Rewrite Optional#orElse to Optional#orElseGet for non-constant expressions

error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/OptionalRules.java

@@ -8,6 +8,7 @@
 import com.google.errorprone.refaster.annotation.AfterTemplate;
 import com.google.errorprone.refaster.annotation.BeforeTemplate;
 import com.google.errorprone.refaster.annotation.MayOptionallyUse;
+import com.google.errorprone.refaster.annotation.NotMatches;
 import com.google.errorprone.refaster.annotation.Placeholder;
 import com.google.errorprone.refaster.annotation.UseImportPolicy;
 import java.util.Comparator;
@@ -19,6 +20,7 @@
 import java.util.stream.Stream;
 import org.jspecify.annotations.Nullable;
 import tech.picnic.errorprone.refaster.annotation.OnlineDocumentation;
+import tech.picnic.errorprone.refaster.matchers.IsLikelyTrivialComputation;
 
 /** Refaster rules related to expressions dealing with {@link Optional}s. */
 @OnlineDocumentation
@@ -118,7 +120,7 @@ Optional<T> after(Iterator<T> it) {
   /** Prefer {@link Optional#filter(Predicate)} over usage of the ternary operator. */
   // XXX: This rule may introduce a compilation error: the `test` expression may reference a
   // non-effectively final variable, which is not allowed in the replacement lambda expression.
-  // Maybe our `Refaster` checker should test `compilesWithFix`?
+  // Review whether a `@Matcher` can be used to avoid this.
   abstract static class TernaryOperatorOptionalPositiveFiltering<T> {
     @Placeholder
     abstract boolean test(T value);
@@ -138,7 +140,7 @@ Optional<T> after(T input) {
   /** Prefer {@link Optional#filter(Predicate)} over usage of the ternary operator. */
   // XXX: This rule may introduce a compilation error: the `test` expression may reference a
   // non-effectively final variable, which is not allowed in the replacement lambda expression.
-  // Maybe our `Refaster` checker should test `compilesWithFix`?
+  // Review whether a `@Matcher` can be used to avoid this.
   abstract static class TernaryOperatorOptionalNegativeFiltering<T> {
     @Placeholder
     abstract boolean test(T value);
@@ -161,6 +163,7 @@ Optional<T> after(T input) {
    */
   static final class MapOptionalToBoolean<T> {
     @BeforeTemplate
+    @SuppressWarnings("OptionalOrElseGet" /* Rule is confused by `Refaster#anyOf` usage. */)
     boolean before(Optional<T> optional, Function<? super T, Boolean> predicate) {
       return optional.map(predicate).orElse(Refaster.anyOf(false, Boolean.FALSE));
     }
@@ -224,6 +227,28 @@ T after(Optional<T> o1, Optional<T> o2) {
     }
   }
 
+  /**
+   * Prefer {@link Optional#orElseGet(Supplier)} over {@link Optional#orElse(Object)} if the
+   * fallback value is not the result of a trivial computation.
+   */
+  // XXX: This rule may introduce a compilation error: the `value` expression may reference a
+  // non-effectively final variable, which is not allowed in the replacement lambda expression.
+  // Review whether a `@Matcher` can be used to avoid this.
+  // XXX: Once `MethodReferenceUsage` is "production ready", replace
+  // `@NotMatches(IsLikelyTrivialComputation.class)` with `@Matches(RequiresComputation.class)` (and
+  // reimplement the matcher accordingly).
+  static final class OptionalOrElseGet<T> {
+    @BeforeTemplate
+    T before(Optional<T> optional, @NotMatches(IsLikelyTrivialComputation.class) T value) {
+      return optional.orElse(value);
+    }
+
+    @AfterTemplate
+    T after(Optional<T> optional, T value) {
+      return optional.orElseGet(() -> value);
+    }
+  }
+
   /**
    * Flatten a stream of {@link Optional}s using {@link Optional#stream()}, rather than using one of
    * the more verbose alternatives.
@@ -325,6 +350,9 @@ static final class OptionalOrOtherOptional<T> {
     Optional<T> before(Optional<T> optional1, Optional<T> optional2) {
       // XXX: Note that rewriting the first and third variant will change the code's behavior if
       // `optional2` has side-effects.
+      // XXX: Note that rewriting the first and third variant will introduce a compilation error if
+      // `optional2` is not effectively final. Review whether a `@Matcher` can be used to avoid
+      // this.
       return Refaster.anyOf(
           optional1.map(Optional::of).orElse(optional2),
           optional1.map(Optional::of).orElseGet(() -> optional2),

