Implement getField CEL function (#225)

I think it has to be a global overload, a member overload would require
a generic on the receiver which doesn't seem possible.

I'm proposing this as an eventual replacement (before 1.0) of our hack
around the fact that the `in` identifier is reserved in CEL. This is
especially urgent for protovalidate-cc which is currently carrying
patches to the CEL implementation in order to enable it, since cel-cpp
doesn't allow this sort of functionality to be added in at runtime.

Protovalidate PR: bufbuild/protovalidate#352

Author: jchadwick-buf

cel/library.go

@@ -78,6 +78,24 @@ func (l library) CompileOptions() []cel.EnvOption { //nolint:funlen,gocyclo
 			l.uniqueMemberOverload(cel.StringType, l.uniqueScalar),
 			l.uniqueMemberOverload(cel.BytesType, l.uniqueBytes),
 		),
+		cel.Function("getField",
+			cel.Overload(
+				"get_field_any_string",
+				[]*cel.Type{cel.AnyType, cel.StringType},
+				cel.AnyType,
+				cel.FunctionBinding(func(values ...ref.Val) ref.Val {
+					message, ok := values[0].(traits.Indexer)
+					if !ok {
+						return types.UnsupportedRefValConversionErr(values[0])
+					}
+					fieldName, ok := values[1].Value().(string)
+					if !ok {
+						return types.UnsupportedRefValConversionErr(values[1])
+					}
+					return message.Get(types.String(fieldName))
+				}),
+			),
+		),
 		cel.Function("isNan",
 			cel.MemberOverload(
 				"double_is_nan_bool",

cel/library_test.go

@@ -17,7 +17,10 @@ package cel
 import (
 	"testing"
 
+	"github.com/bufbuild/protovalidate-go/internal/gen/buf/validate/conformance/cases"
 	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
 	"github.com/google/cel-go/interpreter"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -26,15 +29,31 @@ import (
 func TestCELLib(t *testing.T) {
 	t.Parallel()
 
-	env, err := cel.NewEnv(cel.Lib(NewLibrary()))
+	testValue := cases.StringConst_builder{Val: "test_string"}.Build()
+
+	activation, err := interpreter.NewActivation(map[string]any{
+		"test": testValue,
+	})
+	require.NoError(t, err)
+
+	env, err := cel.NewEnv(
+		cel.Lib(NewLibrary()),
+		cel.Variable(
+			"test",
+			cel.ObjectType(
+				string(testValue.ProtoReflect().Descriptor().FullName()),
+			),
+		),
+	)
+
 	require.NoError(t, err)
 
 	t.Run("ext", func(t *testing.T) {
 		t.Parallel()
 
 		tests := []struct {
 			expr string
-			ex   bool
+			ex   any
 		}{
 			{"0.0.isInf()", false},
 			{"0.0.isNan()", false},
@@ -197,18 +216,34 @@ func TestCELLib(t *testing.T) {
 				"'[email protected]    '.isEmail()",
 				false,
 			},
+			{
+				"getField(test, 'val')",
+				"test_string",
+			},
+			{
+				"getField(test, 'lav')",
+				types.NewErrFromString("no such field"),
+			},
+			{
+				"getField(0, 'val')",
+				types.NewErrFromString("unsupported conversion"),
+			},
 		}
 
 		for _, tc := range tests {
 			test := tc
 			t.Run(test.expr, func(t *testing.T) {
 				t.Parallel()
 				prog := buildTestProgram(t, env, test.expr)
-				val, _, err := prog.Eval(interpreter.EmptyActivation())
-				require.NoError(t, err)
-				isUnique, ok := val.Value().(bool)
-				require.True(t, ok)
-				assert.Equal(t, test.ex, isUnique)
+				val, _, err := prog.Eval(activation)
+				if refEx, ok := test.ex.(ref.Val); ok && types.IsError(refEx) {
+					refErr, ok := refEx.Value().(error)
+					require.True(t, ok)
+					assert.ErrorContains(t, err, refErr.Error())
+				} else {
+					require.NoError(t, err)
+					assert.Equal(t, test.ex, val.Value())
+				}
 			})
 		}
 	})