Huh, I thought Rust already used jemalloc by default. I looked it up and it looks like it was removed relatively recently. When I was doing this experiment, I was using a version of Rust that included jemalloc by default so that 10% number already uses jemalloc. I remember this because I also thought of trying to speed up the allocator.
Like I said above, I profiled both Go and Rust and the 10% slowdown with Rust appeared to be running destructors for the AST. I think the appropriate solution to this would be some form of arena allocator instead of changing the system allocator. But that gets even more complicated with lifetimes and stuff.
> Can you work around this by using separate functions for the branches?
Yeah, I could have tried restructuring my code to try to avoid compiler issues. But this would have been even more time spent working around issues with Rust. Go was better than Rust by pretty much every metric that mattered for me, so I went with Go instead.
It's too bad because I was initially super excited about the promise of Rust. Being able to avoid the overhead of GC while keeping memory safety and performance is really appealing. But Rust turned out to be not a productive enough language for me.