFrom`] is not implemented for transmutations
/// that might violate the language-level bit-validity invariant of the
/// destination type; e.g.:
///
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
#[cfg_attr(not(bootstrap), doc = "```compile_fail,E0277")]
/// #![feature(transmutability)]
/// use core::mem::TransmuteFrom;
///
/// let src: u8 = 3;
///
/// // SAFETY: No safety obligations.
/// let dst: bool = unsafe {
/// <_ as TransmuteFrom<_>>::transmute(src)
/// };
/// ```
///
/// When `true`, [`TransmuteFrom`] assumes that *you* have ensured
/// that the value being transmuted is a bit-valid instance of the
/// transmuted value; e.g.:
///
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
#[cfg_attr(not(bootstrap), doc = "```rust")]
/// #![feature(transmutability)]
/// use core::mem::{Assume, TransmuteFrom};
///
/// let src: u8 = 1;
///
/// let maybe_dst: Option<bool> = if src == 0 || src == 1 {
/// // SAFETY: We have checked above that the value of `src` is a bit-valid
/// // instance of `bool`.
/// Some(unsafe {
/// <_ as TransmuteFrom<_, { Assume::VALIDITY }>>::transmute(src)
/// })
/// } else {
/// None
/// };
///
/// assert_eq!(maybe_dst, Some(true));
/// ```
pub validity: bool,
}
#[unstable(feature = "transmutability", issue = "99571")]
impl ConstParamTy_ for Assume {}
#[unstable(feature = "transmutability", issue = "99571")]
impl UnsizedConstParamTy for Assume {}
impl Assume {
/// With this, [`TransmuteFrom`] does not assume you have ensured any safety
/// obligations are met, and relies only upon its own analysis to (dis)prove
/// transmutability.
#[unstable(feature = "transmutability", issue = "99571")]
pub const NOTHING: Self =
Self { alignment: false, lifetimes: false, safety: false, validity: false };
/// With this, [`TransmuteFrom`] assumes only that you have ensured that
/// references in the transmuted value satisfy the alignment requirements of
/// their referent types. See [`Assume::alignment`] for examples.
#[unstable(feature = "transmutability", issue = "99571")]
pub const ALIGNMENT: Self = Self { alignment: true, ..Self::NOTHING };
/// With this, [`TransmuteFrom`] assumes only that you have ensured that
/// references in the transmuted value do not outlive their referents. See
/// [`Assume::lifetimes`] for examples.
#[unstable(feature = "transmutability", issue = "99571")]
pub const LIFETIMES: Self = Self { lifetimes: true, ..Self::NOTHING };
/// With this, [`TransmuteFrom`] assumes only that you have ensured that
/// undefined behavior does not arise from using the transmuted value. See
/// [`Assume::safety`] for examples.
#[unstable(feature = "transmutability", issue = "99571")]
pub const SAFETY: Self = Self { safety: true, ..Self::NOTHING };
/// With this, [`TransmuteFrom`] assumes only that you have ensured that the
/// value being transmuted is a bit-valid instance of the transmuted value.
/// See [`Assume::validity`] for examples.
#[unstable(feature = "transmutability", issue = "99571")]
pub const VALIDITY: Self = Self { validity: true, ..Self::NOTHING };
/// Combine the assumptions of `self` and `other_assumptions`.
///
/// This is especially useful for extending [`Assume`] in generic contexts;
/// e.g.:
///
#[cfg_attr(bootstrap, doc = "```rust,ignore not runnable on bootstrap")]
#[cfg_attr(not(bootstrap), doc = "```rust")]
#[unstable(feature = "transmutability", issue = "99571")]
/// #![feature(
/// adt_const_params,
/// generic_const_exprs,
/// pointer_is_aligned_to,
/// transmutability,
/// )]
/// #![allow(incomplete_features)]
/// use core::mem::{align_of, Assume, TransmuteFrom};
///
/// /// Attempts to transmute `src` to `&Dst`.
/// ///
/// /// Returns `None` if `src` violates the alignment requirements of `&Dst`.
/// ///
/// /// # Safety
/// ///
/// /// The caller guarantees that the obligations required by `ASSUME`, except
/// /// alignment, are satisfied.
/// unsafe fn try_transmute_ref<'a, Src, Dst, const ASSUME: Assume>(src: &'a Src) -> Option<&'a Dst>
/// where
/// &'a Dst: TransmuteFrom<&'a Src, { ASSUME.and(Assume::ALIGNMENT) }>,
/// {
/// if <*const _>::is_aligned_to(src, align_of::<Dst>()) {
/// // SAFETY: By the above dynamic check, we have ensured that the address
/// // of `src` satisfies the alignment requirements of `&Dst`. By contract
/// // on the caller, the safety obligations required by `ASSUME` have also
/// // been satisfied.
/// Some(unsafe {
/// <_ as TransmuteFrom<_, { ASSUME.and(Assume::ALIGNMENT) }>>::transmute(src)
/// })
/// } else {
/// None
/// }
/// }
///
/// let src: &[u8; 2] = &[0xFF, 0xFF];
///
/// // SAFETY: No safety obligations.
/// let maybe_dst: Option<&u16> = unsafe {
/// try_transmute_ref::<_, _, { Assume::NOTHING }>(src)
/// };
///```
pub const fn and(self, other_assumptions: Self) -> Self {
Self {
alignment: self.alignment || other_assumptions.alignment,
lifetimes: self.lifetimes || other_assumptions.lifetimes,
safety: self.safety || other_assumptions.safety,
validity: self.validity || other_assumptions.validity,
}
}
/// Remove `other_assumptions` the obligations of `self`; e.g.:
///
#[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 {
alignment: self.alignment && !other_assumptions.alignment,
lifetimes: self.lifetimes && !other_assumptions.lifetimes,
safety: self.safety && !other_assumptions.safety,
validity: self.validity && !other_assumptions.validity,
}
}
}
// FIXME(jswrenn): This const op is not actually usable. Why?
// https://github.com/rust-lang/rust/pull/100726#issuecomment-1219928926
#[unstable(feature = "transmutability", issue = "99571")]
impl core::ops::Add for Assume {
type Output = Assume;
fn add(self, other_assumptions: Assume) -> Assume {
self.and(other_assumptions)
}
}
// FIXME(jswrenn): This const op is not actually usable. Why?
// https://github.com/rust-lang/rust/pull/100726#issuecomment-1219928926
#[unstable(feature = "transmutability", issue = "99571")]
impl core::ops::Sub for Assume {
type Output = Assume;
fn sub(self, other_assumptions: Assume) -> Assume {
self.but_not(other_assumptions)
}
}