[-Wunterminated-string-initialization] Handle C string literals ending with explicit '\0'

In C, a char array needs no "nonstring" attribute, if its initializer
is a string literal that 1) explicitly ends with '\0' and 2) fits in
the array after a possible truncation.

For example
`char a[4] = "ABC\0"; // fine, needs no "nonstring" attr`

rdar://152506883

Author: ziqingluo-90

clang/lib/Sema/SemaInit.cpp

@@ -260,6 +260,11 @@ 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 warn:
+      if (const auto *SL = dyn_cast<StringLiteral>(Str->IgnoreParens()))
+        if (SL->isOrdinary() && SL->getBytes().back() == 0)
+          return;
       // If the entity being initialized has the nonstring attribute, then
       // silence the "missing nonstring" diagnostic. If there's no entity,
       // check whether we're initializing an array of arrays; if so, walk the

clang/test/Sema/attr-nonstring_safe.c

@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wunterminated-string-initialization %s -x c
+// RUN: %clang_cc1 -fsyntax-only -verify -Wunterminated-string-initialization %s -x c++
+
+
+// In C, the following examples are fine:
+#if __cplusplus
+char foo[3] = "fo\0"; // expected-error {{initializer-string for char array is too long, array size is 3 but initializer has size 4 (including the null terminating character)}}
+
+struct S {
+  char buf[3];
+  char fub[3];
+} s = { "ba\0", "bo\0" }; // expected-error 2{{initializer-string for char array is too long, array size is 3 but initializer has size 4 (including the null terminating character)}}
+
+signed char scfoo[3] = "fo\0"; // expected-error {{initializer-string for char array is too long, array size is 3 but initializer has size 4 (including the null terminating character)}}
+unsigned char ucfoo[3] = "fo\0"; // expected-error {{initializer-string for char array is too long, array size is 3 but initializer has size 4 (including the null terminating character)}}
+
+#else
+//expected-no-diagnostics
+char foo[3] = "fo\0";
+
+struct S {
+  char buf[3];
+  char fub[3];
+} s = { "ba\0", "bo\0" };
+
+signed char scfoo[3] = "fo\0";
+unsigned char ucfoo[3] = "fo\0";
+#endif