Adds `AbslStringify` support to `absl::Hex` and `absl::Dec`

`absl::Hex` and `absl::Dec` are now stringifiable through `AbslStringify`. This means that they can be passed to logging, `absl::StrFormat`, `absl::StrCat` and `absl::Substitute`. Note that this change will break unsupported usages of `absl::AlphaNum`. `absl::AlphaNum` is not intended to be instantiated as a stack variable due to lifetime issues. Unsupported usages include:

* constructing an `absl::AlphaNum` as a local variable.
  * For example, `absl::AlphaNum a = absl::Hex(...);`
* declaring an `absl::AlphaNum` as a member variable

Usage of `absl::AlphaNum` as a function parameter will continue to be supported.

PiperOrigin-RevId: 505158534
Change-Id: Idecdf0b3d137482e86b393c7480229343d68eb36

2 files changed, 55 insertions, 52 deletions

diff --git a/absl/strings/str_cat.cc b/absl/strings/str_cat.cc
index 114a2ff2..6c198f85 100644
--- a/absl/strings/str_cat.cc
+++ b/absl/strings/str_cat.cc
@@ -30,55 +30,6 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-AlphaNum::AlphaNum(Hex hex) {
-  static_assert(numbers_internal::kFastToBufferSize >= 32,
-                "This function only works when output buffer >= 32 bytes long");
-  char* const end = &digits_[numbers_internal::kFastToBufferSize];
-  auto real_width =
-      absl::numbers_internal::FastHexToBufferZeroPad16(hex.value, end - 16);
-  if (real_width >= hex.width) {
-    piece_ = absl::string_view(end - real_width, real_width);
-  } else {
-    // Pad first 16 chars because FastHexToBufferZeroPad16 pads only to 16 and
-    // max pad width can be up to 20.
-    std::memset(end - 32, hex.fill, 16);
-    // Patch up everything else up to the real_width.
-    std::memset(end - real_width - 16, hex.fill, 16);
-    piece_ = absl::string_view(end - hex.width, hex.width);
-  }
-}
-
-AlphaNum::AlphaNum(Dec dec) {
-  assert(dec.width <= numbers_internal::kFastToBufferSize);
-  char* const end = &digits_[numbers_internal::kFastToBufferSize];
-  char* const minfill = end - dec.width;
-  char* writer = end;
-  uint64_t value = dec.value;
-  bool neg = dec.neg;
-  while (value > 9) {
-    *--writer = '0' + (value % 10);
-    value /= 10;
-  }
-  *--writer = '0' + static_cast<char>(value);
-  if (neg) *--writer = '-';
-
-  ptrdiff_t fillers = writer - minfill;
-  if (fillers > 0) {
-    // Tricky: if the fill character is ' ', then it's <fill><+/-><digits>
-    // But...: if the fill character is '0', then it's <+/-><fill><digits>
-    bool add_sign_again = false;
-    if (neg && dec.fill == '0') {  // If filling with '0',
-      ++writer;                    // ignore the sign we just added
-      add_sign_again = true;       // and re-add the sign later.
-    }
-    writer -= fillers;
-    std::fill_n(writer, fillers, dec.fill);
-    if (add_sign_again) *--writer = '-';
-  }
-
-  piece_ = absl::string_view(writer, static_cast<size_t>(end - writer));
-}
-
 // ----------------------------------------------------------------------
 // StrCat()
 //    This merges the given strings or integers, with no delimiter. This
diff --git a/absl/strings/str_cat.h b/absl/strings/str_cat.h
index 8b68dac0..fcd48c4e 100644
--- a/absl/strings/str_cat.h
+++ b/absl/strings/str_cat.h
@@ -87,8 +87,10 @@
 #ifndef ABSL_STRINGS_STR_CAT_H_
 #define ABSL_STRINGS_STR_CAT_H_
 
+#include <algorithm>
 #include <array>
 #include <cstdint>
+#include <cstring>
 #include <string>
 #include <type_traits>
 #include <utility>
@@ -202,6 +204,27 @@ struct Hex {
   explicit Hex(Pointee* v, PadSpec spec = absl::kNoPad)
       : Hex(spec, reinterpret_cast<uintptr_t>(v)) {}
 
+  template <typename S>
+  friend void AbslStringify(S& sink, Hex hex) {
+    static_assert(
+        numbers_internal::kFastToBufferSize >= 32,
+        "This function only works when output buffer >= 32 bytes long");
+    char buffer[numbers_internal::kFastToBufferSize];
+    char* const end = &buffer[numbers_internal::kFastToBufferSize];
+    auto real_width =
+        absl::numbers_internal::FastHexToBufferZeroPad16(hex.value, end - 16);
+    if (real_width >= hex.width) {
+      sink.Append(absl::string_view(end - real_width, real_width));
+    } else {
+      // Pad first 16 chars because FastHexToBufferZeroPad16 pads only to 16 and
+      // max pad width can be up to 20.
+      std::memset(end - 32, hex.fill, 16);
+      // Patch up everything else up to the real_width.
+      std::memset(end - real_width - 16, hex.fill, 16);
+      sink.Append(absl::string_view(end - hex.width, hex.width));
+    }
+  }
+
  private:
   Hex(PadSpec spec, uint64_t v)
       : value(v),
@@ -236,6 +259,38 @@ struct Dec {
                                              : spec - absl::kZeroPad2 + 2),
         fill(spec >= absl::kSpacePad2 ? ' ' : '0'),
         neg(v < 0) {}
+
+  template <typename S>
+  friend void AbslStringify(S& sink, Dec dec) {
+    assert(dec.width <= numbers_internal::kFastToBufferSize);
+    char buffer[numbers_internal::kFastToBufferSize];
+    char* const end = &buffer[numbers_internal::kFastToBufferSize];
+    char* const minfill = end - dec.width;
+    char* writer = end;
+    uint64_t val = dec.value;
+    while (val > 9) {
+      *--writer = '0' + (val % 10);
+      val /= 10;
+    }
+    *--writer = '0' + static_cast<char>(val);
+    if (dec.neg) *--writer = '-';
+
+    ptrdiff_t fillers = writer - minfill;
+    if (fillers > 0) {
+      // Tricky: if the fill character is ' ', then it's <fill><+/-><digits>
+      // But...: if the fill character is '0', then it's <+/-><fill><digits>
+      bool add_sign_again = false;
+      if (dec.neg && dec.fill == '0') {  // If filling with '0',
+        ++writer;                    // ignore the sign we just added
+        add_sign_again = true;       // and re-add the sign later.
+      }
+      writer -= fillers;
+      std::fill_n(writer, fillers, dec.fill);
+      if (add_sign_again) *--writer = '-';
+    }
+
+    sink.Append(absl::string_view(writer, static_cast<size_t>(end - writer)));
+  }
 };
 
 // -----------------------------------------------------------------------------
@@ -283,9 +338,6 @@ class AlphaNum {
   AlphaNum(double f)  // NOLINT(runtime/explicit)
       : piece_(digits_, numbers_internal::SixDigitsToBuffer(f, digits_)) {}
 
-  AlphaNum(Hex hex);  // NOLINT(runtime/explicit)
-  AlphaNum(Dec dec);  // NOLINT(runtime/explicit)
-
   template <size_t size>
   AlphaNum(  // NOLINT(runtime/explicit)
       const strings_internal::AlphaNumBuffer<size>& buf