It can be helpful to show the monomorphized versions of pick, either before talking about the generic pick in order to show how generics can reduce code duplication, or after talking about generics to show how monomorphization works.

#![allow(unused)]
fn main() {
fn pick_i32(cond: bool, left: i32, right: i32) -> i32 {
    if cond {
        left
    } else {
        right
    }
}

fn pick_char(cond: bool, left: char, right: char) -> char {
    if cond {
        left
    } else {
        right
    }
}
}

Rust infers a type for T based on the types of the arguments and return value.

In this example we only use the primitive types i32 and char for T, but we can use any type here, including user-defined types:

struct Foo {
    val: u8,
}

pick(123, Foo { val: 7 }, Foo { val: 456 });

This is similar to C++ templates, but Rust partially compiles the generic function immediately, so that function must be valid for all types matching the constraints. For example, try modifying pick to return even + odd if n == 0. Even if only the pick instantiation with integers is used, Rust still considers it invalid. C++ would let you do this.

Generic code is turned into non-generic code based on the call sites. This is a zero-cost abstraction: you get exactly the same result as if you had hand-coded the data structures without the abstraction.