nix run: Use packages/legacyPackages as fallback if there is no app definition

'nix run' will try to run $out/bin/<name>, where <name> is the
derivation name (excluding the version). This often works well:

  $ nix run nixpkgs#hello
  Hello, world!

  $ nix run nix -- --version
  nix (Nix) 2.4pre20200626_adf2fbb

  $ nix run patchelf -- --version
  patchelf 0.11.20200623.e61654b

  $ nix run nixpkgs#firefox -- --version
  Mozilla Firefox 77.0.1

  $ nix run nixpkgs#gimp -- --version
  GNU Image Manipulation Program version 2.10.14

though not always:

  $ nix run nixpkgs#git
  error: unable to execute '/nix/store/kp7wp760l4gryq9s36x481b2x4rfklcy-git-2.25.4/bin/git-minimal': No such file or directory

1 files changed, 9 insertions, 3 deletions

diff --git a/src/nix/run.cc b/src/nix/run.cc
index 204937cbc..59a6c7252 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -172,12 +172,18 @@ struct CmdRun : InstallableCommand, RunCommon
 
     Strings getDefaultFlakeAttrPaths() override
     {
-        return {"defaultApp." + settings.thisSystem.get()};
+        Strings res{"defaultApp." + settings.thisSystem.get()};
+        for (auto & s : SourceExprCommand::getDefaultFlakeAttrPaths())
+            res.push_back(s);
+        return res;
     }
 
     Strings getDefaultFlakeAttrPathPrefixes() override
     {
-        return {"apps." + settings.thisSystem.get() + "."};
+        Strings res{"apps." + settings.thisSystem.get() + ".", "packages"};
+        for (auto & s : SourceExprCommand::getDefaultFlakeAttrPathPrefixes())
+            res.push_back(s);
+        return res;
     }
 
     void run(ref<Store> store) override
@@ -186,7 +192,7 @@ struct CmdRun : InstallableCommand, RunCommon
 
         auto app = installable->toApp(*state);
 
-        state->realiseContext(app.context);
+        state->store->buildPaths(app.context);
 
         Strings allArgs{app.program};
         for (auto & i : args) allArgs.push_back(i);