Understand error E0500

[occyou]

Hi everyone,

I came across the capture modes of the Closure types in the Rust Reference and I took the example from the docs, changed the commented lines a bit, and used it in a simple example to better understand what's going on.

use  std::collections::HashSet;

#[derive(Debug)]
struct SetVec {
    set: HashSet<u32>,
    vec: Vec<u32>
}

impl SetVec {
    fn populate(&mut self) {
        // let vec = &mut self.vec;
        self.set.iter().for_each(|&n| {
            self.vec.push(n);   // use `vec` to compile successfully
        })
    }
}

// My code example
fn main() {
	let mut set_vec = SetVec {
		set: [1,2,3].iter().cloned().collect(),
		vec: vec![]
	};

	set_vec.populate();
	
    println!("set_vec = {:?}", set_vec);
}

(Playground)

The code above does not compile because I use self inside the closure as in self.vec.push(n). This tells the closure that it should fully capture self.

The way I understand it is that the Rust compiler is complaining about this situation:

• self.set is an immutable shared-borrow : read-only (because of the iterator)
• self is a mutable borrow : exclusive to closure (because of the self.vec.push(n))

Is my understanding correct ?

Thank you!

[crumplecup]

You are taking a mutable reference to self to create an iterator, and then using a mutable reference to self inside the iterator to populate the field 'vec'. If you split the sequence of operations so you are only mutating self one filed at a time, it will pass the compiler. Others may be able to offer a more idiomatic approach.

impl SetVec {
    fn populate(&mut self) {
        let mut vec = Vec::new();
        self.set.iter().for_each(|&n| {
            vec.push(n);   // use `vec` to compile successfully
        });
        self.vec = vec;
    }
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c362b3475a8c448dceabebff1bbe25d9

[geeklint]

Note that in the upcoming (soon!) Rust 2021 edition, the original code will work, due to the new "Disjoint captures in closures" feature.

[steffahn]

@geeklint

the original code will work

As can already be demonstrated in the playground. (Edition selection can be found under the “advanced compilation options”, i.e. the three dots to the right of Stable/Beta/Nightly selection.) By the way, just 4 more days until it’s on stable