Documented REQUIRE under CATCH_CONFIG_FAST_COMPILE

docs/configuration.md

@@ -74,20 +74,33 @@ All C++11 support can be disabled with `CATCH_CONFIG_NO_CPP11`
 
     CATCH_CONFIG_COUNTER                    // Use __COUNTER__ to generate unique names for test cases
     CATCH_CONFIG_WINDOWS_SEH                // Enable SEH handling on Windows
-    CATCH_CONFIG_FAST_COMPILE               // Sacrifices some (extremely minor) features for compilation speed
+    CATCH_CONFIG_FAST_COMPILE               // Sacrifices some (rather minor) features for compilation speed
     CATCH_CONFIG_POSIX_SIGNALS              // Enable handling POSIX signals
     CATCH_CONFIG_WINDOWS_CRTDBG             // Enable leak checking using Windows's CRT Debug Heap
 
 Currently Catch enables `CATCH_CONFIG_WINDOWS_SEH` only when compiled with MSVC, because some versions of MinGW do not have the necessary Win32 API support.
 
-At this moment, `CATCH_CONFIG_FAST_COMPILE` changes only the behaviour of the `-b` (`--break`) flag, making it break into debugger in a stack frame *below* the actual test, unlike the default behaviour, where the break into debugger occurs in the same stack frame as the actual test. `CATCH_CONFIG_FAST_COMPILE` has to be either defined, or not defined, in all translation units that are linked into single test binary, or the behaviour of setting `-b` flag will be unpredictable.
-
 `CATCH_CONFIG_POSIX_SIGNALS` is on by default, except when Catch is compiled under `Cygwin`, where it is disabled by default (but can be force-enabled by defining `CATCH_CONFIG_POSIX_SIGNALS`).
 
 `CATCH_CONFIG_WINDOWS_CRTDBG` is off by default. If enabled, Windows's CRT is used to check for memory leaks, and displays them after the tests finish running.
 
 Just as with the C++11 conformance toggles, these toggles can be disabled by using `_NO_` form of the toggle, e.g. `CATCH_CONFIG_NO_WINDOWS_SEH`.
 
+## `CATCH_CONFIG_FAST_COMPILE`
+Defining this flag speeds up compilation of test files by ~20%, by making 2 changes:
+* The `-b` (`--break`) flag no longer makes Catch break into debugger in the same stack frame as the failed test, but rather in a stack frame *below*.
+* The `REQUIRE` family of macros (`REQUIRE`, `REQUIRE_FALSE` and `REQUIRE_THAT`) no longer uses local try-catch block. This creates minor chance for false negative under certain conditions, when expression tested inside `REQUIRE` throws exception, that is then caught by user's code before it gets to the test runner. Example:
+```cpp
+TEST_CASE("False negative") {
+    try {
+        REQUIRE(throws() == "");
+    } catch (...) {}
+}
+```
+This test case will succeed, reporting no assertions checked, instead of failing as it would without `CATCH_CONFIG_FAST_COMPILE`.
+
+`CATCH_CONFIG_FAST_COMPILE` has to be either defined, or not defined, in all translation units that are linked into single test binary, or the behaviour of setting `-b` flag will be unpredictable.
+
 # Windows header clutter
 
 On Windows Catch includes `windows.h`. To minimize global namespace clutter in the implementation file, it defines `NOMINMAX` and `WIN32_LEAN_AND_MEAN` before including it. You can control this behaviour via two macros: