I'm confused that this compiles:

fn either_string<'a>(b: bool, str1: &'a str, str2: &'a str) -> &'a str {
    if b { str1 } else { str2 }
}

fn test_subtyping<'a, 'b>(b: bool, str1: &'a str, str2: &'b str) -> String {
    let x = either_string(b, str1, str2);
    String::from(x)
}

fn main() {
    println!("{}", test_subtyping(true, "foo", "bar"));
}

What's the magic that makes either_string(b, str1, str2) compile even though str1 and str2 have different lifetimes without a bound between them? There is no type annotation I could put on x that would keep it compiling, right? Is there some kind of heuristic that turns this into fn test_subtyping<'c, 'a: 'c, 'b: 'c> { let x : &'c str = ... }?

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