RFC 2203 specifically calls out Vec::new as a motivating example, however this does not work on the current nightly.

fn main() {
    let _: [Vec<i32>; 4] = [Vec::new(); 4];
}

There are two reasons for this. First, promotion in array repeat expressions currently uses the "implicit" rules for promotability, which forbids promotion of function calls without #[rustc_promotable]. Second, the result of Vec::new() is marked as NeedsDrop since Vec<T> is Drop, which disqualifies it from promotion, even in an explicit context.

cc #49147