Shape of errors to come
I really appreciate the user-friendliness of Rust's error messages -- I can't remember seeing a compiler tell me "maybe try this instead?" before (perhaps something from Clang, but never with the specificity of, e.g., a suggested lifetime annotation). And from a parsing / compiler-hacking perspective, it seems <i>really</i> hard to get the heuristics good enough to produce the "right" root cause. Kudos to the Rust team for this continued focus!