The formalization work for Rust was done mostly at the MIR level, which is one step lower than the THIR level we use here. See, for example, the https://plv.mpi-sws.org/rustbelt/ project. MIR should be more amenable to formal specification, as this language is more compact than THIR and aims to be more stable.

However, we also lose some information going to MIR, as there are no expressions/loops from the original code anymore. There are still ways to reconstruct these, but we preferred to use the THIR representation directly.