Struct Assume

pub struct Assume {
    pub alignment: bool,
    pub lifetimes: bool,
    pub safety: bool,
    pub validity: bool,
}

🔬This is a nightly-only experimental API. (transmutability #99571)

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.

Fields

alignment: bool

🔬This is a nightly-only experimental API. (transmutability #99571)

When false, TransmuteFrom is not implemented for transmutations that might violate the alignment requirements of references; e.g.:

#![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.:

#![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));

lifetimes: bool

🔬This is a nightly-only experimental API. (transmutability #99571)

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.

safety: bool

🔬This is a nightly-only experimental API. (transmutability #99571)

When false, TransmuteFrom is not implemented for transmutations that might violate the library safety invariants of the destination type; e.g.:

#![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.:

#![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 })));

validity: bool

🔬This is a nightly-only experimental API. (transmutability #99571)

When false, TransmuteFrom is not implemented for transmutations that might violate the language-level bit-validity invariant of the destination type; e.g.:

#![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.:

#![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));

Implementations

impl Assume

pub const NOTHING: Self

🔬This is a nightly-only experimental API. (transmutability #99571)

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.

pub const ALIGNMENT: Self

🔬This is a nightly-only experimental API. (transmutability #99571)

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.

pub const LIFETIMES: Self

🔬This is a nightly-only experimental API. (transmutability #99571)

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.

pub const SAFETY: Self

🔬This is a nightly-only experimental API. (transmutability #99571)

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.

pub const VALIDITY: Self

🔬This is a nightly-only experimental API. (transmutability #99571)

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.

pub const fn and(self, other_assumptions: Self) -> Self

🔬This is a nightly-only experimental API. (transmutability #99571)

Combine the assumptions of self and other_assumptions.

This is especially useful for extending Assume in generic contexts; e.g.:

#![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 but_not(self, other_assumptions: Self) -> Self

🔬This is a nightly-only experimental API. (transmutability #99571)

Remove other_assumptions the obligations of self; e.g.:

#![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,
);

Trait Implementations

impl Add for Assume

type Output = Assume

The resulting type after applying the + operator.

fn add(self, other_assumptions: Assume) -> Assume

Performs the + operation.

impl Clone for Assume

fn clone(&self) -> Assume

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Assume

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl PartialEq for Assume

fn eq(&self, other: &Assume) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Sub for Assume

type Output = Assume

The resulting type after applying the - operator.

fn sub(self, other_assumptions: Assume) -> Assume

Performs the - operation.

impl ConstParamTy_ for Assume

impl Copy for Assume

impl Eq for Assume

impl StructuralPartialEq for Assume

impl UnsizedConstParamTy for Assume