Guide behavior changes towards CompatChanges.

Each SDK level often has dozens of different behavior changes, which
can be difficult for large app developers to adjust to during preview
or beta releases. For this reason, android.app.compat.CompatChanges
was introduced as a new best-practice for adding behavior changes.

During a preview or beta release, developers can temporarily opt-out
of each individual change to aid debugging. This opt-out is only
available during preview of beta releases, and cannot be adjusted on
finalized builds.

Bug: 169879376
Test: atest error_prone_android_framework_test
Change-Id: Ib3b2e2139e084b0fa1bcbb5e89dd55e7ca4bfa00

4 files changed, 209 insertions, 5 deletions

diff --git a/errorprone/java/com/google/errorprone/bugpatterns/android/CompatChangeChecker.java b/errorprone/java/com/google/errorprone/bugpatterns/android/CompatChangeChecker.java
new file mode 100644
index 000000000000..8829b0d3c649
--- /dev/null
+++ b/errorprone/java/com/google/errorprone/bugpatterns/android/CompatChangeChecker.java
@@ -0,0 +1,109 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * 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.errorprone.bugpatterns.android;
+
+import static com.google.errorprone.BugPattern.SeverityLevel.WARNING;
+import static com.google.errorprone.matchers.FieldMatchers.anyFieldInClass;
+import static com.google.errorprone.matchers.FieldMatchers.staticField;
+import static com.google.errorprone.matchers.Matchers.allOf;
+import static com.google.errorprone.matchers.Matchers.anyOf;
+import static com.google.errorprone.matchers.Matchers.anything;
+import static com.google.errorprone.matchers.Matchers.binaryTree;
+import static com.google.errorprone.matchers.Matchers.not;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.bugpatterns.BugChecker.BinaryTreeMatcher;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.sun.source.tree.BinaryTree;
+import com.sun.source.tree.ExpressionTree;
+
+/**
+ * Each SDK level often has dozens of different behavior changes, which can be
+ * difficult for large app developers to adjust to during preview or beta
+ * releases. For this reason, {@code android.app.compat.CompatChanges} was
+ * introduced as a new best-practice for adding behavior changes.
+ * <p>
+ * During a preview or beta release, developers can temporarily opt-out of each
+ * individual change to aid debugging. This opt-out is only available during
+ * preview of beta releases, and cannot be adjusted on finalized builds.
+ */
+@AutoService(BugChecker.class)
+@BugPattern(
+    name = "AndroidFrameworkCompatChange",
+    summary = "Verifies that behavior changes use the modern compatibility framework",
+    severity = WARNING)
+public final class CompatChangeChecker extends BugChecker implements BinaryTreeMatcher {
+    private static final Matcher<ExpressionTree> VERSION_CODE =
+            anyFieldInClass("android.os.Build.VERSION_CODES");
+
+    // Ship has already sailed on these SDK levels; not worth fixing
+    private static final Matcher<ExpressionTree> LEGACY_VERSION_CODE = anyOf(
+            staticField("android.os.Build.VERSION_CODES", "BASE"),
+            staticField("android.os.Build.VERSION_CODES", "BASE_1_1"),
+            staticField("android.os.Build.VERSION_CODES", "CUPCAKE"),
+            staticField("android.os.Build.VERSION_CODES", "DONUT"),
+            staticField("android.os.Build.VERSION_CODES", "ECLAIR"),
