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The `ffi_pure` attribute was used on a non-foreign function.
Erroneous code example:
```compile_fail,E0755
#![feature(ffi_pure)]
#[ffi_pure] // error!
pub fn foo() {}
# fn main() {}
```
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" {
#[ffi_pure] // ok!
pub fn strlen(s: *const i8) -> isize;
}
# fn main() {}
```
You can find more information about it in the [unstable Rust Book].
[unstable Rust Book]: https://doc.rust-lang.org/unstable-book/language-features/ffi-pure.html
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