A method or constant was implemented on a primitive type.

Erroneous code example:

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 or constant is a good solution. Example:

struct Foo {
    x: i32
}

trait Bar {
    fn bar();
}

impl Bar for *mut Foo {
    fn bar() {} // ok!
}