Giving Sly syntax data from the Lisp side?

[swapneils]

Is there a way to give Sly completion and syntax-checking knowledge from the Lisp side?

For instance, if I have a library with a (mylang (&key one two) &body body) macro where one and two each only allow a certain set of symbols, and body allows only a subset of possible functions/macros, could I tell Slynk to only give completions for those restricted values rather than all symbols in the Lisp image, and point out errors when user code doesn't use that restricted set?

If not, I guess this would be a feature request rather than a General discussion.
There's some tricky edge cases, for example a (myotherlang &body otherbody) macro where every (mylang () body) form in otherbody has body transformed from CL to mylang code.
But I expect a sufficiently flexible configuration setup would allow the library authors to be the ones taking care of those edges. And if they're restricted to configuring only their own forms (plus arguments to those), and users can opt-out with a Emacs/CL configuration, mistakes from one library author shouldn't impact functionality elsewhere in the code.