What Are the Stages of Rust Compilation? (Parsing, HIR, MIR, LLVM IR)

Rust compilation transforms source code into machine code through four distinct stages: Parsing, HIR, MIR, and LLVM IR. First, the compiler parses text into an Abstract Syntax Tree (AST), then expands macros and checks types to create the High-Level Intermediate Representation (HIR). Next, it lowers HIR into the Mid-Level Intermediate Representation (MIR) for borrow checking and optimization. Finally, MIR is translated into LLVM IR, which LLVM compiles into optimized machine code. You can inspect these stages using the rustc compiler with specific flags:

rustc --pretty=expanded src/main.rs
rustc -Z unpretty=hir src/main.rs
rustc -Z unpretty=mir src/main.rs
rustc -C llvm-args=-print-after-all src/main.rs

Note: The -Z flags require the nightly toolchain (rustup override set nightly).

Rust compiles your code in four steps to ensure it is safe and fast. It first reads your text, then checks for logic errors and memory safety rules, and finally translates the code into a universal format that a specialized engine turns into executable instructions. Think of it like a chef who reads a recipe, checks for missing ingredients, writes a simplified cooking plan, and then hands it to a sous-chef to actually cook the meal.