E0753: include_str!("./error_codes/E0753.md"),
E0754: include_str!("./error_codes/E0754.md"),
E0755: include_str!("./error_codes/E0755.md"),
+E0756: include_str!("./error_codes/E0756.md"),
E0758: include_str!("./error_codes/E0758.md"),
E0759: include_str!("./error_codes/E0759.md"),
E0760: include_str!("./error_codes/E0760.md"),
E0722, // Malformed `#[optimize]` attribute
E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
// E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
- E0756, // `#[ffi_const]` is only allowed on foreign functions
E0757, // `#[ffi_const]` functions cannot be `#[ffi_pure]`
E0772, // `'static' obligation coming from `impl dyn Trait {}` or `impl Foo for dyn Bar {}`.
}
--- /dev/null
+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