Earlier quoted context omitted.
Perhaps my examples were a little too trivial. I do a decent amount of game/graphics coding, so there's a lot of "hmm, does this look good 20 pixels over? How about 18?" and I don't know how you'd get around that without recompiling. (OK, you could read preferences from a file, but then you'd have to optimistically write every value you'd ever want to recompile to a file. I've tried this, but it's far too much overhe…
Why not have controls in your program to do these immediate, non-logic changing modifications? It can report the ideal value through a debug interface. No recompiles needed. I'd also recommend getting used to serialisation/deserialisation. Serde for Rust makes this remarkably easy. Writing every setting to a file is simple if the compiler can do it for you, rather than you walking the long way around.
Also there are also a shit-ton of minor logic-changing modifications when you're writing prototype gamedev code, (for example, moving an if statement here or there to tweak the physics of your platforming). These cannot be easily represented in config files, and this is why people attach lightweight scripting languages like Lua to their game engines (so that you don't have to wait for those atrocious C++ compile times when tweaking your gameplay code).
(Ironically, serde is known for its atrocious compile times, which really makes the situation even worse. Because of this some frustrated Rust gamedevs wrote their own serialization libraries such as nanoserde (https://github.com/not-fl3/nanoserde)... but you get the point.)