transmutability.rs

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`.
///
/// Specifically, this trait models (and
/// [implements](BikeshedIntrinsicFrom::transmute)) the semantics of
/// transmute-via-union; i.e.:
///
/// ```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 supports some transmutations forbidden by
/// [`mem::transmute_copy`](super::transmute_copy); namely, transmutations that
/// extend the bits of `Src` with trailing padding to fill trailing
/// uninitialized bytes of `Self`; e.g.:
///
/// ```
/// #![feature(transmutability)]
///
/// use core::mem::{Assume, BikeshedIntrinsicFrom};
///
/// let src = 42u8; // size = 1
///
/// #[repr(C, align(2))]
/// struct Dst(u8); // size = 2
//
/// let _ = unsafe { <Dst as BikeshedIntrinsicFrom<u8, { Assume::SAFETY
///     }>>::transmute(src) };
/// ```
///
/// ## 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 BikeshedIntrinsicFrom<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 [`BikeshedIntrinsicFrom`].
///
/// When `false`, the respective proof obligation belongs to the compiler. When
/// `true`, the onus of the safety proof belongs to the programmer.
/// [`BikeshedIntrinsicFrom`].
#[unstable(feature = "transmutability", issue = "99571")]
#[lang = "transmute_opts"]
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
pub struct Assume {
    /// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
    /// transmutations that might violate the the alignment requirements of
    /// references.
    ///
    /// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
    /// that that references in the transmuted value satisfy the alignment
    /// requirements of their referent types.
    pub alignment: bool,

    /// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
    /// transmutations that extend the lifetimes of references.
    ///
    /// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
    /// that that references in the transmuted value do not outlive their
    /// referents.
    pub lifetimes: bool,

    /// When `false`,  [`BikeshedIntrinsicFrom`] is not implemented for
    /// transmutations that might violate the library safety invariants of the
    /// return type.
    ///
    /// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
    /// that undefined behavior does not arise from using the returned value.
    pub safety: bool,

    /// When `false`, [`BikeshedIntrinsicFrom`] is not implemented for
    /// transmutations that might violate the language-level bit-validity
    /// invariant of the return type.
    ///
    /// When `true`, [`BikeshedIntrinsicFrom`] assumes that *you* have ensured
    /// that the value being transmuted is a bit-valid instance of the return
    /// type.
    pub validity: bool,
}

#[unstable(feature = "transmutability", issue = "99571")]
impl ConstParamTy_ for Assume {}
#[unstable(feature = "transmutability", issue = "99571")]
impl UnsizedConstParamTy for Assume {}

impl Assume {
    /// Do not assume that *you* have ensured any safety properties are met.
    #[unstable(feature = "transmutability", issue = "99571")]
    pub const NOTHING: Self =
        Self { alignment: false, lifetimes: false, safety: false, validity: false };

    /// Assume only that alignment conditions are met.
    #[unstable(feature = "transmutability", issue = "99571")]
    pub const ALIGNMENT: Self = Self { alignment: true, ..Self::NOTHING };

    /// Assume only that lifetime conditions are met.
    #[unstable(feature = "transmutability", issue = "99571")]
    pub const LIFETIMES: Self = Self { lifetimes: true, ..Self::NOTHING };

    /// Assume only that safety conditions are met.
    #[unstable(feature = "transmutability", issue = "99571")]
    pub const SAFETY: Self = Self { safety: true, ..Self::NOTHING };

    /// Assume only that dynamically-satisfiable validity conditions are met.
    #[unstable(feature = "transmutability", issue = "99571")]
    pub const VALIDITY: Self = Self { validity: true, ..Self::NOTHING };

    /// Assume both `self` and `other_assumptions`.
    #[unstable(feature = "transmutability", issue = "99571")]
    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,
        }
    }

    /// Assume `self`, excepting `other_assumptions`.
    #[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)
    }
}