A method 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 is a good solution. Example:
struct Foo { x: i32 } trait Bar { fn bar(); } impl Bar for *mut Foo { fn bar() {} // ok! }