Selective Closure Capture in Rust

Written 2020-08-11 — Updated 2020-08-17

Sometimes in Rust you may have a closure that needs to move some values but not others. If all the values satisfy the Copy this is easy, but that it usually not the case.
Sometimes we need to move some values into a closure, but want to borrow others.
You can solve this problem by creating new bindings for the by-reference values, so that the closure moves the references instead of the original value. It's not pretty but it works well.

let threads = vec![("referrers", "s"), ("referrals", "o")];
thread::scope(|s| {
    for (description, field) in threads {
        // Get variables explicitly as references so that the closure
        // moves only the references.
        let idmapper = &idmapper;
        let input_path = &input_path;
        let output_path = &output_path;
        let logger = &logger;
        let file_prefix = &file_prefix;
        s.spawn(move |_| {
            write_relationships_table(
                &logger,
                &idmapper,
                description,
                &input_path,
                &output_path,
                file_prefix.as_str(),
                field,
                max_year,
            );
        });
    }
})
.unwrap();

This is done by making it a move closure, and rebinding the variables that we want to borrow as references.

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