[clang][modules] Lazily load by name lookups in module maps (#132853)

Instead of eagerly populating the `clang::ModuleMap` when looking up a
module by name, this patch changes `HeaderSearch` to only load the
modules that are actually used.

This introduces `ModuleMap::findOrLoadModule` which will load modules
from parsed but not loaded module maps. This cannot be used anywhere
that the module loading code calls into as it can create infinite
recursion.

This currently just reparses module maps when looking up a module by
header. This is fine as redeclarations are allowed from the same file,
but future patches will also make looking up a module by header lazy.

This patch changes the shadow.m test to use explicitly built modules and
`#import`. This test and the shadow feature are very brittle and do not
work in general. The test relied on pcm files being left behind by prior
failing clang invocations that were then reused by the last invocation.
If you clean the cache then the last invocation will always fail. This
is because the input module map and the `-fmodule-map-file=` module map
are parsed in the same module scope, and `-fmodule-map-file=` is
forwarded to implicit module builds. That means you are guaranteed to
hit a module redeclaration error if the TU actually imports the module
it is trying to shadow.

This patch changes when we load A2's module map to after the `A` module
has been loaded, which sets the `IsFromModuleFile` bit on `A`. This
means that A2's `A` is skipped entirely instead of creating a shadow
module, and we get textual inclusion. It is possible to construct a case
where this would happen before this patch too.

An upcoming patch in this series will rework shadowing to work in the
general case, but that's only possible once header -> module lookup is
lazy too.

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index 4bba571..bf1b76b 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -395,7 +395,7 @@ Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
   case ModuleDeclKind::PartitionInterface: {
     // We can't have parsed or imported a definition of this module or parsed a
     // module map defining it already.
-    if (auto *M = Map.findModule(ModuleName)) {
+    if (auto *M = Map.findOrLoadModule(ModuleName)) {
       Diag(Path[0].getLoc(), diag::err_module_redefinition) << ModuleName;
       if (M->DefinitionLoc.isValid())
         Diag(M->DefinitionLoc, diag::note_prev_module_definition);