How does boxing affect borrow checking?

Question

This code works as expected in Rust 1.27.2:

struct X {
    a: String,
    b: String,
}

impl X {
    fn func(mut self: Self) -> String {
        let _a = self.a;
        self.a = "".to_string();
        self.b
    }
}

fn main() {
    let x = X {
        a: "".to_string(),
        b: "".to_string(),
    };
    x.func();
}

Using the same compiler version, the X instance is now boxed. Even without trying to mutate the partial object, the borrow checker stops reasoning behind the self:

struct X {
    a: String,
    b: String,
}

impl X {
    fn func(self: Box<Self>) -> String {
        let _a = self.a;
        self.b
    }
}

fn main() {
    let x = Box::new(X {
        a: "".to_string(),
        b: "".to_string(),
    });
    x.func();
}

Error message:

error[E0382]: use of moved value: `self`
 --> src/main.rs:9:9
  |
8 |         let _a = self.a;
  |             -- value moved here
9 |         self.b
  |         ^^^^^^ value used here after move
  |
  = note: move occurs because `self.a` has type `std::string::String`, which does not implement the `Copy` trait

Is it by design that the boxed objects cannot be partially borrowed or is it a regression in the ongoing refactoring of the borrow checker?