Function transmute

pub const unsafe extern "rust-intrinsic" fn transmute<Src, Dst>(
    src: Src,
) -> Dst

Reinterprets the bits of a value of one type as another type.

Both types must have the same size. Compilation will fail if this is not guaranteed.

transmute is semantically equivalent to a bitwise move of one type into another. It copies the bits from the source value into the destination value, then forgets the original. Note that source and destination are passed by-value, which means if Src or Dst contain padding, that padding is not guaranteed to be preserved by transmute.

Both the argument and the result must be valid at their given type. Violating this condition leads to undefined behavior. The compiler will generate code assuming that you, the programmer, ensure that there will never be undefined behavior. It is therefore your responsibility to guarantee that every value passed to transmute is valid at both types Src and Dst. Failing to uphold this condition may lead to unexpected and unstable compilation results. This makes transmute incredibly unsafe. transmute should be the absolute last resort.

Because transmute is a by-value operation, alignment of the transmuted values themselves is not a concern. As with any other function, the compiler already ensures both Src and Dst are properly aligned. However, when transmuting values that point elsewhere (such as pointers, references, boxes…), the caller has to ensure proper alignment of the pointed-to values.

The nomicon has additional documentation.

Transmutation between pointers and integers

Special care has to be taken when transmuting between pointers and integers, e.g. transmuting between *const () and usize.

Transmuting pointers to integers in a const context is undefined behavior, unless the pointer was originally created from an integer. (That includes this function specifically, integer-to-pointer casts, and helpers like dangling, but also semantically-equivalent conversions such as punning through repr(C) union fields.) Any attempt to use the resulting value for integer operations will abort const-evaluation. (And even outside const, such transmutation is touching on many unspecified aspects of the Rust memory model and should be avoided. See below for alternatives.)

Transmuting integers to pointers is a largely unspecified operation. It is likely not equivalent to an as cast. Doing non-zero-sized memory accesses with a pointer constructed this way is currently considered undefined behavior.

All this also applies when the integer is nested inside an array, tuple, struct, or enum. However, MaybeUninit<usize> is not considered an integer type for the purpose of this section. Transmuting *const () to MaybeUninit<usize> is fine—but then calling assume_init() on that result is considered as completing the pointer-to-integer transmute and thus runs into the issues discussed above.

In particular, doing a pointer-to-integer-to-pointer roundtrip via transmute is not a lossless process. If you want to round-trip a pointer through an integer in a way that you can get back the original pointer, you need to use as casts, or replace the integer type by MaybeUninit<$int> (and never call assume_init()). If you are looking for a way to store data of arbitrary type, also use MaybeUninit<T> (that will also handle uninitialized memory due to padding). If you specifically need to store something that is “either an integer or a pointer”, use *mut (): integers can be converted to pointers and back without any loss (via as casts or via transmute).

Examples

There are a few things that transmute is really useful for.

Turning a pointer into a function pointer. This is not portable to machines where function pointers and data pointers have different sizes.

fn foo() -> i32 {
    0
}
// Crucially, we `as`-cast to a raw pointer before `transmute`ing to a function pointer.
// This avoids an integer-to-pointer `transmute`, which can be problematic.
// Transmuting between raw pointers and function pointers (i.e., two pointer types) is fine.
let pointer = foo as *const ();
let function = unsafe {
    std::mem::transmute::<*const (), fn() -> i32>(pointer)
};
assert_eq!(function(), 0);

Extending a lifetime, or shortening an invariant lifetime. This is advanced, very unsafe Rust!

struct R<'a>(&'a i32);
unsafe fn extend_lifetime<'b>(r: R<'b>) -> R<'static> {
    std::mem::transmute::<R<'b>, R<'static>>(r)
}

unsafe fn shorten_invariant_lifetime<'b, 'c>(r: &'b mut R<'static>)
                                             -> &'b mut R<'c> {
    std::mem::transmute::<&'b mut R<'static>, &'b mut R<'c>>(r)
}

Alternatives

Don’t despair: many uses of transmute can be achieved through other means. Below are common applications of transmute which can be replaced with safer constructs.

Turning raw bytes ([u8; SZ]) into u32, f64, etc.:

let raw_bytes = [0x78, 0x56, 0x34, 0x12];

let num = unsafe {
    std::mem::transmute::<[u8; 4], u32>(raw_bytes)
};

// use `u32::from_ne_bytes` instead
let num = u32::from_ne_bytes(raw_bytes);
// or use `u32::from_le_bytes` or `u32::from_be_bytes` to specify the endianness
let num = u32::from_le_bytes(raw_bytes);
assert_eq!(num, 0x12345678);
let num = u32::from_be_bytes(raw_bytes);
assert_eq!(num, 0x78563412);

Turning a pointer into a usize:

let ptr = &0;
let ptr_num_transmute = unsafe {
    std::mem::transmute::<&i32, usize>(ptr)
};

// Use an `as` cast instead
let ptr_num_cast = ptr as *const i32 as usize;

Note that using transmute to turn a pointer to a usize is (as noted above) undefined behavior in const contexts. Also outside of consts, this operation might not behave as expected – this is touching on many unspecified aspects of the Rust memory model. Depending on what the code is doing, the following alternatives are preferable to pointer-to-integer transmutation:

• If the code just wants to store data of arbitrary type in some buffer and needs to pick a type for that buffer, it can use MaybeUninit.
• If the code actually wants to work on the address the pointer points to, it can use as casts or ptr.addr().

