| #### 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 |
