feature: add option to extend CEL environment

proto/tests/example/v1/example.proto

@@ -19,7 +19,14 @@ package tests.example.v1;
 import "buf/validate/validate.proto";
 
 message Person {
-  uint64 id = 1 [(buf.validate.field).uint64.gt = 999];
+  uint64 id = 1 [
+    (buf.validate.field).cel = {
+      id: "person.id",
+      expression:
+        "this <= PersonMinId"
+        "? 'value must be greater than %s'.format([PersonMinId]) : ''"
+    }
+  ];
 
   string email = 2 [(buf.validate.field).string.email = true];
 

validator.go

@@ -22,6 +22,7 @@ import (
 	"github.com/bufbuild/protovalidate-go/internal/errors"
 	"github.com/bufbuild/protovalidate-go/internal/evaluator"
 	"github.com/bufbuild/protovalidate-go/resolver"
+	"github.com/google/cel-go/cel"
 	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/reflect/protoreflect"
 )
@@ -70,6 +71,12 @@ func New(options ...ValidatorOption) (*Validator, error) {
 			"failed to construct CEL environment: %w", err)
 	}
 
+	env, err = env.Extend(cfg.celEnvOpts...)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"failed to extend CEL environment: %w", err)
+	}
+
 	bldr := evaluator.NewBuilder(
 		env,
 		cfg.disableLazy,
@@ -104,6 +111,7 @@ type config struct {
 	disableLazy bool
 	desc        []protoreflect.MessageDescriptor
 	resolver    StandardConstraintResolver
+	celEnvOpts  []cel.EnvOption
 }
 
 // A ValidatorOption modifies the default configuration of a Validator. See the
@@ -183,3 +191,10 @@ func WithStandardConstraintInterceptor(interceptor StandardConstraintInterceptor
 		c.resolver = interceptor(c.resolver)
 	}
 }
+
+// WithExtendedCelEnv allows to extend CEL environment used for validation with user provided options
+func WithExtendedCelEnv(celEnvOpts ...cel.EnvOption) ValidatorOption {
+	return func(cfg *config) {
+		cfg.celEnvOpts = celEnvOpts
+	}
+}

validator_example_test.go

@@ -20,11 +20,19 @@ import (
 	"log"
 
 	pb "github.com/bufbuild/protovalidate-go/internal/gen/tests/example/v1"
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/types"
 	"google.golang.org/protobuf/reflect/protoregistry"
 )
 
+var (
+	PersonMinId = 999
+)
+
 func Example() {
-	validator, err := New()
+	validator, err := New(WithExtendedCelEnv(
+		cel.Constant("PersonMinId", cel.UintType, types.Uint(PersonMinId)),
+	))
 	if err != nil {
 		log.Fatal(err)
 	}