Add a mechanism for derivation attributes to reference the derivation's outputs

For example, you can now say:

  configureFlags = "--prefix=${placeholder "out"} --includedir=${placeholder "dev"}";

The strings returned by the ‘placeholder’ builtin are replaced at
build time by the actual store paths corresponding to the specified
outputs.

Previously, you had to work around the inability to self-reference by doing stuff like:

  preConfigure = ''
    configureFlags+=" --prefix $out --includedir=$dev"
  '';

or rely on ad-hoc variable interpolation semantics in Autoconf or Make
(e.g. --prefix=\$(out)), which doesn't always work.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index 7dcf71d46..f051f10bd 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -390,4 +390,11 @@ Sink & operator << (Sink & out, const BasicDerivation & drv)
 }
 
 
+std::string hashPlaceholder(const std::string & outputName)
+{
+    // FIXME: memoize?
+    return "/" + printHash32(hashString(htSHA256, "nix-output:" + outputName));
+}
+
+
 }