Auto merge of #123240 - compiler-errors:assert-args-compat, r=fmease

Assert that args are actually compatible with their generics, rather than just their count

Right now we just check that the number of args is right, rather than actually checking the kinds. Uplift a helper fn that I wrote from trait selection to do just that. Found a couple bugs along the way.

r? `@lcnr` or `@fmease` (or anyone really lol)

Author: bors