Adds support for "%v" in absl::StrFormat and related functions for string-like…

Adds support for "%v" in absl::StrFormat and related functions for string-like types (support for other builtin types will follow in future changes). Rather than specifying %s for strings, users may specify %v and have the format specifier deduced. Notably, %v does not work for `const char*` because we cannot be certain if %s or %p was intended (nor can we be certain if the `const char*` was properly null-terminated). If you have a `const char*` you know is null-terminated and would like to work with %v, please wrap it in a `string_view` before using it.

PiperOrigin-RevId: 471321055
Change-Id: Ifbb50082d301baecc7edc277975f12e7ad3ecc8a

absl/strings/internal/str_format/arg.h

@@ -110,8 +110,8 @@ constexpr FormatConversionCharSet ExtractCharSet(ArgConvertResult<C>) {
   return C;
 }
 
-using StringConvertResult =
-    ArgConvertResult<FormatConversionCharSetInternal::s>;
+using StringConvertResult = ArgConvertResult<FormatConversionCharSetUnion(
+    FormatConversionCharSetInternal::s, FormatConversionCharSetInternal::v)>;
 ArgConvertResult<FormatConversionCharSetInternal::p> FormatConvertImpl(
     VoidPtr v, FormatConversionSpecImpl conv, FormatSinkImpl* sink);
 

absl/strings/internal/str_format/bind.h

@@ -235,8 +235,9 @@ class StreamedWrapper {
 
  private:
   template <typename S>
-  friend ArgConvertResult<FormatConversionCharSetInternal::s> FormatConvertImpl(
-      const StreamedWrapper<S>& v, FormatConversionSpecImpl conv,
+  friend ArgConvertResult<FormatConversionCharSetUnion(
+      FormatConversionCharSetInternal::s, FormatConversionCharSetInternal::v)>
+  FormatConvertImpl(const StreamedWrapper<S>& v, FormatConversionSpecImpl conv,
                     FormatSinkImpl* out);
   const T& v_;
 };

absl/strings/internal/str_format/checker_test.cc

@@ -39,7 +39,7 @@ std::string ConvToString(FormatConversionCharSet conv) {
 
 TEST(StrFormatChecker, ArgumentToConv) {
   FormatConversionCharSet conv = ArgumentToConv<std::string>();
-  EXPECT_EQ(ConvToString(conv), "s");
+  EXPECT_EQ(ConvToString(conv), "sv");
 
   conv = ArgumentToConv<const char*>();
   EXPECT_EQ(ConvToString(conv), "sp");

absl/strings/internal/str_format/extension.h

@@ -169,7 +169,7 @@ inline std::ostream& operator<<(std::ostream& os, Flags v) {
   X_VAL(f) X_SEP X_VAL(F) X_SEP X_VAL(e) X_SEP X_VAL(E) X_SEP \
   X_VAL(g) X_SEP X_VAL(G) X_SEP X_VAL(a) X_SEP X_VAL(A) X_SEP \
   /* misc */ \
-  X_VAL(n) X_SEP X_VAL(p)
+  X_VAL(n) X_SEP X_VAL(p) X_SEP X_VAL(v)
 // clang-format on
 
 // This type should not be referenced, it exists only to provide labels
@@ -191,7 +191,7 @@ struct FormatConversionCharInternal {
     c, s,                    // text
     d, i, o, u, x, X,        // int
     f, F, e, E, g, G, a, A,  // float
-    n, p,                    // misc
+    n, p, v,                    // misc
     kNone
   };
   // clang-format on

absl/strings/internal/str_format/parser.cc

@@ -56,7 +56,7 @@ ABSL_CONST_INIT const ConvTag kTags[256] = {
     CC::X,  {},    {},    {},    {},    {},     {},    {},     // XYZ[\]^_
     {},     CC::a, {},    CC::c, CC::d, CC::e,  CC::f, CC::g,  // `abcdefg
     LM::h,  CC::i, LM::j, {},    LM::l, {},     CC::n, CC::o,  // hijklmno
-    CC::p,  LM::q, {},    CC::s, LM::t, CC::u,  {},    {},     // pqrstuvw
+    CC::p,  LM::q, {},    CC::s, LM::t, CC::u,  CC::v, {},     // pqrstuvw
     CC::x,  {},    LM::z, {},    {},    {},     {},    {},     // xyz{|}!
     {},     {},    {},    {},    {},    {},     {},    {},     // 80-87
     {},     {},    {},    {},    {},    {},     {},    {},     // 88-8f

absl/strings/str_format.h

@@ -637,7 +637,7 @@ enum class FormatConversionChar : uint8_t {
   c, s,                    // text
   d, i, o, u, x, X,        // int
   f, F, e, E, g, G, a, A,  // float
-  n, p                     // misc
+  n, p, v                  // misc
 };
 // clang-format on
 
@@ -757,6 +757,7 @@ enum class FormatConversionCharSet : uint64_t {
   // misc
   n = str_format_internal::FormatConversionCharToConvInt('n'),
   p = str_format_internal::FormatConversionCharToConvInt('p'),
+  v = str_format_internal::FormatConversionCharToConvInt('v'),
 
   // Used for width/precision '*' specification.
   kStar = static_cast<uint64_t>(