LSC: Fix null safety issues diagnosed by Clang’s -Wnonnull and -Wnullability.

**Note**: These changes are generated by hand. Review with care.

This CL changes the `absl_internal` implementations of `AsciiStrToLower()` /
`AsciiStrToUpper()` to allow `src` to be null. This can, in fact, happen if the
`string_view` passed to the public API is empty, and the implementations handle
it correctly. I have added comments noting that `src` is allowed to be null
iff the size is zero.

`-Wnonnull` diagnoses cases where a `nullptr` literal is passed to a parameter
annotated nonnull, or where `nullptr` is returned from a function whose return
type is annotated nonnull.

`-Wnullability` diagnoses cases where nullability annotations conflict, for
example between the declaration and definition of a function.

PiperOrigin-RevId: 673846759
Change-Id: I6cf3490ce13837eba9814156c420598000ecc596

absl/strings/ascii.cc

@@ -178,9 +178,10 @@ constexpr bool AsciiInAZRange(unsigned char c) {
 }
 
 // Force-inline so the compiler won't merge the short and long implementations.
+// `src` may be null iff `size` is zero.
 template <bool ToUpper>
 ABSL_ATTRIBUTE_ALWAYS_INLINE inline constexpr void AsciiStrCaseFoldImpl(
-    absl::Nonnull<char*> dst, absl::Nonnull<const char*> src, size_t size) {
+    absl::Nonnull<char*> dst, absl::Nullable<const char*> src, size_t size) {
   // The upper- and lowercase versions of ASCII characters differ by only 1 bit.
   // When we need to flip the case, we can xor with this bit to achieve the
   // desired result. Note that the choice of 'a' and 'A' here is arbitrary. We
@@ -199,28 +200,30 @@ ABSL_ATTRIBUTE_ALWAYS_INLINE inline constexpr void AsciiStrCaseFoldImpl(
 constexpr size_t kCaseFoldThreshold = 16;
 
 // No-inline so the compiler won't merge the short and long implementations.
+// `src` may be null iff `size` is zero.
 template <bool ToUpper>
 ABSL_ATTRIBUTE_NOINLINE constexpr void AsciiStrCaseFoldLong(
-    absl::Nonnull<char*> dst, absl::Nonnull<const char*> src, size_t size) {
+    absl::Nonnull<char*> dst, absl::Nullable<const char*> src, size_t size) {
   ABSL_ASSUME(size >= kCaseFoldThreshold);
   AsciiStrCaseFoldImpl<ToUpper>(dst, src, size);
 }
 
 // Splitting to short and long strings to allow vectorization decisions
 // to be made separately in the long and short cases.
+// `src` may be null iff `size` is zero.
 template <bool ToUpper>
 constexpr void AsciiStrCaseFold(absl::Nonnull<char*> dst,
-                                absl::Nonnull<const char*> src, size_t size) {
+                                absl::Nullable<const char*> src, size_t size) {
   size < kCaseFoldThreshold ? AsciiStrCaseFoldImpl<ToUpper>(dst, src, size)
                             : AsciiStrCaseFoldLong<ToUpper>(dst, src, size);
 }
 
-void AsciiStrToLower(absl::Nonnull<char*> dst, absl::Nonnull<const char*> src,
+void AsciiStrToLower(absl::Nonnull<char*> dst, absl::Nullable<const char*> src,
                      size_t n) {
   return AsciiStrCaseFold<false>(dst, src, n);
 }
 
-void AsciiStrToUpper(absl::Nonnull<char*> dst, absl::Nonnull<const char*> src,
+void AsciiStrToUpper(absl::Nonnull<char*> dst, absl::Nullable<const char*> src,
                      size_t n) {
   return AsciiStrCaseFold<true>(dst, src, n);
 }

absl/strings/ascii.h

@@ -76,10 +76,10 @@ ABSL_DLL extern const char kToUpper[256];
 // Declaration for the array of characters to lower-case characters.
 ABSL_DLL extern const char kToLower[256];
 
-void AsciiStrToLower(absl::Nonnull<char*> dst, absl::Nonnull<const char*> src,
+void AsciiStrToLower(absl::Nonnull<char*> dst, absl::Nullable<const char*> src,
                      size_t n);
 
-void AsciiStrToUpper(absl::Nonnull<char*> dst, absl::Nonnull<const char*> src,
+void AsciiStrToUpper(absl::Nonnull<char*> dst, absl::Nullable<const char*> src,
                      size_t n);
 
