https://github.com/mozilla/rust/pull/12764 landed, but didn't include docs for the feature. The manual and tutorial should mention it in the context of type inference, and give a proper explanation: - `_` in a type is a "type placeholder" and has the meaning "infer this type". - Type placeholders are not allowed in item signatures, because global inference is not allowed by design. - Patterns like `Vec<_>` are called "partial type hints" and are useful in combination with things like `collect()` on iterators.