Ensure a Tree is planted for atleast one app variant.

* Added tests for the lint rule.

timber-lint/src/main/java/timber/lint/PlantATreeDetector.kt

@@ -0,0 +1,80 @@
+@file:Suppress("UnstableApiUsage")
+
+package timber.lint
+
+import com.android.tools.lint.detector.api.*
+import com.intellij.psi.PsiMethod
+import org.jetbrains.uast.UCallExpression
+import java.util.*
+
+/**
+ * A [Detector] which makes sure than anytime Timer APIs are used, there is at-least a single tree
+ * planted.
+ */
+class PlantATreeDetector : Detector(), SourceCodeScanner {
+  companion object {
+    val ISSUE = Issue.create(
+      id = "MustPlantATimberTree",
+      briefDescription = "A Timber tree needs to be planted",
+      explanation = """
+                When using Timber's logging APIs, a `Tree` must be planted on at least a single \
+                variant of the app.
+            """,
+      androidSpecific = true,
+      category = Category.CORRECTNESS,
+      severity = Severity.ERROR,
+      implementation = Implementation(
+        PlantATreeDetector::class.java,
+        EnumSet.of(Scope.JAVA_FILE)
+      )
+    )
+
+    // plant() is a @JvmStatic
+    // It can be resolved via both Timber & Forest.
+    private const val TIMBER = "timber.log.Timber"
+    private const val FOREST = "timber.log.Timber.Forest"
+  }
+
+  // Do we need to check if a Tree is planted
+  private var checkForPlantedTrees = false
+  private var hasPlantedTree = false
+  private var location: Location? = null
+
+  override fun getApplicableMethodNames() = listOf("v", "d", "i", "w", "e", "wtf", "plant")
+
+  override fun visitMethodCall(context: JavaContext, node: UCallExpression, method: PsiMethod) {
+    val methodName = method.name
+    when (context.driver.phase) {
+      1 -> {
+        if (methodName.matches(Regex("(v|d|i|w|e|wtf)"))
+          && context.evaluator.isMemberInClass(method, FOREST)
+        ) {
+          if (!checkForPlantedTrees) {
+            location = context.getLocation(node)
+            checkForPlantedTrees = true
+            // Request a second scan with the same scope
+            context.driver.requestRepeat(this, null)
+          }
+        }
+      }
+      else -> {
+        if (methodName.matches(Regex("plant")) &&
+          (context.evaluator.isMemberInClass(method, TIMBER) ||
+              context.evaluator.isMemberInClass(method, FOREST))
+        ) {
+          hasPlantedTree = true
+        }
+      }
+    }
+  }
+
+  override fun afterCheckRootProject(context: Context) {
+    if (checkForPlantedTrees && !hasPlantedTree && context.driver.phase > 1) {
+      context.report(
+        issue = ISSUE,
+        location = location ?: Location.create(context.file),
+        message = "A `Tree` must be planted for at least a single variant of the application."
+      )
+    }
+  }
+}

timber-lint/src/main/java/timber/lint/TimberIssueRegistry.kt

@@ -10,7 +10,7 @@ import com.google.auto.service.AutoService
 @AutoService(value = [IssueRegistry::class])
 class TimberIssueRegistry : IssueRegistry() {
   override val issues: List<Issue>
-    get() = WrongTimberUsageDetector.issues
+    get() = WrongTimberUsageDetector.issues + PlantATreeDetector.ISSUE
 
   override val api: Int
     get() = CURRENT_API
@@ -27,4 +27,4 @@ class TimberIssueRegistry : IssueRegistry() {
     identifier = "com.jakewharton.timber:timber:{version}",
     feedbackUrl = "https://github.com/JakeWharton/timber/issues",
   )
-}
+}

timber-lint/src/test/java/timber/lint/PlantATreeDetectorTest.kt

@@ -0,0 +1,145 @@
+package timber.lint
+
+import com.android.tools.lint.checks.infrastructure.LintDetectorTest.java
+import com.android.tools.lint.checks.infrastructure.LintDetectorTest.kotlin
+import com.android.tools.lint.checks.infrastructure.TestLintTask.lint
+import org.junit.Test
+
+class PlantATreeDetectorTest {
+
+  private val timber = kotlin("timber/log/Timber.kt", """
+    package timber.log
+    class Timber private constructor() {
+      abstract class Tree {
+        
+      }
+      companion object Forest: Tree() {
+        @JvmStatic
+        fun e(message: String?, vararg args: Any?) {
+
+        }
+        @JvmStatic
+        fun w(message: String?, vararg args: Any?) {
+
+        }
+        @JvmStatic
+        fun i(message: String?, vararg args: Any?) {
+
+        }
+        @JvmStatic
+        fun d(message: String?, vararg args: Any?) {
+
+        }
+        @JvmStatic
+        fun v(message: String?, vararg args: Any?) {
+
+        }
+        @JvmStatic
+        fun plant(tree: Tree) {
+
+        }
+      }
+    }
+  """).indented().within("src")
+
+  @Test
+  fun testNoTimberLoggingApisAreUsed() {
+    val application = kotlin("com/example/App.kt", """
+      package com.example
+
+      import timber.log.Timber
+
+      class App {
+        fun onCreate() {
+        }
+      }
+    """).indented().within("src")
+
+    lint()
+        .files(timber, application)
+        .issues(PlantATreeDetector.ISSUE)
+        .run()
+        .expectClean()
+  }
+
+  @Test
+  fun testWhenTimberApisAreUsed() {
+    val application = kotlin("com/example/App.kt", """
+      package com.example
+
+      import timber.log.Timber
+
+      class App {
+        fun onCreate() {
+          Timber.d("Log something")
+        }
+      }
+    """).indented().within("src")
+
+    lint()
+        .files(timber, application)
+        .issues(PlantATreeDetector.ISSUE)
+        .run()
+        .expect("""
+            src/com/example/App.kt:7: Error: A Tree must be planted for at least a single variant of the application. [MustPlantATimberTree]
+                Timber.d("Log something")
+                ~~~~~~~~~~~~~~~~~~~~~~~~~
+            1 errors, 0 warnings
+        """.trimIndent())
+  }
+
+  @Test
+  fun testWhenTimberApisAreUsedAndTreeIsPlanted() {
+    val application = kotlin("com/example/App.kt", """
+      package com.example
+
+      import timber.log.Timber
+
+      class App {
+        fun onCreate() {
+          plantTree()
+          Timber.d("Log something")
+        }
+
+        private fun plantTree() {
+          val tree = Timber.Tree()
+          Timber.plant(tree)
+        }
+      }
+    """).indented().within("src")
+
+    lint()
+        .files(timber, application)
+        .issues(PlantATreeDetector.ISSUE)
+        .run()
+        .expectClean()
+  }
+
+  @Test
+  fun testWhenTimberApisAreUsedAndTreeIsPlanted_java() {
+    val application = java("com/example/App.java", """
+      package com.example;
+
+      import timber.log.Timber;
+
+      class App {
+        void onCreate() {
+          plantTree();
+          Timber.d("Log something");
+        }
+
+        void plantTree() {
+          val tree = Timber.Tree();
+          Timber.plant(tree);
+        }
+      }
+    """).indented().within("src")
+
+    lint()
+      .files(timber, application)
+      .issues(PlantATreeDetector.ISSUE)
+      .run()
+      .expectClean()
+  }
+
+}