rustc is written in Rust - for many years, start based on OCaml
Close! Depends what you call 100% I suppose. Even C isnt 100% C
I suspect they are arguing that it is not because the default codegen backend produces LLVM IR, which C++ code turns into the final executable. The Cranelift backend which is pure Rust exists, but because it is not the default some people discount it. I personally believe that leveraging existing codegen infrastructure is sound engineering. Having a "pure" implementation is not a goal of the project, having solid tooling is. That might translate in the future to tackling codegen more directly, but it isn't a huge pain point today.
The real contention is how do you know it’s memory safe if Rust itself isn’t pure safe Rust?
Because that’s not really relevant; Java is a memory safe language and the JVM is written in C++, Ruby is a memory safe language and it’s written in C.
Both of those have had memory related exploits, as I recall