fix: retain JsonAdapter generic signature for R8 full mode

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## Unreleased
 
-* None yet.
+* Keep `JsonAdapter`'s generic signature in the embedded R8 rules so `@ToJson` and `@FromJson` delegate parameters stay parameterized under R8 full mode.
 
 
 ## [2.0.0-alpha.1] - 2026-01-28

moshi/src/main/resources/META-INF/proguard/moshi.pro

@@ -25,3 +25,15 @@
   @com.squareup.moshi.FromJson <methods>;
   @com.squareup.moshi.ToJson <methods>;
 }
+
+# R8 full mode retains a generic signature only for types a keep rule matches. Matching
+# JsonAdapter keeps JsonAdapter<T> on @ToJson/@FromJson methods that already reference it, so
+# Method.getGenericParameterTypes() stays a ParameterizedType and bind() can read the delegate
+# type argument. A raw JsonAdapter has no type argument to recover.
+# allowshrinking still allows unused members to be removed, so this does not pin code into the
+# app. allowobfuscation still allows the class to be renamed; parametersAreJsonAdapters compares
+# rawType to JsonAdapter, and R8 rewrites both the library reference and the signature.
+# The rule names this single class, with no member wildcard and no com.squareup.moshi.**
+# pattern, so subclasses and other Moshi types stay unmatched. allowoptimization is omitted so
+# the rule matches R8 full-mode signature guidance and Retrofit's retrofit2.Response rule.
+-keep,allowobfuscation,allowshrinking class com.squareup.moshi.JsonAdapter

moshi/src/test/java/com/squareup/moshi/MoshiProguardRulesTest.kt

@@ -0,0 +1,33 @@
+/*
+ * 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.moshi
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.Test
+
+class MoshiProguardRulesTest {
+  @Test
+  fun jsonAdapterKeepRuleIsAnExactLine() {
+    val rules =
+      requireNotNull(javaClass.classLoader.getResource("META-INF/proguard/moshi.pro")) {
+          "Missing META-INF/proguard/moshi.pro"
+        }
+        .readText()
+    // Exact line: a member wildcard or a com.squareup.moshi.** pattern must not match.
+    assertThat(rules.lines())
+      .contains("-keep,allowobfuscation,allowshrinking class com.squareup.moshi.JsonAdapter")
+  }
+}