* Allow the output/expression id to be forced to a certain

value; this potentially dangerous feature enables better 
  sharing for those paths for which the content is known in 
  advance (e.g., because a content hash is given).
* Fast builds: if we can expand all output paths of a derive 
  expression, we don't have to build.

corepkgs/fetchurl/fetchurl.fix

@@ -4,6 +4,7 @@ Function(["url", "md5"],
     , ("url", Var("url"))
     , ("md5", Var("md5"))
     , ("name", BaseName(Var("url")))
+    , ("id", Var("md5"))
     ]
   )
 )

corepkgs/fetchurl/fetchurl.sh

@@ -4,9 +4,7 @@ echo "downloading $url into $out..."
 wget "$url" -O "$out" || exit 1
 
 actual=$(md5sum -b $out | cut -c1-32)
-if ! test "$md5" == "ignore"; then
-    if ! test "$actual" == "$md5"; then
-	echo "hash is $actual, expected $md5"
-	exit 1
-    fi
+if ! test "$actual" == "$md5"; then
+    echo "hash is $actual, expected $md5"
+    exit 1
 fi