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Is that even valid C code?

Yes, if PRI_LL_PREFIX expands to a string literal, the literals get merged together.

Ruby supports this too but it will be removed in the future (3.0). The preview doesn't show a warning but 2.0 will.

Btw, PRI_LL_PREFIX is defined in Ruby's config.h.

Not as written, but it's possible that PRI_LL_PREFIX is some sort of macro expanding to a string (similar to PRIi64 which is standard C).