internal/cuetxtar: allow inline archives

It is often handy allow testing of an isolated test
for debugging purposes. This change allows any
txtar test to be turned into such a test. This avoids
having to write a separate TestX-like test, like we have
now. TestX-style may deviate a little bit from a txtar-
based test, making it hard to track down bugs.

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I707197788c4be941a317145e107aff9da6605794
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194650
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

internal/cuetxtar/txtar.go

@@ -74,6 +74,10 @@ type TxTarTest struct {
 	// If Matrix is non-nil, the tests are run for each configuration in the
 	// matrix.
 	Matrix cuetdtest.Matrix
+
+	// DebugArchive, if set, is loaded instead of the on-disk archive. This allows
+	// a test to be used for debugging.
+	DebugArchive string
 }
 
 // A Test represents a single test based on a .txtar file.
@@ -358,6 +362,31 @@ func (t *Test) Context() *cue.Context {
 func (x *TxTarTest) run(t *testing.T, f func(tc *Test)) {
 	t.Helper()
 
+	if x.DebugArchive != "" {
+		archive := txtar.Parse([]byte(x.DebugArchive))
+
+		t.Run("", func(t *testing.T) {
+			tc := &Test{
+				T:       t,
+				Archive: archive,
+				Dir:     "/tmp",
+
+				prefix:     path.Join("out", x.Name),
+				LoadConfig: x.LoadConfig,
+			}
+			// Don't let Env be nil, as the tests shouldn't depend on os.Environ.
+			if tc.LoadConfig.Env == nil {
+				tc.LoadConfig.Env = []string{}
+			}
+
+			f(tc)
+
+			// Unconditionally output as an error.
+			t.Error(tc.buf.String())
+		})
+		return
+	}
+
 	dir, err := os.Getwd()
 	if err != nil {
 		t.Fatal(err)