 }  // namespace ascii_internal

absl/strings/ascii_test.cc

@@ -200,6 +200,10 @@ TEST(AsciiStrTo, Lower) {
   EXPECT_EQ("abcdefghijklmnopqrstuvwxyz1!a", absl::AsciiStrToLower(long_str));
   EXPECT_EQ("pqrstu", absl::AsciiStrToLower(fun()));
 
+  // An empty `string_view` specifically exercises the case where a null data
+  // pointer is passed to internal functions.
+  EXPECT_EQ("", absl::AsciiStrToLower(absl::string_view()));
+
   absl::AsciiStrToLower(&mutable_str);
   EXPECT_EQ("_`?@[{amnopqrstuvwxyz", mutable_str);
 
@@ -223,6 +227,10 @@ TEST(AsciiStrTo, Upper) {
   EXPECT_EQ("ABCDEFGHIJKLMNOPQRSTUVWXYZ1!A", absl::AsciiStrToUpper(long_str));
   EXPECT_EQ("PQRSTU", absl::AsciiStrToUpper(fun()));
 
+  // An empty `string_view` specifically exercises the case where a null data
+  // pointer is passed to internal functions.
+  EXPECT_EQ("", absl::AsciiStrToUpper(absl::string_view()));
+
   char mutable_buf[] = "Mutable";
   std::transform(mutable_buf, mutable_buf + strlen(mutable_buf),
                  mutable_buf, absl::ascii_toupper);

absl/strings/str_format_test.cc

@@ -516,7 +516,13 @@ TEST_F(FormatEntryPointTest, SNPrintF) {
   EXPECT_EQ(result, 17);
   EXPECT_EQ(std::string(buffer), "NUMBER: 1234567");
 
-  result = SNPrintF(nullptr, 0, "Just checking the %s of the output.", "size");
+  // The `output` parameter is annotated nonnull, but we want to test that
+  // it is never written to if the size is zero.
+  // Use a variable instead of passing nullptr directly to avoid a `-Wnonnull`
+  // warning.
+  char* null_output = nullptr;
+  result =
+      SNPrintF(null_output, 0, "Just checking the %s of the output.", "size");
   EXPECT_EQ(result, 37);
 }
 
@@ -545,7 +551,13 @@ TEST_F(FormatEntryPointTest, SNPrintFWithV) {
 
   std::string size = "size";
 
-  result = SNPrintF(nullptr, 0, "Just checking the %v of the output.", size);
+  // The `output` parameter is annotated nonnull, but we want to test that
+  // it is never written to if the size is zero.
+  // Use a variable instead of passing nullptr directly to avoid a `-Wnonnull`
+  // warning.
+  char* null_output = nullptr;
+  result =
+      SNPrintF(null_output, 0, "Just checking the %v of the output.", size);
   EXPECT_EQ(result, 37);
 }
 

absl/strings/string_view_test.cc

@@ -894,7 +894,11 @@ TEST(StringViewTest, NULLInput) {
   EXPECT_EQ(s.size(), 0u);
 
 #ifdef ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
-  s = absl::string_view(nullptr);
+  // The `str` parameter is annotated nonnull, but we want to test the defensive
+  // null check. Use a variable instead of passing nullptr directly to avoid a
+  // `-Wnonnull` warning.
+  char* null_str = nullptr;
+  s = absl::string_view(null_str);
   EXPECT_EQ(s.data(), nullptr);
   EXPECT_EQ(s.size(), 0u);
 
@@ -1076,8 +1080,21 @@ TEST(StringViewTest, ConstexprNullSafeStringView) {
 
 TEST(StringViewTest, ConstexprCompiles) {
   constexpr absl::string_view sp;
+  // With `-Wnonnull` turned on, there is no way to test the defensive null
+  // check in the `string_view(const char*)` constructor in a constexpr context,
+  // as the argument needs to be constexpr. The compiler will therefore always
+  // know at compile time that the argument is nullptr and complain because the
+  // parameter is annotated nonnull. We hence turn the warning off for this
+  // test.
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wnonnull"
+#endif
 #ifdef ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
   constexpr absl::string_view cstr(nullptr);
+#endif
+#if defined(__clang__)
+#pragma clang diagnostic pop
 #endif
   constexpr absl::string_view cstr_len("cstr", 4);