This allows two overriding args to interact with each other mid-line.
This was broken in 7673dfc / #1154. Before that, we duplicated the override
logic in several places.
Now we plug into the start of a new arg which allows us to do this
incrementally without making the logic complex or inefficient, thanks to
prior refactors.
Fixes#3217
This is prep for moving the derive tests. Besides organizing the test
folder for each API, this should reduce link time at the cost of
re-compiling more when a test changes.
