Implement predefined field constraints (#246)

- Renames `buf.validate.priv.FieldConstraints` to
`buf.validate.PredefinedConstraint`
- Renames `buf.validate.priv.field` to `buf.validate.predefined`
- Removes package `buf.validate.priv` to `buf.validate`
- Merges `buf/validate/expression.proto` to
`buf/validate/validate.proto`
- Removes `//proto/protovalidate/buf/validate:expression_proto`
- Switches `buf/validate/validate.proto` from `proto3` syntax to
`proto2` syntax to enable usage of extensions
- Adds `extension 1000 to max` to each `...Rules` message
- Adds test cases for valid and invalid predefined constraints of every
possible rule type for both proto2 and protobuf edition 2023
- Added the CEL constant `rule` that can be used by predefined
constraints to refer to themselves _specifically_.

A predefined constraint in this schema looks like this:

```proto
edition = "2023";

package example.v1;

import "buf/validate/validate.proto";

extend buf.validate.StringRules {
  bool valid_path = 1162 [
    (buf.validate.predefined).cel = {
      id: "string.valid_path"
      expression: "!rule && !this.matches('^([^/.][^/]?|[^/][^/.]|[^/]{3,})(/([^/.][^/]?|[^/][^/.]|[^/]{3,}))*$') ? 'not a valid path: `%s`'.format([this]) : ''"
    }
  ];
}

message File {
  string path = 1 [(buf.validate.field).string.(valid_path) = true];
}
```

**This is a breaking change.**

docs/README.md

@@ -27,6 +27,12 @@ library.
       This guide will help you understand when and how to use these standard
       constraints effectively.
 
+    - [Predefined Constraints](predefined-constraints.md): This section
+      discusses how to extend `protovalidate` with custom reusable rules that
+      behave similarly to the standard constraints. This can be useful in order
+      to share similar custom validation logic across multiple fields or
+      messages.
+
 - [Errors](errors.md): This section explains the error system in `protovalidate`
   and provides guidance on how to handle them effectively.
 
@@ -38,7 +44,7 @@ designed to help you implement new language support and assist in migrating your
 existing projects to `protovalidate`.
 
 - [Conformance](conformance.md): This document is dedicated to explaining the
-  Conformance tool. Learn how to use this tool to ensure all implementations 
+  Conformance tool. Learn how to use this tool to ensure all implementations
   align with `protovalidate`'s rules and constraints effectively.
 
