pragma Import with 'Address clause

[Lutz Donnerhacke <lutz@iks-jena.de>]

A common technique to reinterpret data is:
  a : AT;
  [...]
  b : BT;
  for b'Address use a'Address;
  pragma Import(Ada, b);

Is the "pragma Import" really necessary? Of course, the ARM says, that
elaboration and definition of the imported object is externally defined. So
they are omitted from the Ada code itself.

Assume BT is a record without any default expressions. Does this mean, the
pragma can be omitted from the source code?

Background: Spark prohibits the use of Pragma Import on objects.