Add BugPattern to rewrite assertThat(...).isEqualTo(null) to `asser…

…tThat(...).isNull()`

Run google-java-format

Add some XXXs

Suggest fix in AsserThatIsNullCheck

Add javadoc

Make test class final

Add typecheck for literal tree

fmt

Improve method invocation matching

error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/AssertThatIsNullCheck.java

@@ -0,0 +1,45 @@
+package tech.picnic.errorprone.bugpatterns;
+
+import static com.google.errorprone.BugPattern.LinkType.NONE;
+import static com.google.errorprone.BugPattern.SeverityLevel.SUGGESTION;
+import static com.google.errorprone.BugPattern.StandardTags.SIMPLIFICATION;
+
+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.MethodInvocationTreeMatcher;
+import com.google.errorprone.fixes.SuggestedFixes;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.util.ASTHelpers;
+import com.sun.source.tree.MethodInvocationTree;
+import com.sun.source.tree.Tree;
+import com.sun.tools.javac.code.Symbol.MethodSymbol;
+import org.assertj.core.api.AbstractAssert;
+
+/**
+ * A {@link BugChecker} which flags {@code asserThat(someValue).isEqualTo(null)} and suggests {@link
+ * AbstractAssert#isNull()}.
+ */
+@AutoService(BugChecker.class)
+@BugPattern(
+    name = "AssertThatIsNull",
+    summary = "asserThat(...).isEqualTo(null) should be assertThat(...).isNull()",
+    linkType = NONE,
+    severity = SUGGESTION,
+    tags = SIMPLIFICATION)
+public final class AssertThatIsNullCheck extends BugChecker implements MethodInvocationTreeMatcher {
+  private static final long serialVersionUID = 1L;
+
+  @Override
+  public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState state) {
+    MethodSymbol symbol = ASTHelpers.getSymbol(tree);
+    if (symbol == null
+        || tree.getArguments().size() != 1
+        || tree.getArguments().get(0).getKind() != Tree.Kind.NULL_LITERAL) {
+      return Description.NO_MATCH;
+    }
+
+    return describeMatch(tree, SuggestedFixes.renameMethodInvocation(tree, "isNull()", state));
+  }
+}

error-prone-contrib/src/test/java/tech/picnic/errorprone/bugpatterns/AssertThatIsNullCheckTest.java

@@ -0,0 +1,70 @@
+package tech.picnic.errorprone.bugpatterns;
+
+import com.google.errorprone.BugCheckerRefactoringTestHelper;
+import com.google.errorprone.CompilationTestHelper;
+import org.junit.jupiter.api.Test;
+
+public final class AssertThatIsNullCheckTest {
+  private final CompilationTestHelper compilationTestHelper =
+      CompilationTestHelper.newInstance(AssertThatIsNullCheck.class, getClass());
+
+  private final BugCheckerRefactoringTestHelper refactoringTestHelper =
+      BugCheckerRefactoringTestHelper.newInstance(AssertThatIsNullCheck.class, getClass());
+
+  // XXX: Methods always start with lowercase. I thought we had a Checkstyle thing for this.
+  @Test
+  void identification() {
+    compilationTestHelper
+        .addSourceLines(
+            "A.java",
+            "import static org.assertj.core.api.Assertions.assertThat;",
+            "import com.google.common.collect.ImmutableSortedSet;",
+            "import org.junit.jupiter.api.Test;",
+            "public final class A {",
+            "  @Test",
+            "  public void testAssertThat() {",
+            "    String nullValue = null;",
+            "    assertThat(12).isEqualTo(12);",
+            "    // BUG: Diagnostic contains: assertThat(...).isNull()",
+            "    assertThat(\"value\").isEqualTo(null);",
+            "    // BUG: Diagnostic contains: assertThat(...).isNull()",
+            "    assertThat(nullValue).isEqualTo(null);",
+            "  }",
+            "} ")
+        .doTest();
+  }
+
+  @Test
+  void replacement() {
+    refactoringTestHelper
+        .addInputLines(
+            "A.java",
+            "import static org.assertj.core.api.Assertions.assertThat;",
+            "import com.google.common.collect.ImmutableSortedSet;",
+            "import org.junit.jupiter.api.Test;",
+            "public final class A {",
+            "  @Test",
+            "  public void testAssertThat() {",
+            "    String nullValue = null;",
+            "    assertThat(12).isEqualTo(12);",
+            "    assertThat(\"value\").isEqualTo(null);",
+            "    assertThat(nullValue).isEqualTo(null);",
+            "  }",
+            "} ")
+        .addOutputLines(
+            "A.java",
+            "import static org.assertj.core.api.Assertions.assertThat;",
+            "import com.google.common.collect.ImmutableSortedSet;",
+            "import org.junit.jupiter.api.Test;",
+            "public final class A {",
+            "  @Test",
+            "  public void testAssertThat() {",
+            "    String nullValue = null;",
+            "    assertThat(12).isEqualTo(12);",
+            "    assertThat(\"value\").isNull();",
+            "    assertThat(nullValue).isNull();",
+            "  }",
+            "} ")
+        .doTest();
+  }
+}