 - [Migrate](migrate.md): If you're planning to migrate your existing project to

docs/predefined-constraints.md

@@ -0,0 +1,67 @@
+# Predefined Constraints
+
+Custom constraints in `protovalidate` afford a lot of power, but can become
+cumbersome and repetitive when the same kind of custom constraints are needed
+across multiple fields or messages. To this end, `protovalidate` provides a
+mechanism for creating reusable constraints that can be applied on multiple
+fields.
+
+Predefined constraints require Protobuf extensions, which are not available in
+proto3. Either proto2 or at least Protobuf 2023 Edition must be used to define
+predefined constraints. Predefined constraints defined in proto2 or Protobuf
+2023 Edition or later can be imported and utilized in proto3 files.
+
+## Predefined Field Constraints
+
+To create a predefined field constraint, extend one of the standard rules
+messages. For example, to define a new rule for `float` fields, extend
+`buf.validate.FloatRules`, as follows:
+
+```proto
+import "buf/validate/validate.proto";
+
+extend buf.validate.FloatRules {
+  float abs_range = 80048952 [(buf.validate.predefined).cel = {
+    id: "float.abs_range"
+    expression: "this >= -rule && this <= rule"
+    message: "float value is out of range"
+  }];
+}
+```
+
+> [!TIP]
+> Constraints can refer to their own value with the `rule` constant. Rules apply
+> when they are set, so a boolean constraint in the form of `is_...` should
+> always check to ensure that `rule` is `true`.
+
+> [!WARNING]
+> Be mindful that extension numbers must not conflict with any other extension
+> to the same message across _all_ Protobuf files in a given process. This
+> restriction also applies to users that consume Protobuf files indirectly as
+> dependencies. The same extension number may be re-used across different kinds
+> of constraint, e.g. `1000` in `FloatRules` is distinct from `1000` in
+> `Int32Rules`.
+>
+> Extension numbers may be from 1000 to 536870911, inclusive. Values from 1000
+> to 99999 are reserved for [Protobuf Global Extension Registry][1] entries, and
+> values from 100000 to 536870911 are reserved for integers that are not
+> explicitly assigned. It is discouraged to use the latter range for rules that
+> are defined in public schemas due to the risk of conflicts. 
+
+[1]: https://github.com/protocolbuffers/protobuf/blob/main/docs/options.md "Protobuf Global Extension Registry"
+
+Similarly to the standard constraints, a rule will take effect when it is set on
+the options of a field. Here is how one might use a predefined constraint:
+
+```proto
+message MyMessage {
+  float normal_value = 1 [(buf.validate.field).float.(abs_range) = 1.0];
+}
+```
+
+> [!TIP]
+> Extensions are always qualified by the package they are defined in. In this
+> example, we assume that `abs_range` is defined in the same package it is used
+> in, so no further qualification is needed. In other cases, you will need to
+> qualify the package name of the extension, e.g.
+> `(buf.validate.field).float.(foo.bar.abs_range)`

proto/protovalidate-testing/buf/validate/conformance/cases/BUILD.bazel

@@ -33,6 +33,9 @@ proto_library(
         "messages.proto",
         "numbers.proto",
         "oneofs.proto",
+        "predefined_rules_proto2.proto",
+        "predefined_rules_proto3.proto",
+        "predefined_rules_proto_editions.proto",
         "repeated.proto",
         "required_field_proto2.proto",
         "required_field_proto3.proto",

proto/protovalidate-testing/buf/validate/conformance/cases/predefined_rules_proto2.proto

