config: allow exact multi-resource references outside slices

config/config.go

@@ -455,7 +455,7 @@ func (c *Config) validateVarContextFn(
 			return
 		}
 
-		if rv.Multi && loc != reflectwalk.SliceElem {
+		if rv.Multi && rv.Index == -1 && loc != reflectwalk.SliceElem {
 			*errs = append(*errs, fmt.Errorf(
 				"%s: multi-variable must be in a slice", source))
 		}

config/config_test.go

@@ -221,6 +221,13 @@ func TestConfigValidate_varDefaultInterpolate(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_varMultiExactNonSlice(t *testing.T) {
+	c := testConfig(t, "validate-var-multi-exact-non-slice")
+	if err := c.Validate(); err != nil {
+		t.Fatalf("should be valid: %s", err)
+	}
+}
+
 func TestConfigValidate_varMultiNonSlice(t *testing.T) {
 	c := testConfig(t, "validate-var-multi-non-slice")
 	if err := c.Validate(); err == nil {

config/test-fixtures/validate-var-multi-exact-non-slice/main.tf

@@ -0,0 +1,7 @@
+resource "aws_instance" "foo" {
+    count = 3
+}
+
+resource "aws_instance" "bar" {
+    foo = "${aws_instance.foo.0.id}"
+}