[Ada] Confusing pragma unreferenced

[Arnaud Charlet <charlet at adacore dot com>]

Two pragmas exist - Unmodified and Unreferenced which issue warnings if the
respective entities contained get written or read repectivly. Additionally,
pragma Unreferenced will surpress compiler generated warnings for unread
variables. However, this can lead to confusion about pragma Unreferenced
whereby the assumed meaning would encompass writing as well as reading and
to achive this effect both pragmas would have to be utilized which is
inefficient. This patch adds a new pragma "Unused" to serve as a combination
of Unmodified and Unreferenced.

------------
-- Source --
------------

--  main.adb

with Ada.Text_IO;

--  Context clause
pragma Unused (Ada.Text_IO);        --  Warn Unused
pragma Unmodified (Ada.Text_IO);    --  Warn Unmodified
pragma Unreferenced (Ada.Text_IO);  --  Valid

procedure Main is

   --  Improper use
   X, Y, Z : Boolean := False;

   --  Non-variable
   procedure Test is begin null; end;
   pragma Unmodified (Test);        --  Warn Unmodified
   pragma Unused (Test);            --  Warn Unused
   pragma Unreferenced (Test);      --  Valid

   --  Equivalence of Unused to Unmodified + Unreferenced
   pragma Unmodified (X);           --  Valid
   pragma Unmodified (X);           --  Warn Unmodified
   pragma Unreferenced (X);         --  Valid
   pragma Unused (Y);               --  Valid

   --  Duplicate error messages
   pragma Unreferenced (X);         --  Warn Unreferenced
   pragma Unused (X);               --  Warn Unmodified and Unreferenced
   pragma Unused (Y);               --  Warn Unused
   pragma Unmodified (Y);           --  Warn Unused
   pragma Unreferenced (Y);         --  Warn Unused

   --  Proper use
   A, B, C, D : Boolean := True;
   pragma Unmodified (A);           --  Valid
   pragma Unreferenced (B);         --  Valid
   pragma Unmodified (C);           --  Valid
   pragma Unreferenced (C);         --  Valid
   pragma Unused (D);               --  Valid

begin
   X := True;                       --  Warn Unmodified
   Z := X;                          --  Warn Unreferenced
   Y := True;                       --  Warn Unused
   Z := Y;                          --  Warn Unused
   Z := A;                          --  Valid
   B := False;                      --  Valid
end Main;

----------------------------
-- Compilation and output --
----------------------------

$ gcc -gnatl -c main.adb

[...]

     1. with Ada.Text_IO;
     2.
     3. --  Context clause
     4. pragma Unused (Ada.Text_IO);        --  Warn Unused
                          |
        >>> pragma "Unused" argument must be in same declarative part

     5. pragma Unmodified (Ada.Text_IO);    --  Warn Unmodified
                              |
        >>> pragma "Unmodified" argument must be in same declarative part

     6. pragma Unreferenced (Ada.Text_IO);  --  Valid
     7.
     8. procedure Main is
     9.
    10.    --  Improper use
    11.    X, Y, Z : Boolean := False;
    12.
    13.    --  Non-variable
    14.    procedure Test is begin null; end;
    15.    pragma Unmodified (Test);        --  Warn Unmodified
                              |
        >>> pragma "Unmodified" can only be applied to a variable

    16.    pragma Unused (Test);            --  Warn Unused
                          |
        >>> pragma "Unused" can only be applied to a variable

    17.    pragma Unreferenced (Test);      --  Valid
    18.
    19.    --  Equivalence of Unused to Unmodified + Unreferenced
    20.    pragma Unmodified (X);           --  Valid
    21.    pragma Unmodified (X);           --  Warn Unmodified
                              |
        >>> warning: pragma Unmodified given for "X"

    22.    pragma Unreferenced (X);         --  Valid
    23.    pragma Unused (Y);               --  Valid
    24.
    25.    --  Duplicate error messages
    26.    pragma Unreferenced (X);         --  Warn Unreferenced
                                |
        >>> warning: pragma Unreferenced given for "X"

    27.    pragma Unused (X);               --  Warn Unmodified and Unreferenced
                          |
        >>> warning: pragma Unmodified given for "X"
        >>> warning: pragma Unreferenced given for "X"

    28.    pragma Unused (Y);               --  Warn Unused
                          |
        >>> warning: pragma Unused given for "Y"

    29.    pragma Unmodified (Y);           --  Warn Unused
                              |
        >>> warning: pragma Unused given for "Y"

    30.    pragma Unreferenced (Y);         --  Warn Unused
                                |
        >>> warning: pragma Unused given for "Y"

    31.
    32.    --  Proper use
    33.    A, B, C, D : Boolean := True;
    34.    pragma Unmodified (A);           --  Valid
    35.    pragma Unreferenced (B);         --  Valid
    36.    pragma Unmodified (C);           --  Valid
    37.    pragma Unreferenced (C);         --  Valid
    38.    pragma Unused (D);               --  Valid
    39.
    40. begin
    41.    X := True;                       --  Warn Unmodified
           |
        >>> warning: pragma Unmodified given for "X"

    42.    Z := X;                          --  Warn Unreferenced
                |
        >>> warning: pragma Unreferenced given for "X"

    43.    Y := True;                       --  Warn Unused
           |
        >>> warning: pragma Unused given for "Y"

    44.    Z := Y;                          --  Warn Unused
                |
        >>> warning: pragma Unused given for "Y"

    45.    Z := A;                          --  Valid
    46.    B := False;                      --  Valid
    47. end Main;

 47 lines: 4 errors, 11 warnings

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

2016-07-04  Justin Squirek  <squirek@adacore.com>

	* einfo.adb (Has_Pragma_Unused): Create this function as a setter
	for a new flag294 (Set_Has_Pragma_Unused): Create this procedure
	as a getter for flag294 (Write_Entity_Flags): Register the new
	flag with an alias
	* einfo.ads Add comment documenting Has_Pragma_Unused (flag294)
	and subsequent getter and setter declarations.
	* lib-xref.adb (Generate_Reference): Recognize Has_Pragma_Unused
	flag to print appropriate warning messages.
	* par-prag.adb (Prag): Classify Pragma_Unused into "All Other
	Pragmas."
	* snames.ads-tmpl Add a new name to the name constants and a
	new pramga to Pragma_Id for pramga Unused.
	* sem_prag.adb (Analyze_Pragma): Create case for Pragma_Unused
	and move the block for Pragma_Unmodified and Pragma_Unreferenced
	out and into local subprograms.
	(Analyze_Unmodified, Analyze_Unreferenced): From the old pragma blocks
	that have been separated in to local subprograms add a parameter to
	indicate the if they are being called in the context of Pragma_Unused
	and handle it accordingly.
	(Is_Non_Significant_Pragma_Reference): Add an entry for Pragma_Unused
	and correct the position of Pragma_Unevaluated_Use_Of_Old.
	* sem_util.adb (Note_Possible_Modification): Recognize
	Has_Pragma_Unused flag to print appropriate warning messages.

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