Proposal: Add application hooks

One theme at DConf was the unification of the frontends.
("One frontend to rule them all")
One source of differences in the frontend are compiler-specific
extensions. An easy way to support this is the addition of hooks.
An application may (or may not) use these hooks for additional
processing.

This hook was inspired by a current PR (ldc-developers/ldc#1434).
LDC-specific pragmas could be realized with the same approach.

Author: redstar

src/appl.d

@@ -0,0 +1,33 @@
+// Compiler implementation of the D programming language
+// Copyright (c) 1999-2016 by Digital Mars
+// All Rights Reserved
+// written by Walter Bright
+// http://www.digitalmars.com
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+module ddmd.appl;
+
+import ddmd.dscope;
+import ddmd.expression;
+
+struct Hooks
+{
+    Expression function(TraitsExp e, Scope* sc) semanticTraits;
+}
+
+void setHooks(Hooks hooks)
+{
+    .hooks = hooks;
+}
+
+Expression semanticTraitsHook(TraitsExp e, Scope* sc)
+{
+    if (hooks.semanticTraitsHook)
+        return hooks.semanticTraits(e, sc);
+    return null;
+}
+
+package:
+
+__gshared Hooks hooks;

src/posix.mak

@@ -197,7 +197,7 @@ endif
 endif
 
 
-FRONT_SRCS=$(addsuffix .d,access aggregate aliasthis apply argtypes arrayop	\
+FRONT_SRCS=$(addsuffix .d,access aggregate aliasthis appl apply argtypes arrayop	\
 	arraytypes attrib builtin canthrow clone complex cond constfold		\
 	cppmangle ctfeexpr dcast dclass declaration delegatize denum dimport	\
 	dinifile dinterpret dmacro dmangle dmodule doc dscope dstruct dsymbol	\

src/traits.d

@@ -12,6 +12,7 @@ import core.stdc.stdio;
 import core.stdc.string;
 import ddmd.aggregate;
 import ddmd.arraytypes;
+import ddmd.appl;
 import ddmd.canthrow;
 import ddmd.dclass;
 import ddmd.declaration;
@@ -1186,6 +1187,10 @@ extern (C++) Expression semanticTraits(TraitsExp e, Scope* sc)
     {
         return pointerBitmap(e);
     }
+    if (Expression ret = semanticTraitsHook(e, sc))
+    {
+        return ret;
+    }
 
     extern (D) void* trait_search_fp(const(char)* seed, ref int cost)
     {

src/win32.mak

@@ -141,7 +141,7 @@ DMDMAKE=$(MAKE) -fwin32.mak C=$C TK=$(TK) ROOT=$(ROOT) MAKE="$(MAKE)" HOST_DC="$
 ############################### Rule Variables ###############################
 
 # D front end
-FRONT_SRCS=access.d aggregate.d aliasthis.d apply.d argtypes.d arrayop.d	\
+FRONT_SRCS=access.d aggregate.d aliasthis.d appl.d apply.d argtypes.d arrayop.d	\
 	arraytypes.d attrib.d builtin.d canthrow.d clone.d complex.d		\
 	cond.d constfold.d cppmangle.d ctfeexpr.d dcast.d dclass.d		\
 	declaration.d delegatize.d denum.d dimport.d dinifile.d dinterpret.d	\