closure_matching: wat! I figure out that "|_| Some(2) =>" in a match arm is just a disguise for the alternation "_ | Some(2)" with prefix separator, but what d

closure_matching: wat! I figure out that "|_| Some(2) =>" in a match arm is just a disguise for the alternation "_ | Some(2)" with prefix separator, but what does the `let` do there?