config: validate output only has "value" field

config/config.go

@@ -146,6 +146,19 @@ func (c *Config) Validate() error {
 	}
 
 	// Check that all outputs are valid
+	for _, o := range c.Outputs {
+		invalid := false
+		for k, _ := range o.RawConfig.Raw {
+			if k != "value" {
+				invalid = true
+				break
+			}
+		}
+		if invalid {
+			errs = append(errs, fmt.Errorf(
+				"%s: output should only have 'value' field", o.Name))
+		}
+	}
 
 	if len(errs) > 0 {
 		return &multierror.Error{Errors: errs}

config/config_test.go

@@ -15,6 +15,13 @@ func TestConfigValidate(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_outputBadField(t *testing.T) {
+	c := testConfig(t, "validate-output-bad-field")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_unknownResourceVar(t *testing.T) {
 	c := testConfig(t, "validate-unknown-resource-var")
 	if err := c.Validate(); err == nil {

config/test-fixtures/validate-output-bad-field/main.tf

@@ -0,0 +1,7 @@
+resource "aws_instance" "web" {
+}
+
+output "ip" {
+  value = "foo"
+  another = "nope"
+}