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+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.