Rust isn't quite portable enough yet. There are lots of small environments or specialized toolchains that are based on gcc that'll compile bash just fine, but once you require Rust, you're eliminating architectures like Alpha (which includes familial descendents like the Shenwei architecture in the Sunway TaihuLight), m68k, SuperH and others.
Rust is getting better, but they're not quite there yet.
> C2Rust helps you migrate C99-compliant code to Rust. The translator (or transpiler), c2rust transpile, produces unsafe Rust code that closely mirrors the input C code. The primary goal of the translator is to preserve functionality; test suites should continue to pass after translation.
Rust is getting better, but they're not quite there yet.