command: validate backend config

The validation itself was added a couple weeks ago but I forgot to
actually call it. :sad:

command/meta_backend.go

@@ -237,6 +237,12 @@ func (m *Meta) backendConfig(opts *BackendOpts) (*config.Backend, error) {
 		backend.RawConfig = backend.RawConfig.Merge(rc)
 	}
 
+	// Validate the backend early. We have to do this before the normal
+	// config validation pass since backend loading happens earlier.
+	if errs := backend.Validate(); len(errs) > 0 {
+		return nil, multierror.Append(nil, errs...)
+	}
+
 	// Return the configuration which may or may not be set
 	return backend, nil
 }

command/meta_backend_test.go

@@ -267,6 +267,24 @@ func TestMetaBackend_emptyLegacyRemote(t *testing.T) {
 	}
 }
 
+// Verify that interpolations result in an error
+func TestMetaBackend_configureInterpolation(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("backend-new-interp"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	// Setup the meta
+	m := testMetaBackend(t, nil)
+
+	// Get the backend
+	_, err := m.Backend(&BackendOpts{Init: true})
+	if err == nil {
+		t.Fatal("should error")
+	}
+}
+
 // Newly configured backend
 func TestMetaBackend_configureNew(t *testing.T) {
 	// Create a temporary working directory that is empty

command/test-fixtures/backend-new-interp/main.tf

@@ -0,0 +1,7 @@
+variable "foo" { default = "bar" }
+
+terraform {
+    backend "local" {
+        path = "${var.foo}"
+    }
+}