Rust Infers Closure Types on First Call

If you attempt to define a closure without concrete types, Rust will infer the types the first time the closure is called. In the following example, Rust will infer that the type of x in |x| x + 1 is an i32, and, if you attempt to use this closure with any other type, it will fail to compile.

fn main() {
    let add_one = |x| x + 1;
    let x = 5;
    println!("{} + 1 = {}", x, add_one(x));
    let y = 10.0;
    println!("{} + 1 = {}", y, add_one(y));
}

The second call to add_one fails because the type i32 has been locked into the closure.