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/OptionalRulesTestInput.java

@@ -119,4 +119,11 @@ ImmutableSet<Optional<String>> testOptionalFilter() {
   Optional<String> testOptionalMap() {
     return Optional.of(1).stream().map(String::valueOf).findAny();
   }
+
+  ImmutableSet<String> testOptionalOrElseGet() {
+    return ImmutableSet.of(
+        Optional.of("foo").orElse("bar"),
+        Optional.of("baz").orElse(toString()),
+        Optional.of("qux").orElse(String.valueOf(true)));
+  }
 }

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/OptionalRulesTestOutput.java

@@ -112,4 +112,11 @@ ImmutableSet<Optional<String>> testOptionalFilter() {
   Optional<String> testOptionalMap() {
     return Optional.of(1).map(String::valueOf);
   }
+
+  ImmutableSet<String> testOptionalOrElseGet() {
+    return ImmutableSet.of(
+        Optional.of("foo").orElse("bar"),
+        Optional.of("baz").orElse(toString()),
+        Optional.of("qux").orElseGet(() -> String.valueOf(true)));
+  }
 }

refaster-support/src/main/java/tech/picnic/errorprone/refaster/matchers/IsLikelyTrivialComputation.java

@@ -0,0 +1,92 @@
+package tech.picnic.errorprone.refaster.matchers;
+
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.matchers.Matcher;
+import com.sun.source.tree.ArrayAccessTree;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.IdentifierTree;
+import com.sun.source.tree.LambdaExpressionTree;
+import com.sun.source.tree.LiteralTree;
+import com.sun.source.tree.MemberReferenceTree;
+import com.sun.source.tree.MemberSelectTree;
+import com.sun.source.tree.MethodInvocationTree;
+import com.sun.source.tree.ParenthesizedTree;
+import com.sun.source.tree.TypeCastTree;
+import com.sun.source.tree.UnaryTree;
+
+/** A matcher of expressions that likely require little to no computation. */
+public final class IsLikelyTrivialComputation implements Matcher<ExpressionTree> {
+  private static final long serialVersionUID = 1L;
+
+  /** Instantiates a new {@link IsLikelyTrivialComputation} instance. */
+  public IsLikelyTrivialComputation() {}
+
+  @Override
+  public boolean matches(ExpressionTree expressionTree, VisitorState state) {
+    if (expressionTree instanceof MethodInvocationTree) {
+      // XXX: Method invocations are generally *not* trivial computations, but we make an exception
+      // for nullary method invocations on the result of a trivial computation. This exception
+      // allows this `Matcher` to by the `OptionalOrElseGet` Refaster rule, such that it does not
+      // suggest the introduction of lambda expressions that are better expressed as method
+      // references. Once the `MethodReferenceUsage` bug checker is production-ready, this exception
+      // should be removed. (But at that point, instead defining a `RequiresComputation` matcher may
+      // be more appropriate.)
+      MethodInvocationTree methodInvocation = (MethodInvocationTree) expressionTree;
+      if (methodInvocation.getArguments().isEmpty()
+          && matches(methodInvocation.getMethodSelect())) {
+        return true;
+      }
+    }
+
+    return matches(expressionTree);
+  }
+
+  // XXX: Some `BinaryTree`s may represent what could be considered "trivial computations".
+  // Depending on feedback such trees may be matched in the future.
+  private static boolean matches(ExpressionTree expressionTree) {
+    if (expressionTree instanceof ArrayAccessTree) {
+      return matches(((ArrayAccessTree) expressionTree).getExpression())
+          && matches(((ArrayAccessTree) expressionTree).getIndex());
+    }
+
+    if (expressionTree instanceof LiteralTree) {
+      return true;
+    }
+
+    if (expressionTree instanceof LambdaExpressionTree) {
+      /*
+       * Lambda expressions encapsulate computations, but their definition does not involve
+       * significant computation.
+       */
+      return true;
+    }
+
+    if (expressionTree instanceof IdentifierTree) {
+      return true;
+    }
+
+    if (expressionTree instanceof MemberReferenceTree) {
+      return matches(((MemberReferenceTree) expressionTree).getQualifierExpression());
+    }
+
+    if (expressionTree instanceof MemberSelectTree) {
+      return matches(((MemberSelectTree) expressionTree).getExpression());
+    }
+
+    if (expressionTree instanceof ParenthesizedTree) {
+      return matches(((ParenthesizedTree) expressionTree).getExpression());
+    }
+
+    if (expressionTree instanceof TypeCastTree) {
+      return matches(((TypeCastTree) expressionTree).getExpression());
+    }
+
+    if (expressionTree instanceof UnaryTree) {
+      // XXX: Arguably side-effectful options such as pre- and post-increment and -decrement are not
+      // trivial.
+      return matches(((UnaryTree) expressionTree).getExpression());
+    }
+
+    return false;
+  }
+}

