Transmute copy

In Rust it is possible to directly convert a byte slice into a struct. This can be done with the transmute_copy function—type arguments are required.

By using try_into and unwrap, we can convert a range of bytes into the required argument. The transmute_copy function is unsafe, and the resulting struct may be invalid.

Example code

To begin, we have a Node struct that has two 32-bit unsigned integers in it. Each Node is 8 bytes. We have the "packed" repr to ensure the data has a consistent layout in memory.

Note In main() we have a source vector with 16 bytes—this could be read in from a file.

Next We invoke transmute_copy() twice on the source data. The offset local indicates where we start reading the bytes.

Finally We take a slice of the data in the argument to transmute_copy. This is fast as it references existing data.

use std::mem;

#[repr(packed)]
#[derive(Debug)]
struct Node {
    id: u32,
    data: u32,
}

fn main() {
    // The input data containing the 2 structs in byte form.
    let source = vec![0, 2, 10, 4, 200, 100, 2, 4,
    0, 40, 10, 2, 1, 50, 20, 7];

    // Use transmute_copy to convert the raw bytes into structs.
    unsafe {
        // First struct.
        let offset = 0;
        let node1 = mem::transmute_copy::<[u8; 8], Node>(&source[offset..offset+8].try_into().unwrap());
        // Second struct (offset is 8).
        let offset = 8;
        let node2 = mem::transmute_copy::<[u8; 8], Node>(&source[offset..offset+8].try_into().unwrap());
        // Print the structs.
        println!("{:?}", node1);
        println!("{:?}", node2);
    }
}Node { id: 67764736, data: 67265736 }
Node { id: 34220032, data: 118764033 }

Warning

The transmute_copy function does not ensure the resulting struct will be valid. But if you have complete control over the data source, the program will work correctly.

However Using a function like from_ne_bytes() to read in the data would avoid the need for unsafe code. It would likely be slower.

With transmute_copy, we can convert bytes to a struct in an extremely fast and direct way. But this conversion is unsafe—the compiler cannot prove it is correct.