Understanding Rust Error E0756: Misusing the ffi_const Attribute

Rust Error E0756

This error occurs when the ffi_const attribute is applied to an item other than a foreign function declaration. The ffi_const attribute in Rust is used to indicate that a foreign function has no side effects except for its return value, which allows certain optimizations to be performed.

Erroneous code example

#![feature(ffi_const)]

#[ffi_const] // error!
pub fn foo() {}
fn main() {}

In the example above, the ffi_const attribute has been applied to a regular function named foo. This will result in an E0756 error because the ffi_const attribute is meant to be used exclusively with foreign function declarations.

Correct usage

To correct this issue, make sure to only apply the ffi_const attribute to foreign function declarations. A correct usage of the ffi_const attribute looks like this:

#![feature(ffi_const)]

extern "C" {
    #[ffi_const] // ok!
    pub fn strlen(s: *const i8) -> i32;
}
fn main() {}

In the corrected example, the ffi_const attribute is appropriately applied to the foreign function strlen, which allows the optimizations that the attribute is intended for.

Additional Considerations

Keep in mind that using the ffi_const attribute might cause undefined behavior if the foreign function does have side effects. Therefore, it's important to have a thorough understanding of the foreign function's behaviors before using the attribute.

The ffi_const attribute is currently an unstable feature and can only be used in nightly builds of the Rust compiler. You should include the following line in your code to use it:

#![feature(ffi_const)]

As the ffi_const attribute may change in the future, keeping an eye on the official Rust documentation and tracking its development in the Rust Book is recommended for staying up to date.