Why is the Rust compiler so slow?

So there's this guy you may have heard of called Ryan Fleury who makes the RAD debugger for Epic. The whole thing is made with 278k lines of C and is built as a unity build (all the code is included into one file that is compiled as a single translation unit). On a decent windows machine it takes 1.5 seconds to do a clean compile. This seems like a clear case-study that compilation can be incredibly fast and makes me wonder why other languages like Rust and Swift can't just do something similar to achieve similar speeds.

Because Russt and Swift are doing much more work than a C compiler would? The analysis necessary for the borrow checker is not free, likewise with a lot of other compile-time checks in both languages. C can be fast because it effectively does no compile-time checking of things beyond basic syntax so you can call foo(char) with foo(int) and other unholy things.

The borrow checker is usually a blip on the overall graph of compilation time.

The overall principle is sound though: it's true that doing some work is more than doing no work. But the borrow checker and other safety checks are not the root of compile time performance in Rust.

While the borrow checker is one big difference, it's certainly not the only thing the rust compiler offers on top of C that takes more work.

Stuff like inserting bounds checking puts more work on the optimization passes and codegen backend as it simply has to deal with more instructions. And that then puts more symbols and larger sections in the input to the linker, slowing that down. Even if the frontend "proves" it's unnecessary that calculation isn't free. Many of those features are related to "safety" due to the goals of the language. I doubt the syntax itself really makes much of a difference as the parser isn't normally high on the profiled times either.

Generally it provides stricter checks that are normally punted to a linter tool in the c/c++ world - and nobody has accused clang-tidy of being fast :P