I am not familiar with the compilers for all of these, but I can make a couple of interesting comparisons: * GHC does exactly what I suggested rustc needs to d

I am not familiar with the compilers for all of these, but I can make a couple of interesting comparisons: * GHC does exactly what I suggested rustc needs to do- it does the flattening on an earlier, higher-level IR than LLVM's. (It is also not exactly a fast compiler, though!) * I'm not sure if this applies to F#, but in C#, LINQ compiles faster by not doing as much (if any) flattening in the first place.