[clang] [-Wunsafe-buffer-usage] Fix false positives for constant cases (PR #92432)

[Ziqing Luo via cfe-commits]

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@@ -420,25 +420,64 @@ AST_MATCHER(ArraySubscriptExpr, isSafeArraySubscript) {
   //    already duplicated
   //  - call both from Sema and from here
 
-  const auto *BaseDRE =
-      dyn_cast<DeclRefExpr>(Node.getBase()->IgnoreParenImpCasts());
-  if (!BaseDRE)
+  if (const auto *BaseDRE =
+      dyn_cast<DeclRefExpr>(Node.getBase()->IgnoreParenImpCasts())) {
+    if (!BaseDRE->getDecl())
+      return false;
+    if (const auto *CATy = Finder->getASTContext().getAsConstantArrayType(
+      BaseDRE->getDecl()->getType())) {
+      if (const auto *IdxLit = dyn_cast<IntegerLiteral>(Node.getIdx())) {
+        const APInt ArrIdx = IdxLit->getValue();
+        // FIXME: ArrIdx.isNegative() we could immediately emit an error as that's a
+        // bug
+        if (ArrIdx.isNonNegative() &&
+            ArrIdx.getLimitedValue() < CATy->getLimitedSize())
+          return true;
+      }
+    }
+  }
+
+  if (const auto *BaseSL =
+      dyn_cast<StringLiteral>(Node.getBase()->IgnoreParenImpCasts())) {
+    if (const auto *CATy = Finder->getASTContext().getAsConstantArrayType(
+      BaseSL->getType())) {
+      if (const auto *IdxLit = dyn_cast<IntegerLiteral>(Node.getIdx())) {
+          const APInt ArrIdx = IdxLit->getValue();
+          // FIXME: ArrIdx.isNegative() we could immediately emit an error as that's a
+          // bug
+          if (ArrIdx.isNonNegative() &&
+              ArrIdx.getLimitedValue() < CATy->getLimitedSize())
+            return true;
+      }
+    }
+  }
+
+  return false;
+}
+
+AST_MATCHER(BinaryOperator, isSafePtrArithmetic) {
+  if (Node.getOpcode() != BinaryOperatorKind::BO_Add)
     return false;
-  if (!BaseDRE->getDecl())
+
+  const auto *LHSDRE =
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ziqingluo-90 wrote:

For both the AST matchers, I'm just curious if we really need to make sure the left-hand side is a DRE?  Could we just try to test if its' type is a constant array type regardless of its' expression kind?

https://github.com/llvm/llvm-project/pull/92432