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Daniel Imfeld
Daniel Imfeld

Posted on • Originally published at imfeld.dev

Selective Closure Capture in Rust

Sometimes in Rust you have a closure that needs to move some values but not others. If all the values satisfy the Copy trait this isn't an issue, but that is usually not the case.

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")];

crossbeam::thread::scope(|s| {
    for &(description, field) in &threads {
        // Get variables that we want to borrow explicitly as references so
        // that the closure moves only the references.
        let input_path = &input_path;
        let output_path = &output_path;
        let logger = &logger;
        let file_prefix = &file_prefix;
        s.spawn(move |_| {
            write_data(
                &logger,
                description,
                &input_path,
                &output_path,
                file_prefix.as_str(),
                field
            );
        });
    }
})
.unwrap();
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