allow `@:useAstSource`

ast.ml

@@ -171,6 +171,7 @@ module Meta = struct
 		| Unreflective
 		| Unsafe
 		| Usage
+		| UseAstSource
 		| Used
 		| Value
 		| Void

common.ml

@@ -503,6 +503,7 @@ module MetaInfo = struct
 		| Unreflective -> ":unreflective",("",[Platform Cpp])
 		| Unsafe -> ":unsafe",("Declares a class, or a method with the C#'s 'unsafe' flag",[Platform Cs; UsedOnEither [TClass;TClassField]])
 		| Usage -> ":usage",("?",[])
+		| UseAstSource -> ":useAstSource",("Copies the expression of the annotated field",[UsedOn TExpr])
 		| Used -> ":used",("Internally used by DCE to mark a class or field as used",[Internal])
 		| Value -> ":value",("Used to store default values for fields and function arguments",[UsedOn TClassField])
 		| Void -> ":void",("Use Cpp native 'void' return type",[Platform Cpp])

tests/unit/src/unit/issues/Issue3285.hx

@@ -0,0 +1,16 @@
+package unit.issues;
+
+private class C {
+	@:astSource
+	public function doSomething(s:String) {
+		s += s;
+	}
+}
+
+class Issue3285 extends Test {
+	function test() {
+		var s = "foo";
+		@:useAstSource C.doSomething;
+		eq("foofoo", s);
+	}
+}

typer.ml

@@ -3642,6 +3642,36 @@ and type_expr ctx (e,p) (with_type:with_type) =
 			| (Meta.NoPrivateAccess,_,_) ->
 				ctx.meta <- List.filter (fun(m,_,_) -> m <> Meta.PrivateAccess) ctx.meta;
 				e()
+			| (Meta.UseAstSource,_,_) ->
+				let e1,core_api,name = match fst e1 with
+					| EField(e1,name) -> e1,false,name
+					| EMeta((Meta.CoreApi,[],_),(EField(e1,name),_)) -> e1,true,name
+					| _ -> error "Field expression expected as argument to @:useAstSource" (pos e1)
+				in
+				let e1 = type_expr ctx e1 NoValue in
+				let cf = match e1.eexpr with
+					| TTypeExpr (TClassDecl c) ->
+						let c = if core_api then Typeload.load_core_class ctx c else c in
+						begin try
+							PMap.find name c.cl_statics
+						with Not_found -> try
+							PMap.find name c.cl_fields
+						with Not_found ->
+							error ((s_type_path c.cl_path) ^ " has no field " ^ name) p;
+						end
+					| _ ->
+						error "Expected path to class" e1.epos
+				in
+				let e = try
+					begin match Meta.get Meta.AstSource cf.cf_meta with
+						| _,[e],_ -> e
+						| _ -> raise Not_found
+					end
+				with Not_found ->
+					display_error ctx ("Field " ^ cf.cf_name ^ " requires @:astSource metadata to be used as source") p;
+					error "Location of field was here" cf.cf_pos;
+				in
+				type_expr ctx e with_type
 			| _ -> e()
 		in
 		ctx.meta <- old;