What is the orphan rule

The orphan rule is a Rust coherence rule that prevents you from implementing an external trait on an external type within your crate. You can only implement a trait for a type if either the trait or the type is local to your crate.

// ❌ Error: Both `Display` and `Vec<T>` are external
impl std::fmt::Display for Vec<i32> { }

// ✅ OK: `MyStruct` is local, so we can implement external `Display`
impl std::fmt::Display for MyStruct { }

// ✅ OK: `MyTrait` is local, so we can implement it on external `Vec<T>`
impl MyTrait for Vec<i32> { }

The orphan rule stops different parts of your code from accidentally defining the same behavior for the same data type. It ensures that only the creator of a type or the creator of a trait can decide how they work together. Think of it like a rule that says you can't add a new instruction manual to a product you didn't make unless you also wrote the manual yourself.