DOC: Slightly extend to docs to note that we assume no-copy buffer protocol

Author: seberg

numpy/_globals.py

@@ -106,6 +106,9 @@ class _CopyMode(enum.Enum):
     - NEVER: This means that the deep copy will never be taken.
              If a copy cannot be avoided then a `ValueError` will be
              raised.
+
+    Note that the buffer-protocol could in theory do copies.  NumPy currently
+    assumes an object exporting the buffer protocol will never do this.
     """
 
     ALWAYS = True

numpy/core/include/numpy/ndarraytypes.h

@@ -868,7 +868,7 @@ typedef int (PyArray_FinalizeFunc)(PyArrayObject *, PyObject *);
 
 /*
  * Always copy the array. Returned arrays are always CONTIGUOUS,
- * ALIGNED, and WRITEABLE.
+ * ALIGNED, and WRITEABLE. See also: NPY_ARRAY_ENSURENOCOPY = 0x4000.
  *
  * This flag may be requested in constructor functions.
  */
@@ -937,6 +937,11 @@ typedef int (PyArray_FinalizeFunc)(PyArrayObject *, PyObject *);
 #define NPY_ARRAY_UPDATEIFCOPY    0x1000 /* Deprecated in 1.14 */
 #define NPY_ARRAY_WRITEBACKIFCOPY 0x2000
 
+/*
+ * No copy may be made while converting from an object/array (result is a view)
+ *
+ * This flag may be requested in constructor functions.
+ */
 #define NPY_ARRAY_ENSURENOCOPY 0x4000
 
 /*