[analyzer] Improve FieldRegion descriptive name (llvm#112313)

The current implementation of MemRegion::getDescriptiveName fails for
FieldRegions whose SuperRegion is an ElementRegion. As outlined below:
```Cpp
struct val_struct { int val; };
extern struct val_struct val_struct_array[3];

void func(){
  // FieldRegion with ElementRegion as SuperRegion.
  val_struct_array[0].val;
}
```

For this special case, the expression cannot be pretty printed and must
therefore be obtained separately.

Author: T-Gruber

clang/lib/StaticAnalyzer/Core/MemRegion.cpp

@@ -722,6 +722,13 @@ std::string MemRegion::getDescriptiveName(bool UseQuotes) const {
   SmallString<50> buf;
   llvm::raw_svector_ostream os(buf);
 
+  // Enclose subject with single quotes if needed.
+  auto QuoteIfNeeded = [UseQuotes](const Twine &Subject) -> std::string {
+    if (UseQuotes)
+      return ("'" + Subject + "'").str();
+    return Subject.str();
+  };
+
   // Obtain array indices to add them to the variable name.
   const ElementRegion *ER = nullptr;
   while ((ER = R->getAs<ElementRegion>())) {
@@ -751,12 +758,20 @@ std::string MemRegion::getDescriptiveName(bool UseQuotes) const {
   }
 
   // Get variable name.
-  if (R && R->canPrintPrettyAsExpr()) {
-    R->printPrettyAsExpr(os);
-    if (UseQuotes)
-      return (llvm::Twine("'") + os.str() + ArrayIndices + "'").str();
-    else
-      return (llvm::Twine(os.str()) + ArrayIndices).str();
+  if (R) {
+    // MemRegion can be pretty printed.
+    if (R->canPrintPrettyAsExpr()) {
+      R->printPrettyAsExpr(os);
+      return QuoteIfNeeded(llvm::Twine(os.str()) + ArrayIndices);
+    }
+
+    // FieldRegion may have ElementRegion as SuperRegion.
+    if (const auto *FR = R->getAs<FieldRegion>()) {
+      std::string Super = FR->getSuperRegion()->getDescriptiveName(false);
+      if (Super.empty())
+        return "";
+      return QuoteIfNeeded(Super + "." + FR->getDecl()->getName());
+    }
   }
 
   return VariableName;

clang/unittests/StaticAnalyzer/MemRegionDescriptiveNameTest.cpp

@@ -12,7 +12,6 @@
 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/MemRegion.h"
 #include "gtest/gtest.h"
-#include <fstream>
 
 using namespace clang;
 using namespace ento;
@@ -143,4 +142,32 @@ void top() {
   EXPECT_EQ(Output, "DescriptiveNameChecker: array[x]\n");
 }
 
+TEST(MemRegionDescriptiveNameTest, FieldRegWithSuperElementReg) {
+  StringRef Code = R"cpp(
+void reportDescriptiveName(int *p);
+struct val_struct { int val; };
+extern struct val_struct val_struct_array[3];
+void top() {
+  reportDescriptiveName(&val_struct_array[0].val);
+})cpp";
+
+  std::string Output;
+  ASSERT_TRUE(runChecker(Code, Output));
+  EXPECT_EQ(Output, "DescriptiveNameChecker: val_struct_array[0].val\n");
+}
+
+TEST(MemRegionDescriptiveNameTest, FieldRegWithSuperMultidimElementReg) {
+  StringRef Code = R"cpp(
+void reportDescriptiveName(int *p);
+struct val_struct { int val; };
+extern struct val_struct val_struct_array[3][4];
+void top() {
+  reportDescriptiveName(&val_struct_array[1][2].val);
+})cpp";
+
+  std::string Output;
+  ASSERT_TRUE(runChecker(Code, Output));
+  EXPECT_EQ(Output, "DescriptiveNameChecker: val_struct_array[1][2].val\n");
+}
+
 } // namespace