Add an Env.Extend method. (#271)

* Introduce a new method to create a new Env configured with additional options.
* Update proto formatting to match best practices.

cel/cel_test.go

@@ -564,6 +564,21 @@ func Benchmark_EvalOptions(b *testing.B) {
 	}
 }
 
+func Test_EnvExtension(t *testing.T) {
+	e, _ := NewEnv(
+		Container("google.api.expr.v1alpha1"),
+		Types(&exprpb.Expr{}),
+		Declarations(
+			decls.NewIdent("expr",
+				decls.NewObjectType("google.api.expr.v1alpha1.Expr"), nil),
+		),
+	)
+	e2, _ := e.Extend()
+	if e == e2 {
+		t.Error("Got object equality, wanted separate objects")
+	}
+}
+
 func Test_ParseAndCheckConcurrently(t *testing.T) {
 	e, _ := NewEnv(
 		Container("google.api.expr.v1alpha1"),

cel/env.go

@@ -61,6 +61,9 @@ type Ast interface {
 // constants, variables, and functions. The Env interface also defines a method for generating
 // evaluable programs from parsed and checked Asts.
 type Env interface {
+	// Extend the current environment with additional options to produce a new Env.
+	Extend(opts ...EnvOption) (Env, error)
+
 	// Check performs type-checking on the input Ast and yields a checked Ast and/or set of Issues.
 	//
 	// Checking has failed if the returned Issues value and its Issues.Err() value is non-nil.
@@ -109,15 +112,26 @@ type Issues interface {
 // See the EnvOptions for the options that can be used to configure the environment.
 func NewEnv(opts ...EnvOption) (Env, error) {
 	registry := types.NewRegistry()
-	e := &env{
+	return (&env{
 		declarations:                   checker.StandardDeclarations(),
 		macros:                         parser.AllMacros,
 		pkg:                            packages.DefaultPackage,
 		provider:                       registry,
 		adapter:                        registry,
 		enableBuiltins:                 true,
 		enableDynamicAggregateLiterals: true,
-	}
+	}).configure(opts...)
+}
+
+// Extend the current environment with additional options to produce a new Env.
+func (e *env) Extend(opts ...EnvOption) (Env, error) {
+	ext := &env{}
+	*ext = *e
+	return ext.configure(opts...)
+}
+
+// configure applies a series of EnvOptions to the current environment.
+func (e *env) configure(opts ...EnvOption) (Env, error) {
 	// Customized the environment using the provided EnvOption values. If an error is
 	// generated at any step this, will be returned as a nil Env with a non-nil error.
 	var err error