Overhaul completions, redo #6693 (#8131)

As I complained in
https://github.com/NixOS/nix/pull/6784#issuecomment-1421777030 (a
comment on the wrong PR, sorry again!), #6693 introduced a second
completions mechanism to fix a bug. Having two completion mechanisms
isn't so nice.

As @thufschmitt also pointed out, it was a bummer to go from `FlakeRef`
to `std::string` when collecting flake refs. Now it is `FlakeRefs`
again.

The underlying issue that sought to work around was that completion of
arguments not at the end can still benefit from the information from
latter arguments.

To fix this better, we rip out that change and simply defer all
completion processing until after all the (regular, already-complete)
arguments have been passed.

In addition, I noticed the original completion logic used some global
variables. I do not like global variables, because even if they save
lines of code, they also obfuscate the architecture of the code.

I got rid of them  moved them to a new `RootArgs` class, which now has
`parseCmdline` instead of `Args`. The idea is that we have many argument
parsers from subcommands and what-not, but only one root args that owns
the other per actual parsing invocation. The state that was global is
now part of the root args instead.

This did, admittedly, add a bunch of new code. And I do feel bad about
that. So I went and added a lot of API docs to try to at least make the
current state of things clear to the next person.

--

This is needed for RFC 134 (tracking issue #7868). It was very hard to
modularize `Installable` parsing when there were two completion
arguments. I wouldn't go as far as to say it is *easy* now, but at least
it is less hard (and the completions test finally passed).

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libcmd/command.hh

@@ -97,8 +97,6 @@ struct MixFlakeOptions : virtual Args, EvalCommand
 {
     flake::LockFlags lockFlags;
 
-    std::optional<std::string> needsFlakeInputCompletion = {};
-
     MixFlakeOptions();
 
     /**
@@ -109,12 +107,8 @@ struct MixFlakeOptions : virtual Args, EvalCommand
      * command is operating with (presumably specified via some other
      * arguments) so that the completions for these flags can use them.
      */
-    virtual std::vector<std::string> getFlakesForCompletion()
+    virtual std::vector<FlakeRef> getFlakeRefsForCompletion()
     { return {}; }
-
-    void completeFlakeInput(std::string_view prefix);
-
-    void completionHook() override;
 };
 
 struct SourceExprCommand : virtual Args, MixFlakeOptions
@@ -137,7 +131,13 @@ struct SourceExprCommand : virtual Args, MixFlakeOptions
     /**
      * Complete an installable from the given prefix.
      */
-    void completeInstallable(std::string_view prefix);
+    void completeInstallable(AddCompletions & completions, std::string_view prefix);
+
+    /**
+     * Convenience wrapper around the underlying function to make setting the
+     * callback easier.
+     */
+    CompleterClosure getCompleteInstallable();
 };
 
 /**
@@ -170,7 +170,7 @@ struct RawInstallablesCommand : virtual Args, SourceExprCommand
 
     bool readFromStdIn = false;
 
-    std::vector<std::string> getFlakesForCompletion() override;
+    std::vector<FlakeRef> getFlakeRefsForCompletion() override;
 
 private:
 
@@ -199,10 +199,7 @@ struct InstallableCommand : virtual Args, SourceExprCommand
 
     void run(ref<Store> store) override;
 
-    std::vector<std::string> getFlakesForCompletion() override
-    {
-        return {_installable};
-    }
+    std::vector<FlakeRef> getFlakeRefsForCompletion() override;
 
 private:
 
@@ -329,9 +326,10 @@ struct MixEnvironment : virtual Args {
     void setEnviron();
 };
 
-void completeFlakeRef(ref<Store> store, std::string_view prefix);
+void completeFlakeRef(AddCompletions & completions, ref<Store> store, std::string_view prefix);
 
 void completeFlakeRefWithFragment(
+    AddCompletions & completions,
     ref<EvalState> evalState,
     flake::LockFlags lockFlags,
     Strings attrPathPrefixes,