portable APT?

There is a "Rust to C converter", or rather a Rust compiler that emits C (mrustc). More importantly, there are two different projects to bring Rust to GCC (gccrs and rustc_codegen_gcc).

If you can compile Rust to any architecture that supports GCC, you certainly have enough portability for APT.

Rust is already portable to the Debian architectures that people actually use. The ones that it does not support are not up-to-date anyway so shipping with an older version of APT would not be the end of the world.

As above though, there is no reason you cannot compile APT, even with Rust in it, for any of these platforms.