Ex., this code: ``` #[cfg_attr(target_arch = "x86", path = "x86.rs")] #[cfg_attr(target_arch = "x86_64", path = "x86_64.rs")] mod imp; ``` results in this on `rustc 1.12.0-nightly (576f76659 2016-08-09)`: ``` warning: unused attribute, #[warn(unused_attributes)] on by default --> src/arch/mod.rs:7:36 | 7 | #[cfg_attr(target_arch = "x86", path = "x86.rs")] | ^^^^^^^^^^^^^^^^ ``` (this is when building with `target_arch = "x86"`, of course)