Underscores in Rust

The thing is, it's not just renaming a variable. '_' is not a variable name, it is an entirely different syntactic entity: it is a *pattern*. Other examples of patterns are 'Some(x)' and '(a, b, c)'. The meaning of the '_' pattern is "discard the value matched against this pattern as soon as possible".

That said, I agree this is quite surprising, and I've been bitten by this myself in the past.