Can't directly destructure boxed values

[jdm]

This compiles:

struct Foo {
  s: String,
  s2: String,
}

impl Foo {
    fn bar(self: Box<Self>) {
        let f = *self;
        let _a = f.s;
        let _b = f.s2;
    }
}

fn main() {
    let f = Foo { s: "hi".to_owned(), s2: "bye".to_owned() };
    Box::new(f).bar();
}

This does not:

struct Foo {
  s: String,
  s2: String,
}

impl Foo {
    fn bar(self: Box<Self>) {
        let Foo { s, s2 } = *self;
    }
}

fn main() {
    let f = Foo { s: "hi".to_owned(), s2: "bye".to_owned() };
    Box::new(f).bar();
}

Due to this error:

<anon>:8:22: 8:24 error: use of moved value: `self` [E0382]
<anon>:8         let Foo { s, s2 } = *self;
                              ^~
<anon>:8:22: 8:24 help: see the detailed explanation for E0382
<anon>:8:19: 8:20 note: `self` moved here (through moving `self.s`) because it has type `collections::string::String`, which is moved by default
<anon>:8         let Foo { s, s2 } = *self;
                           ^
<anon>:8:19: 8:20 help: if you would like to borrow the value instead, use a `ref` binding as shown:
<anon>:          let Foo { ref s, s2 } = *self;
error: aborting due to previous error
playpen: application terminated with error code 101

I don't understand why one form is legal and the other is not, and the error message does not make it clear to me.

[eefriedman]

Duplicate of #30564, I think. (It's not legal because of rust-lang/rfcs#130.)