extract_fn_is_mut.md

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title: "`take_extract` now returns `dyn FnMut` instead of `dyn Fn`"
pull_requests: [19926]

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Previously, set_extract accepted any Fn. Now we accept any FnMut. For callers of set_extract, there is no difference since Fn: FnMut.

However, callers of take_extract will now be returned Option<Box<dyn FnMut(&mut World, &mut World) + Send>> instead.