cmd/nixbuild: instantiated deduplicate stat

Use case for this came up while writing tests.

cmd/nixbuild/main.go

@@ -118,6 +118,32 @@ func main() {
 	}).
 		Flag(&resolveFlagOut, "o", command.StringFlag("store-paths"), "Path to write collected store paths")
 
+	c.Command("stat", "Compute the instantiated deduplication stat of an installable", func(args []string) error {
+		var installable string
+		if err := formatInstallable("instantiated", &installable, flagNixOS, args); err != nil {
+			return err
+		}
+
+		log.Println("initialising evaluator")
+		var collective []string
+		if eval, err := nixbuild.NewInstantiatedEvaluator(ctx, installable); err != nil {
+			return commandHandlerError(fmt.Sprintf("cannot initialise evaluator: %v", err))
+		} else {
+			log.Println("collecting paths")
+			collective = slices.Collect(eval.Instantiated())
+			if err := eval.Err(); err != nil {
+				return commandHandlerError(fmt.Sprintf("cannot collect: %v", err))
+			}
+		}
+
+		l := len(collective)
+		slices.Sort(collective)
+		collective = slices.Compact(collective)
+		log.Printf("`%s`: %d paths instantiated, %d duplicate, dedup %.1f%%",
+			installable, l, l-len(collective), float64(l-len(collective))/float64(l)*100)
+		return nil
+	})
+
 	c.Command("instantiated", "Evaluate an installable and output all derivations instantiated during evaluation", func(args []string) error {
 		var installable string
 		if err := formatInstallable("instantiated", &installable, flagNixOS, args); err != nil {