`log/internal/check_op`: Add ABSL_ATTRIBUTE_UNUSED to CHECK macros when STRIP_LOG is enabled

When `STRIP_LOG` is off, the internal variable `absl_log_internal_check_op_result` is passed to `absl::log_internal::LogMessageFatal()` and used in the failure message.
When `STRIP_LOG` is on, the variable is truly unused.

Applying a `ABSL_ATTRIBUTE_UNUSED` on the variable triggers `-Wused-but-marked-unused` when `STRIP_LOG` is off, not applying the attribute triggers `-Wunused-but-set-variable` when `STRIP_LOG` is on.

Define a new internal macro `ABSL_LOG_INTERNAL_ATTRIBUTE_UNUSED_IF_STRIP_LOG` that evaluates to `ABSL_ATTRIBUTE_UNUSED` when `STRIP_LOG` is on and nothing when `STRIP_LOG` is off to address both of these scenarios.

PiperOrigin-RevId: 625049155
Change-Id: Ia3f8a6ca916dd67a287bbda4b9bd6c574c92247a

absl/log/CMakeLists.txt

@@ -290,6 +290,7 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::core_headers
     absl::log_internal_message
     absl::log_internal_nullstream
     absl::log_severity

absl/log/internal/BUILD.bazel

@@ -266,6 +266,7 @@ cc_library(
     deps = [
         ":log_message",
         ":nullstream",
+        "//absl/base:core_headers",
         "//absl/base:log_severity",
     ],
 )

absl/log/internal/check_op.h

@@ -58,12 +58,13 @@
 #endif
 
 #define ABSL_LOG_INTERNAL_CHECK_OP(name, op, val1, val1_text, val2, val2_text) \
-  while (::std::string* absl_log_internal_check_op_result =                    \
-             ::absl::log_internal::name##Impl(                                 \
-                 ::absl::log_internal::GetReferenceableValue(val1),            \
-                 ::absl::log_internal::GetReferenceableValue(val2),            \
-                 ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(                       \
-                     val1_text " " #op " " val2_text)))                        \
+  while (::std::string* absl_log_internal_check_op_result                      \
+             ABSL_LOG_INTERNAL_ATTRIBUTE_UNUSED_IF_STRIP_LOG =                 \
+                 ::absl::log_internal::name##Impl(                             \
+                     ::absl::log_internal::GetReferenceableValue(val1),        \
+                     ::absl::log_internal::GetReferenceableValue(val2),        \
+                     ABSL_LOG_INTERNAL_STRIP_STRING_LITERAL(                   \
+                         val1_text " " #op " " val2_text)))                    \
     ABSL_LOG_INTERNAL_CONDITION_FATAL(STATELESS, true)                         \
   ABSL_LOG_INTERNAL_CHECK(*absl_log_internal_check_op_result).InternalStream()
 #define ABSL_LOG_INTERNAL_QCHECK_OP(name, op, val1, val1_text, val2, \

absl/log/internal/strip.h

@@ -20,6 +20,7 @@
 #ifndef ABSL_LOG_INTERNAL_STRIP_H_
 #define ABSL_LOG_INTERNAL_STRIP_H_
 
+#include "absl/base/attributes.h"  // IWYU pragma: keep
 #include "absl/base/log_severity.h"
 #include "absl/log/internal/log_message.h"
 #include "absl/log/internal/nullstream.h"
@@ -29,6 +30,9 @@
 // of defines comes in three flavors: vanilla, plus two variants that strip some
 // logging in subtly different ways for subtly different reasons (see below).
 #if defined(STRIP_LOG) && STRIP_LOG
+
+#define ABSL_LOG_INTERNAL_ATTRIBUTE_UNUSED_IF_STRIP_LOG ABSL_ATTRIBUTE_UNUSED
+
 #define ABSL_LOGGING_INTERNAL_LOG_INFO ::absl::log_internal::NullStream()
 #define ABSL_LOGGING_INTERNAL_LOG_WARNING ::absl::log_internal::NullStream()
 #define ABSL_LOGGING_INTERNAL_LOG_ERROR ::absl::log_internal::NullStream()
@@ -48,7 +52,11 @@
 #define ABSL_LOG_INTERNAL_CHECK(failure_message) ABSL_LOGGING_INTERNAL_LOG_FATAL
 #define ABSL_LOG_INTERNAL_QCHECK(failure_message) \
   ABSL_LOGGING_INTERNAL_LOG_QFATAL
+
 #else  // !defined(STRIP_LOG) || !STRIP_LOG
+
+#define ABSL_LOG_INTERNAL_ATTRIBUTE_UNUSED_IF_STRIP_LOG
+
 #define ABSL_LOGGING_INTERNAL_LOG_INFO \
   ::absl::log_internal::LogMessage(    \
       __FILE__, __LINE__, ::absl::log_internal::LogMessage::InfoTag{})