Add Installable::toDrvPaths()

This is needed to get the path of a derivation that might not exist
(e.g. for 'nix store copy-log').

InstallableStorePath::toDerivedPaths() cannot be used for this because
it calls readDerivation(), so it fails if the store doesn't have the
derivation.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nix/app.cc b/src/nix/app.cc
index 2fcf4752c..e104cc9c1 100644
--- a/src/nix/app.cc
+++ b/src/nix/app.cc
@@ -19,7 +19,7 @@ struct InstallableDerivedPath : Installable
     }
 
 
-    std::string what() override { return derivedPath.to_string(*store); }
+    std::string what() const override { return derivedPath.to_string(*store); }
 
     DerivedPaths toDerivedPaths() override
     {