+            staticField("android.os.Build.VERSION_CODES", "ECLAIR_0_1"),
+            staticField("android.os.Build.VERSION_CODES", "ECLAIR_MR1"),
+            staticField("android.os.Build.VERSION_CODES", "FROYO"),
+            staticField("android.os.Build.VERSION_CODES", "GINGERBREAD"),
+            staticField("android.os.Build.VERSION_CODES", "GINGERBREAD_MR1"),
+            staticField("android.os.Build.VERSION_CODES", "HONEYCOMB"),
+            staticField("android.os.Build.VERSION_CODES", "HONEYCOMB_MR1"),
+            staticField("android.os.Build.VERSION_CODES", "HONEYCOMB_MR2"),
+            staticField("android.os.Build.VERSION_CODES", "ICE_CREAM_SANDWICH"),
+            staticField("android.os.Build.VERSION_CODES", "ICE_CREAM_SANDWICH_MR1"),
+            staticField("android.os.Build.VERSION_CODES", "JELLY_BEAN"),
+            staticField("android.os.Build.VERSION_CODES", "JELLY_BEAN_MR1"),
+            staticField("android.os.Build.VERSION_CODES", "JELLY_BEAN_MR2"),
+            staticField("android.os.Build.VERSION_CODES", "KITKAT"),
+            staticField("android.os.Build.VERSION_CODES", "KITKAT_WATCH"),
+            staticField("android.os.Build.VERSION_CODES", "L"),
+            staticField("android.os.Build.VERSION_CODES", "LOLLIPOP"),
+            staticField("android.os.Build.VERSION_CODES", "LOLLIPOP_MR1"),
+            staticField("android.os.Build.VERSION_CODES", "M"),
+            staticField("android.os.Build.VERSION_CODES", "N"),
+            staticField("android.os.Build.VERSION_CODES", "N_MR1"),
+            staticField("android.os.Build.VERSION_CODES", "O"),
+            staticField("android.os.Build.VERSION_CODES", "O_MR1"),
+            staticField("android.os.Build.VERSION_CODES", "P"),
+            staticField("android.os.Build.VERSION_CODES", "Q"));
+
+    private static final Matcher<ExpressionTree> MODERN_VERSION_CODE =
+            allOf(VERSION_CODE, not(LEGACY_VERSION_CODE));
+
+    private static final Matcher<BinaryTree> INVALID = anyOf(
+            binaryTree(MODERN_VERSION_CODE, anything()),
+            binaryTree(anything(), MODERN_VERSION_CODE));
+
+    @Override
+    public Description matchBinary(BinaryTree tree, VisitorState state) {
+        if (INVALID.matches(tree, state)) {
+            return buildDescription(tree)
+                    .setMessage("Behavior changes should use CompatChanges.isChangeEnabled() "
+                            + "instead of direct SDK checks to ease developer transitions; "
+                            + "see go/compat-framework for more details")
+                    .build();
+
+        }
+        return Description.NO_MATCH;
+    }
+}
diff --git a/errorprone/java/com/google/errorprone/matchers/FieldMatchers.java b/errorprone/java/com/google/errorprone/matchers/FieldMatchers.java
index 46f0fb2e534c..08969d60671a 100644
--- a/errorprone/java/com/google/errorprone/matchers/FieldMatchers.java
+++ b/errorprone/java/com/google/errorprone/matchers/FieldMatchers.java
@@ -36,7 +36,8 @@ public final class FieldMatchers {
     return new FieldReferenceMatcher() {
       @Override
       boolean classIsAppropriate(ClassSymbol classSymbol) {
-        return classSymbol.getQualifiedName().contentEquals(className);
+        return classSymbol != null
+                && classSymbol.getQualifiedName().contentEquals(className);
       }
 
