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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
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