Avoid use of temporary bytes object in string_caster for UTF-8

Fixes #3252

Author: jbms

include/pybind11/cast.h

@@ -377,6 +377,22 @@ template <typename StringType, bool IsView = false> struct string_caster {
 #endif
         }
 
+#if PY_VERSION_HEX >= 0x03030000
+        // On Python >= 3.3, for UTF-8 we avoid the need for a temporary `bytes`
+        // object by using `PyUnicode_AsUTF8AndSize`.
+        if (PYBIND11_SILENCE_MSVC_C4127(UTF_N == 8)) {
+            Py_ssize_t size; // NOLINT(cppcoreguidelines-init-variables)
+            const auto *buffer
+                = reinterpret_cast<const CharT *>(PyUnicode_AsUTF8AndSize(load_src.ptr(), &size));
+            if (!buffer) {
+                PyErr_Clear();
+                return false;
+            }
+            value = StringType(buffer, static_cast<size_t>(size));
+            return true;
+        }
+#endif
+
         auto utfNbytes = reinterpret_steal<object>(PyUnicode_AsEncodedString(
             load_src.ptr(), UTF_N == 8 ? "utf-8" : UTF_N == 16 ? "utf-16" : "utf-32", nullptr));
         if (!utfNbytes) { PyErr_Clear(); return false; }