Cross-compilation is orthogonal to bootstrapping, which is the major motivating factor for having something like what is described earlier in this thread: a small compiler written in C which can compile a subset of Rust, used to compile a larger, feature complete Rust compiler in that Rust subset -- versus what we have right now, which is a Rust compiler written in Rust which requires an existing Rust compiler binary, which means we have an unmitigated supply chain attack vector.
If you change your question to "why does anyone care about bootstrapping?", the answer would revolve around that aforementioned supply chain attack vector.
Perhaps you're comfortable with the lack of assurances that non-boostrapable builds entails (everyone has a different appetite for risk); some others aren't though, and so they have an interest in efforts to mitigate the security risks inherent in trusting a supply chain of opaque binaries.
If you change your question to "why does anyone care about bootstrapping?", the answer would revolve around that aforementioned supply chain attack vector.
For details, you could check out:
- Reading the 1984 paper "Reflections on Trusting Trust", for which Ken Thompson was given the ACM Turing award: https://www.cs.cmu.edu/~rdriley/487/papers/Thompson_1984_Ref...
- Or watching this short Computerphile video on the above: https://www.youtube.com/watch?v=SJ7lOus1FzQ
- You can read about the GNU Guix endeavor to achieve a 100% bootstrapped Linux development environment (using zero pre-compiled binaries), starting from a 357-byte HEX file: https://guix.gnu.org/en/blog/2023/the-full-source-bootstrap-...
Perhaps you're comfortable with the lack of assurances that non-boostrapable builds entails (everyone has a different appetite for risk); some others aren't though, and so they have an interest in efforts to mitigate the security risks inherent in trusting a supply chain of opaque binaries.