Cannot move out of boxed value during deconstruction

[jonhoo]

On stable, the following code does not compile:

enum X {
    A(String, Box<X>),
}

fn foo(x: Box<X>) {
    let X::A(a, b) = *x;
}

The compiler says that extracting b is a "use of moved value: x", and that x was moved when extracting a. The issue also occurs if using match, or when using the box_patterns feature. However, this clearly should be legal to do.

The following code also breaks in a similar way, but with a different error message ("use of collaterally moved value: (x:X::A).1"; which is not even mentioned by --explain E0382):

enum X {
    A(String, Box<X>),
    C,
}

fn foo(x: Box<X>) {
    if let X::A(a, b) = *x {
    }
}

It was pointed out on IRC that this can be worked around by writing {*x} instead of *x, but that's not particularly satisfactory. Someone then said that this was likely a bug with the deref-move special case for Box, where it doesn't handle partial moves correctly.

[jonhoo]

Should also mention that the code works correctly is s/String/i32/, presumably because i32 is Copy, and so only b is really moved.

[apasel422]

Thanks for the report! Closing as duplicate of #30564 (and a few other closed issues, e.g. #33550).