functions: Improve marks support for length

Similar to cty's implementation, we only need to preserve marks from the
value itself, not any nested values it may contain. This means that
taking the length of an umarked list with marked elements results in an
unmarked number.

lang/funcs/collection.go

@@ -20,6 +20,7 @@ var LengthFunc = function.New(&function.Spec{
 			Type:             cty.DynamicPseudoType,
 			AllowDynamicType: true,
 			AllowUnknown:     true,
+			AllowMarked:      true,
 		},
 	},
 	Type: func(args []cty.Value) (cty.Type, error) {
@@ -34,15 +35,16 @@ var LengthFunc = function.New(&function.Spec{
 	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
 		coll := args[0]
 		collTy := args[0].Type()
+		marks := coll.Marks()
 		switch {
 		case collTy == cty.DynamicPseudoType:
-			return cty.UnknownVal(cty.Number), nil
+			return cty.UnknownVal(cty.Number).WithMarks(marks), nil
 		case collTy.IsTupleType():
 			l := len(collTy.TupleElementTypes())
-			return cty.NumberIntVal(int64(l)), nil
+			return cty.NumberIntVal(int64(l)).WithMarks(marks), nil
 		case collTy.IsObjectType():
 			l := len(collTy.AttributeTypes())
-			return cty.NumberIntVal(int64(l)), nil
+			return cty.NumberIntVal(int64(l)).WithMarks(marks), nil
 		case collTy == cty.String:
 			// We'll delegate to the cty stdlib strlen function here, because
 			// it deals with all of the complexities of tokenizing unicode

lang/funcs/collection_test.go

@@ -122,6 +122,54 @@ func TestLength(t *testing.T) {
 			cty.DynamicVal,
 			cty.UnknownVal(cty.Number),
 		},
+		{ // Marked collections return a marked length
+			cty.ListVal([]cty.Value{
+				cty.StringVal("hello"),
+				cty.StringVal("world"),
+			}).Mark("secret"),
+			cty.NumberIntVal(2).Mark("secret"),
+		},
+		{ // Marks on values in unmarked collections do not propagate
+			cty.ListVal([]cty.Value{
+				cty.StringVal("hello").Mark("a"),
+				cty.StringVal("world").Mark("b"),
+			}),
+			cty.NumberIntVal(2),
+		},
+		{ // Marked strings return a marked length
+			cty.StringVal("hello world").Mark("secret"),
+			cty.NumberIntVal(11).Mark("secret"),
+		},
+		{ // Marked tuples return a marked length
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("hello"),
+				cty.StringVal("world"),
+			}).Mark("secret"),
+			cty.NumberIntVal(2).Mark("secret"),
+		},
+		{ // Marks on values in unmarked tuples do not propagate
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("hello").Mark("a"),
+				cty.StringVal("world").Mark("b"),
+			}),
+			cty.NumberIntVal(2),
+		},
+		{ // Marked objects return a marked length
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("hello"),
+				"b": cty.StringVal("world"),
+				"c": cty.StringVal("nice to meet you"),
+			}).Mark("secret"),
+			cty.NumberIntVal(3).Mark("secret"),
+		},
+		{ // Marks on object attribute values do not propagate
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("hello").Mark("a"),
+				"b": cty.StringVal("world").Mark("b"),
+				"c": cty.StringVal("nice to meet you").Mark("c"),
+			}),
+			cty.NumberIntVal(3),
+		},
 	}
 
 	for _, test := range tests {