Turning a *mut T into a &mut T:

let ptr: *mut i32 = &mut 0;
let ref_transmuted = unsafe {
    std::mem::transmute::<*mut i32, &mut i32>(ptr)
};

// Use a reborrow instead
let ref_casted = unsafe { &mut *ptr };

Turning a &mut T into a &mut U:

let ptr = &mut 0;
let val_transmuted = unsafe {
    std::mem::transmute::<&mut i32, &mut u32>(ptr)
};

// Now, put together `as` and reborrowing - note the chaining of `as`
// `as` is not transitive
let val_casts = unsafe { &mut *(ptr as *mut i32 as *mut u32) };

Turning a &str into a &[u8]:

// this is not a good way to do this.
let slice = unsafe { std::mem::transmute::<&str, &[u8]>("Rust") };
assert_eq!(slice, &[82, 117, 115, 116]);

// You could use `str::as_bytes`
let slice = "Rust".as_bytes();
assert_eq!(slice, &[82, 117, 115, 116]);

// Or, just use a byte string, if you have control over the string
// literal
assert_eq!(b"Rust", &[82, 117, 115, 116]);

Turning a Vec<&T> into a Vec<Option<&T>>.

To transmute the inner type of the contents of a container, you must make sure to not violate any of the container’s invariants. For Vec, this means that both the size and alignment of the inner types have to match. Other containers might rely on the size of the type, alignment, or even the TypeId, in which case transmuting wouldn’t be possible at all without violating the container invariants.

let store = [0, 1, 2, 3];
let v_orig = store.iter().collect::<Vec<&i32>>();

// clone the vector as we will reuse them later
let v_clone = v_orig.clone();

// Using transmute: this relies on the unspecified data layout of `Vec`, which is a
// bad idea and could cause Undefined Behavior.
// However, it is no-copy.
let v_transmuted = unsafe {
    std::mem::transmute::<Vec<&i32>, Vec<Option<&i32>>>(v_clone)
};

let v_clone = v_orig.clone();

// This is the suggested, safe way.
// It may copy the entire vector into a new one though, but also may not.
let v_collected = v_clone.into_iter()
                         .map(Some)
                         .collect::<Vec<Option<&i32>>>();

let v_clone = v_orig.clone();

// This is the proper no-copy, unsafe way of "transmuting" a `Vec`, without relying on the
// data layout. Instead of literally calling `transmute`, we perform a pointer cast, but
// in terms of converting the original inner type (`&i32`) to the new one (`Option<&i32>`),
// this has all the same caveats. Besides the information provided above, also consult the
// [`from_raw_parts`] documentation.
let v_from_raw = unsafe {
    // Ensure the original vector is not dropped.
    let mut v_clone = std::mem::ManuallyDrop::new(v_clone);
    Vec::from_raw_parts(v_clone.as_mut_ptr() as *mut Option<&i32>,
                        v_clone.len(),
                        v_clone.capacity())
};

Implementing split_at_mut:

use std::{slice, mem};

// There are multiple ways to do this, and there are multiple problems
// with the following (transmute) way.
fn split_at_mut_transmute<T>(slice: &mut [T], mid: usize)
                             -> (&mut [T], &mut [T]) {
    let len = slice.len();
    assert!(mid <= len);
    unsafe {
        let slice2 = mem::transmute::<&mut [T], &mut [T]>(slice);
        // first: transmute is not type safe; all it checks is that T and
        // U are of the same size. Second, right here, you have two
        // mutable references pointing to the same memory.
        (&mut slice[0..mid], &mut slice2[mid..len])
    }
}

// This gets rid of the type safety problems; `&mut *` will *only* give
// you a `&mut T` from a `&mut T` or `*mut T`.
fn split_at_mut_casts<T>(slice: &mut [T], mid: usize)
                         -> (&mut [T], &mut [T]) {
    let len = slice.len();
    assert!(mid <= len);
    unsafe {
        let slice2 = &mut *(slice as *mut [T]);
        // however, you still have two mutable references pointing to
        // the same memory.
        (&mut slice[0..mid], &mut slice2[mid..len])
    }
}

// This is how the standard library does it. This is the best method, if
// you need to do something like this
fn split_at_stdlib<T>(slice: &mut [T], mid: usize)
                      -> (&mut [T], &mut [T]) {
    let len = slice.len();
    assert!(mid <= len);
    unsafe {
        let ptr = slice.as_mut_ptr();
        // This now has three mutable references pointing at the same
        // memory. `slice`, the rvalue ret.0, and the rvalue ret.1.
        // `slice` is never used after `let ptr = ...`, and so one can
        // treat it as "dead", and therefore, you only have two real
        // mutable slices.
        (slice::from_raw_parts_mut(ptr, mid),
         slice::from_raw_parts_mut(ptr.add(mid), len - mid))
    }
}