Export of internal Abseil changes

--
d857e6e1f9b09a3eb5abd890677a98b23346f07a by Abseil Team <absl-team@google.com>:

Simplify internal TryAcquireWithSpinning.

No point declaring the `result` variable: we can just return the results
directly.

PiperOrigin-RevId: 307045800

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421952252bc23be51f47f7d23f3422bad1ed382c by Derek Mauro <dmauro@google.com>:

Add custom sink support for `absl::Format()` through an ADL extension mechanism.

Users can now define
`void AbslFormatFlush(MySink* dest, absl::string_view part)`
to allow `absl::Format()` to append to a custom sink.

PiperOrigin-RevId: 306929052

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c73d5cdb62cd58ea421ed1aeeab78a0ffcfeeefb by Matt Calabrese <calabrese@google.com>:

Internal-only conformance-testing macro ABSL_INTERNAL_ASSERT_CONFORMANCE_OF for compile-time and runtime checks of a specified type, expected properties of that type, and a logically-ordered series of equivalence classes of that type.

PiperOrigin-RevId: 306885512

--
a8c2495a07f37d68907855e3f0535bd5c27a3b52 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 306766753
GitOrigin-RevId: d857e6e1f9b09a3eb5abd890677a98b23346f07a
Change-Id: Ic23c92ac74f9ffcbb2471ff8c6691f4b7b20354b

1 files changed, 5 insertions, 7 deletions

diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 6ee5f235ac93..8cda5a1ce093 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc
@@ -1439,20 +1439,18 @@ void Mutex::AssertNotHeld() const {
 // may spin for a short while if the lock cannot be acquired immediately.
 static bool TryAcquireWithSpinning(std::atomic<intptr_t>* mu) {
   int c = mutex_globals.spinloop_iterations;
-  int result = -1;  // result of operation:  0=false, 1=true, -1=unknown
-
   do {  // do/while somewhat faster on AMD
     intptr_t v = mu->load(std::memory_order_relaxed);
-    if ((v & (kMuReader|kMuEvent)) != 0) {  // a reader or tracing -> give up
-      result = 0;
+    if ((v & (kMuReader|kMuEvent)) != 0) {
+      return false;  // a reader or tracing -> give up
     } else if (((v & kMuWriter) == 0) &&  // no holder -> try to acquire
                mu->compare_exchange_strong(v, kMuWriter | v,
                                            std::memory_order_acquire,
                                            std::memory_order_relaxed)) {
-      result = 1;
+      return true;
     }
-  } while (result == -1 && --c > 0);
-  return result == 1;
+  } while (--c > 0);
+  return false;
 }
 
 ABSL_XRAY_LOG_ARGS(1) void Mutex::Lock() {