Rust 编译器不期望可变引用，而应该期望可变引用

Question

We are writing a function to erase a node from a sorted tree in Rust, but are stuck with the Rust compiler complaining that it expects no mutable reference when the called function does need a mutable reference:

error[E0308]: mismatched types
   --> src/tree.rs:111:17
    |
111 |                 SortedContainer::node_find_parent(&mut Some(node), &age, &name)
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `std::option::Option`, found mutable reference
    |
    = note: expected type `std::option::Option<std::boxed::Box<tree::Node>>`
               found type `&mut std::option::Option<std::boxed::Box<tree::Node>>`

This is our code snippet in the impl block of SortedContainer (Our tree structure):

fn node_find_parent<'a>(parent_option: &'a mut Option<Box<Node>>, age: &i32, name: &String) -> &'a mut Option<Box<Node>> {
    // TODO: implement
    parent_option
}

pub fn find_parent(&mut self, age: &i32, name: &String) -> &mut Option<Box<Node>> {
    &mut match self.root {
        Some(node) => if node.name == *name && node.age == *age {
            self.root
        } else {
            SortedContainer::node_find_parent(&mut Some(node), &age, &name)
        }
        None => &mut None,
    }
}

The SortedContainer::node_find_parent should return a mutable reference, since the return type is a mutable reference

What are we are doing wrong?

Answer 1

Changing the &mut match to match &mut solved the problems:

pub fn find_parent(&mut self, age: &i32, name: &String) -> &mut Option<Box<Node>> {
    match &mut self.root {
        Some(node) => {
            if node.name == *name && node.age == *age {
                &mut self.root
            } else {
                SortedContainer::node_find_parent(&mut self.root, &age, &name)
            }
        }
        None => &mut None,
    }
}