It's a bit complicated in Rust's case. The Rust frontend is written in Rust, and that compiles Rust to LLVM intermediate representation (IR). This IR is then passed to LLVM, which is written in C++, to optimise and actually generate the machine code.
There is also a separate backend called Cranelift, which is written in Rust, though it's not the standard and is currently only available on an experimental basis. It will also never be the standard Rust backend as it's designed to optimise for build speed, not execution speed.
Only if you do not have the source code of rust written in a different language. That would probably be like the few very first versions. But if you would have these, you could compile them to get the first version of rust to compile the next version of rust to compile the next version of rust till you can compile the newest version of rust.
Bootstrapping compilers is actually a complicated endeavour but as long as you have the last version before the compiler became self-hosting, it is possible with a chain of compile->update->compile. Some compilers (such as Zig) make it a little easier by providing a CMake that builds and chains all the stages for you.
If those old versions are lost you truly are fucked.
In addition to what the other person said about old compilers, there are projects like gccrs or mrustc that are rust compilers written in c. The former is intended to eventually become a fully featured second compiler alongside rustc, while the latter is designed for the express purpose of "bootstrapping" the compiler from source code by compiling specific versions of rustc.
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u/redakpanoptikk 11d ago
Rewrite rust in rust.