foo.rs

struct ExternalNode {
    key: int,
    value: int
    }

fn main() {
    let mut boxed_node = box ExternalNode{ key : 1, value: 2};

    match boxed_node {
        box ExternalNode{key : ref fkey, value: ref mut val} => {
         *val+=1;
        }
    }
         
}

foo.rs:12:49: 12:60 error: cannot borrow `boxed_node.value` as mutable because `boxed_node.key` is also borrowed as immutable
foo.rs:12         box ExternalNode{key : ref fkey, value: ref mut val} => {
                                                          ^~~~~~~~~~~
foo.rs:12:32: 12:40 note: previous borrow of `boxed_node.key` occurs here; the immutable borrow prevents subsequent moves or mutable borrows of `boxed_node.key` until the borrow ends
foo.rs:12         box ExternalNode{key : ref fkey, value: ref mut val} => {
                                         ^~~~~~~~
foo.rs:15:6: 15:6 note: previous borrow ends here
foo.rs:11     match boxed_node {
foo.rs:12         box ExternalNode{key : ref fkey, value: ref mut val} => {
foo.rs:13          *val+=1;
foo.rs:14         }
foo.rs:15     }
              ^
error: aborting due to previous error