Cannot create a generic function that transmutes a slice of bytes into an integer because the size is not known at compile time

user_8003030 0
Viewed 845

I'm trying to create a generic function that transmutes a slice of bytes into an integer. 

fn i_from_slice<T>(slice: &[u8]) -> Option<T>
where
    T: Sized,
{
    match slice.len() {
        std::mem::size_of::<T>() => {
            let mut buf = [0; std::mem::size_of::<T>()];
            buf.copy_from_slice(slice);
            Some(unsafe { std::mem::transmute_copy(&buf) })
        }
        _ => None,
    }
}

Rust won't let me do that:

error[E0532]: expected tuple struct/variant, found function `std::mem::size_of`
 --> src/lib.rs:6:9
  |
6 |         std::mem::size_of::<T>() => {
  |         ^^^^^^^^^^^^^^^^^^^^^^ not a tuple struct/variant

error[E0277]: the size for values of type `T` cannot be known at compilation time
 --> src/lib.rs:7:31
  |
7 |             let mut buf = [0; std::mem::size_of::<T>()];
  |                               ^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `T`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = help: consider adding a `where T: std::marker::Sized` bound
  = note: required by `std::mem::size_of`

Is there a way that I can statically know the size of T?

rust
3 Answers

If your T is an integer, you don't need any unsafe code, since there is from_ne_bytes.

If you absolutely want a generic function, you can add a trait:

use std::convert::TryInto;

trait FromBytes: Sized {
    fn from_ne_bytes_(bytes: &[u8]) -> Option<Self>;
}

impl FromBytes for i32 {
    fn from_ne_bytes_(bytes: &[u8]) -> Option<Self> {
        bytes.try_into().map(i32::from_ne_bytes).ok()
    }
}

// Etc. for the other numeric types.

fn main() {
    let i1: i32 = i_from_slice(&[1, 2, 3, 4]).unwrap();
    let i2 = i32::from_ne_bytes_(&[1, 2, 3, 4]).unwrap();

    assert_eq!(i1, i2);
}

// This `unsafe` usage is invalid, but copied from the original post
// to compare the result with my implementation.
fn i_from_slice<T>(slice: &[u8]) -> Option<T> {
    if slice.len() == std::mem::size_of::<T>() {
        Some(unsafe { std::mem::transmute_copy(&slice[0]) })
    } else {
        None
    }
}
user_44988310

Is there a way that i can know statically the size of T?

Yes, you do know the size at compile time. But the size can vary and is not a constant. Instead of using a fixed-size array, you can use a vector which is a contiguous growable array.

Also, Sized is an opt-out marker trait. All type parameters have an implicit Sized bound. You don't need to spell that fact out.

You need a match arm guard to use pattern matching the way you did, but it is more straightforward to use if-else expression here.

All in all, this works:

fn i_from_slice<T>(slice: &[u8]) -> Option<T> {
    let n = std::mem::size_of::<T>();
    if slice.len() == n {
        let mut buf = vec![0; n];
        buf.copy_from_slice(slice);
        Some(unsafe { std::mem::transmute_copy(&buf) })
    } else {
        None
    }
}
user_7944570

You don't need your intermediate buffer, you can call transmute_copy directly on the input slice. Moreover, as pointed out by @BenjaminLindley in the comments, you need to make sure that you transmute from the first item in the slice and not the fat pointer that is the slice itself:

fn i_from_slice<T>(slice: &[u8]) -> Option<T> {
    if slice.len() == std::mem::size_of::<T>() {
        Some(unsafe { std::mem::transmute_copy(&slice[0]) })
    } else {
        None
    }
}
user_53970090
Related