Add field and rule value to violations (#215)

Adds the ability to access the captured rule and field value from a
Violation.

There's a bit of refactoring toil, so I tried to split the commits
cleanly to make this easier to review.
- The first commit adds the `Violation` wrapper class and plumbs it
through all uses of `Violation`.
- The second commit makes `Value` non-internal so we can use it to
expose protobuf values in a cleaner fashion.
- The third commit actually implements filling the `fieldValue` and
`ruleValue`fields.

**This is a breaking change.** The API changes in the following ways:
- `ValidationResult` now provides a new wrapper `Violation` type instead
of the `buf.validate.Violation` message. This new wrapper has a
`getProto()` method to return a `buf.validate.Violation` message, and
`ValidationResult` now has a `toProto()` method to return a
`buf.validate.Violations` message.

conformance/src/main/java/build/buf/protovalidate/conformance/Main.java

@@ -20,7 +20,6 @@
 import build.buf.protovalidate.exceptions.CompilationException;
 import build.buf.protovalidate.exceptions.ExecutionException;
 import build.buf.validate.ValidateProto;
-import build.buf.validate.Violation;
 import build.buf.validate.Violations;
 import build.buf.validate.conformance.harness.TestConformanceRequest;
 import build.buf.validate.conformance.harness.TestConformanceResponse;
@@ -100,11 +99,11 @@ static TestResult testCase(
   private static TestResult validate(Validator validator, DynamicMessage dynamicMessage) {
     try {
       ValidationResult result = validator.validate(dynamicMessage);
-      List<Violation> violations = result.getViolations();
-      if (violations.isEmpty()) {
+      if (result.isSuccess()) {
         return TestResult.newBuilder().setSuccess(true).build();
       }
-      Violations error = Violations.newBuilder().addAllViolations(violations).build();
+      Violations error =
+          Violations.newBuilder().addAllViolations(result.toProto().getViolationsList()).build();
       return TestResult.newBuilder().setValidationError(error).build();
     } catch (CompilationException e) {
       return TestResult.newBuilder().setCompilationError(e.getMessage()).build();

src/main/java/build/buf/protovalidate/ValidationResult.java

@@ -14,7 +14,8 @@
 
 package build.buf.protovalidate;
 
-import build.buf.validate.Violation;
+import build.buf.validate.Violations;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 
@@ -71,12 +72,27 @@ public String toString() {
     builder.append("Validation error:");
     for (Violation violation : violations) {
       builder.append("\n - ");
-      if (!violation.getFieldPath().isEmpty()) {
-        builder.append(violation.getFieldPath());
+      if (!violation.toProto().getFieldPath().isEmpty()) {
+        builder.append(violation.toProto().getFieldPath());
         builder.append(": ");
       }
-      builder.append(String.format("%s [%s]", violation.getMessage(), violation.getConstraintId()));
+      builder.append(
+          String.format(
+              "%s [%s]", violation.toProto().getMessage(), violation.toProto().getConstraintId()));
     }
     return builder.toString();
   }
+
+  /**
+   * Converts the validation result to its equivalent protobuf form.
+   *
+   * @return The protobuf form of this validation result.
+   */
+  public build.buf.validate.Violations toProto() {
+    List<build.buf.validate.Violation> protoViolations = new ArrayList<>();
+    for (Violation violation : violations) {
+      protoViolations.add(violation.toProto());
+    }
+    return Violations.newBuilder().addAllViolations(protoViolations).build();
+  }
 }

src/main/java/build/buf/protovalidate/Validator.java

@@ -17,12 +17,14 @@
 import build.buf.protovalidate.exceptions.CompilationException;
 import build.buf.protovalidate.exceptions.ValidationException;
 import build.buf.protovalidate.internal.celext.ValidateLibrary;
-import build.buf.protovalidate.internal.errors.FieldPathUtils;
+import build.buf.protovalidate.internal.errors.ConstraintViolation;
 import build.buf.protovalidate.internal.evaluator.Evaluator;
 import build.buf.protovalidate.internal.evaluator.EvaluatorBuilder;
 import build.buf.protovalidate.internal.evaluator.MessageValue;
 import com.google.protobuf.Descriptors.Descriptor;
 import com.google.protobuf.Message;
+import java.util.ArrayList;
+import java.util.List;
 import org.projectnessie.cel.Env;
 import org.projectnessie.cel.Library;
 
