Ban the var keyword via error-prone VarUsage (#1788)

Ban the var keyword via error-prone `VarUsage`

baseline-error-prone/src/main/java/com/palantir/baseline/errorprone/VarUsage.java

@@ -0,0 +1,68 @@
+/*
+ * (c) Copyright 2020 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.palantir.baseline.errorprone;
+
+import com.google.auto.service.AutoService;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.BugPattern.LinkType;
+import com.google.errorprone.BugPattern.SeverityLevel;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.fixes.SuggestedFixes;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.util.ASTHelpers;
+import com.google.errorprone.util.ErrorProneToken;
+import com.sun.source.tree.Tree;
+import com.sun.source.tree.VariableTree;
+import com.sun.tools.javac.parser.Tokens.TokenKind;
+
+@AutoService(BugChecker.class)
+@BugPattern(
+        name = "VarUsage",
+        link = "https://github.com/palantir/gradle-baseline#baseline-error-prone-checks",
+        linkType = LinkType.CUSTOM,
+        severity = SeverityLevel.ERROR,
+        summary = "The `var` keyword results in illegible code in most cases and should not be used.")
+public final class VarUsage extends BugChecker implements BugChecker.VariableTreeMatcher {
+
+    @Override
+    public Description matchVariable(VariableTree tree, VisitorState state) {
+        Tree typeTree = tree.getType();
+        // The AST doesn't differentiate between 'var' and a concrete type, so we check the source
+        // prior to tokenizing the variable tree.
+        String sourceType = state.getSourceForNode(typeTree);
+        if (sourceType != null) {
+            return Description.NO_MATCH;
+        }
+        for (ErrorProneToken token : state.getOffsetTokensForNode(tree)) {
+            if (token.kind() == TokenKind.IDENTIFIER
+                    && token.hasName()
+                    && token.name().contentEquals("var")) {
+                SuggestedFix.Builder fix = SuggestedFix.builder();
+                return buildDescription(typeTree)
+                        .addFix(fix.replace(
+                                        token.pos(),
+                                        token.endPos(),
+                                        SuggestedFixes.prettyType(state, fix, ASTHelpers.getType(typeTree)))
+                                .build())
+                        .build();
+            }
+        }
+        return Description.NO_MATCH;
+    }
+}

baseline-error-prone/src/test/java/com/palantir/baseline/errorprone/VarUsageTest.java

@@ -0,0 +1,72 @@
+/*
+ * (c) Copyright 2021 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.palantir.baseline.errorprone;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledForJreRange;
+import org.junit.jupiter.api.condition.JRE;
+
+class VarUsageTest {
+
+    @Test
+    @EnabledForJreRange(min = JRE.JAVA_10)
+    void testSimple() {
+        fix().addInputLines(
+                        "Test.java",
+                        // format
+                        "class Test {",
+                        "  void function() {",
+                        "    var x = 3;",
+                        "  }",
+                        "}")
+                .addOutputLines(
+                        "Test.java",
+                        // format
+                        "class Test {",
+                        "  void function() {",
+                        "    int x = 3;",
+                        "  }",
+                        "}")
+                .doTest();
+    }
+
+    @Test
+    @EnabledForJreRange(min = JRE.JAVA_10)
+    void testWithFinalModifier() {
+        fix().addInputLines(
+                        "Test.java",
+                        // format
+                        "class Test {",
+                        "  void function() {",
+                        "    final var x = 3;",
+                        "  }",
+                        "}")
+                .addOutputLines(
+                        "Test.java",
+                        // format
+                        "class Test {",
+                        "  void function() {",
+                        "    final int x = 3;",
+                        "  }",
+                        "}")
+                .doTest();
+    }
+
+    private RefactoringValidator fix() {
+        return RefactoringValidator.of(VarUsage.class, getClass());
+    }
+}

changelog/@unreleased/pr-1788.v2.yml

@@ -0,0 +1,5 @@
+type: improvement
+improvement:
+  description: Ban the var keyword via error-prone `VarUsage`
+  links:
+  - https://github.com/palantir/gradle-baseline/pull/1788

gradle-baseline-java/src/main/groovy/com/palantir/baseline/extensions/BaselineErrorProneExtension.java

@@ -71,6 +71,7 @@ public class BaselineErrorProneExtension {
             // TODO(ckozak): re-enable pending scala check
             // "ThrowSpecificity",
             "UnsafeGaugeRegistration",
+            "VarUsage",
 
             // Built-in checks
             "ArrayEquals",