Add --inputs-from to use flake inputs as registry entries

This allows you to refer to an input from another flake. For example,

  $ nix run --inputs-from /path/to/hydra nixpkgs#hello

runs 'hello' from the 'nixpkgs' inputs of the 'hydra' flake.

Fixes #3769.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libfetchers/registry.cc b/src/libfetchers/registry.cc
index 914a0e1e8..d4134ce29 100644
--- a/src/libfetchers/registry.cc
+++ b/src/libfetchers/registry.cc
@@ -204,6 +204,8 @@ std::pair<Input, Attrs> lookupInRegistries(
     if (!input.isDirect())
         throw Error("cannot find flake '%s' in the flake registries", input.to_string());
 
+    debug("looked up '%s' -> '%s'", _input.to_string(), input.to_string());
+
     return {input, extraAttrs};
 }