Inline the `getResultCapture` helper into header.

We still keep the error check in the function, but hide it in an
outlined function inside a .cpp file, to promote inlining of the
retrieval part.

In the future, we should explore two things

1) Skipping over the context retrieval here, allowing direct access.
   I currently do not see a way to do this while keeping the
   "greppability" of mutable vs immutable accesses that is there now,
   but it would help a lot when inlining is not enabled.
2) Removing the error check, to make the function trivially inlinable,
   and without branches.

**runtime difference**

| --------- | Debug | Release |
|:----------|------:|--------:|
| Slow path |  0.98 |    1.07 |
| Fast path |  1.04 |    1.08 |

We lost bit of performance on the assertion slow path in debug mode,
but together with the previous commit, it comes out at net zero.
For other combinations, we see 5-10% perf improvement across the
two commits.

1 files changed, 8 insertions, 1 deletions

diff --git a/src/catch2/interfaces/catch_interfaces_capture.cpp b/src/catch2/interfaces/catch_interfaces_capture.cpp
index 9b40ee5d..492c5bd9 100644
--- a/src/catch2/interfaces/catch_interfaces_capture.cpp
+++ b/src/catch2/interfaces/catch_interfaces_capture.cpp
@@ -7,7 +7,14 @@
 // SPDX-License-Identifier: BSL-1.0
 
 #include <catch2/interfaces/catch_interfaces_capture.hpp>
+#include <catch2/internal/catch_enforce.hpp>
 
 namespace Catch {
+    namespace Detail {
+        void missingCaptureInstance() {
+            CATCH_INTERNAL_ERROR( "No result capture instance" );
+        }
+    } // namespace Detail
+
     IResultCapture::~IResultCapture() = default;
-}
+} // namespace Catch