[clang] [clang][modules] Only compute affecting module maps with implicit search (PR #87849)

[Jan Svoboda via cfe-commits]

================
@@ -161,8 +161,13 @@ static TypeCode getTypeCodeForTypeClass(Type::TypeClass id) {
 
 namespace {
 
-std::set<const FileEntry *> GetAffectingModuleMaps(const Preprocessor &PP,
-                                                   Module *RootModule) {
+std::optional<std::set<const FileEntry *>>
+GetAffectingModuleMaps(const Preprocessor &PP, Module *RootModule) {
+  // Without implicit module map search, there's no good reason to know about
+  // any module maps that are not affecting.
+  if (!PP.getHeaderSearchInfo().getHeaderSearchOpts().ImplicitModuleMaps)
+    return std::nullopt;
----------------
jansvoboda11 wrote:

I wasn't aware that you're passing unused module map files to the compiler. In that case I can introduce a separate flag to control this functionality and only disable the computation of affecting module maps for our flavor of explicit modules where this is a no-op. Sounds good?

https://github.com/llvm/llvm-project/pull/87849