Adds format support for logging.

This makes it possible to use the C++20 formatting library next to the
existing stream operator when creating log messages. This feature is
only available when the standard library used has format support.

README.md

@@ -1024,7 +1024,7 @@ All tests passed (2 asserts in 2 tests)
 <details open><summary>&nbsp;&nbsp;&nbsp;&nbsp;Misc</summary>
 <p>
 
-<details open><summary>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Logging</summary>
+<details open><summary>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Logging using streams</summary>
 <p>
 
 ```cpp
@@ -1053,6 +1053,36 @@ asserts: 1 | 0 passed | 1 failed
 </p>
 </details>
 
+<details open><summary>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Logging using formatting</summary>
+<p>
+This requires using C++20 with a standard library with std::format support.
+
+```cpp
+"logging"_test = [] {
+  log("\npre  {} == {}\n", 42, 43);
+  expect(42_i == 43) << "message on failure";
+  log("\npost {} == {} -> {}\n", 42, 43, 42 == 43);
+};
+```
+
+```
+Running "logging"...
+pre  42 == 43
+  logging.cpp:8:FAILED [42 == 43] message on failure
+post 42 == 43 -> false
+FAILED
+
+===============================================================================
+
+tests:   1 | 1 failed
+asserts: 1 | 0 passed | 1 failed
+```
+
+> https://godbolt.org/z/26fPSY
+
+</p>
+</details>
+
 <details open><summary>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Matchers</summary>
 <p>
 

example/log.cpp

@@ -32,4 +32,12 @@ int main() {
     expect(e.value() == true);
   };
 #endif
+
+#if defined(BOOST_UT_HAS_FORMAT)
+  "log format"_test = [] {
+    boost::ut::log("\npre: {}\n", 42);
+    expect(42_i == 42) << "message on failure";
+    boost::ut::log("\npost\n");
+  };
+#endif
 }

include/boost/ut.hpp

@@ -18,6 +18,17 @@ export import std;
 #include <iso646.h>  // and, or, not, ...
 #endif
 
+#include <version>
+// Before libc++ 17 had experimental support for format and it required a
+// special build flag. Currently libc++ has not implemented all C++20 chrono
+// improvements. Therefore doesn't define __cpp_lib_format, instead query the
+// library version to detect the support status.
+//
+// MSVC STL and libstdc++ provide __cpp_lib_format.
+#if defined(__cpp_lib_format) or (defined(_LIBCPP_VERSION) and _LIBCPP_VERSION >= 170000)
+#define BOOST_UT_HAS_FORMAT
+#endif
+
 #if not defined(__cpp_rvalue_references)
 #error "[Boost::ext].UT requires support for rvalue references";
 #elif not defined(__cpp_decltype)
@@ -80,6 +91,9 @@ export import std;
 #include <exception>
 #endif
 
+#if __has_include(<format>)
+#include <format>
+#endif
 #if __has_include(<source_location>)
 #include <source_location>
 #endif
@@ -2249,6 +2263,14 @@ struct log {
     on<TMsg>(events::log{msg});
     return next{};
   }
+
+#if defined(BOOST_UT_HAS_FORMAT)
+  template <class... Args>
+  void operator()(std::format_string<Args...> fmt, Args&&... args) {
+    on<std::string>(
+        events::log{std::vformat(fmt.get(), std::make_format_args(args...))});
+  }
+#endif
 };
 
 template <class TExpr>

test/ut/ut.cpp

@@ -18,6 +18,10 @@
 #include <string_view>
 #include <vector>
 
+#if __has_include(<format>)
+#include <format>
+#endif
+
 namespace ut = boost::ut;
 
 namespace test_has_member_object {
@@ -895,7 +899,22 @@ int main() {
       test_assert(' ' == std::any_cast<char>(test_cfg.log_calls[6]));
       test_assert(42 == std::any_cast<int>(test_cfg.log_calls[7]));
     }
+#if defined(BOOST_UT_HAS_FORMAT)
+    {
+      test_cfg = fake_cfg{};
 
+      "logging with format"_test = [] {
+        boost::ut::log("{:<10} {:#x}\n{:<10} {}\n", "expected:", 42,
+                       std::string("actual:"), 99);
+      };
+
+      test_assert(1 == std::size(test_cfg.run_calls));
+      test_assert("logging with format"sv == test_cfg.run_calls[0].name);
+      test_assert(1 == std::size(test_cfg.log_calls));
+      test_assert("expected:  0x2a\nactual:    99\n"sv ==
+                  std::any_cast<std::string>(test_cfg.log_calls[0]));
+    }
+#endif
     {
       test_cfg = fake_cfg{};
       expect(true) << "true msg" << true;