[Swift-ObjC-Interop] Speculative cyclic lookup fix

[NuriAmari]

The lookup to resolve an ObjC forward declaration
to its potential native Swift definition within
a mixed module is becoming cyclic somehow.

The old uncached lookup still works, so it seems
this is an issue with cache invalidation somehow.
Until the root of the issue is resolved, never mark the cache as populated.
This is the same behaviour as before the cached lookup was introduced, but
we can easily turn the caching back on later.

[NuriAmari]

@swift-ci Please test

[NuriAmari]

@swift-ci Please test

[NuriAmari]

@swift-ci Please test

[slavapestov]

Can we do this by writing out a lookup table instead of calling getTopLevelDeclsWhereAttributesMatch(), which performs an O(n) walk and a lot of unnecessary deserialization?

[NuriAmari]

Can we do this by writing out a lookup table instead of calling getTopLevelDeclsWhereAttributesMatch(), which performs an O(n) walk and a lot of unnecessary deserialization?

In some sense that was the old behavior, but apparently it caused infinite recursion when compiling some Apple framework. I suspect that would work, but we apparently need to re-populate the lookup table at some point, but I don't have a repro for the issue so I can't figure that out.

For what its worth, this change is a partial revert of #61606, so the lookup strategy restored by this patch has been in use for quite some time.

[slavapestov]

In some sense that was the old behavior, but apparently it caused infinite recursion when compiling some Apple framework. I suspect that would work, but we apparently need to re-populate the lookup table at some point, but I don't have a repro for the issue so I can't figure that out.

I debugged and diagnosed the request cycle a few weeks ago. It was caused by excessive deserialization. The problem is not the caching, it's the fact that ultimately this code path was triggering deserialization of large numbers of top-level decls in the cache miss case.

You need to find a way of doing this without calling getTopLevelDeclsWhereAttributesMatch(). What I'm suggesting is to serialize an on-disk lookup table mapping identifiers to declarations, so that you can avoid the O(n) walk that triggers deserialization of all top-level declarations when you want to find a single identifier.

[slavapestov]

Oh I see, getTopLevelDeclsWhereAttributesMatch() does skip some work if the predicate fails, and the predicate is narrower with this PR. My apologies. We should still find a way of doing this that doesn't involve getTopLevelDeclsWhereAttributesMatch(), but maybe this fix will address the issue.

[NuriAmari]

Oh I see, getTopLevelDeclsWhereAttributesMatch() does skip some work if the predicate fails, and the predicate is narrower with this PR. My apologies. We should still find a way of doing this that doesn't involve getTopLevelDeclsWhereAttributesMatch(), but maybe this fix will address the issue.

Right. It worked this way up until #61606, landed (including Swift5.8), so I imagine it should continue to work.

Can you also approve the pick into Swift5.9: #66195? I will work on the proposed serialized lookup table, but in the immediate future I am trying to as best possible make sure the change remains in Swift5.9.