Pointer metadata (RFC 2580) is not supported

```Rust
#![feature(ptr_metadata)]

use core::ptr::Pointee;

fn main() {
    let x: <[&str] as Pointee>::Metadata = todo!();
    let mut y = x;
    y = 2;
}
```

Rustc accepts the above code but r-a reports a type mismatch between `<[&str] as Pointee>::Metadata` and `i32`. It is probably due magic implementation of `Pointee` trait for types.