I guess the confusion happens because MIR: the representation, mir: the stage, stable MIR: the potential future API interface for hooking into the compiler stage, and miri: the MIR interpreter all share pretty much the same name. Const evaluation uses MIR and that's the most likely culprit. Miri is an interpreter (as you found out on your own now), and it is not meant for use in production workloads due to the slowdown it introduces, so it limits its use to use in test suites and during debugging.

From my understanding Fil-C is an LLVM operation, so it should be possible to build integration to have a Fil-Rust binary that is slower but gives you some of the benefits of miri. I see value in doing something like that. There are plenty of other languages that would be well served by this too!