@@ -75,11 +77,15 @@ public ValidationResult validate(Message msg) throws ValidationException {
     }
     Descriptor descriptor = msg.getDescriptorForType();
     Evaluator evaluator = evaluatorBuilder.load(descriptor);
-    ValidationResult result = evaluator.evaluate(new MessageValue(msg), failFast);
-    if (result.isSuccess()) {
-      return result;
+    List<ConstraintViolation.Builder> result = evaluator.evaluate(new MessageValue(msg), failFast);
+    if (result.isEmpty()) {
+      return ValidationResult.EMPTY;
+    }
+    List<Violation> violations = new ArrayList<>(result.size());
+    for (ConstraintViolation.Builder builder : result) {
+      violations.add(builder.build());
     }
-    return new ValidationResult(FieldPathUtils.calculateFieldPathStrings(result.getViolations()));
+    return new ValidationResult(violations);
   }
 
   /**

src/main/java/build/buf/protovalidate/Violation.java

@@ -0,0 +1,65 @@
+// Copyright 2023-2024 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.
+
+package build.buf.protovalidate;
+
+import com.google.protobuf.Descriptors;
+import javax.annotation.Nullable;
+
+/**
+ * {@link Violation} provides all of the collected information about an individual constraint
+ * violation.
+ */
+public interface Violation {
+  /** {@link FieldValue} represents a Protobuf field value inside a Protobuf message. */
+  interface FieldValue {
+    /**
+     * Gets the value of the field, which may be null, a primitive, a Map or a List.
+     *
+     * @return The value of the protobuf field.
+     */
+    @Nullable
+    Object getValue();
+
+    /**
+     * Gets the field descriptor of the field this value is from.
+     *
+     * @return A FieldDescriptor pertaining to this field.
+     */
+    Descriptors.FieldDescriptor getDescriptor();
+  }
+
+  /**
+   * Gets the protobuf form of this violation.
+   *
+   * @return The protobuf form of this violation.
+   */
+  build.buf.validate.Violation toProto();
+
+  /**
+   * Gets the value of the field this violation pertains to, or null if there is none.
+   *
+   * @return Value of the field associated with the violation, or null if there is none.
+   */
+  @Nullable
+  FieldValue getFieldValue();
+
+  /**
+   * Gets the value of the rule this violation pertains to, or null if there is none.
+   *
+   * @return Value of the rule associated with the violation, or null if there is none.
+   */
+  @Nullable
+  FieldValue getRuleValue();
+}

src/main/java/build/buf/protovalidate/internal/constraints/ConstraintCache.java

@@ -17,6 +17,8 @@
 import build.buf.protovalidate.Config;
 import build.buf.protovalidate.exceptions.CompilationException;
 import build.buf.protovalidate.internal.errors.FieldPathUtils;
+import build.buf.protovalidate.internal.evaluator.ObjectValue;
+import build.buf.protovalidate.internal.evaluator.Value;
 import build.buf.protovalidate.internal.expression.AstExpression;
 import build.buf.protovalidate.internal.expression.CompiledProgram;
 import build.buf.protovalidate.internal.expression.Expression;
@@ -142,6 +144,7 @@ public List<CompiledProgram> compile(
       Env ruleEnv = getRuleEnv(fieldDescriptor, message, rule.field, forItems);
       Variable ruleVar = Variable.newRuleVariable(message, message.getField(rule.field));
       ProgramOption globals = ProgramOption.globals(ruleVar);
+      Value ruleValue = new ObjectValue(rule.field, message.getField(rule.field));
       try {
         Program program = ruleEnv.program(rule.astExpression.ast, globals, PARTIAL_EVAL_OPTIONS);
         Program.EvalResult evalResult = program.eval(Activation.emptyActivation());
@@ -158,13 +161,17 @@ public List<CompiledProgram> compile(
         Ast residual = ruleEnv.residualAst(rule.astExpression.ast, evalResult.getEvalDetails());
         programs.add(
             new CompiledProgram(
-                ruleEnv.program(residual, globals), rule.astExpression.source, rule.rulePath));
+                ruleEnv.program(residual, globals),
+                rule.astExpression.source,
+                rule.rulePath,
+                ruleValue));
       } catch (Exception e) {
         programs.add(
             new CompiledProgram(
                 ruleEnv.program(rule.astExpression.ast, globals),
                 rule.astExpression.source,
-                rule.rulePath));
+                rule.rulePath,
+                ruleValue));
       }
     }
     return Collections.unmodifiableList(programs);