Incremental compilation flags #[target_feature] as an unused attribute

[alexcrichton]

Given this code:

#[target_feature(enable = "sse2")]
unsafe fn foo() {
}

fn main() {
    unsafe { foo(); }
}

it compiles as:

$ rustc +nightly foo.rs -C incremental=tmp
$ rustc +nightly foo.rs -C incremental=tmp
warning: unused attribute
 --> foo.rs:1:1
  |
1 | #[target_feature(enable = "sse2")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
= note: #[warn(unused_attributes)] on by default

How odd!

[alexcrichton]

Not technically a regression but this needs to be fixed before 1.27 hits stable, so tagging as a nightly regression