Type inference for return types - Rust for C-Programmers

8.14 Type Inference for Function Return Types

Rust’s type inference applies chiefly to local variables. Typically, you must specify a function’s return type explicitly:

#![allow(unused)]
fn main() {
fn add(a: i32, b: i32) -> i32 {
    a + b
}
}

When returning more complex or anonymous types (like closures), you can use impl Trait to let the compiler infer the exact type:

#![allow(unused)]
fn main() {
fn make_adder(x: i32) -> impl Fn(i32) -> i32 {
    move |y| x + y
}
}

This returns “some closure that implements Fn(i32) -> i32,” without forcing you to name the closure’s type.