[Ada] Minimize parts of Ada.Strings.Fixed marked SPARK_Mode => Off

[Pierre-Marie de Rodat <derodat@adacore.com>]

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Replace use of SPARK_Mode Off on callers of procedure Move by stronger
preconditions, as mandated by Ada RM A.4.3, so that calls to the
corresponding functions inside the procedure bodies are provably
correct.

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

gcc/ada/

	* libgnat/a-strfix.adb (Delete, Insert, Overwrite,
	Replace_Slice): Remove SPARK_Mode Off.
	* libgnat/a-strfix.ads (Insert, Overwrite, Replace_Slice):
	Strengthen precondition.

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diff --git a/gcc/ada/libgnat/a-strfix.adb b/gcc/ada/libgnat/a-strfix.adb
--- a/gcc/ada/libgnat/a-strfix.adb
+++ b/gcc/ada/libgnat/a-strfix.adb
@@ -214,7 +214,8 @@ package body Ada.Strings.Fixed with SPARK_Mode is
       -- Lemma_Split --
       -----------------
 
-      procedure Lemma_Split (Result : String) is
+      procedure Lemma_Split (Result : String)
+      is
       begin
          for K in Ptr + 1 .. Ptr + Right'Length loop
             Lemma_Mod (K - 1);
@@ -307,7 +308,8 @@ package body Ada.Strings.Fixed with SPARK_Mode is
       From    : Positive;
       Through : Natural;
       Justify : Alignment := Left;
-      Pad     : Character := Space) with SPARK_Mode => Off is
+      Pad     : Character := Space)
+   is
    begin
       Move (Source  => Delete (Source, From, Through),
             Target  => Source,
@@ -403,7 +405,8 @@ package body Ada.Strings.Fixed with SPARK_Mode is
      (Source   : in out String;
       Before   : Positive;
       New_Item : String;
-      Drop     : Truncation := Error) with SPARK_Mode => Off is
+      Drop     : Truncation := Error)
+   is
    begin
       Move (Source => Insert (Source, Before, New_Item),
             Target => Source,
@@ -419,7 +422,8 @@ package body Ada.Strings.Fixed with SPARK_Mode is
       Target  : out String;
       Drop    : Truncation := Error;
       Justify : Alignment  := Left;
-      Pad     : Character  := Space) with SPARK_Mode => Off
+      Pad     : Character  := Space)
+   with SPARK_Mode => Off
    is
       Sfirst  : constant Integer := Source'First;
       Slast   : constant Integer := Source'Last;
@@ -571,7 +575,8 @@ package body Ada.Strings.Fixed with SPARK_Mode is
      (Source   : in out String;
       Position : Positive;
       New_Item : String;
-      Drop     : Truncation := Right) with SPARK_Mode => Off is
+      Drop     : Truncation := Right)
+   is
    begin
       Move (Source => Overwrite (Source, Position, New_Item),
             Target => Source,
@@ -648,7 +653,8 @@ package body Ada.Strings.Fixed with SPARK_Mode is
       By       : String;
       Drop     : Truncation := Error;
       Justify  : Alignment  := Left;
-      Pad      : Character  := Space) with SPARK_Mode => Off is
+      Pad      : Character  := Space)
+   is
    begin
       Move (Replace_Slice (Source, Low, High, By), Source, Drop, Justify, Pad);
    end Replace_Slice;


diff --git a/gcc/ada/libgnat/a-strfix.ads b/gcc/ada/libgnat/a-strfix.ads
--- a/gcc/ada/libgnat/a-strfix.ads
+++ b/gcc/ada/libgnat/a-strfix.ads
@@ -904,7 +904,15 @@ package Ada.Strings.Fixed with SPARK_Mode is
       Justify : Alignment  := Left;
       Pad     : Character  := Space)
    with
-     Pre    => Low - 1 <= Source'Last,
+     Pre    =>
+       Low - 1 <= Source'Last
+         and then High >= Source'First - 1
+         and then (if High >= Low
+                   then Natural'Max (0, Low - Source'First)
+                        <= Natural'Last
+                           - By'Length
+                           - Natural'Max (Source'Last - High, 0)
+                   else Source'Length <= Natural'Last - By'Length),
 
    --  Incomplete contract
 
@@ -966,7 +974,9 @@ package Ada.Strings.Fixed with SPARK_Mode is
       New_Item : String;
       Drop     : Truncation := Error)
    with
-     Pre    => Before - 1 in Source'First - 1 .. Source'Last,
+     Pre    =>
+       Before - 1 in Source'First - 1 .. Source'Last
+         and then Source'Length <= Natural'Last - New_Item'Length,
 
      --  Incomplete contract
 
@@ -1033,7 +1043,11 @@ package Ada.Strings.Fixed with SPARK_Mode is
       New_Item : String;
       Drop     : Truncation := Right)
    with
-     Pre    => Position - 1 in Source'First - 1 .. Source'Last,
+     Pre    =>
+       Position - 1 in Source'First - 1 .. Source'Last
+         and then
+           (if Position - Source'First >= Source'Length - New_Item'Length
+            then Position - Source'First <= Natural'Last - New_Item'Length),
 
      --  Incomplete contract