[Lvl999Noob]

I disagree with you on this. In another language with explicit public / private separation, the compiler can have access to the internal layout of a type (and thus optimise on it) without letting the developer mess around with it directly. I am assuming static compilation of course. Across a dynamic boundary, I would expect this compiler to behave like a normal C compiler and not use that layout.

In a header file, the information for the compiler and the user are the exact same which means you can't reduce your public interface without straight up hiding more of yourself.

[uecker]

A compiler can still have access to the internal layout in C via link-time optimization.

[johannes1234321]

Linker is something different from Compiler (even if often called via same Frontend)

[uecker]

The linker invokes the compiler again.

[TheNewAndy]

Personally, I'm happy to just let a Sufficiently Advanced Compiler do link time optimizations to deal with that level of optimization and either take the hit, or make more things public while that compiler doesn't exist.

Let the header files be written for people to read first, and only if there is actually a big performance issue, and the problem is the interface do you need to revisit it (and I'm not just saying this - I will frequently and happily go back and modify interfaces to allow for less data movement etc, but most of the time it really isn't important).

I think you are probably right to disagree with me though - I think I should have said that it is more of a limitation on how object files work, rather than how machines work. Object files aren't the only way things can work.