From 1ed55234d9efce7d40c97ef1215090015d18498c Mon Sep 17 00:00:00 2001
From: Shea Levy <shea@shealevy.com>
Date: Thu, 23 Jul 2015 09:34:06 -0400
Subject: [PATCH] Allow derivations-as-srcs in the context of builtins.toFile
 files

---
 src/libexpr/primops.cc | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 6a4b5b042..f555de73e 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -872,8 +872,12 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
 
     for (auto path : context) {
         if (path.at(0) == '=') path = string(path, 1);
-        if (isDerivation(path))
-            throw EvalError(format("in ‘toFile’: the file ‘%1%’ cannot refer to derivation outputs, at %2%") % name % pos);
+        if (isDerivation(path)) {
+            /* See prim_unsafeDiscardOutputDependency. */
+            if (path.at(0) != '~')
+                throw EvalError(format("in ‘toFile’: the file ‘%1%’ cannot refer to derivation outputs, at %2%") % name % pos);
+            path = string(path, 1);
+	}
         refs.insert(path);
     }