Understanding Rust Error E0591

What is Rust Error E0591?

Rust Error E0591 occurs when using the `transmute` function to convert a function item (with a zero-sized type, such as `typeof(foo)`) into a non-zero-sized function pointer (like `fn(S)`). This is an incorrect pattern because the source type is zero-sized and the target type is not. In this article, we will discuss what causes this error, how to identify it, and how to resolve it in Rust programs.

Causes of Rust Error E0591

The error occurs when you attempt to use `transmute` to convert the types of function arguments in an FFI (Foreign Function Interface) call. The following code example demonstrates this:

extern "C" fn foo(userdata: Box<i32>) {
  /* ... */

fn callback(_: extern "C" fn(*mut i32)) {}
use std::mem::transmute;
unsafe {
  let f: extern "C" fn(*mut i32) = transmute(foo);

How to Resolve Rust Error E0591

To resolve this error, you should rewrite the code pattern. Here are a few possible ways to do this:

1. Change the original `fn` declaration to match the expected signature, and do the cast in the `fn` body. This is the preferred option.

2. Cast the function item to a function pointer before calling `transmute`. Here's an example:

extern "C" fn foo(_: Box<i32>) {}
use std::mem::transmute;
unsafe {
  let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
  let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works too


Rust Error E0591 occurs when attempting to use `transmute` incorrectly to convert a zero-sized function item into a non-zero-sized function pointer. The solution is to change the original `fn` declaration or cast the function item before using `transmute`. By understanding and implementing these solutions, you can resolve this error in your Rust programs.

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