concat string using format!() and return to &'static str [duplicate]

Question

I'll get error when compiled: 'returns value referencing data owned by the current function'

here's the code:

fn return_static_str(a: &'static str, b: &'static str) -> &'static str {
    format!("{} {}", a,b).as_str()
}

fn main() {
    let s = return_static_str("Hello ", "World!");
}

using return as_ref() get same issue.