Comment by compiler-guy
6 hours ago
It's simply an ordering problem. When building the entire world from scratch, usually the C and C++ toolchains are built near the very first, and Rust toolchains built somewhat later. Anything written in Rust must come after the Rust toolchain is built. You need a linker as part of your C++ toolchain, so it must be written in a language ready to go at that point. If it is written in C, you are done. If it is written in Rust you have to wait. So a Rust-based linker can no longer be used from that very, very early C bootstrap.
It's not a big deal for normal users, where you have Rust ready to go. Kind of a bummer in this case, but this is a specialized one.
Can't you just use a statically linked mold binary for the initial bootstrap, just like you use some kind of pre-existing, basic C compiler?
You could. But now you are trusting an entirely new toolchain that can build the Rust-based linker, even if it is statically linked. This toolchain is much larger than a comparatively small and much more easily understood C bootstrap toolchain.
This effectively triples or quadruples (maybe even more) the amount of code you need to trust for the cold bootstrap.
Why do you keep saying C? Mold was C++ with some dependencies like TBB or zstd.
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Maybe that's about to flip. Maybe Rust will be built first, followed by C.
Sorry to break it to everyone but Rust depends on python which depends on perl and openssl and the majority of any lean full source bootstrapped toolchain tree.
Unless the gcc rust engine is mature any time soon (lol), we have no path to use rust until very late game in a distro build.
The earliest we can bootstrap a go compiler is about 10 minutes. It builds directly from tinycc. Add 6 hours for our fastest compile of the shortest path to rust, with 192 cores.
I am a rust fan too, but it is the worst language to bootstrap, which is why for systems programming I still must often revert to C to have a small and reviewable and fast to build dependency surface.
Rust requires LLVM (or GCC) which requires C++, not just C.
LLVM is the default yes, there is also an experimental cranelift backend, which is all Rust. Not sure if it is good enough to build the compiler itself using it.
(There is also a GCC backend called codegen_gcc that is pretty far along. And a separate reimplementation of both the frontend and backend using gcc and C++, called gccrs, which is not nearly as far along.)
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Maybe it should be rewritten in Rust then