The idea of bootstrapping is that you start with nothing, or at least with almost nothing. In case of stage0 (the first stage of live-bootstrap) this a small program that converts a file with hexadecimal characters (in pairs) to a binary file. From there you have to build everything. To compile Rust, you need a Rust or C compiler, I understand. I have no idea how hard it would be to build a minimal Rust compiler that could compile itself, one that not necessary performs all the checks that the (full) Rust compiler would do.
I think that Rust is build as part of live-bootstrap, but at a much later stage when the latest version of the GNU C compiler has been build.
There was a project [0] to write a bare bones rust compiler (no borrow checking) in C that aims to be able to build rustc in order to simplify bootstrapping, but it seems to have stalled.
The latest version of g++ is written in C++. If I am not mistaken g++ 4.7.4 is the oldest version that is written in C. But it does not support the C++ version in which the latest version of g++ is written. For that you have to compile g++ 10.4.0 first. And g++ 4.7.4 cannot be compiled by the Tiny C Compiler (tcc). The total chain then becomes:
