Improve error message for unknown fields

Author: jchadwick-buf

internal/constraints/cache.go

@@ -63,7 +63,7 @@ func (c *Cache) Build(
 		return nil, errors.NewCompilationErrorf("error reparsing message: %w", err)
 	}
 	if !allowUnknownFields && len(constraints.GetUnknown()) > 0 {
-		return nil, errors.NewCompilationErrorf("unknown constraints in %s", constraints.Descriptor().FullName())
+		return nil, errors.NewCompilationErrorf("unknown constraints in %s; see protovalidate.WithExtensionTypeResolver", constraints.Descriptor().FullName())
 	}
 
 	env, err = c.prepareEnvironment(env, fieldDesc, constraints, forItems)

validator.go

@@ -195,6 +195,10 @@ func WithStandardConstraintInterceptor(interceptor StandardConstraintInterceptor
 // WithExtensionTypeResolver specifies a resolver to use when reparsing unknown
 // extension types. When dealing with dynamic file descriptor sets, passing this
 // option will allow extensions to be resolved using a custom resolver.
+//
+// To ignore unknown extension fields, use the [WithAllowUnknownFields] option.
+// Note that this may result in messages being treated as valid even though not
+// all constraints are being applied.
 func WithExtensionTypeResolver(extensionTypeResolver protoregistry.ExtensionTypeResolver) ValidatorOption {
 	return func(c *config) {
 		c.extensionTypeResolver = extensionTypeResolver