When macros expand

Decided

D-0074, settled by the language’s author: part of spec §18. Older text cites it as 18.74.

On syntax alone: after parsing and #if, before name resolution. A macro sees tokens, never types, so each module expands on its own, the editor parses an invocation as it is typed, and nothing depends on checking order. Chosen over expanding after checking, which would let a macro iterate T::fields but would make expansion depend on types that depend on expansion. A macro that needs a type’s fields takes its declaration (§12.11, §13.4)