The bundling of the form of a request (e.g., the storage that makes up a request)
with the function that evaluates the request value requires us to perform
ad hoc indirection to address the AST —> Sema layering violation. For
example, ClassDecl::getSuperclass() calls through the LazyResolver (when
available) to form the appropriate request. This means that we cannot
use the the request-evaluator’s cache when LazyResolver is null, forcing
all cached state into the AST.
Provide the evaluator with a zone-based registration system, where each
request “zone” (e.g., the type checker’s requests) registers
callbacks to evaluate each kind of request within that zone. The
evaluator indirects through this table of function pointers, allowing
the request classes themselves to be available at a lower level (AST)
than the functions that perform the computation when the value isn’t
in the cache (e.g., Sema).
We are not taking advantage of the indirection yet; that’ll come in a
follow-up commit.
Simplify the definition of new request kinds by pulling the parameterization
of caching logic into SimpleRequest itself, so that the caching-related
parts of the contract cannot easily be forgotten.
