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Showing content from https://doc.rust-lang.org/nightly/std/iter/trait.Extend.html below:

Extend in std::iter - Rust

pub trait Extend<A> {
    // Required method
    fn extend<T>(&mut self, iter: T)
       where T: IntoIterator<Item = A>;

    // Provided methods
    fn extend_one(&mut self, item: A) { ... }
    fn extend_reserve(&mut self, additional: usize) { ... }
}

Extend a collection with the contents of an iterator.

Iterators produce a series of values, and collections can also be thought of as a series of values. The Extend trait bridges this gap, allowing you to extend a collection by including the contents of that iterator. When extending a collection with an already existing key, that entry is updated or, in the case of collections that permit multiple entries with equal keys, that entry is inserted.

Basic usage:

let mut message = String::from("The first three letters are: ");

message.extend(&['a', 'b', 'c']);

assert_eq!("abc", &message[29..32]);

Implementing Extend:

#[derive(Debug)]
struct MyCollection(Vec<i32>);

impl MyCollection {
    fn new() -> MyCollection {
        MyCollection(Vec::new())
    }

    fn add(&mut self, elem: i32) {
        self.0.push(elem);
    }
}

impl Extend<i32> for MyCollection {

    fn extend<T: IntoIterator<Item=i32>>(&mut self, iter: T) {

        for elem in iter {
            self.add(elem);
        }
    }
}

let mut c = MyCollection::new();

c.add(5);
c.add(6);
c.add(7);

c.extend(vec![1, 2, 3]);

assert_eq!("MyCollection([5, 6, 7, 1, 2, 3])", format!("{c:?}"));

Extends a collection with the contents of an iterator.

As this is the only required method for this trait, the trait-level docs contain more details.

let mut message = String::from("abc");

message.extend(['d', 'e', 'f'].iter());

assert_eq!("abcdef", &message);

Extends a collection with exactly one element.

Reserves capacity in a collection for the given number of additional elements.

The default implementation does nothing.

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

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