Support custom message validators

buildSrc/src/main/kotlin/io/spine/dependency/Dependency.kt

@@ -58,7 +58,7 @@ abstract class Dependency {
     /**
      * The [modules] given with the [version].
      */
-    final val artifacts: Map<String, String> by lazy {
+    val artifacts: Map<String, String> by lazy {
         modules.associateWith { "$it:$version" }
     }
 
@@ -114,3 +114,19 @@ private fun ResolutionStrategy.forceWithLogging(
     force(artifact)
     project.log { "Forced the version of `$artifact` in " + configuration.diagSuffix(project) }
 }
+
+/**
+ * Obtains full Maven coordinates for the requested [module].
+ *
+ * This extension allows referencing properties of the [Dependency],
+ * upon which it is invoked.
+ *
+ * An example usage:
+ *
+ * ```
+ * // Supposing there is `Ksp.symbolProcessingApi: String` property declared.
+ * Ksp.artifact { symbolProcessingApi }
+ * ```
+ */
+fun <T : Dependency> T.artifact(module: T.() -> String): String =
+    artifact(module())

buildSrc/src/main/kotlin/io/spine/dependency/test/KotlinCompileTesting.kt

@@ -33,7 +33,7 @@ package io.spine.dependency.test
  */
 @Suppress("unused", "ConstPropertyName")
 object KotlinCompileTesting {
-    private const val version = "0.7.0"
+    private const val version = "0.7.1"
     private const val group = "dev.zacsweers.kctfork"
     const val libCore = "$group:core:$version"
     const val libKsp = "$group:ksp:$version"

buildSrc/src/main/kotlin/module.gradle.kts

@@ -28,6 +28,7 @@ import io.spine.dependency.boms.BomsPlugin
 import io.spine.dependency.build.Dokka
 import io.spine.dependency.build.ErrorProne
 import io.spine.dependency.build.JSpecify
+import io.spine.dependency.build.Ksp
 import io.spine.dependency.lib.Grpc
 import io.spine.dependency.lib.Kotlin
 import io.spine.dependency.lib.Protobuf
@@ -144,8 +145,10 @@ fun Module.forceConfigurations() {
         all {
             resolutionStrategy {
                 Grpc.forceArtifacts(project, this@all, this@resolutionStrategy)
+                Ksp.forceArtifacts(project, this@all, this@resolutionStrategy)
                 force(
                     Kotlin.bom,
+                    Kotlin.Compiler.embeddable,
                     Reflect.lib,
                     Base.lib,
                     Protobuf.compiler,

java-api/src/main/kotlin/io/spine/validation/api/DetectedViolation.kt

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2025, TeamDev. All rights reserved.
+ *
+ * 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
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Redistribution and use in source and/or binary forms, with or without
+ * modification, must retain the above copyright notice and the following
+ * disclaimer.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package io.spine.validation.api
+
+import io.spine.base.FieldPath
+import io.spine.validate.TemplateString
+
+/**
+ * Abstract base for violations detected by [MessageValidator]s.
+ *
+ * @param message The error message describing the violation.
+ * @param fieldPath The path to the field where the violation occurred, if applicable.
+ * @param fieldValue The field value that caused the violation, if any.
+ */
+public abstract class DetectedViolation(
+    public val message: TemplateString,
+    public val fieldPath: FieldPath?,
+    public val fieldValue: Any?,
+)
+
+/**
+ * A violation tied to a specific field in a message.
+ *
+ * @param message The error message describing the violation.
+ * @param fieldPath The path to the field where the violation occurred.
+ * @param fieldValue The field value that caused the violation, if any.
+ */
+public class FieldViolation(
+    message: TemplateString,
+    fieldPath: FieldPath,
+    fieldValue: Any? = null,
+) : DetectedViolation(message, fieldPath, fieldValue)
+
+/**
+ * A violation related to the message level (not tied to a specific field).
+ *
+ * @param message The error message describing the violation.
+ */
+public class MessageViolation(
+    message: TemplateString
+) : DetectedViolation(message, fieldPath = null, fieldValue = null)

java-api/src/main/kotlin/io/spine/validation/api/DiscoveredValidators.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2025, TeamDev. All rights reserved.
+ *
+ * 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
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Redistribution and use in source and/or binary forms, with or without
+ * modification, must retain the above copyright notice and the following
+ * disclaimer.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package io.spine.validation.api
+
+import io.spine.annotation.Internal
+
+/**
+ * Holds a path to a file with the discovered validators.
+ *
+ * The KSP processor generates a resource file using this path.
+ * Then, the Java codegen plugin picks up this file.
+ */
+@Internal
+public object DiscoveredValidators {
+
+    /**
+     * The path to the file with the discovered message validators.
+     *
+     * The path is relative to the output directory of the KSP processor.
+     */
+   public const val RESOURCES_LOCATION: String = "spine/validation/message-validators"
+}

