Inconsistent borrowck behavior between sync/async `fn`s when reassigning function arguments with local borrows

lib.rs

// This doesn't compile (with good reason!):
#[allow(unused)]
fn reassign_and_return_arg(mut arg: &()) -> &() {
    let local = ();
    arg = &local;
    arg
    //~^ ERROR cannot return value referencing local variable `local`
}

// This doesn't compile either (seemingly without good reason, but consistent with the previous code):
#[allow(unused)]
fn reassign_arg(mut arg: &()) {
    let local = ();
    //~^ ERROR `local` does not live long enough
    arg = &local;
}

// But this compiles (WHAT):
#[allow(unused)]
async fn reassign_arg_async(mut arg: &()) {
    let local = ();
    arg = &local;
}