> Macro expansion is slow, but only noticeably in the specific situation of a) third-party proc macros, b) a debug build, and c) a few thousand invocations of s

> Macro expansion is slow, but only noticeably in the specific situation of a) third-party proc macros, b) a debug build, and c) a few thousand invocations of said proc macros. Oh that's interesting. But note that it's an issue caused by (from the perspective of the language) user code being slow, and also only if you compile in debug mode. You probably don't want to unconditionally compile proc macros in release mode, as most times they probably don't get invoked thousands of times. Ideally you would probably a) invoke macros in a parallel fashion, with multiple expansion threads, and b) maybe have rustc signal to cargo somehow that there are tons of invocations of a specific macro and that it might make more sense to recompile the proc macro with optimizations turned on.