libexpr: Migrate makePositionThunks to return Value tuple

part of #1136 and a step towards #744

Change-Id: Ib2ccea4c098bbb66eaa47645b6fa59e56a6a6964

lix/libexpr/eval.cc

@@ -857,7 +857,9 @@ void EvalState::mkPos(Value & v, PosIdx p)
     if (auto path = std::get_if<CheckedSourcePath>(&origin)) {
         auto attrs = ctx.buildBindings(3);
         attrs.alloc(ctx.symbols.sym_file) = {NewValueAs::string, path->to_string()};
-        makePositionThunks(*this, p, attrs.alloc(ctx.symbols.sym_line), attrs.alloc(ctx.symbols.sym_column));
+        Value & line = attrs.alloc(ctx.symbols.sym_line);
+        Value & col = attrs.alloc(ctx.symbols.sym_column);
+        std::tie(line, col) = makePositionThunks(*this, p);
         v = {NewValueAs::attrs, attrs};
     } else
         v.mkNull();

lix/libexpr/primops.cc

@@ -26,6 +26,7 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <tuple>
 #include <unistd.h>
 
 #include <algorithm>
@@ -1931,17 +1932,18 @@ static struct LazyPosAcessors {
     Value lineOfPos = {NewValueAs::primop, primop_lineOfPos},
           columnOfPos = {NewValueAs::primop, primop_columnOfPos};
 
-    void operator()(EvalState & state, const PosIdx pos, Value & line, Value & column)
+    std::tuple<Value, Value> operator()(EvalState & state, const PosIdx pos)
     {
         Value posV{NewValueAs::integer, NixInt{pos.id}};
-        line = {NewValueAs::app, state.ctx.mem, lineOfPos, posV};
-        column = {NewValueAs::app, state.ctx.mem, columnOfPos, posV};
+        Value line = {NewValueAs::app, state.ctx.mem, lineOfPos, posV};
+        Value column = {NewValueAs::app, state.ctx.mem, columnOfPos, posV};
+        return std::make_tuple(line, column);
     }
 } makeLazyPosAccessors;
 
-void makePositionThunks(EvalState & state, const PosIdx pos, Value & line, Value & column)
+std::tuple<Value, Value> makePositionThunks(EvalState & state, const PosIdx pos)
 {
-    makeLazyPosAccessors(state, pos, line, column);
+    return makeLazyPosAccessors(state, pos);
 }
 
 /* Dynamic version of the `?' operator. */

lix/libexpr/primops.hh

@@ -55,6 +55,5 @@ void prim_importNative(EvalState & state, Value * * args, Value & v);
  */
 void prim_exec(EvalState & state, Value * * args, Value & v);
 
-void makePositionThunks(EvalState & state, const PosIdx pos, Value & line, Value & column);
-
+std::tuple<Value, Value> makePositionThunks(EvalState & state, const PosIdx pos);
 }