Fix flake input completion for `InstallablesCommand`s

Defers completion of flake inputs until the whole command line is parsed
so that we know what flakes we need to complete the inputs of.

Previously, `nix build flake --update-input <Tab>` always behaved like
`nix build . --update-input <Tab>`.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/nix/main.cc b/src/nix/main.cc
index f398e3118..f6138cbe6 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -342,7 +342,10 @@ void mainWrapped(int argc, char * * argv)
         if (!completions) throw;
     }
 
-    if (completions) return;
+    if (completions) {
+        args.completionHook();
+        return;
+    }
 
     if (args.showVersion) {
         printVersion(programName);