Hold a conversion buffer string on the VtEngine to prevent hundreds o…

…f thousands of transient allocations as we convert things to emit them as a UTF-8 string.

src/renderer/vt/state.cpp

@@ -50,7 +50,10 @@ VtEngine::VtEngine(_In_ wil::unique_hfile pipe,
     _deferredCursorPos{ INVALID_COORDS },
     _inResizeRequest{ false },
     _trace{},
-    _bufferLine{}
+    _bufferLine{},
+    _buffer{},
+    _formatBuffer{},
+    _conversionBuffer{}
 {
 #ifndef UNIT_TESTING
     // When unit testing, we can instantiate a VtEngine without a pipe.
@@ -146,8 +149,11 @@ VtEngine::VtEngine(_In_ wil::unique_hfile pipe,
 {
     try
     {
-        const auto converted = ConvertToA(CP_UTF8, wstr);
-        return _Write(converted);
+        // We're likely going to be converting a ton of strings, so we'll use
+        // a buffer string stored on this class so we don't end up alloc/freeing
+        // in a tight loop and wasting a bunch of time and energy on that.
+        ConvertToA(CP_UTF8, wstr, _conversionBuffer);
+        return _Write(_conversionBuffer);
     }
     CATCH_RETURN();
 }

src/renderer/vt/vtrenderer.hpp

@@ -113,6 +113,7 @@ namespace Microsoft::Console::Render
         std::string _buffer;
 
         std::string _formatBuffer;
+        std::string _conversionBuffer;
 
         TextAttribute _lastTextAttributes;