Allow JetBrains @contract annotations

Contract annotations allow us to write contracts for methods such as:
"null -> true" (TextUtils.isEmpty()), this allows further lint warnings
on Android framework methods, which should be annotated, but aren't yet.

These should be written inline when possible, but we also include a
folder of annotations for our external libraries

Further instructions for external annotations are detailed in README.md

AnkiDroid/build.gradle

@@ -129,6 +129,7 @@ tasks.withType(JavaCompile) {
 apply from: "./jacoco.gradle"
 
 dependencies {
+    compileOnly 'org.jetbrains:annotations:16.0.1'
     compileOnly "com.google.auto.service:auto-service-annotations:1.0-rc6"
     annotationProcessor "com.google.auto.service:auto-service:1.0-rc6"
 

AnkiDroid/src/main/java/com/ichi2/utils/Assert.java

@@ -17,7 +17,10 @@
 
 package com.ichi2.utils;
 
+import org.jetbrains.annotations.Contract;
+
 public class Assert {
+    @Contract("false, _, _ -> fail")
     public static void that(boolean condition, String message, Object... args) {
         if (!condition) {
             String msg = String.format(message, args);

annotations/README.md

@@ -0,0 +1,49 @@
+# External Annotations for Android Standard Library
+
+## Rationale
+
+`@Contract` annotations for Android methods allow the removal of tautotlogical lint checks. 
+
+For example: if `TextUtils.isEmpty(str)` returns `false`, then `str` is non-null. `@Contract` allows us to specify this invariant to the linter, which reduces lint warnings and/or unnecessary code.
+
+**@Contract annotations should be inline whenever possible**. This folder exists so we can define our own contract annotations on our dependencies.
+
+## Syntax
+
+See: https://www.jetbrains.com/help/idea/contract-annotations.html
+
+## Installation Instructions
+
+* Open an Android Source file in Android Studio
+* Move the Cursor over a method and bring up the Quick Fixes Menu (Alt + Enter)
+* Select "Add Method Contract to `[method]`"
+* Enter a dummy annotation: `null -> null`
+* Select the folder containing this file as the annotations root
+
+Annotations should now appear
+
+## Modificiation Instructions
+
+* In Android Studio Settings: `jdk.table.xml`
+* You can find the element under one of the `<jdk>` elements as a `file://` URL: 
+* Removing this line will allow the selection of another annotations root.
+
+Sample:
+```xml
+<jdk version="2">
+      <name value="Android API 28 Platform" />
+      <type value="Android SDK" />
+      <version value="java version &quot;1.8.0_112-release&quot;" />
+      <homePath value="C:\Users\David\AppData\Local\Android\Sdk" />
+      <roots>
+        <annotationsPath>
+          <root type="composite">
+            <root url="jar://$USER_HOME$/AppData/Local/Android/Sdk/platforms/android-28/data/annotations.zip!/" type="simple" />
+            <root url="file://C:/GitHub/Anki-Android-David/annotations" type="simple" />
+```
+
+## Future Goals
+
+It would be ideal if these could be set on a per-project basis. I haven't had the time to determine whether this is possible.
+
+These annotations are not yet supported by our automated tooling.

annotations/android/text/annotations.xml

@@ -0,0 +1,24 @@
+<!--
+  Copyright (c) 2020 David Allison <[email protected]>
+
+  This program is free software; you can redistribute it and/or modify it under
+  the terms of the GNU General Public License as published by the Free Software
+  Foundation; either version 3 of the License, or (at your option) any later
+  version.
+
+  This program is distributed in the hope that it will be useful, but WITHOUT ANY
+  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+  PARTICULAR PURPOSE. See the GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License along with
+  this program.  If not, see <http://www.gnu.org/licenses/>.
+  -->
+
+<root>
+    <item name='android.text.TextUtils boolean isEmpty(java.lang.CharSequence)'>
+        <annotation name='org.jetbrains.annotations.Contract'>
+            <val name="value" val="&quot;null -&gt; true&quot;" />
+            <val name="pure" val="true" />
+        </annotation>
+    </item>
+</root>