The `ffi_const` attribute was used on something other than a foreign function
declaration.

Erroneous code example:

```compile_fail,E0756
#![feature(ffi_const)]

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

The `ffi_const` attribute can only be used on foreign function declarations
which have no side effects except for their return value:

```
#![feature(ffi_const)]

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

You can get more information about it in the [unstable Rust Book].

[unstable Rust Book]: https://doc.rust-lang.org/nightly/unstable-book/language-features/ffi-const.html