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, 7 insertions, 0 deletions

diff --git a/src/libutil/args.hh b/src/libutil/args.hh
index 07c017719..84866f12b 100644
--- a/src/libutil/args.hh
+++ b/src/libutil/args.hh
@@ -148,6 +148,11 @@ protected:
        argument (if any) have been processed. */
     virtual void initialFlagsProcessed() {}
 
+    /* Called after the command line has been processed if we need to generate
+       completions. Useful for commands that need to know the whole command line
+       in order to know what completions to generate. */
+    virtual void completionHook() { }
+
 public:
 
     void addFlag(Flag && flag);
@@ -223,6 +228,8 @@ public:
 
     bool processArgs(const Strings & args, bool finish) override;
 
+    void completionHook() override;
+
     nlohmann::json toJSON() override;
 };