Understanding Rust Error E0139: Type Parameter Restrictions in Transmuting

Introduction to Rust Error E0139

Rust Error E0139 occurs when attempting to use the transmute function with types that have unsubstituted type parameters. The Rust compiler restricts transmuting between types as they must have the same size, among other conditions. This article discusses the error in depth, including why it occurs, examples of problematic code, and solutions to fix the issue.

Circumstances leading to E0139 error

In the presence of type parameters, the Rust compiler cannot always perform a safe transmutation and may produce the E0139 error. The following code example illustrates this scenario:

use std::mem::transmute;

struct Foo<T>(Vec<T>);

fn foo<T>(x: Vec<T>) {
    let y: Foo<T> = unsafe { transmute(x) };
    // do something with y
}

In this example, transmuting between Vec<T> and Foo<T> is performed. Although the transmute may be harmless, Rust does not guarantee that. The sizes might not match when considering alignment and enum optimizations; therefore, transmute only works with types without any unsubstituted type parameters.

Solutions to Rust Error E0139

If you encounter the E0139 error, it may signify that something is wrong with your implementation. As Rust does not guarantee much about struct layouts, consider adjusting your solution to avoid the transmute entirely.

One approach is to hand-monomorphize the function using traits for each possible type substitution, as presented in this example:

use std::mem::transmute;

struct Foo<T>(Vec<T>);

trait MyTransmutableType: Sized {
    fn transmute(_: Vec<Self>) -> Foo<Self>;
}

impl MyTransmutableType for u8 {
    fn transmute(x: Vec<u8>) -> Foo<u8> {
        unsafe { transmute(x) }
    }
}

impl MyTransmutableType for String {
    fn transmute(x: Vec<String>) -> Foo<String> {
        unsafe { transmute(x) }
    }
}

// ... more impls for the types you intend to transmute

fn foo<T: MyTransmutableType>(x: Vec<T>) {
    let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
    // do something with y
}

Transmutations using manual methods:

Another solution is to perform a manual transmute using pointer manipulation, like the following code example:

use std::ptr;
let v = Some("value");
type SomeType = &'static [u8];
unsafe {
    ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
}
;

In this example, the value 'v' is transmuted to 'SomeType' without using the transmute function directly. Keep in mind that this method does not move 'v' and that mem::forget(v) might be necessary to avoid calling destructors.