(Moved from #69098 (comment) with an added PoC.)

The following program (playground) demonstrates how current implementation of target_feature_11 allows using a target_feature from safe code without ensuring it's actually available:

#![feature(target_feature_11)]

#[target_feature(enable="avx")]
fn use_avx() {
    println!("Hello from AVX")
}

fn call_it(f: impl FnOnce()) {
    f();
}

fn main() {
    call_it(use_avx);
}

This is unsound because it allows executing (e.g.) AVX instructions on CPUs that do not implement them, which is UB. It only works because "safe fns with target_features" are erroneously considered to implement the FnOnce/FnMut/Fn traits.