[clang] [-Wunterminated-string-initialization] Handle C string literals ending with explicit '\0' (PR #143487)

[Eli Friedman via cfe-commits]

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@@ -260,6 +260,12 @@ static void CheckStringInit(Expr *Str, QualType &DeclT, const ArrayType *AT,
              diag::ext_initializer_string_for_char_array_too_long)
           << Str->getSourceRange();
     else if (StrLength - 1 == ArrayLen) {
+      // If the string literal is null-terminated explicitly, e.g., `char a[4] =
+      // "ABC\0"`, there should be no warning:
+      if (const auto *SL = dyn_cast<StringLiteral>(Str->IgnoreParens());
+          SL && SL->getLength() > 0 &&
+          SL->getCodeUnit(SL->getLength() - 1) == 0)
+        return;
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efriedma-quic wrote:

I think the early return suppresses warn_initializer_string_for_char_array_too_long_for_cpp warnings?

https://github.com/llvm/llvm-project/pull/143487