config: friendlier error message on resource arity mismatch

closes #2072

config/loader_hcl.go

@@ -406,8 +406,9 @@ func loadResourcesHcl(list *ast.ObjectList) ([]*Resource, error) {
 	// all of the actual resources.
 	for _, item := range list.Items {
 		if len(item.Keys) != 2 {
-			// TODO: bad error message
-			return nil, fmt.Errorf("resource needs exactly 2 names")
+			return nil, fmt.Errorf(
+				"position %s: resource must be followed by exactly two strings, a type and a name",
+				item.Pos())
 		}
 
 		t := item.Keys[0].Token.Value().(string)

config/loader_test.go

@@ -45,6 +45,17 @@ func TestLoadFile_badType(t *testing.T) {
 	}
 }
 
+func TestLoadFile_resourceArityMistake(t *testing.T) {
+	_, err := LoadFile(filepath.Join(fixtureDir, "resource-arity-mistake.tf"))
+	if err == nil {
+		t.Fatal("should have error")
+	}
+	expected := "Error loading test-fixtures/resource-arity-mistake.tf: position 2:10: resource must be followed by exactly two strings, a type and a name"
+	if err.Error() != expected {
+		t.Fatalf("expected:\n%s\ngot:\n%s", expected, err)
+	}
+}
+
 func TestLoadFileWindowsLineEndings(t *testing.T) {
 	testFile := filepath.Join(fixtureDir, "windows-line-endings.tf")
 

config/test-fixtures/resource-arity-mistake.tf

@@ -0,0 +1,5 @@
+# I forgot the resource name!
+resource "aws_instance" {
+  ami = "ami-abc123"
+  instance_type = "t2.micro"
+}