If you ever have to change lifetimes of existing functions/structs on a non-trivial Rust code base, you're screwed.
I forgot that I had also written a small cycle-by-cycle emulator of a simple CPU (8-bit data bus, 16-bit addr bus, 8 16-bit GPRs + a 16-bit PC + a single condition flag + an interrupt flag, timer interrupts, conditional jumps, call, ret, iret, mov, add, terminal I/O). That was also surprisingly easy.
I would like to use generators or something similar for a more realistic cycle-by-cycle emulator but I haven't gotten around to playing with that yet.
Generators and various kinds of coroutines are, yet again, just formalizations of the way I have been thinking for years.
Maybe other domains would be more challenging but I seriously doubt it. I do know that it will take me a couple more months before I'm really proficient in Rust -- including multi-threading -- and no longer fear macros.
Perhaps it's more a question of background? Rust contains many features I already know from the ML family + it formalizes the way I've already been thinking about memory since the early 90's.