Fill in long diagnostic

src/librustc_typeck/diagnostics.rs

@@ -4698,6 +4698,55 @@ element type `T`. Also note that the error is conservatively reported even when
 the alignment of the zero-sized type is less than or equal to the data field's
 alignment.
 "##,
+
+
+E0908: r##"
+A method was called on a raw pointer whose inner type wasn't completely known.
+
+For example, you may have done something like:
+
+```compile_fail
+# #![deny(warnings)]
+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.
+"##,
+
 }
 
 register_diagnostics! {
@@ -4777,5 +4826,4 @@ register_diagnostics! {
     E0641, // cannot cast to/from a pointer with an unknown kind
     E0645, // trait aliases not finished
     E0907, // type inside generator must be known in this context
-    E0908, // methods on raw pointers can only be called if the pointer type is fully known
 }