@@ -0,0 +1,288 @@
+// Copyright 2023 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto2";
+
+package buf.validate.conformance.cases;
+
+import "buf/validate/validate.proto";
+import "google/protobuf/duration.proto";
+import "google/protobuf/timestamp.proto";
+
+extend buf.validate.FloatRules {
+  optional float float_abs_range_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "float.abs_range.proto2"
+    expression: "this >= -rule && this <= rule"
+    message: "float value is out of range"
+  }];
+}
+
+extend buf.validate.DoubleRules {
+  optional double double_abs_range_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "double.abs_range.proto2"
+    expression: "this >= -rule && this <= rule"
+    message: "double value is out of range"
+  }];
+}
+
+extend buf.validate.Int32Rules {
+  optional bool int32_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "int32.even.proto2"
+    expression: "this % 2 == 0"
+    message: "int32 value is not even"
+  }];
+}
+
+extend buf.validate.Int64Rules {
+  optional bool int64_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "int64.even.proto2"
+    expression: "this % 2 == 0"
+    message: "int64 value is not even"
+  }];
+}
+
+extend buf.validate.UInt32Rules {
+  optional bool uint32_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "uint32.even.proto2"
+    expression: "this % 2u == 0u"
+    message: "uint32 value is not even"
+  }];
+}
+
+extend buf.validate.UInt64Rules {
+  optional bool uint64_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "uint64.even.proto2"
+    expression: "this % 2u == 0u"
+    message: "uint64 value is not even"
+  }];
+}
+
+extend buf.validate.SInt32Rules {
+  optional bool sint32_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "sint32.even.proto2"
+    expression: "this % 2 == 0"
+    message: "sint32 value is not even"
+  }];
+}
+
+extend buf.validate.SInt64Rules {
+  optional bool sint64_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "sint64.even.proto2"
+    expression: "this % 2 == 0"
+    message: "sint64 value is not even"
+  }];
+}
+
+extend buf.validate.Fixed32Rules {
+  optional bool fixed32_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "fixed32.even.proto2"
+    expression: "this % 2u == 0u"
+    message: "fixed32 value is not even"
+  }];
+}
+
+extend buf.validate.Fixed64Rules {
+  optional bool fixed64_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "fixed64.even.proto2"
+    expression: "this % 2u == 0u"
+    message: "fixed64 value is not even"
+  }];
+}
+
+extend buf.validate.SFixed32Rules {
+  optional bool sfixed32_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "sfixed32.even.proto2"
+    expression: "this % 2 == 0"
+    message: "sfixed32 value is not even"
+  }];
+}
+
+extend buf.validate.SFixed64Rules {
+  optional bool sfixed64_even_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "sfixed64.even.proto2"
+    expression: "this % 2 == 0"
+    message: "sfixed64 value is not even"
+  }];
+}
+
+extend buf.validate.BoolRules {
+  optional bool bool_false_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "bool.false.proto2"
+    expression: "this == false"
+    message: "bool value is not false"
+  }];
+}
+
+extend buf.validate.StringRules {
+  optional bool string_valid_path_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "string.valid_path.proto2"
+    expression: "!this.matches('^([^/.][^/]?|[^/][^/.]|[^/]{3,})(/([^/.][^/]?|[^/][^/.]|[^/]{3,}))*$') ? 'not a valid path: `%s`'.format([this]) : ''"
+  }];
+}
+
+extend buf.validate.BytesRules {
+  optional bool bytes_valid_path_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "bytes.valid_path.proto2"
+    expression: "!string(this).matches('^([^/.][^/]?|[^/][^/.]|[^/]{3,})(/([^/.][^/]?|[^/][^/.]|[^/]{3,}))*$') ? 'not a valid path: `%s`'.format([this]) : ''"
+  }];
+}
+
+extend buf.validate.EnumRules {
+  optional bool enum_non_zero_proto2 = 1161 [(buf.validate.predefined).cel = {
+    id: "enum.non_zero.proto2"
+    expression: "int(this) != 0"
+    message: "enum value is not non-zero"
+  }];
+}
+
+extend buf.validate.RepeatedRules {
+  optional bool repeated_at_least_five_proto2 = 1161 [(predefined).cel = {
+    id: "repeated.at_least_five.proto2"
+    expression: "uint(this.size()) >= 5u"
