[marshallward]

I have only looked at the GCC Fortran frontend. I would not say it is necessarily difficult but rather very ad-hoc and disconnected from the rest of gcc. I don't see many tools for specifying a grammar or tokens. Instead, there are a lot of constructs custom-written to handle Fortran. Looking around, the Go and Rust frontends feel equally disconnected.

I'm told that you eventually make your way to the "GCC IR", but it seems like you are largely on your own with how to get there.

I did not invest a lot of time here, so I could be wildly off base, but thought I'd at least try to give a very-slightly-hands-on perspective.