(Almost) Everything You Should Know About The Compiler Frontend

Rustc’s frontend has various stages, phases, contexts and IRs, with the code going through transformations and acquiring invariants that build up to ensure the compiler is well equipped to do its analysis that underpins Rust’s guarantee of soundness and correctness. While many contributors interact directly with the compiler’s frontend, it’s not always easy to understand why things are set up the way they are, and reverse-engineering the bigger picture is a daunting task to do alone. Come spelunking through the surprisingly deep caves that are the compiler’s frontend — from parsing to MIR optimization — while I try to motivate each step’s existence, explain its general philosophy, constraints and problem-solving approaches, point out its quirks (in the code!), and hopefully help you appreciate the complexity of the compiler a bit more.