java-api/src/main/kotlin/io/spine/validation/api/MessageValidator.kt

@@ -0,0 +1,129 @@
+/*
+ * Copyright 2025, TeamDev. All rights reserved.
+ *
+ * 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
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Redistribution and use in source and/or binary forms, with or without
+ * modification, must retain the above copyright notice and the following
+ * disclaimer.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package io.spine.validation.api
+
+import com.google.protobuf.Message
+import io.spine.annotation.SPI
+
+/**
+ * A custom validator for Protobuf messages of type [M] that are defined externally.
+ *
+ * External messages are those that come from dependencies.
+ * Protobuf [well-known](https://protobuf.dev/reference/protobuf/google.protobuf/)
+ * messages should also be considered external. For such messages, it is impossible
+ * to declare validation constraints using the validation options because there's
+ * no access to their proto definitions.
+ *
+ * To be able to validate external messages, one must implement this interface
+ * and annotate the implementing class with the [Validator] annotation, specifying
+ * the type of the message to validate.
+ *
+ * An example of the validator declaration for the `Earphones` message:
+ *
+ * ```kotlin
+ * @Validator(Earphones::class)
+ * public class EarphonesValidator : MessageValidator<Earphones> {
+ *     public override fun validate(message: Earphones): List<DetectedViolation> {
+ *         return emptyList() // Always valid.
+ *     }
+ * }
+ * ```
+ *
+ * ## Applicability
+ *
+ * A validator is applied only to the local messages of the module it is declared in.
+ * For each local message that has a field of type [M], the validation library
+ * will invoke a validator when validating that message.
+ *
+ * The following field types are supported:
+ *
+ * 1. Singular fields of type [M].
+ * 2. Repeated field of type [M].
+ * 3. Map field with values of type [M].
+ *
+ * Please note that standalone instances of [M] and fields of [M] type that occur in
+ * external messages will not be validated using the validator.
+ *
+ * Consider the following example:
+ *
+ * ```proto
+ * import "earphones.proto"; // Brings the `Earphones` message from dependencies.
+ *
+ * // A locally-declared message.
+ * message WorkingSetup {
+ *
+ *     // The field of external message type.
+ *     Earphones earphones = 1;
+ * }
+ * ```
+ *
+ * Supposing that `WorkingSetup` and `EarphonesValidator` are declared within
+ * the same module, then every instance of `WorkingSetup.earphones` will be validated
+ * with the validator.
+ *
+ * Also note the restrictions:
+ *
+ * - Standalone instantiations of `Earphones` will not be validated.
+ * - External messages that use `Earphones` as a field will not be validated.
+ *
+ * ## Implementation
+ *
+ * The message validator does not have restrictions upon how exactly the message
+ * must be validated. It can validate a particular field or several fields,
+ * the whole message instance (for example, checking the field relations) and
+ * perform a deep validation.
+ *
+ * It is a responsibility of the validator to provide the correct instances
+ * of [DetectedViolation]. Before reporting to the user, the validation library
+ * converts [DetectedViolation] to a [ConstraintViolation][io.spine.validate.ConstraintViolation].
+ * Returning of an empty list of violations means that the message is valid.
+ *
+ * Please keep in mind that for each invocation a new instance of [MessageValidator]
+ * is created. Every implementation of [MessageValidator] must have a public,
+ * no-args constructor.
+ *
+ * ## Restrictions
+ *
+ * A [MessageValidator] will be rejected by the validation library in the following cases:
+ *
+ * 1) It is used to validate a local message. Only external messages are allowed
+ *    to have a validator.
+ * 2) There already exists a validator for the validated message type. Having several
+ *    validators for the same message type is prohibited.
+ *
+ * @param M the type of Protobuf [Message] being validated.
+ */
+@SPI
+public interface MessageValidator<M : Message> {
+
+    /**
+     * Validates the given [message].
+     *
+     * @return the detected violations or empty list.
+     */
+    public fun validate(message: M): List<DetectedViolation>
+}

java-api/src/main/kotlin/io/spine/validation/api/Validator.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2025, TeamDev. All rights reserved.
+ *
+ * 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
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Redistribution and use in source and/or binary forms, with or without
+ * modification, must retain the above copyright notice and the following
+ * disclaimer.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package io.spine.validation.api
+
+import com.google.protobuf.Message
+import kotlin.annotation.AnnotationRetention.SOURCE
+import kotlin.annotation.AnnotationTarget.CLASS
+import kotlin.reflect.KClass
+
+/**
+ * Marks the class as a message validator.
+ *
+ * Applying this annotation to an implementation of [MessageValidator]
+ * makes the class visible to the validation library.
+ *
+ * Please note that the following requirements are imposed to the marked class:
+ *
+ * 1. The class must implement the [MessageValidator] interface.
+ * 2. The class must have a public, no-args constructor.
+ * 3. The class cannot be `inner`, but nested classes are allowed.
+ * 4. The message type of [Validator.value] and [MessageValidator] must match.
+ */
+@Target(CLASS)
+@Retention(SOURCE)
+public annotation class Validator(
+
+    /**
+     * The class of the validated external message.
+     */
+    val value: KClass<out Message>
+)

java-api/src/main/kotlin/io/spine/validation/api/expression/FieldPaths.kt

@@ -26,6 +26,7 @@
 
 package io.spine.validation.api.expression
 
+import com.google.protobuf.Message
 import io.spine.base.FieldPath
 import io.spine.protodata.ast.FieldName
 import io.spine.protodata.ast.OneofName
@@ -53,6 +54,16 @@ public fun Expression<FieldPath>.resolve(field: FieldName): Expression<FieldPath
         .chainAdd("field_name", StringLiteral(field.value))
         .chainBuild()
 
+/**
+ * Returns an expression that merges the provided [FieldPath] expression into this one.
+ *
+ * To perform merging, this method uses the [Message.Builder.mergeFrom] method.
+ */
+public fun Expression<FieldPath>.mergeFrom(other: Expression<FieldPath>): Expression<FieldPath> =
+    toBuilder()
+        .chain<Message.Builder>("mergeFrom", other)
+        .chainBuild()
+
 /**
  * Returns an expression that yields a new instance of [FieldPath] by appending
  * the provided [oneof] group name to this parental [FieldPath] expression.