Error code E0755

The ffi_pure attribute was used on a non-foreign function.

Erroneous code example:

#![feature(ffi_pure)]

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

The ffi_pure attribute can only be used on foreign functions which do not have side effects or infinite loops:

#![feature(ffi_pure)]

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

You can find more information about it in the unstable Rust Book.

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