From ad513d5a82dc803dcc5179ffcdf7073d43430623 Mon Sep 17 00:00:00 2001
From: Kurt Alfred Kluever <kak@google.com>
Date: Mon, 26 Feb 2024 10:45:09 -0800
Subject: [PATCH] Recommend using `var` for `var unused = ...;` and `var thrown
 = assertThrows(MyException.class, () -> ...);`

PiperOrigin-RevId: 610457244
---
 .../errorprone/bugpatterns/Varifier.java      | 11 +++++
 .../errorprone/bugpatterns/VarifierTest.java  | 46 +++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/Varifier.java b/core/src/main/java/com/google/errorprone/bugpatterns/Varifier.java
index 03a61264e23..f7ff65a1e4a 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/Varifier.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/Varifier.java
@@ -67,6 +67,9 @@ public final class Varifier extends BugChecker implements VariableTreeMatcher {
                 && isSameType(((MethodSymbol) symbol).getReturnType(), symbol.owner.type, s);
           });
 
+  private static final Matcher<ExpressionTree> ASSERT_THROWS =
+      staticMethod().onClass("org.junit.Assert").named("assertThrows");
+
   @Override
   public Description matchVariable(VariableTree tree, VisitorState state) {
     var symbol = getSymbol(tree);
@@ -77,6 +80,14 @@ public Description matchVariable(VariableTree tree, VisitorState state) {
         || hasImplicitType(tree, state)) {
       return NO_MATCH;
     }
+    // Foo unused = ...;
+    if (symbol.getSimpleName().contentEquals("unused")) {
+      return fix(tree);
+    }
+    // MyException exception = assertThrows(MyException.class, () -> ...);
+    if (ASSERT_THROWS.matches(initializer, state)) {
+      return fix(tree);
+    }
     // Foo foo = (Foo) bar;
     if (initializer instanceof TypeCastTree
         && isSameType(
diff --git a/core/src/test/java/com/google/errorprone/bugpatterns/VarifierTest.java b/core/src/test/java/com/google/errorprone/bugpatterns/VarifierTest.java
index 499fa36de50..e9155c2823f 100644
--- a/core/src/test/java/com/google/errorprone/bugpatterns/VarifierTest.java
+++ b/core/src/test/java/com/google/errorprone/bugpatterns/VarifierTest.java
@@ -176,4 +176,50 @@ public void fromFactoryMethod() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void varUnused() {
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  public void trim(String string) {",
+            "    String unused = string.trim();",
+            "  }",
+            "}")
+        .addOutputLines(
+            "Test.java",
+            "class Test {",
+            "  public void trim(String string) {",
+            "    var unused = string.trim();",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void assertThrows() {
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            "import static org.junit.Assert.assertThrows;",
+            "class Test {",
+            "  public void testFoo() {",
+            "    Object nil = null;",
+            "    NullPointerException thrown = ",
+            "        assertThrows(NullPointerException.class, () -> nil.toString());",
+            "  }",
+            "}")
+        .addOutputLines(
+            "Test.java",
+            "import static org.junit.Assert.assertThrows;",
+            "class Test {",
+            "  public void testFoo() {",
+            "    Object nil = null;",
+            "    var thrown = ",
+            "        assertThrows(NullPointerException.class, () -> nil.toString());",
+            "  }",
+            "}")
+        .doTest();
+  }
 }