Doug Gregor f96240d2d8 [Macros] Do not look into macro expansions while resolving macro arguments. ...

Macros introduced a significant wrinkle into Swift's name lookup mechanism.
Specifically, when resolving names (and, really, anything else) within the
arguments to a macro expansion, name lookup must not try to expand any
macros, because doing so trivially creates a cyclic dependency amongst the
macro expansions that will be detected by the request-evaluator.

Our lookup requests don't always have enough information to answer the
question "is this part of an argument to a macro?", so we do a much simpler,
more efficient, and not-entirely-sound hack based on the request-evaluator.
Specifically, if we are in the process of resolving a macro (which is
determined by checking for the presence of a `ResolveMacroRequest` in the
request-evaluator stack), then we adjust the options used for the name
lookup request we are forming to exclude macro expansions. The evaluation
of that request will then avoid expanding any macros, and not produce any
results that involve entries in already-expanded macros. By adjusting the
request itself, we still distinguish between requests that can and cannot
look into macro expansions, so it doesn't break caching for those immediate
requests.

Over time, we should seek to replace this heuristic with a location-based
check, where we use ASTScope to determine whether we are inside a macro
argument. This existing check might still be useful because it's going to
be faster than a location-based query, but the location-based query can be
fully correct.

This addresses a class of cyclic dependencies that we've been seeing
with macros, and aligns the lookup behavior for module-level lookups
with that specified in the macros proposals. It is not fully complete
because lookup until nominal types does not yet support excluding
results from macro expansions.

2023-04-06 16:04:41 -07:00

17 KiB

Raw Blame History

View Raw