+    message: "repeated field must have at least five values"
+  }];
+}
+
+extend buf.validate.DurationRules {
+  optional bool duration_too_long_proto2 = 1161 [(predefined).cel = {
+    id: "duration.too_long.proto2"
+    expression: "this <= duration('10s')"
+    message: "duration can't be longer than 10 seconds"
+  }];
+}
+
+extend buf.validate.TimestampRules {
+  optional bool timestamp_in_range_proto2 = 1161 [(predefined).cel = {
+    id: "timestamp.time_range.proto2"
+    expression: "int(this) >= 1049587200 && int(this) <= 1080432000"
+    message: "timestamp out of range"
+  }];
+}
+
+message PredefinedFloatRuleProto2 {
+  optional float val = 1 [(buf.validate.field).float.(float_abs_range_proto2) = 1.0];
+}
+
+message PredefinedDoubleRuleProto2 {
+  optional double val = 1 [(buf.validate.field).double.(double_abs_range_proto2) = 1.0];
+}
+
+message PredefinedInt32RuleProto2 {
+  optional int32 val = 1 [(buf.validate.field).int32.(int32_even_proto2) = true];
+}
+
+message PredefinedInt64RuleProto2 {
+  optional int64 val = 1 [(buf.validate.field).int64.(int64_even_proto2) = true];
+}
+
+message PredefinedUInt32RuleProto2 {
+  optional uint32 val = 1 [(buf.validate.field).uint32.(uint32_even_proto2) = true];
+}
+
+message PredefinedUInt64RuleProto2 {
+  optional uint64 val = 1 [(buf.validate.field).uint64.(uint64_even_proto2) = true];
+}
+
+message PredefinedSInt32RuleProto2 {
+  optional sint32 val = 1 [(buf.validate.field).sint32.(sint32_even_proto2) = true];
+}
+
+message PredefinedSInt64RuleProto2 {
+  optional sint64 val = 1 [(buf.validate.field).sint64.(sint64_even_proto2) = true];
+}
+
+message PredefinedFixed32RuleProto2 {
+  optional fixed32 val = 1 [(buf.validate.field).fixed32.(fixed32_even_proto2) = true];
+}
+
+message PredefinedFixed64RuleProto2 {
+  optional fixed64 val = 1 [(buf.validate.field).fixed64.(fixed64_even_proto2) = true];
+}
+
+message PredefinedSFixed32RuleProto2 {
+  optional sfixed32 val = 1 [(buf.validate.field).sfixed32.(sfixed32_even_proto2) = true];
+}
+
+message PredefinedSFixed64RuleProto2 {
+  optional sfixed64 val = 1 [(buf.validate.field).sfixed64.(sfixed64_even_proto2) = true];
+}
+
+message PredefinedBoolRuleProto2 {
+  optional bool val = 1 [(buf.validate.field).bool.(bool_false_proto2) = true];
+}
+
+message PredefinedStringRuleProto2 {
+  optional string val = 1 [(buf.validate.field).string.(string_valid_path_proto2) = true];
+}
+
+message PredefinedBytesRuleProto2 {
+  optional bytes val = 1 [(buf.validate.field).bytes.(bytes_valid_path_proto2) = true];
+}
+
+message PredefinedEnumRuleProto2 {
+  enum EnumProto2 {
+    ENUM_PROTO2_ZERO_UNSPECIFIED = 0;
+    ENUM_PROTO2_ONE = 1;
+  }
+  optional EnumProto2 val = 1 [(buf.validate.field).enum.(enum_non_zero_proto2) = true];
+}
+
+message PredefinedRepeatedRuleProto2 {
+  repeated uint64 val = 1 [(buf.validate.field).repeated.(repeated_at_least_five_proto2) = true];
+}
+
+message PredefinedDurationRuleProto2 {
+  optional google.protobuf.Duration val = 1 [(buf.validate.field).duration.(duration_too_long_proto2) = true];
+}
+
+message PredefinedTimestampRuleProto2 {
+  optional google.protobuf.Timestamp val = 1 [(buf.validate.field).timestamp.(timestamp_in_range_proto2) = true];
+}
+
+message PredefinedAndCustomRuleProto2 {
+  optional int32 a = 1 [
+    (field).cel = {
+      id: "predefined_and_custom_rule_scalar_proto2"
+      expression: "this > 24 ? '' : 'a must be greater than 24'"
+    },
+    (field).int32.(int32_even_proto2) = true
+  ];
+
+  optional Nested b = 2 [(field).cel = {
+    id: "predefined_and_custom_rule_embedded_proto2"
+    message: "b.c must be a multiple of 3"
+    expression: "this.c % 3 == 0"
+  }];
+
+  message Nested {
+    optional int32 c = 1 [
+      (field).cel = {
+        id: "predefined_and_custom_rule_nested_proto2"
+        expression: "this > 0 ? '' : 'c must be positive'"
+      },
+      (field).int32.(int32_even_proto2) = true
+    ];
+  }
+}
+
+message StandardPredefinedAndCustomRuleProto2 {
+  optional int32 a = 1 [
+    (field).int32.lt = 28,
+    (field).int32.(int32_even_proto2) = true,
+    (field).cel = {
+      id: "standard_predefined_and_custom_rule_scalar_proto2"
+      expression: "this > 24 ? '' : 'a must be greater than 24'"
+    }
+  ];
+}