A request is intended to be a pure function of its inputs. That function could, in theory, fail. In practice, there were basically no requests taking advantage of this ability - the few that were using it to explicitly detect cycles can just return reasonable defaults instead of forwarding the error on up the stack.
This is because cycles are checked by *the Evaluator*, and are unwound by the Evaluator.
Therefore, restore the idea that the evaluate functions are themselves pure, but keep the idea that *evaluation* of those requests may fail. This model enables the best of both worlds: we not only keep the evaluator flexible enough to handle future use cases like cancellation and diagnostic invalidation, but also request-based dependencies using the values computed at the evaluation points. These aforementioned use cases would use the llvm::Expected interface and the regular evaluation-point interface respectively.
IDE functionality needs some internal type checking logics, e.g. checking
whether an extension is applicable to a concrete type. We used to directly
expose an header from sema called IDETypeChecking.h so that IDE functionalities
could invoke these APIs. The goal of the commit and following commits is to
expose evaluator requests instead of directly exposing function entry points from
sema so that we could later move IDETypeChecking.h to libIDE and implement these functions
by internally evaluating these requests.
Resolving cursor info is a necessary step both when checking the availability
of a refactoring kind and performing the refactoring action. These
two steps are two high-level sourcekitd requests issued from IDE.
By caching the result of the resolved cursor info in the evaluator of
the ASTContext, we can reuse the cursor info result across these separate
IDE requests.
