format: wrap nix command

format.go

@@ -1,5 +1,23 @@
 package nixbuild
 
+import (
+	"context"
+	"os/exec"
+)
+
+const (
+	nixExtraExperimentalFeatures  = "--extra-experimental-features"
+	nixExperimentalFeaturesFlakes = "nix-command flakes"
+)
+
+// since flakes are supposedly experimental
+var nixEnableFlakes = []string{nixExtraExperimentalFeatures, nixExperimentalFeaturesFlakes}
+
+// Nix returns the [exec.Cmd] struct to execute a nix command.
+func Nix(ctx context.Context, arg ...string) *exec.Cmd {
+	return exec.CommandContext(ctx, "nix", append(nixEnableFlakes, arg...)...)
+}
+
 const (
 	nixosSuffix0 = "#nixosConfigurations."
 	nixosSuffix1 = ".config.system.build.toplevel"

format_test.go

@@ -1,11 +1,34 @@
 package nixbuild_test
 
 import (
+	"slices"
 	"testing"
 
 	"git.gensokyo.uk/yonah/nixbuild"
 )
 
+func TestNix(t *testing.T) {
+	testCases := []struct {
+		name string
+		arg  []string
+		want []string
+	}{
+		{"build", []string{"build"},
+			[]string{"nix", "--extra-experimental-features", "nix-command flakes", "build"}},
+		{"build workflow", []string{"build", `--out-link "result"`, "--print-out-paths", "--print-build-logs"},
+			[]string{"nix", "--extra-experimental-features", "nix-command flakes", "build", `--out-link "result"`, "--print-out-paths", "--print-build-logs"}},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := nixbuild.Nix(t.Context(), tc.arg...)
+			if !slices.Equal(got.Args, tc.want) {
+				t.Errorf("Nix: %#v, want %#v",
+					got.Args, tc.want)
+			}
+		})
+	}
+}
+
 func TestInstallable(t *testing.T) {
 	testCases := []struct {
 		name, flake, host, want string

instantiated.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"errors"
 	"io"
-	"os/exec"
 	"path"
 	"slices"
 	"strings"
@@ -105,10 +104,7 @@ func EvalInstantiated(ctx context.Context, installable string) ([]string, error)
 	c, cancel := context.WithCancel(ctx)
 	defer cancel()
 
-	cmd := exec.CommandContext(c,
-		"nix", "build", installable,
-		// since flakes are supposedly experimental
-		"--extra-experimental-features", "nix-command flakes",
+	cmd := Nix(c, "build", installable,
 		// 'instantiated' messages are only emitted when actually evaluating something
 		"--option", "eval-cache", "false",
 		// do not actually build anything

instantiated_test.go

@@ -2,6 +2,7 @@ package nixbuild_test
 
 import (
 	"errors"
+	"os"
 	"runtime"
 	"slices"
 	"strings"
@@ -42,6 +43,12 @@ func TestDecodeInstantiated(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			if tc.wantErr != nil {
+				w := nixbuild.Stderr
+				nixbuild.Stderr = os.Stderr
+				t.Cleanup(func() { nixbuild.Stderr = w })
+			}
+
 			stderr := strings.NewReader(tc.out)
 			got, err := nixbuild.DecodeInstantiated(stderr)
 			if !errors.Is(err, tc.wantErr) {
@@ -70,6 +77,16 @@ func TestDecodeInstantiated(t *testing.T) {
 				t.Error("unexpected MalformedInstantiatedError equivalence")
 			}
 		})
+
+		t.Run("unreachable", func(t *testing.T) {
+			defer func() {
+				wantPanic := "unreachable"
+				if r := recover(); r != wantPanic {
+					t.Errorf("Error: panic = %q, want %q", r, wantPanic)
+				}
+			}()
+			_ = (&nixbuild.MalformedInstantiatedError{Type: -1}).Error()
+		})
 	})
 }