Function nom::lib::std::mem::transmute

pub unsafe extern "rust-intrinsic" fn transmute<T, U>(e: T) -> U

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

Both types must have the same size. Neither the original, nor the result, may be an invalid value.

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. It's equivalent to C's memcpy under the hood, just like transmute_copy.

transmute is incredibly unsafe. There are a vast number of ways to cause undefined behavior with this function. transmute should be the absolute last resort.

The nomicon has additional documentation.

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
}
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 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;

Turning a *mut T into an &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 an &mut T into an &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 an &str into an &[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>>:

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

// Using transmute: this is Undefined Behavior, and a bad idea.
// However, it is no-copy.
let v_transmuted = unsafe {
    std::mem::transmute::<Vec<&i32>, Vec<Option<&i32>>>(
        v_orig.clone())
};

// This is the suggested, safe way.
// It does copy the entire vector, though, into a new array.
let v_collected = v_orig.clone()
                        .into_iter()
                        .map(|r| Some(r))
                        .collect::<Vec<Option<&i32>>>();

// The no-copy, unsafe way, still using transmute, but not UB.
// This is equivalent to the original, but safer, and reuses the
// same `Vec` internals. Therefore, the new inner type must have the
// exact same size, and the same alignment, as the old type.
// The same caveats exist for this method as transmute, for
// the original inner type (`&i32`) to the converted inner type
// (`Option<&i32>`), so read the nomicon pages linked above.
let v_from_raw = unsafe {
    Vec::from_raw_parts(v_orig.as_mut_ptr() as *mut Option<&i32>,
                        v_orig.len(),
                        v_orig.capacity())
};
std::mem::forget(v_orig);

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 typesafe; 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 typesafety problems; `&mut *` will *only* give
// you an `&mut T` from an `&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))
    }
}