Comment by weinzierl

This is sometimes called amalgamation and you can do it Rust as well. Either manually or with tools. The point is that apart from very specific niches it is just not a practical approach.

It's not that it can't be done but that it usually is not worth the hassle and our goal should be for compilation to be fast despite not everything being in one file.

Turbo Pascal is a prime example for a compiler that won the market not least because of its - for the time - outstanding compilation speed.

In the same vein, a language can be designed for fast compilation. Pascal in general was designed for single-pass compilation which made it naturally fast. All the necessary forward declarations were a pain though and the victory of languages that are not designed for single-pass compilation proofs that while doable it was not worth it in the end.