December 2025 focused on strengthening performance profiling workflows and documentation across rust-lang/rustc-dev-guide and rust-lang/rust. The work delivered feature-focused documentation improvements and workflow clarifications to reduce onboarding friction and improve reproducibility of benchmarks. No major bug fixes were required this month; changes center on enhancing guidance and correctness of profiling steps rather than code changes. Impact: clearer, example-driven guidance for profiling external crates, standardized usage of the Rust performance collector with cargo, and explicit cross-repo context (SHAs from rustc-repo while commands run in rustc-perf) that enable consistent performance analysis. Technologies/skills demonstrated include Rust tooling, cargo command ergonomics, performance profiling practices, PR-driven documentation updates, and cross-repo collaboration for reproducible benchmarks.