From ef6827158b8a66676d4ca4f70da2610416bc5a0b Mon Sep 17 00:00:00 2001
From: Adrian Prantl <aprantl@apple.com>
Date: Fri, 19 Dec 2025 16:00:58 -0800
Subject: [PATCH] [LLDB] Add an opt-out setting for the explicit-only import
 mode

rdar://166747831
---
 lldb/include/lldb/Target/Target.h             |  2 ++
 .../TypeSystem/Swift/SwiftASTContext.cpp      |  3 +++
 lldb/source/Target/Target.cpp                 | 13 ++++++++++++
 lldb/source/Target/TargetProperties.td        |  3 +++
 ...estSwiftExplicitModulesImplicitFallback.py | 21 +++++++++++++++++++
 5 files changed, 42 insertions(+)

diff --git a/lldb/include/lldb/Target/Target.h b/lldb/include/lldb/Target/Target.h
index 3d92629e4da1f..9834d3f61b724 100644
--- a/lldb/include/lldb/Target/Target.h
+++ b/lldb/include/lldb/Target/Target.h
@@ -206,6 +206,8 @@ class TargetProperties : public Properties {
 
   bool GetSwiftAllowExplicitModules() const;
 
+  bool GetSwiftAllowImplicitModules() const;
+
   AutoBool GetSwiftPCMValidation() const;
 
   bool GetSwiftUseTasksPlugin() const;
diff --git a/lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp b/lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp
index 7de8d4bb4cc20..682ff2fea1983 100644
--- a/lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp
+++ b/lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp
@@ -9421,6 +9421,9 @@ bool SwiftASTContext::GetCompileUnitImportsImpl(
   // If EBM is enabled, disable implicit modules during contextual imports.
   // fixme nullptr!
   bool turn_off_implicit = m_has_explicit_modules;
+  if (Target::GetGlobalProperties().GetSwiftAllowImplicitModules())
+    turn_off_implicit = false;
+  
   auto reset = llvm::make_scope_exit([&] {
     if (turn_off_implicit) {
       LOG_PRINTF(GetLog(LLDBLog::Types), "Turning on implicit modules");
diff --git a/lldb/source/Target/Target.cpp b/lldb/source/Target/Target.cpp
index c776d5394220d..fcaf215b141fc 100644
--- a/lldb/source/Target/Target.cpp
+++ b/lldb/source/Target/Target.cpp
@@ -4708,6 +4708,19 @@ bool TargetProperties::GetSwiftAllowExplicitModules() const {
   return true;
 }
 
+bool TargetProperties::GetSwiftAllowImplicitModules() const {
+  const Property *exp_property =
+      m_collection_sp->GetPropertyAtIndex(ePropertyExperimental);
+  OptionValueProperties *exp_values =
+      exp_property->GetValue()->GetAsProperties();
+  if (exp_values)
+    return exp_values
+        ->GetPropertyAtIndexAs<bool>(ePropertySwiftAllowImplicitModules)
+        .value_or(false);
+
+  return false;
+}
+
 AutoBool TargetProperties::GetSwiftPCMValidation() const {
   const Property *exp_property =
       m_collection_sp->GetPropertyAtIndex(ePropertyExperimental);
diff --git a/lldb/source/Target/TargetProperties.td b/lldb/source/Target/TargetProperties.td
index 7d7263ece5c48..796d17b6323f6 100644
--- a/lldb/source/Target/TargetProperties.td
+++ b/lldb/source/Target/TargetProperties.td
@@ -26,6 +26,9 @@ let Definition = "target_experimental" in {
   def SwiftAllowExplicitModules: Property<"swift-allow-explicit-modules", "Boolean">,
     DefaultTrue,
     Desc<"Allows explicit module flags to be passed through to ClangImporter.">;
+  def SwiftAllowImplicitModules: Property<"swift-allow-implicit-modules", "Boolean">,
+    DefaultFalse,
+    Desc<"Allows implicit module imports in an explicitly-built target.">;
   def SwiftPCMValidation: Property<"swift-pcm-validation", "Enum">,
     Global,
     DefaultEnumValue<"llvm::to_underlying(AutoBool::Auto)">,
diff --git a/lldb/test/API/lang/swift/explicit_modules/implicit_fallback/TestSwiftExplicitModulesImplicitFallback.py b/lldb/test/API/lang/swift/explicit_modules/implicit_fallback/TestSwiftExplicitModulesImplicitFallback.py
index f9acedd9fee22..66f42bf8cd3b2 100644
--- a/lldb/test/API/lang/swift/explicit_modules/implicit_fallback/TestSwiftExplicitModulesImplicitFallback.py
+++ b/lldb/test/API/lang/swift/explicit_modules/implicit_fallback/TestSwiftExplicitModulesImplicitFallback.py
@@ -6,6 +6,7 @@
 
 
 class TestCase(lldbtest.TestBase):
+    NO_DEBUG_INFO_TESTCASE = True
     @swiftTest
     def test_missing_explicit_modules(self):
         """Test missing explicit Swift modules and fallback to implicit modules."""
@@ -53,3 +54,23 @@ def test_sanity(self):
         # CHECK-SANITY: Explicit modules : true
         # CHECK-SANITY: Turning off implicit modules
         # CHECK-SANITY: Turning on implicit modules
+
+    @swiftTest
+    def test_setting(self):
+        """Check the normal behavior."""
+        self.build()
+
+        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("settings set target.experimental.swift-allow-implicit-modules true")
+
+        self.expect("expression c")
+
+        self.filecheck(f"platform shell cat {log}", __file__,
+                       '--check-prefix=CHECK-SETTING')
+        # CHECK-SETTING: Explicit modules : true
+        # CHECK-SETTING-NOT: Turning {{.*}} implicit modules