Rust error E0755: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

Note: this error code is no longer emitted by the compiler.

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

Erroneous code example:

#![feature(ffi_pure)]

#[unsafe(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" {
    #[unsafe(ffi_pure)] // ok!
    pub fn strlen(s: *const std::ffi::c_char) -> usize;
}

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

From rustc --explain E0755 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.