Rollup merge of #136497 - Jarcho:fn_ctxt, r=compiler-errors

Report generic mismatches when calling bodyless trait functions

Don't know if there's an open issue for this. Just happened to notice this when working in that area.

The awkward extra spans added to the diagnostics of some tests (e.g. `trait-with-missing-associated-type-restriction`) is consistent with what happens for normal functions. Should probably be removed since that span doesn't seem to note anything useful.

First and third commit are both cleanups removing some unnecessary work. Second commit has the actual fix.

fixes #135124

Author: matthiaskrgr