FieldMask: runtime + schema awareness

wire-runtime/api/wire-runtime.api

@@ -73,6 +73,18 @@ public final class com/squareup/wire/FieldEncoding : java/lang/Enum {
 public final class com/squareup/wire/FieldEncoding$Companion {
 }
 
+public final class com/squareup/wire/FieldMask {
+	public fun <init> ()V
+	public fun <init> (Ljava/util/List;)V
+	public synthetic fun <init> (Ljava/util/List;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public final fun copy (Ljava/util/List;)Lcom/squareup/wire/FieldMask;
+	public static synthetic fun copy$default (Lcom/squareup/wire/FieldMask;Ljava/util/List;ILjava/lang/Object;)Lcom/squareup/wire/FieldMask;
+	public fun equals (Ljava/lang/Object;)Z
+	public final fun getPaths ()Ljava/util/List;
+	public fun hashCode ()I
+	public fun toString ()Ljava/lang/String;
+}
+
 public final class com/squareup/wire/InstantKt {
 	public static final fun ofEpochSecond (JJ)Ljava/time/Instant;
 }

wire-runtime/src/commonMain/kotlin/com/squareup/wire/FieldMask.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2026 Square, 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
+ *
+ *      https://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 com.squareup.wire
+
+/**
+ * A set of symbolic field paths.
+ *
+ * Field masks are used to specify a subset of fields on a target message. Each path uses proto
+ * field names, separated by dots for nested fields.
+ */
+class FieldMask(paths: List<String> = emptyList()) {
+  val paths: List<String> = paths.toList()
+
+  fun copy(paths: List<String> = this.paths): FieldMask = FieldMask(paths)
+
+  override fun equals(other: Any?): Boolean {
+    if (other === this) return true
+    return other is FieldMask && paths == other.paths
+  }
+
+  override fun hashCode(): Int = paths.hashCode()
+
+  override fun toString(): String = "FieldMask{paths=$paths}"
+}

wire-runtime/src/commonTest/kotlin/com/squareup/wire/FieldMaskTest.kt

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2026 Square, 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
+ *
+ *      https://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 com.squareup.wire
+
+import assertk.assertThat
+import assertk.assertions.containsExactly
+import assertk.assertions.isEqualTo
+import kotlin.test.Test
+
+class FieldMaskTest {
+  @Test fun storesPaths() {
+    val fieldMask = FieldMask(listOf("user.display_name", "photo"))
+
+    assertThat(fieldMask.paths).containsExactly("user.display_name", "photo")
+  }
+
+  @Test fun defaultPathsIsEmpty() {
+    assertThat(FieldMask().paths).isEqualTo(emptyList())
+  }
+
+  @Test fun pathsAreImmutableCopy() {
+    val paths = mutableListOf("user.display_name")
+    val fieldMask = FieldMask(paths)
+
+    paths += "photo"
+
+    assertThat(fieldMask.paths).containsExactly("user.display_name")
+  }
+
+  @Test fun copy() {
+    val fieldMask = FieldMask(listOf("user.display_name"))
+
+    assertThat(fieldMask.copy(paths = listOf("photo"))).isEqualTo(FieldMask(listOf("photo")))
+  }
+}

wire-schema/api/wire-schema.api

@@ -648,6 +648,7 @@ public final class com/squareup/wire/schema/ProtoType {
 	public static final field DOUBLE_VALUE Lcom/squareup/wire/schema/ProtoType;
 	public static final field DURATION Lcom/squareup/wire/schema/ProtoType;
 	public static final field EMPTY Lcom/squareup/wire/schema/ProtoType;
+	public static final field FIELD_MASK Lcom/squareup/wire/schema/ProtoType;
 	public static final field FIXED32 Lcom/squareup/wire/schema/ProtoType;
 	public static final field FIXED64 Lcom/squareup/wire/schema/ProtoType;
 	public static final field FLOAT Lcom/squareup/wire/schema/ProtoType;

wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/ProtoType.kt

@@ -138,6 +138,8 @@ class ProtoType {
 
     @JvmField val EMPTY = ProtoType(false, "google.protobuf.Empty")
 
+    @JvmField val FIELD_MASK = ProtoType(false, "google.protobuf.FieldMask")
+
     @JvmField val STRUCT_MAP = ProtoType(false, "google.protobuf.Struct")
 
     @JvmField val STRUCT_VALUE = ProtoType(false, "google.protobuf.Value")

wire-schema/src/commonTest/kotlin/com/squareup/wire/schema/ProtoTypeTest.kt

@@ -117,6 +117,12 @@ class ProtoTypeTest {
     assertThat(phoneType.toString()).isEqualTo("squareup.protos.person.Person.PhoneType")
   }
 
+  @Test
+  fun fieldMask() {
+    assertThat(ProtoType.FIELD_MASK).isEqualTo(ProtoType.get("google.protobuf.FieldMask"))
+    assertThat(ProtoType.FIELD_MASK.typeUrl).isEqualTo("type.googleapis.com/google.protobuf.FieldMask")
+  }
+
   @Test
   fun enclosingTypeOrPackage() {
     assertThat(ProtoType.STRING.enclosingTypeOrPackage).isNull()