The `Sized` trait was implemented explicitly.

Erroneous code example:

```compile_fail,E0322
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.
