> Could the plugin be modified to print warnings about pointers to non-const
> instances being passed to functions that take pointers to const instances ?
it's surely possible but i'm not sure this is what you really want as such typecasts are allowed by C and extensively used everywhere. now i assume you'd really want this detection for ops structures only in which case the problem is how the plugin would know of them (i assume you wouldn't want to use the current 'constify by default' approach). you could make use of the do_const attribute (without calling constify_type) then in the FINISH_TYPE callback you can check whether the pointed to structure is already const or not (and has the do_const attr, although on second thought, i forget now whether attrs are carried with types or have to be acted upon in the attr callback).
with all that said, i would not go the 'check the pointer' route but rather i'd constify the variable instances instead (but not function arguments) and then rely on gcc's existing warning system (if you take the address of a const object to initialize a ptr to the non-const type then you'll get a warning).