       @Override
@@ -50,12 +51,14 @@ public final class FieldMatchers {
     return new FieldReferenceMatcher() {
       @Override
       boolean classIsAppropriate(ClassSymbol classSymbol) {
-        return classSymbol.getQualifiedName().contentEquals(className);
+        return classSymbol != null
+                && classSymbol.getQualifiedName().contentEquals(className);
       }
 
       @Override
       boolean fieldSymbolIsAppropriate(Symbol symbol) {
-        return symbol.isStatic() && symbol.getSimpleName().contentEquals(fieldName);
+        return symbol != null
+                && symbol.isStatic() && symbol.getSimpleName().contentEquals(fieldName);
       }
     };
   }
@@ -64,12 +67,14 @@ public final class FieldMatchers {
     return new FieldReferenceMatcher() {
       @Override
       boolean classIsAppropriate(ClassSymbol classSymbol) {
-        return classSymbol.getQualifiedName().contentEquals(className);
+        return classSymbol != null
+                && classSymbol.getQualifiedName().contentEquals(className);
       }
 
       @Override
       boolean fieldSymbolIsAppropriate(Symbol symbol) {
-        return !symbol.isStatic() && symbol.getSimpleName().contentEquals(fieldName);
+        return symbol != null
+                && !symbol.isStatic() && symbol.getSimpleName().contentEquals(fieldName);
       }
     };
   }
diff --git a/errorprone/tests/java/com/google/errorprone/bugpatterns/android/CompatChangeCheckerTest.java b/errorprone/tests/java/com/google/errorprone/bugpatterns/android/CompatChangeCheckerTest.java
new file mode 100644
index 000000000000..105633e4de28
--- /dev/null
+++ b/errorprone/tests/java/com/google/errorprone/bugpatterns/android/CompatChangeCheckerTest.java
@@ -0,0 +1,89 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * 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.errorprone.bugpatterns.android;
+
+import com.google.errorprone.CompilationTestHelper;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class CompatChangeCheckerTest {
+    private CompilationTestHelper compilationHelper;
+
+    @Before
+    public void setUp() {
+        compilationHelper = CompilationTestHelper.newInstance(
+                CompatChangeChecker.class, getClass());
+    }
+
+    @Test
+    public void testSimple() {
+        compilationHelper
+                .addSourceFile("/android/os/Build.java")
+                .addSourceLines("Example.java",
+                        "import android.os.Build;",
+                        "public class Example {",
+                        "  void test(int targetSdkVersion) {",
+                        "    // BUG: Diagnostic contains:",
+                        "    if (targetSdkVersion < Build.VERSION_CODES.R) { }",
+                        "    // BUG: Diagnostic contains:",
+                        "    if (targetSdkVersion <= Build.VERSION_CODES.R) { }",
+                        "    // BUG: Diagnostic contains:",
+                        "    if (targetSdkVersion > Build.VERSION_CODES.R) { }",
+                        "    // BUG: Diagnostic contains:",
+                        "    if (targetSdkVersion >= Build.VERSION_CODES.R) { }",
+                        "    // BUG: Diagnostic contains:",
+                        "    if (targetSdkVersion == Build.VERSION_CODES.R) { }",
+                        "    // BUG: Diagnostic contains:",
+                        "    if (targetSdkVersion != Build.VERSION_CODES.R) { }",
+                        "  }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testObscure() {
+        compilationHelper
+                .addSourceFile("/android/os/Build.java")
+                .addSourceLines("Example.java",
+                        "import static android.os.Build.VERSION_CODES.R;",
+                        "public class Example {",
+                        "  void test(int targetSdkVersion) {",
+                        "    // BUG: Diagnostic contains:",
+                        "    boolean indirect = R > targetSdkVersion;",
+                        "  }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    public void testLegacyIgnored() {
+        compilationHelper
+                .addSourceFile("/android/os/Build.java")
+                .addSourceLines("Example.java",
+                        "import android.os.Build;",
+                        "public class Example {",
+                        "  void test(int targetSdkVersion) {",
+                        "    if (targetSdkVersion < Build.VERSION_CODES.DONUT) { }",
+                        "  }",
+                        "}")
+                .doTest();
+    }
+}
diff --git a/errorprone/tests/res/android/os/Build.java b/errorprone/tests/res/android/os/Build.java
index bbf7ef2172b5..0b51bdba0731 100644
--- a/errorprone/tests/res/android/os/Build.java
+++ b/errorprone/tests/res/android/os/Build.java
@@ -19,5 +19,6 @@ package android.os;
 public class Build {
     public static class VERSION_CODES {
         public static final int DONUT = 4;
+        public static final int R = 30;
     }
 }