internal/core/export: keep open lists open

When no default resolution is selected.

In the future we may make lists open by default if
they are not in definitions or within the scope of
a close builtin (like with structs).

Issue #867

Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>

Change-Id: I2c946b3cc18894f96f83f788684618cd2ebfadfd
Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/528464
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>

Author: mpvl

internal/core/export/testdata/adt.txtar

@@ -312,8 +312,8 @@ _|_ // e3: index out of range [2] with length 2
 		bar
 		line
 		"""
-	l1: [3]
-	l2: []
+	l1: [3, ...int]
+	l2: [...int]
 	l3: []
 	l4: [1, 2]
 	l5: {

internal/core/export/testdata/issue1284.txtar

@@ -82,22 +82,22 @@ _|_ // a2.l: conflicting values ["a","b",...] and {} (mismatched types list and
 == All
 {
 	a0: {
-		l: ["a", "b"]
+		l: ["a", "b", ...]
 	}
 	a1: {
-		l: ["a", "b"]
+		l: ["a", "b", ...]
 	}
 	a2: {
 		l: _|_ // a2.l: conflicting values ["a","b",...] and {} (mismatched types list and struct)
 	}
 	a3: {
-		l: ["a", "b"]
+		l: ["a", "b", ...]
 	}
 	a4: {
 		l: {
 			@decl(1)
 			#foo: 1
-			["a", "b"]
+			["a", "b", ...]
 		}
 	}
 }

internal/core/export/value.go

@@ -344,6 +344,21 @@ func (e *exporter) listComposite(v *adt.Vertex) ast.Expr {
 
 		l.Elts = append(l.Elts, elem)
 	}
+	m, ok := v.BaseValue.(*adt.ListMarker)
+	if !e.cfg.TakeDefaults && ok && m.IsOpen {
+		ellipsis := &ast.Ellipsis{}
+		typ := &adt.Vertex{
+			Parent: v,
+			Label:  adt.AnyIndex,
+		}
+		v.MatchAndInsert(e.ctx, typ)
+		typ.Finalize(e.ctx)
+		if typ.Kind() != adt.TopKind {
+			ellipsis.Type = e.value(typ)
+		}
+
+		l.Elts = append(l.Elts, ellipsis)
+	}
 	return l
 }