```
use aggregator::{Summary, Tweet};
fn main() {
let tweet = Tweet {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
reply: false,
retweet: false,
};
println!("1 new tweet: {}", tweet.summarize());
}
```
This code prints 1 new tweet: horse_ebooks: of course, as you probably already know, people.
Other crates that depend on the aggregator crate can also bring the Summary trait into scope to implement Summary on their own types. One restriction to note is that we can implement a trait on a type only if at least one of the trait or the type is local to our crate. For example, we can implement standard library traits like Display on a custom type like Tweet as part of our aggregator crate functionality, because the type Tweet is local to our aggregator crate. We can also implement Summary on Vec<T> in our aggregator crate, because the trait Summary is local to our aggregator crate.
But we can’t implement external traits on external types. For example, we can’t implement the Display trait on Vec<T> within our aggregator crate, because Display and Vec<T> are both defined in the standard library and aren’t local to our aggregator crate. This restriction is part of a property called coherence, and more specifically the orphan rule, so named because the parent type is not present. This rule ensures that other people’s code can’t break your code and vice versa. Without the rule, two crates could implement the same trait for the same type, and Rust wouldn’t know which implementation to use.
{
Two trait implementations overlap when there is a non-empty intersection of the traits the implementation is for, the implementations can be instantiated with the same type.
Orphan rules
Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only if at least one of the following is true:
Trait is a local trait
All of
At least one of the types T0..=Tn must be a local type. Let Ti be the first such type.
No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
Only the appearance of uncovered type parameters is restricted. Note that for the purposes of coherence, fundamental types are special. The T in Box<T> is not considered covered, and Box<LocalType> is considered local.
}