Except in some rare edge cases, it’s mostly the latter, indirectly: in the average crate the vast majority of the time is spent in LLVM optimization passes and linking. Sometimes IR generation gets a pretty high score, but that’s somewhat inconsistent.
`cargo check` that does all the parsing, type system checks, and lifetime analysis is pretty fast compared to builds.
Rust compilation time spends most time in LLVM, due to verbosity of the IR it outputs, and during linking, due to absurd amount of debug info and objects to link.
When cargo check isn't fast, it's usually due to build scripts and procedural macros, which are slow due to being compiled binaries, so LLVM, linking, and running of an unoptimized ton of code blocks type checking.
