Is the order of proc-macro expansion defined?

[ehuss]

Or is it intentionally left undefined (if so, that should be specified)? Proc-macros can have side-effects. If it is defined, what its it? (In source-sequence, starting from crate root, entire module processed before mod statements, or are mod processed as seen?)

[Centril]

The order is as afar as I know undefined.

cc @eddyb

[eddyb]

It's very much the opposite of defined (well, not UB, but implementation-specific non-deterministic).
This is (in part) so we can have some hope of incremental re-expansion, even with impure macros.