Auto merge of #2484 - alexcrichton:fix-bad-backtrack, r=brson

Fix caching features across backtracking

In the local loop during resolution all variables need to be reset whenever we
backtrack up a frame, but currently the `method` and `features` set are
accidentally not reset whenever we backtrack. Calculate the `method` later and
cache `features` in each frame so we can properly backtrack.

Closes #2472