Export of internal Abseil changes

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a9ac6567c0933d786d68c10011e3f3ff9deedf89 by Greg Falcon <gfalcon@google.com>:

Add absl::FunctionRef, a type analogous to the proposed C++23 std::function_ref.

Like std::function, FunctionRef can be used to type-erase any callable (invokable) object.  However, FunctionRef works by reference: it does not store a copy of the type-erased object.  If the wrapped object is destroyed before the FunctionRef, the reference becomes dangling.

FunctionRef relates to std::function in much the same way that string_view relates to std::string.

Because of these limitations, FunctionRef is best used only as a function argument type, and only where the function will be invoked immediately (rather than saved for later use).  When `const std::function<...>&` is used in this way, `absl::FunctionRef<...>` is a better-performing replacement.

PiperOrigin-RevId: 275484044

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1f7c4df3760f8b93e5a5baf40b070eca1d3f4c98 by Abseil Team <absl-team@google.com>:

Add FastHexToBufferZeroPad16() function for blazingly fast hex encoding of uint64_t.

PiperOrigin-RevId: 275420901

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08d48ac004eba57cf2f1ada827181a2995f74807 by Abseil Team <absl-team@google.com>:

Avoid applying the workaround for MSVC's static initialization problems when using clang-cl.

PiperOrigin-RevId: 275366326

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40be82bd2b34670b5458c0a72a0475086153c2d6 by Abseil Team <absl-team@google.com>:

Added comments to SimpleAtof()/SimpleAtod() that clarify that they
always use the "C" locale, unlike the standard functions strtod()
and strtof() referenced now in the comments.

PiperOrigin-RevId: 275355815

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086779dacb3f6f2b3ab59947e94e79046bdb1fe1 by Jorg Brown <jorg@google.com>:

Move the hex conversion table used by escaping.cc into numbers.h so
that other parts of Abseil can more efficiently access it.

PiperOrigin-RevId: 275331251

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3c4ed1b04e55d96a40cbe70fb70929ffbb0c0432 by Abseil Team <absl-team@google.com>:

Avoid applying the workaround for MSVC's static initialization problems when using clang-cl.

PiperOrigin-RevId: 275323858

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56ceb58ab688c3761978308609b09a1ac2739c9a by Derek Mauro <dmauro@google.com>:

Add script for testing on Alpine Linux (for musl test coverage)

PiperOrigin-RevId: 275321244
GitOrigin-RevId: a9ac6567c0933d786d68c10011e3f3ff9deedf89
Change-Id: I39799fa03768ddb44f3166200c860e1da4461807

1 files changed, 104 insertions, 0 deletions

diff --git a/absl/functional/internal/function_ref.h b/absl/functional/internal/function_ref.h
new file mode 100644
index 000000000000..fcb0496c2c06
--- /dev/null
+++ b/absl/functional/internal/function_ref.h
@@ -0,0 +1,104 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
+#define ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
+
+#include <cassert>
+#include <functional>
+#include <type_traits>
+
+#include "absl/base/internal/invoke.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+namespace functional_internal {
+
+// Like a void* that can handle function pointers as well. The standard does not
+// allow function pointers to round-trip through void*, but void(*)() is fine.
+//
+// Note: It's important that this class remains trivial and is the same size as
+// a pointer, since this allows the compiler to perform tail-call optimizations
+// when the underlying function is a callable object with a matching signature.
+union VoidPtr {
+  const void* obj;
+  void (*fun)();
+};
+
+// Chooses the best type for passing T as an argument.
+// Attempt to be close to SystemV AMD64 ABI. Objects with trivial copy ctor are
+// passed by value.
+template <typename T>
+constexpr bool PassByValue() {
+  return !std::is_lvalue_reference<T>::value &&
+         absl::is_trivially_copy_constructible<T>::value &&
+         absl::is_trivially_copy_assignable<
+             typename std::remove_cv<T>::type>::value &&
+         std::is_trivially_destructible<T>::value &&
+         sizeof(T) <= 2 * sizeof(void*);
+}
+
+template <typename T>
+struct ForwardT : std::conditional<PassByValue<T>(), T, T&&> {};
+
+// An Invoker takes a pointer to the type-erased invokable object, followed by
+// the arguments that the invokable object expects.
+//
+// Note: The order of arguments here is an optimization, since member functions
+// have an implicit "this" pointer as their first argument, putting VoidPtr
+// first allows the compiler to perform tail-call optimization in many cases.
+template <typename R, typename... Args>
+using Invoker = R (*)(VoidPtr, typename ForwardT<Args>::type...);
+
+//
+// InvokeObject and InvokeFunction provide static "Invoke" functions that can be
+// used as Invokers for objects or functions respectively.
+//
+// static_cast<R> handles the case the return type is void.
+template <typename Obj, typename R, typename... Args>
+R InvokeObject(VoidPtr ptr, typename ForwardT<Args>::type... args) {
+  auto o = static_cast<const Obj*>(ptr.obj);
+  return static_cast<R>(
+      absl::base_internal::Invoke(*o, std::forward<Args>(args)...));
+}
+
+template <typename Fun, typename R, typename... Args>
+R InvokeFunction(VoidPtr ptr, typename ForwardT<Args>::type... args) {
+  auto f = reinterpret_cast<Fun>(ptr.fun);
+  return static_cast<R>(
+      absl::base_internal::Invoke(f, std::forward<Args>(args)...));
+}
+
+template <typename Sig>
+void AssertNonNull(const std::function<Sig>& f) {
+  assert(f != nullptr);
+  (void)f;
+}
+
+template <typename F>
+void AssertNonNull(const F&) {}
+
+template <typename F, typename C>
+void AssertNonNull(F C::*f) {
+  assert(f != nullptr);
+  (void)f;
+}
+
+template <bool C>
+using EnableIf = typename ::std::enable_if<C, int>::type;
+
+}  // namespace functional_internal
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_