fix(corpus-sweep): name every missing required flag, in flag order

cmd/internal-tools/corpus-sweep/main.go

@@ -154,16 +154,28 @@ func parseArguments(arguments []string, stderr io.Writer) (config, error) {
 	}
 	configuration.extraArguments = flagSet.Args()
 
-	for name, value := range map[string]string{
+	// Every missing flag is named together, in flag order: stopping at the
-		"--corpus": configuration.corpusRoot,
+	// first turns one rerun into one rerun per missing flag.
-		"--spec":   configuration.specPath,
+	var missing []error
-		"--seeds":  seedSpecification,
+	for _, required := range []struct {
-		"--output": configuration.outputDirectory,
+		name  string
+		value string
+	}{
+		{"--corpus", configuration.corpusRoot},
+		{"--spec", configuration.specPath},
+		{"--seeds", seedSpecification},
+		{"--output", configuration.outputDirectory},
 	} {
-		if value == "" {
+		if required.value == "" {
-			return config{}, fmt.Errorf("%s is required", name)
+			missing = append(
+				missing,
+				fmt.Errorf("%s is required", required.name),
+			)
 		}
 	}
+	if err := errors.Join(missing...); err != nil {
+		return config{}, err
+	}
 	if configuration.maxSteps <= 0 {
 		return config{}, fmt.Errorf(
 			"--max-steps must be positive: every implementation needs the same step budget",

cmd/internal-tools/corpus-sweep/main_test.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"io"
+	"strings"
+	"testing"
+)
+
+// Three flags missing is one rerun, not three: the operator is told about all
+// of them at once, in flag order, whatever order the check happened to walk.
+func TestParseArguments_NamesEveryMissingRequiredFlagInFlagOrder(t *testing.T) {
+	_, err := parseArguments(
+		[]string{"--spec", "s", "--max-steps", "10"},
+		io.Discard,
+	)
+	if err == nil {
+		t.Fatal("got no error, want every missing flag named")
+	}
+	message := err.Error()
+	previous := -1
+	for _, name := range []string{"--corpus", "--seeds", "--output"} {
+		at := strings.Index(message, name)
+		if at < 0 {
+			t.Fatalf("got %q, want %s named", message, name)
+		}
+		if at < previous {
+			t.Errorf("got %q, want the flags named in flag order", message)
+		}
+		previous = at
+	}
+	if strings.Contains(message, "--spec") {
+		t.Errorf("got %q, want the supplied --spec left out", message)
+	}
+}