Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/args/root.hh

@@ -0,0 +1,85 @@
+#pragma once
+
+#include "args.hh"
+
+namespace nix {
+
+/**
+ * The concrete implementation of a collection of completions.
+ *
+ * This is exposed so that the main entry point can print out the
+ * collected completions.
+ */
+struct Completions final : AddCompletions
+{
+    std::set<Completion> completions;
+    Type type = Type::Normal;
+
+    void setType(Type type) override;
+    void add(std::string completion, std::string description = "") override;
+};
+
+/**
+ * The outermost Args object. This is the one we will actually parse a command
+ * line with, whereas the inner ones (if they exists) are subcommands (and this
+ * is also a MultiCommand or something like it).
+ *
+ * This Args contains completions state shared between it and all of its
+ * descendent Args.
+ */
+class RootArgs : virtual public Args
+{
+protected:
+    /**
+     * @brief The command's "working directory", but only set when top level.
+     *
+     * Use getCommandBaseDir() to get the directory regardless of whether this
+     * is a top-level command or subcommand.
+     *
+     * @see getCommandBaseDir()
+     */
+    Path commandBaseDir = ".";
+
+public:
+    /** Parse the command line, throwing a UsageError if something goes
+     * wrong.
+     */
+    void parseCmdline(const Strings & cmdline, bool allowShebang = false);
+
+    std::shared_ptr<Completions> completions;
+
+    Path getCommandBaseDir() const override;
+
+protected:
+
+    friend class Args;
+
+    /**
+     * A pointer to the completion and its two arguments; a thunk;
+     */
+    struct DeferredCompletion {
+        const CompleterClosure & completer;
+        size_t n;
+        std::string prefix;
+    };
+
+    /**
+     * Completions are run after all args and flags are parsed, so completions
+     * of earlier arguments can benefit from later arguments.
+     */
+    std::vector<DeferredCompletion> deferredCompletions;
+
+    /**
+     * Experimental features needed when parsing args. These are checked
+     * after flag parsing is completed in order to support enabling
+     * experimental features coming after the flag that needs the
+     * experimental feature.
+     */
+    std::set<ExperimentalFeature> flagExperimentalFeatures;
+
+private:
+
+    std::optional<std::string> needsCompletion(std::string_view s);
+};
+
+}