Rust is a memory-safe programming language that disallows undefined behavior. Its safety guarantees have been extensively examined by the community through empirical studies, which has led to its remarkable success. However, unsafe code remains a critical concern in Rust. By reviewing the safety design of Rust and analyzing real-world Rust projects, this paper establishes a systematic framework for understanding unsafe code and undefined behavior, and summarizes the soundness criteria for Rust code. It further derives actionable guidance for achieving sound encapsulation.