I was messing around with closures and ran into an odd problem with lifetimes and borrowing.

compile_weirdness.rs

// these two I just don't understand why neither works.
fn it_does_not_works() -> String {
    let mut output_string = String::new(); 

    let do_a_thing: || -> ()  = | | {
        output_string;
    };
    do_a_thing();
    return output_string;
}
// I assume this should compile like the example that follows it, but *shrug*
fn it_also_does_not_works() -> String {
    let mut output_string = String::new(); 
    {
        let do_a_thing: || -> ()  = | | {
            output_string;
        };
        do_a_thing();
    }
    return output_string;
}

// this compiles as expected
fn it_works() -> String {
    let mut output_string = String::new(); 
    {
        let do_a_thing: || -> ()  = | | {
            output_string.push_char('a');
        };
        do_a_thing();
    }
    return output_string;
}

// it seems like this should work if the above does. I specified a slightly
// shorter lifetime in the above example, but the closure's lifetime ends
// here at the function here. I think this is a corner case where the return 
// happens to the borrow checker before it realizes the closure would be out
// of scope.
fn this_does_not_works() -> String {
    let mut output_string = String::new(); 
    let do_a_thing: || -> ()  = | | {
        output_string.push_char('a');
    };
    do_a_thing();
    return output_string;
}