If I recall the reason is "rustc's representation of the AST is an implementation detail and unstable, thus macro code may not rely on it (and it will not be ex

If I recall the reason is "rustc's representation of the AST is an implementation detail and unstable, thus macro code may not rely on it (and it will not be exposed)". Hence "syn" came along and provided a stable API and format. "quote" is the inverse function, turning an AST back into tokens.