Bonkers-wrong commit that recently dropped in systemd: https://github.com/systemd/systemd/commit/686bbf1e7ff7492a86f48d4b9877c2cfb5deca53

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Introduce forward.h header with forward declarations · systemd/systemd@686bbf1

In preparation for adopting forward declarations to reduce unnecessary transitive includes across the tree, let's introduce a forward.h header with forward declarations for all libc, libsystemd...

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@dalias Hm, carrying this logic through would greatly simplify the codebase. One header file is all you really need!