is_x86_feature_detected! does not work with $:literal argument

[dtolnay]

macro_rules! my_is_x86_feature_detected {
    ($feat:literal) => {  // changing to $feat:tt makes it work
        is_x86_feature_detected!($feat)
    };
}

fn main() {
    let _ = is_x86_feature_detected!("mmx");  // works
    let _ = my_is_x86_feature_detected!("mmx");  // does not work
}

error: unknown x86 target feature: mmx
 --> src/main.rs:8:13
  |
8 |     let _ = my_is_x86_feature_detected!("mmx");
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

The std::is_x86_feature_detected! macro appears to work when called with a string literal argument, and works when called with a $:tt argument, but does not work with a $:literal argument. I believe that it should work all three ways.