cmd/cue/cmd: avoid roundtrip when printing non-CUE in eval

Fixes #986

Change-Id: I85961ab2fc23828ad44814479e7057723aed03de
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9862
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/eval.go

@@ -20,6 +20,7 @@ import (
 	"github.com/spf13/cobra"
 
 	"cuelang.org/go/cue"
+	"cuelang.org/go/cue/build"
 	"cuelang.org/go/cue/format"
 	"cuelang.org/go/internal"
 	"cuelang.org/go/internal/encoding"
@@ -117,19 +118,27 @@ func runEval(cmd *Command, args []string) error {
 		}
 		v := iter.value()
 
+		errHeader := func() {
+			if id != "" {
+				fmt.Fprintf(cmd.OutOrStderr(), "// %s\n", id)
+			}
+		}
+		if b.outFile.Encoding != build.CUE {
+			err := e.Encode(v)
+			if err != nil {
+				errHeader()
+				exitOnErr(cmd, err, false)
+			}
+			continue
+		}
+
 		if flagConcrete.Bool(cmd) {
 			syn = append(syn, cue.Concrete(true))
 		}
 		if flagHidden.Bool(cmd) || flagAll.Bool(cmd) {
 			syn = append(syn, cue.Hidden(true))
 		}
 
-		errHeader := func() {
-			if id != "" {
-				fmt.Fprintf(cmd.OutOrStderr(), "// %s\n", id)
-			}
-		}
-
 		if len(b.expressions) > 1 {
 			b, _ := format.Node(b.expressions[i%len(b.expressions)])
 			id = string(b)

cmd/cue/cmd/testdata/script/issue986.txt

@@ -0,0 +1,36 @@
+
+# Hidden values are dropped when outputting CUE. This is fine in eval for
+# debugging, but not when the final result needs to be compiled again to be
+# converted to another format.
+
+cue eval in.cue --out yaml
+cmp stdout expect-stdout
+
+-- in.cue --
+#Foo: {
+	a:  string
+	b:  string
+	ab: "\(a)\(b)"
+}
+
+{
+	a: "aaa"
+	b: "bbb"
+} & #Foo
+
+#Bar: {
+	_c: string
+	_d: string
+	cd: "\(_c)\(_d)"
+}
+
+{
+	_c: "ccc"
+	_d: "ddd"
+} & #Bar
+
+-- expect-stdout --
+a: aaa
+b: bbb
+ab: aaabbb
+cd: cccddd