When were you doing this? There's now a large chunk of the standard library[1] that's explicitly designed to be used in bare-metal/zero-dependency situations, and it even includes "fancy" things like the formatting infrastructure[2] (it has to be hooked up to a writable/IO device, but that's just a trait implementation away) and string processing, certainly more than the zero-allocation subset of libc.

[1]: https://doc.rust-lang.org/core/

[2]: https://doc.rust-lang.org/core/fmt/