It depends entirely on your trust model. If you just want to be able to compile a rust compiler on a new platform that only has a C compiler, you wouldn't care. If you worry about a Thompson-style trojan horse, then you want to be able to audit all sources in the bootstrap chain.
Some idealistic projects like live-bootstrap go a step further and don't even allow generated sources in the full source bootstrap chain, even if they are readable in principle, because the xz debacle showed that a configure script, which is also readable in principle, can contain a backdoor. In those projects you could probably still bootstrap Eurydice from sources, and then do the rust -> c translation of the rust compiler as an ordinary build step, before compiling it with a bootstrapped C compiler.
Some idealistic projects like live-bootstrap go a step further and don't even allow generated sources in the full source bootstrap chain, even if they are readable in principle, because the xz debacle showed that a configure script, which is also readable in principle, can contain a backdoor. In those projects you could probably still bootstrap Eurydice from sources, and then do the rust -> c translation of the rust compiler as an ordinary build step, before compiling it with a bootstrapped C compiler.