A method was called on a raw pointer whose inner type wasn't completely known.

Erroneous code example:

```compile_fail,edition2018,E0699
# #![deny(warnings)]
# fn main() {
let foo = &1;
let bar = foo as *const _;
if bar.is_null() {
    // ...
}
# }
```

Here, the type of `bar` isn't known; it could be a pointer to anything. Instead,
specify a type for the pointer (preferably something that makes sense for the
thing you're pointing to):

```
let foo = &1;
let bar = foo as *const i32;
if bar.is_null() {
    // ...
}
```

Even though `is_null()` exists as a method on any raw pointer, Rust shows this
error because  Rust allows for `self` to have arbitrary types (behind the
arbitrary_self_types feature flag).

This means that someone can specify such a function:

```ignore (cannot-doctest-feature-doesnt-exist-yet)
impl Foo {
    fn is_null(self: *const Self) -> bool {
        // do something else
    }
}
```

and now when you call `.is_null()` on a raw pointer to `Foo`, there's ambiguity.

Given that we don't know what type the pointer is, and there's potential
ambiguity for some types, we disallow calling methods on raw pointers when
the type is unknown.