[Ada] Provide dummy body for big integers library used in reduced runtimes

[Ada] Provide dummy body for big integers library used in reduced runtimes

[Pierre-Marie de Rodat]

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The version of Ada.Numerics.Big_Numbers.Big_Integers used in the light
and embedded runtimes is only meant for proof, not execution. As a
result, all subprograms were previously marked as imported, but this
leads to a spurious compilation error in GNAT. Work around that bug for
now by providing a dummy body for the generics inside that unit.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/a-nbnbin__ghost.adb (Signed_Conversions,
	Unsigned_Conversions): Mark subprograms as not imported.
	* libgnat/a-nbnbin__ghost.ads: Provide a dummy body.

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diff --git /dev/null b/gcc/ada/libgnat/a-nbnbin__ghost.adb
new file mode 100644
--- /dev/null
+++ b/gcc/ada/libgnat/a-nbnbin__ghost.adb
@@ -0,0 +1,76 @@
+------------------------------------------------------------------------------
+--                                                                          --
+--                         GNAT RUN-TIME COMPONENTS                         --
+--                                                                          --
+--                  ADA.NUMERICS.BIG_NUMBERS.BIG_INTEGERS                   --
+--                                                                          --
+--                                 B o d y                                  --
+--                                                                          --
+--            Copyright (C) 2021, Free Software Foundation, Inc.            --
+--                                                                          --
+-- GNAT is free software;  you can  redistribute it  and/or modify it under --
+-- terms of the  GNU General Public License as published  by the Free Soft- --
+-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
+-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
+-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
+-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
+--                                                                          --
+-- As a special exception under Section 7 of GPL version 3, you are granted --
+-- additional permissions described in the GCC Runtime Library Exception,   --
+-- version 3.1, as published by the Free Software Foundation.               --
+--                                                                          --
+-- You should have received a copy of the GNU General Public License and    --
+-- a copy of the GCC Runtime Library Exception along with this program;     --
+-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
+-- <http://www.gnu.org/licenses/>.                                          --
+--                                                                          --
+-- GNAT was originally developed  by the GNAT team at  New York University. --
+-- Extensive contributions were provided by Ada Core Technologies Inc.      --
+--                                                                          --
+------------------------------------------------------------------------------
+
+--  This body is provided as a work-around for a GNAT compiler bug, as GNAT
+--  currently does not compile instantiations of the spec with imported ghost
+--  generics for packages Signed_Conversions and Unsigned_Conversions.
+
+package body Ada.Numerics.Big_Numbers.Big_Integers with
+   SPARK_Mode => Off
+is
+
+   package body Signed_Conversions with
+     SPARK_Mode => Off
+   is
+
+      function To_Big_Integer (Arg : Int) return Valid_Big_Integer is
+      begin
+         raise Program_Error;
+         return (null record);
+      end To_Big_Integer;
+
+      function From_Big_Integer (Arg : Valid_Big_Integer) return Int is
+      begin
+         raise Program_Error;
+         return 0;
+      end From_Big_Integer;
+
+   end Signed_Conversions;
+
+   package body Unsigned_Conversions with
+     SPARK_Mode => Off
+   is
+
+      function To_Big_Integer (Arg : Int) return Valid_Big_Integer is
+      begin
+         raise Program_Error;
+         return (null record);
+      end To_Big_Integer;
+
+      function From_Big_Integer (Arg : Valid_Big_Integer) return Int is
+      begin
+         raise Program_Error;
+         return 0;
+      end From_Big_Integer;
+
+   end Unsigned_Conversions;
+
+end Ada.Numerics.Big_Numbers.Big_Integers;


diff --git a/gcc/ada/libgnat/a-nbnbin__ghost.ads b/gcc/ada/libgnat/a-nbnbin__ghost.ads
--- a/gcc/ada/libgnat/a-nbnbin__ghost.ads
+++ b/gcc/ada/libgnat/a-nbnbin__ghost.ads
@@ -89,12 +89,10 @@ is
 
       function To_Big_Integer (Arg : Int) return Valid_Big_Integer
       with
-        Import,
         Global => null;
 
       function From_Big_Integer (Arg : Valid_Big_Integer) return Int
       with
-        Import,
         Pre    => In_Range (Arg,
                             Low  => To_Big_Integer (Int'First),
                             High => To_Big_Integer (Int'Last))
@@ -108,12 +106,10 @@ is
 
       function To_Big_Integer (Arg : Int) return Valid_Big_Integer
       with
-        Import,
         Global => null;
 
       function From_Big_Integer (Arg : Valid_Big_Integer) return Int
       with
-        Import,
         Pre    => In_Range (Arg,
                             Low  => To_Big_Integer (Int'First),
                             High => To_Big_Integer (Int'Last))