Fix a crash on var in DefaultCharset

Fixes #1563

PiperOrigin-RevId: 350823946

Author: cushon

core/src/main/java/com/google/errorprone/bugpatterns/DefaultCharset.java

@@ -369,21 +369,21 @@ private void variableTypeFix(
         sym.type, state.getTypeFromString(original.getCanonicalName()), state)) {
       return;
     }
-    state
-        .getPath()
-        .getCompilationUnit()
-        .accept(
-            new TreeScanner<Void, Void>() {
-              @Override
-              public Void visitVariable(VariableTree node, Void unused) {
-                if (sym.equals(ASTHelpers.getSymbol(node))) {
-                  fix.replace(node.getType(), replacement.getSimpleName())
-                      .addImport(replacement.getCanonicalName());
-                }
-                return null;
-              }
-            },
-            null);
+    new TreeScanner<Void, Void>() {
+      @Override
+      public Void visitVariable(VariableTree node, Void unused) {
+        if (!sym.equals(ASTHelpers.getSymbol(node))) {
+          return null;
+        }
+        if (ASTHelpers.getStartPosition(node.getType()) == -1) {
+          // ignore synthetic tree nodes for `var`
+          return null;
+        }
+        fix.replace(node.getType(), replacement.getSimpleName())
+            .addImport(replacement.getCanonicalName());
+        return null;
+      }
+    }.scan(state.getPath().getCompilationUnit(), null);
   }
 
   private Description handleFileWriter(NewClassTree tree, VisitorState state) {

core/src/test/java/com/google/errorprone/bugpatterns/DefaultCharsetTest.java

@@ -16,9 +16,12 @@
 
 package com.google.errorprone.bugpatterns;
 
+import static org.junit.Assume.assumeTrue;
+
 import com.google.errorprone.BugCheckerRefactoringTestHelper;
 import com.google.errorprone.BugCheckerRefactoringTestHelper.FixChoosers;
 import com.google.errorprone.CompilationTestHelper;
+import com.google.errorprone.util.RuntimeVersion;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -499,4 +502,31 @@ public void scannerDefaultCharset() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void withVar() {
+    assumeTrue(RuntimeVersion.isAtLeast15());
+    refactoringTest()
+        .addInputLines(
+            "in/Test.java",
+            "import java.io.File;",
+            "import java.io.FileWriter;",
+            "class Test {",
+            "  void f(File file) throws Exception {",
+            "    var fileWriter = new FileWriter(file);",
+            "  }",
+            "}")
+        .addOutputLines(
+            "out/Test.java",
+            "import static java.nio.charset.StandardCharsets.UTF_8;",
+            "import java.io.File;",
+            "import java.io.FileWriter;",
+            "import java.nio.file.Files;",
+            "class Test {",
+            "  void f(File file) throws Exception {",
+            "    var fileWriter = Files.newBufferedWriter(file.toPath(), UTF_8);",
+            "  }",
+            "}")
+        .doTest();
+  }
 }