[vidarh]

A compiler that can't magically know how to find headers that don't exist in the expected directory?

Yes, that is the case for pretty much every compiler. I suppose you could build the headers into the binary, but nobody does that.

[tekne]

Consider: content-addressed headers.

[vidarh]

Then you might as well embed the headers, since in that case you can't update the compiler and headers separately anyway.

[IshKebab]

I guess you've heard of https://www.unison-lang.org/