A built-in trait was implemented explicitly. All implementations of the trait are provided automatically by the compiler.
Erroneous code example:
struct Foo; impl Sized for Foo {} // error!
The Sized
trait is a special trait built-in to the compiler for types with a constant size known at compile-time. This trait is automatically implemented for types as needed by the compiler, and it is currently disallowed to explicitly implement it for a type.