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Explain an obscure behavior of the Default trait
What is the expected outcome of the following example?
struct A {
    i: i32,

impl Default for A {
    fn default() -> Self {
        println!("called A::default()");
        A { i: 42 }

struct B {
    a: A,
    i: i32,

impl B {
    fn new(a: A) -> Self {
        B {

fn main() {
    let a = A { i: 1 };
    let b = B::new(a);
    println!("{}", b.a.i);
The expected output of this example is:
called A::default()
In the main function, an instance of A is created with i being 1. This is passed to B::new function to create an instance of B. Inside B::new, the struct update syntax ..Default::default() is used, which sets the default value for fields of B that are not explicitly specified. However, since a is already specified with the A instance created in main, A::default() is actually not used to give a value to the field a. As a result, b.a.i remains as 1. However, while creating the instance of B, since i is not specified, Default::default will be called for it, which in the case of i32 will make it 0. Also, since A::default() is part of the Default::default for the B struct (although not used in this particular instance), it will also be called. This is why you see the "called A::default()" message. But the value 42 from A::default() is not used for b.a.i in this instance.