Hiding Code from SPARK

Hiding Code from SPARK

[Jeffrey R.Carter]

I've been playing with trying to create a safe-pointer type for SPARK, where 
SPARK doesn't know that access types are involved. So I've tried

private with Ada.Finalization;

generic -- SRC.Pointers
    type Object (<>) is private;
package SRC.Pointers with SPARK_Mode
is
    type Pointer is tagged private with
       Default_Initial_Condition => Pointer.Is_Null;

    function Is_Null ...

    ...

private -- SRC.Pointers
    pragma SPARK_Mode (Off);

    type Safe_Group;

    type Name is access Safe_Group;

    type Pointer is new Ada.Finalization.Controlled with record
       Ptr : Name;
    end record;

    overriding procedure Adjust   (Item : in out Pointer);
    overriding procedure Finalize (Item : in out Pointer);
end SRC.Pointers;

My understanding is that SPARK will not look at the private part or the body. 
All proofs will assume that the operations are correct.

When I instantiate it in a SPARK unit:

package Pointers is new SRC.Pointers (Object => Fixed);

and run

gnatprove --level=4 -Psrc

I get errors such as

value Off was set for SPARK_Mode on "Adjust" at src-pointers.ads:24

implying that SPARK is looking at the private part of SRC.Pointers. I can work 
around that by putting the instantiation in an internal pkg with SPARK_Mode => Off:

    package SPARK_Control with SPARK_Mode => Off
    is
       package Pointers is new SRC.Pointers (Object => Fixed);
    end SPARK_Control;

    package Pointers renames SPARK_Control.Pointers;

but then I get errors such as

"Pointer" is not allowed in SPARK

"Object" is not allowed in SPARK

which don't make much sense. How can I get SPARK to ignore the private part and 
body of SRC.Pointers?

-- 
Jeff Carter
"You cheesy lot of second-hand electric donkey-bottom biters."
Monty Python & the Holy Grail
14