Try harder to find an element we can analyze

Add logic to handle MemberExprs, so we can attempt to find the size
information for the struct member variables. This is probably still
incomplete in terms of the full space of expressions, but eh, just needs
to be better, not perfect.

Author: mxms

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -439,8 +439,16 @@ AST_MATCHER(ArraySubscriptExpr, isSafeArraySubscript) {
       dyn_cast<StringLiteral>(Node.getBase()->IgnoreParenImpCasts());
   uint64_t size;
 
-  if (!BaseDRE && !SLiteral)
-    return false;
+  if (!BaseDRE && !SLiteral) {
+    // Try harder to find something that looks like a DeclRefExpr
+    const auto *Member = dyn_cast<MemberExpr>(Node.getBase()->IgnoreParenImpCasts());
+    if (!Member) return false;
+
+    const auto *Value = Finder->getASTContext().getAsConstantArrayType(Member->getMemberDecl()->getType());
+    if (!Value) return false;
+
+    size = Value->getLimitedSize();
+  }
 
   if (BaseDRE) {
     if (!BaseDRE->getDecl())