You tried to implement methods for a primitive type. Erroneous code example:
```compile_fail,E0390
struct Foo {
x: i32
}
impl *mut Foo {}
// error: only a single inherent implementation marked with
// `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
```
This isn't allowed, but using a trait to implement a method is a good solution.
Example:
```
struct Foo {
x: i32
}
trait Bar {
fn bar();
}
impl Bar for *mut Foo {
fn bar() {} // ok!
}
```