New lint rule that checks if a Tree for planted for an app variant.

* Added tests for the lint rule.

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

@@ -0,0 +1,74 @@
+@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.WARNING,
+        implementation = Implementation(
+            PlantATreeDetector::class.java,
+            EnumSet.of(Scope.JAVA_FILE)
+        )
+    )
+
+    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, 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

@@ -0,0 +1,16 @@
+@file:Suppress("UnstableApiUsage")
+
+package timber.lint
+
+import com.android.tools.lint.client.api.IssueRegistry
+import com.android.tools.lint.detector.api.CURRENT_API
+import com.android.tools.lint.detector.api.Issue
+
+class TimberIssueRegistry : IssueRegistry() {
+  override val api = CURRENT_API
+  override val minApi: Int = CURRENT_API
+  override val issues: List<Issue> = listOf(
+      *WrongTimberUsageDetector.getIssues(),
+      PlantATreeDetector.ISSUE
+  )
+}

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

@@ -0,0 +1,112 @@
+package timber.lint
+
+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() {
+      companion object Forest {
+        fun e(message: String?, vararg args: Any?) {
+
+        }
+        fun w(message: String?, vararg args: Any?) {
+
+        }
+        fun i(message: String?, vararg args: Any?) {
+
+        }
+        fun d(message: String?, vararg args: Any?) {
+
+        }
+        fun v(message: String?, vararg args: Any?) {
+
+        }
+        fun plant(tree: Tree) {
+
+        }
+      }
+
+      open class Tree {
+        // A Tree Stub
+      }
+    }
+  """).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: Warning: A Tree must be planted for at least a single variant of the application. [MustPlantATimberTree]
+              Timber.d("Log something")
+              ~~~~~~~~~~~~~~~~~~~~~~~~~
+          0 errors, 1 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()
+  }
+
+}