* New primop: builtins.toFile, which writes a string into the store

and returns its path.  This can be used to (for instance) write
  builders inside a Nix expression, e.g.,

  stdenv.mkDerivation {
    builder = "
      source $stdenv/setup
      ...
    ";
    ...
  }

src/libexpr/primops.cc

@@ -467,6 +467,9 @@ static Expr primToString(EvalState & state, const ATermVector & args)
 }
 
 
+/* Convert the argument (which can be any Nix expression) to an XML
+   representation returned in a string.  Not all Nix expressions can
+   be sensibly or completely represented (e.g., functions). */
 static Expr primToXML(EvalState & state, const ATermVector & args)
 {
     ostringstream out;
@@ -475,6 +478,16 @@ static Expr primToXML(EvalState & state, const ATermVector & args)
 }
 
 
+/* Store a string in the Nix store as a source file that can be used
+   as an input by derivations. */
+static Expr primToFile(EvalState & state, const ATermVector & args)
+{
+    string s = evalString(state, args[0]);
+    Path storePath = addTextToStore("", s, PathSet());
+    return makePath(toATerm(storePath));
+}
+
+
 /* Boolean constructors. */
 static Expr primTrue(EvalState & state, const ATermVector & args)
 {
@@ -724,6 +737,7 @@ void EvalState::addPrimOps()
     addPrimOp("dirOf", 1, primDirOf);
     addPrimOp("toString", 1, primToString);
     addPrimOp("__toXML", 1, primToXML);
+    addPrimOp("__toFile", 1, primToFile);
     addPrimOp("isNull", 1, primIsNull);
     addPrimOp("dependencyClosure", 1, primDependencyClosure);
     addPrimOp("abort", 1, primAbort);

tests/gc-runtime.nix.in

@@ -1,7 +1,23 @@
-derivation {
+let {
-  name = "gc-runtime";
+
-  system = "@system@";
+  # Test inline source file definitions.
-  builder = "@shell@";
+  builder = builtins.toFile "
-  args = ["-e" "-x" ./gc-runtime.builder.sh];
+mkdir $out
-  PATH = "@testPath@";
+
+cat > $out/program <<EOF
+#! $SHELL
+sleep 10000
+EOF
+
+chmod +x $out/program
+";
+
+  body = derivation {
+    name = "gc-runtime";
+    system = "@system@";
+    builder = "@shell@";
+    args = ["-e" "-x" builder];
+    PATH = "@testPath@";
+  };
+
 }