| #### Note: this error code is no longer emitted by the compiler. | |
| The `ffi_const` attribute was used on something other than a foreign function | |
| declaration. | |
| Erroneous code example: | |
| ```ignore (no longer emitted) | |
| #![feature(ffi_const)] | |
| #[unsafe(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" { | |
| #[unsafe(ffi_const)] // ok! | |
| pub fn strlen(s: *const std::ffi::c_char) -> usize; | |
| } | |
| # 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 |