cue/format: fix simplification bug

Closes Issue #294

Change-Id: If7ac761d9cea5745f273f5c4ced11509d08efd80
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5061
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cue/format/simplify.go b/cue/format/simplify.go
index 410911b..2152ac4 100644
--- a/cue/format/simplify.go
+++ b/cue/format/simplify.go
@@ -95,7 +95,7 @@
 	case *ast.Ident:
 		s.scope[x.Name] = true
 
-	case *ast.ListLit:
+	case *ast.ListLit, *ast.Interpolation:
 		return false
 	}
 	return true
diff --git a/cue/format/testdata/simplify.golden b/cue/format/testdata/simplify.golden
index 53f7a18..84506b9 100644
--- a/cue/format/testdata/simplify.golden
+++ b/cue/format/testdata/simplify.golden
@@ -27,6 +27,9 @@
 // TODO(legacy): Don't simplify "hidden" fields for now.
 "_foo": 3
 
+// Issue #294
+"\("x")": "x"
+
 x: {
 	@tag0(foo)
 	r1: baz1
diff --git a/cue/format/testdata/simplify.input b/cue/format/testdata/simplify.input
index d642ae1..271c6a0 100644
--- a/cue/format/testdata/simplify.input
+++ b/cue/format/testdata/simplify.input
@@ -29,6 +29,9 @@
 // TODO(legacy): Don't simplify "hidden" fields for now.
 "_foo": 3
 
+// Issue #294
+"\("x")": "x"
+
 x: {
 @tag0(foo)
     r1: baz1