59dc2013e2
Extend the `DepthFirstSearch` iterator so that it can be re-used and extended with add'l start nodes. Then replace the FxHashSets of nodes we were using in the fallback analysis with a single iterator. This way we won't re-walk portions of the graph that are reached more than once, and we also do less allocation etc.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.