rust/src/test/ui/functions-closures/closure-expected-type/README.md

Some tests targeted at how we deduce the types of closure arguments. This process is a result of some heuristics aimed at combining the expected type we have with the actual types that we get from inputs. This investigation was kicked off by #38714, which revealed some pretty deep flaws in the ad-hoc way that we were doing things before.

See also src/test/ui/closure-expected-type.