[lldb] Fallback to implicit modules when explicit modules are missing (#8356) (#8427)

Handle the case where an explicit module path does not exist, by falling back to 
implicit modules.

The initial logic is: If any one explicit module is nonexistent, then remove all 
references to explicit modules. Additionally, remove all explicit module maps.

(cherry-picked from commit 2abff47)

Author: kastiglione

lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp

@@ -57,13 +57,15 @@
 #include "clang/Driver/Driver.h"
 
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/MC/TargetRegistry.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/TargetSelect.h"
@@ -1595,9 +1597,50 @@ void SwiftASTContext::AddExtraClangArgs(const std::vector<std::string> &source,
   }
 }
 
+namespace {
+
+bool HasNonexistentExplicitModule(const std::vector<std::string> &args) {
+  for (const std::string &arg : args) {
+    StringRef value = arg;
+    if (!value.consume_front("-fmodule-file="))
+      continue;
+    StringRef path = value;
+    size_t eq = value.find('=');
+    // The value that follows is in one of two formats:
+    //   1. ModuleName=ModulePath
+    //   2. ModulePath
+    if (eq != std::string::npos)
+      // The value appears to be in ModuleName=ModulePath forat.
+      path = value.drop_front(eq + 1);
+    // Check both path and value. This is to handle paths containing '='.
+    if (!llvm::sys::fs::exists(path) && !llvm::sys::fs::exists(value)) {
+      std::string m_description;
+      HEALTH_LOG_PRINTF("Nonexistent explicit module file %s", arg.data());
+      return true;
+    }
+  }
+  return false;
+}
+
+void RemoveExplicitModules(std::vector<std::string> &args) {
+  llvm::erase_if(args, [](const std::string &arg) {
+    if (arg == "-fno-implicit-modules" || arg == "-fno-implicit-module-maps")
+      return true;
+    StringRef s = arg;
+    if (s.starts_with("-fmodule-file=") || s.starts_with("-fmodule-map-file="))
+      return true;
+
+    return false;
+  });
+}
+
+} // namespace
+
 void SwiftASTContext::AddExtraClangArgs(const std::vector<std::string> &ExtraArgs) {
   swift::ClangImporterOptions &importer_options = GetClangImporterOptions();
   AddExtraClangArgs(ExtraArgs, importer_options.ExtraArgs);
+  if (HasNonexistentExplicitModule(importer_options.ExtraArgs))
+    RemoveExplicitModules(importer_options.ExtraArgs);
 }
 
 void SwiftASTContext::AddUserClangArgs(TargetProperties &props) {

lldb/test/API/lang/swift/explicit_modules/implicit_fallback/Makefile

@@ -0,0 +1,4 @@
+SWIFT_SOURCES := main.swift
+SWIFT_ENABLE_EXPLICIT_MODULES := YES
+USE_PRIVATE_MODULE_CACHE := YES
+include Makefile.rules

lldb/test/API/lang/swift/explicit_modules/implicit_fallback/TestSwiftExplicitModulesImplicitFallback.py

@@ -0,0 +1,30 @@
+import shutil
+import lldb
+from lldbsuite.test.decorators import *
+import lldbsuite.test.lldbtest as lldbtest
+import lldbsuite.test.lldbutil as lldbutil
+
+
+class TestCase(lldbtest.TestBase):
+    @swiftTest
+    @skipIf(oslist=["linux"], bugnumber="rdar://124691219")
+    def test_missing_explicit_modules(self):
+        """Test missing explicit Swift modules and fallback to implicit modules."""
+        self.build()
+
+        # This test verifies the case where explicit modules are missing.
+        # Remove explicit modules from their place in the module cache.
+        mod_cache = self.getBuildArtifact("private-module-cache")
+        shutil.rmtree(mod_cache)
+
+        lldbutil.run_to_source_breakpoint(
+            self, "Set breakpoint here", lldb.SBFileSpec("main.swift")
+        )
+
+        log = self.getBuildArtifact("types.log")
+        self.runCmd(f"log enable lldb types -f '{log}'")
+
+        self.expect("expression c", substrs=["hello implicit fallback"])
+
+        self.filecheck(f"platform shell cat {log}", __file__)
+        # CHECK: Nonexistent explicit module file

lldb/test/API/lang/swift/explicit_modules/implicit_fallback/main.swift

@@ -0,0 +1,10 @@
+class Class {
+  var msg : String = "hello implicit fallback"
+}
+
+func main() {
+  let c = Class()
+  print(c) // Set breakpoint here
+}
+
+main()