[lldb] Filter duplicates in Target::GetScratchTypeSystems

`Target::GetScratchTypeSystems` returns the list of scratch TypeSystems. The
current implementation is iterating over all LanguageType values and retrieves
the respective TypeSystem for each LanguageType.

All C/C++/Obj-C LanguageTypes are however mapped to the same
ScratchTypeSystemClang instance, so the current implementation adds this single
TypeSystem instance several times to the list of TypeSystems (once for every
LanguageType that we support).

The only observable effect of this is that `SBTarget.FindTypes` for builtin
types currently queries the ScratchTypeSystemClang several times (and also adds
the same result several times).

Reviewed By: bulbazord, labath

Differential Revision: https://reviews.llvm.org/D111931

Author: Teemperor

lldb/source/Target/Target.cpp

@@ -60,6 +60,7 @@
 #include "lldb/Utility/Timer.h"
 
 #include "llvm/ADT/ScopeExit.h"
+#include "llvm/ADT/SetVector.h"
 
 #include <memory>
 #include <mutex>
@@ -2231,7 +2232,10 @@ std::vector<TypeSystem *> Target::GetScratchTypeSystems(bool create_on_demand) {
   if (!m_valid)
     return {};
 
-  std::vector<TypeSystem *> scratch_type_systems;
+  // Some TypeSystem instances are associated with several LanguageTypes so
+  // they will show up several times in the loop below. The SetVector filters
+  // out all duplicates as they serve no use for the caller.
+  llvm::SetVector<TypeSystem *> scratch_type_systems;
 
   LanguageSet languages_for_expressions =
       Language::GetLanguagesSupportingTypeSystemsForExpressions();
@@ -2247,10 +2251,10 @@ std::vector<TypeSystem *> Target::GetScratchTypeSystems(bool create_on_demand) {
                      "system available",
                      Language::GetNameForLanguageType(language));
     else
-      scratch_type_systems.emplace_back(&type_system_or_err.get());
+      scratch_type_systems.insert(&type_system_or_err.get());
   }
 
-  return scratch_type_systems;
+  return scratch_type_systems.takeVector();
 }
 
 PersistentExpressionState *

lldb/test/API/lang/c/builtin-types/TestCBuiltinTypes.py

@@ -0,0 +1,20 @@
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+class TestCase(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    @no_debug_info_test
+    def test_FindTypes_on_scratch_AST(self):
+        """
+        Tests FindTypes invoked with only LLDB's scratch AST present.
+        """
+        target = self.dbg.GetDummyTarget()
+        # There should be only one instance of 'unsigned long' in our single
+        # scratch AST. Note: FindTypes/SBType hahave no filter by language, so
+        # pick something that is unlikely to also be found in the scratch
+        # TypeSystem of other language plugins.
+        self.assertEqual(len(target.FindTypes("unsigned long")), 1)