Add KSBackingField

api/src/main/kotlin/com/google/devtools/ksp/symbol/KSBackingField.kt

diff --git a/api/src/main/kotlin/com/google/devtools/ksp/symbol/KSBackingField.kt b/api/src/main/kotlin/com/google/devtools/ksp/symbol/KSBackingField.kt
new file mode 100644
index 0000000..5ad1f74
--- /dev/null
+++ b/api/src/main/kotlin/com/google/devtools/ksp/symbol/KSBackingField.kt
@@ -0,0 +1,35 @@
+/*
+ * Copyright 2026 Google LLC
+ * Copyright 2010-2026 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ *
+ * 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 com.google.devtools.ksp.symbol
+
+/**
+ * A backing field for a [KSPropertyDeclaration].
+ */
+interface KSBackingField : KSDeclaration {
+
+    /**
+     * The property that this backing field belongs to.
+     */
+    val property: KSPropertyDeclaration
+
+    /**
+     * The type of the backing field.
+     * This is guaranteed to be a subtype of the type of [property].
+     */
+    val type: KSTypeReference
+}