Destructuring an Option<Box<_>> inside a match statement in Rust

user_11357326 0
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I have a big object that I need boxed in another object but I don't necessarily need it all the time. So I want to use an if statement to get the optional boxed TempStructure but i'm not exactly sure how I can destructure and dereference at the same time.

Example:

pub struct TempStructure {
    lots_of_data: [u64; 64],
}
pub struct Structure {
    pending_removal: Option<Box<(TempStructure, bool)>>,
}
impl Structure {
    pub fn do_somthing(&mut self) {
        // How do I destructure the Option and dereference the Box to get TempStructure?
        if let Some((temp_structure, some_boolean)) = self.pending_removal.take() { 
            // Do something with temp_structure and some_boolean
        }
    }
}

When I do this ^^^ I get an expected struct `std::boxed::Box`, found tuple error.

dereference rust destructuring
2 Answers

Dereference the box after matching:

if let Some(inner) = self.pending_removal.take() {
    let (temp_structure, some_boolean) = *inner;
    // Do something with temp_structure and some_boolean
}

(playground)

If you think this is a bit clunky, you're right. On nightly you can use the unstable box_patterns feature to enable a better syntax for this (although this might never be stabilized):

if let Some(box (temp_structure, some_boolean)) = self.pending_removal.take() {
    // Do something with temp_structure and some_boolean
}

(playground)

user_101132380

You can fix this with adding a .as_deref() after the take():

pub struct TempStructure {
    lots_of_data: [u64; 64],
}
pub struct Structure {
    pending_removal: Option<Box<(TempStructure, bool)>>,
}
impl Structure {
    pub fn do_somthing(&mut self) {
        // How do I destructure the Option and dereference the Box to get TempStructure?
        if let Some((temp_structure, some_boolean)) = self.pending_removal.take().as_deref() { 
            // Do something with temp_structure and some_boolean
        }
    }
}

Box<T> dereferences to &T, as_deref() dereferences the value of the Option, therefore it gives you a &T out of your Option<Box<T>>.

Edit: another option is to dereference the Box to move the value out of it:

        if let Some((temp_structure, some_boolean)) = self.pending_removal.take().map(|boxed| *boxed) { 
            // assignments just for demonstration purposes that we now get
            // owned values rather than references.
            let _: TempStructure = temp_structure;
            let _: bool = some_boolean;
        }
user_106930220
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