Add buffer_info::as_pybuffer() method

include/pybind11/common.h

@@ -231,6 +231,31 @@ struct buffer_info {
     ~buffer_info() {
         if (view) { PyBuffer_Release(view); delete view; }
     }
+
+    Py_buffer& as_pybuffer() const {
+        static Py_buffer buf { };
+        // Py_buffer uses signed sizes, strides and shape!..
+        static std::vector<Py_ssize_t> py_strides { };
+        static std::vector<Py_ssize_t> py_shape { };
+        buf.buf = ptr;
+        buf.itemsize = (Py_ssize_t) itemsize;
+        buf.format = const_cast<char *>(format.c_str());
+        buf.ndim = (int) ndim;
+        buf.len = (Py_ssize_t) size;
+        py_strides.clear();
+        py_shape.clear();
+        for (size_t i = 0; i < ndim; ++i) {
+            py_strides.push_back((Py_ssize_t) strides[i]);
+            py_shape.push_back((Py_ssize_t) shape[i]);
+        }
+        buf.strides = py_strides.data();
+        buf.shape = py_shape.data();
+        buf.suboffsets = nullptr;
+        buf.readonly = false;
+        buf.internal = nullptr;
+        return buf;
+    }
+
 private:
     Py_buffer *view = nullptr;
 };