#[target_feature] is allowed on default implementations

[LeSeulArtichaut]

The compiler currently allows safe default method implementations to be marked with #[target_feature]:

trait Foo {
    #[target_feature(enable = "avx2")] // no error
    fn foo() {}
}

which I don't think is allowed in RFC 2396. For reference, #[target_feature] is not allowed on trait implementations:

struct Bar;

impl Foo for Bar {
    #[target_feature(enable = "avx2")] // error: cannot be applied to safe trait method
    fn foo() {}
}

From my limited testing, this doesn't seem to be unsound however, as the compiler seems to consider all implementations of Foo::foo as having #[target_feature(enable = "avx2")].

cc #69098
@rustbot label T-lang T-compiler C-bug F-target_feature_11

[est31]

A good resolution to this would be to move it into a separate feature gate I guess?

[LeSeulArtichaut]

@rustbot claim