What Are Opaque Types in Rust?

Opaque types in Rust are return types that hide their concrete implementation from the caller while exposing only the traits they implement. Use the impl Trait syntax in function return positions to define an opaque type that the compiler knows but the user does not.

fn create_iterator() -> impl Iterator<Item = i32> {
    (1..10).filter(|x| x % 2 == 0)
}

This allows you to return complex types without exposing their names or requiring the caller to know the exact struct, improving encapsulation and reducing coupling.

Opaque types let you return a value without revealing its specific type name to the user. Think of it like ordering a meal at a restaurant: you know it's "dinner" and what it tastes like, but you don't need to know the exact recipe or ingredients used. This keeps your code clean and prevents users from relying on internal details that might change.