Langkit_Support: add a generic package for iterator interfaces

This will be used as a base for iterators that will yield AST nodes for
the Find primitive and possibly for lazy collections in the properties
DSL.

Change-Id: I54450824dc5066b0ec46004e0f8f2cd6486e3d22
TN: P225-028

langkit/support/langkit_support-iterators.adb

@@ -0,0 +1,21 @@
+package body Langkit_Support.Iterators is
+
+   -------------
+   -- Iterate --
+   -------------
+
+   procedure Iterate
+     (I    : in out Iterator'Class;
+      Proc : access procedure (Element : Element_Type))
+   is
+      Has_Next : Boolean;
+      Element  : Element_Type;
+   begin
+      loop
+         I.Next (Has_Next, Element);
+         exit when not Has_Next;
+         Proc (Element);
+      end loop;
+   end Iterate;
+
+end Langkit_Support.Iterators;

langkit/support/langkit_support-iterators.ads

@@ -0,0 +1,37 @@
+--  This generic package provides for each Element_Type an interface that
+--  used to implement iterators.
+--
+--  Beyond the fact that it's super simple, the difference with standard
+--  iterators is that iteration is destructive: once an element has been
+--  yielded, it's not possible to re-yield it once more.
+--
+--  The analogy with standard Ada containers is that there's no way to keep a
+--  cursor to preserve an iteration state.
+
+with Ada.Unchecked_Deallocation;
+
+generic
+   type Element_Type is private;
+package Langkit_Support.Iterators is
+
+   type Iterator is interface;
+   --  Iterator interface: iterator consumers do not need to mind about
+   --  concrete interator implementations.
+
+   procedure Next
+     (I        : in out Iterator;
+      Has_Next : out Boolean;
+      Element  : out Element_Type) is abstract;
+   --  Get the next iteration element. If there was no element to yield
+   --  anymore, set Has_Next to False. Otherwise, set it to True and set
+   --  Element.
+
+   procedure Iterate
+     (I    : in out Iterator'Class;
+      Proc : access procedure (Element : Element_Type));
+   --  Consume completely the I iterator, calling Proc on all yielded elements
+
+   procedure Destroy is new Ada.Unchecked_Deallocation
+     (Iterator'Class, Iterator_Access);
+
+end Langkit_Support.Iterators;

langkit/support/langkit_support_installed.gpr

@@ -22,6 +22,7 @@ library project Langkit_Support is
       "Langkit_Support.Bump_Ptr",
       "Langkit_Support.Bump_Ptr.Vectors",
       "Langkit_Support.Diagnostics",
+      "Langkit_Support.Iterators",
       "Langkit_Support.Extensions",
       "Langkit_Support.Lexical_Env",
       "Langkit_Support.Packrat",