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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.
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