Auto-dereferencing bug?

[wooque]

Auto-dereferencing is working if &Box<Struct> is in place of &Struct function argument, or in:

let x: &Struct = &_boxed_struct; // &Box<Struct>

but not if Struct reference is destructured:

let &Struct{a: a1, b: b1, c: c1} = &_boxed_struct; // &Box<Struct>

Example:

struct A {
    x: i32,
    y: i32,
}

fn print_a(a: &A) {
    println!("x: {}, y: {}", a.x, a.y);
}

fn main() {
    let boxed_a = &Box::new(A{x: 1, y: 2});
    print_a(boxed_a); // this works

    let ref_a: &A = boxed_a; // this works
    print_a(ref_a);

    let &A{x: x1, y: y1} = boxed_a;
    /*
        this outputs:

        test.rs:17:10: 17:25 error: mismatched types:
        expected `Box<A>`,
            found `A`
        (expected box,
            found struct `A`) [E0308]
        test.rs:17     let &A{x: x1, y: y1} = boxed_a;
                            ^~~~~~~~~~~~~~~
        error: aborting due to previous error
    */
    println!("x: {}, y: {}", x1, y1);
}

Is it a bug or intended behaviour?

[steveklabnik]

/cc @nikomatsakis

[chuck-park]

This still reproduces in rustc 1.14.0-nightly (86affcdf6 2016-09-28).

[Mark-Simulacrum]

I feel like this is intentional behavior. Pattern matching on a Box requires the use of box patterns; that is, let &box A { x: x1, y: y1 } = boxed_a; is the intended way to make this work. Alternatively, you could do let &A { x: x1, y: y1 } = &**boxed_a;, which while ugly, does work on stable.

As such I'm going to close this issue; we simply don't deref in patterns (intentionally, as far as I know) though this could change with the recent match reform RFC.