refaster-support/src/test/java/tech/picnic/errorprone/refaster/matchers/IsLikelyTrivialComputationTest.java

@@ -0,0 +1,147 @@
+package tech.picnic.errorprone.refaster.matchers;
+
+import static com.google.errorprone.BugPattern.SeverityLevel.ERROR;
+
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.CompilationTestHelper;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.sun.source.tree.ReturnTree;
+import org.junit.jupiter.api.Test;
+
+final class IsLikelyTrivialComputationTest {
+  @Test
+  void matches() {
+    CompilationTestHelper.newInstance(MatcherTestChecker.class, getClass())
+        .addSourceLines(
+            "A.java",
+            "import java.util.function.Predicate;",
+            "",
+            "class A {",
+            "  String negative1() {",
+            "    return String.valueOf(1);",
+            "  }",
+            "",
+            "  String negative2() {",
+            "    return toString().toString();",
+            "  }",
+            "",
+            "  String negative3() {",
+            "    return \"foo\" + toString();",
+            "  }",
+            "",
+            "  byte negative4() {",
+            "    return \"foo\".getBytes()[0];",
+            "  }",
+            "",
+            "  int negative5() {",
+            "    int[] arr = new int[0];",
+            "    return arr[hashCode()];",
+            "  }",
+            "",
+            "  int negative6() {",
+            "    return 1 * 2;",
+            "  }",
+            "",
+            "  Predicate<String> negative7() {",
+            "    return toString()::equals;",
+            "  }",
+            "",
+            "  String negative8() {",
+            "    return (toString());",
+            "  }",
+            "",
+            "  Object negative9() {",
+            "    return (Object) toString();",
+            "  }",
+            "",
+            "  int negative10() {",
+            "    return -hashCode();",
+            "  }",
+            "",
+            "  String positive1() {",
+            "    // BUG: Diagnostic contains:",
+            "    return toString();",
+            "  }",
+            "",
+            "  String positive2() {",
+            "    // BUG: Diagnostic contains:",
+            "    return this.toString();",
+            "  }",
+            "",
+            "  int positive3() {",
+            "    int[] arr = new int[0];",
+            "    // BUG: Diagnostic contains:",
+            "    return arr[0];",
+            "  }",
+            "",
+            "  String positive4() {",
+            "    // BUG: Diagnostic contains:",
+            "    return null;",
+            "  }",
+            "",
+            "  boolean positive5() {",
+            "    // BUG: Diagnostic contains:",
+            "    return false;",
+            "  }",
+            "",
+            "  int positive6() {",
+            "    // BUG: Diagnostic contains:",
+            "    return 0;",
+            "  }",
+            "",
+            "  String positive7() {",
+            "    // BUG: Diagnostic contains:",
+            "    return \"foo\" + \"bar\";",
+            "  }",
+            "",
+            "  Predicate<String> positive8() {",
+            "    // BUG: Diagnostic contains:",
+            "    return v -> \"foo\".equals(v);",
+            "  }",
+            "",
+            "  A positive9() {",
+            "    // BUG: Diagnostic contains:",
+            "    return this;",
+            "  }",
+            "",
+            "  Predicate<String> positive10() {",
+            "    // BUG: Diagnostic contains:",
+            "    return \"foo\"::equals;",
+            "  }",
+            "",
+            "  A positive11() {",
+            "    // BUG: Diagnostic contains:",
+            "    return (this);",
+            "  }",
+            "",
+            "  Object positive12() {",
+            "    // BUG: Diagnostic contains:",
+            "    return (Object) this;",
+            "  }",
+            "",
+            "  boolean positive13() {",
+            "    // BUG: Diagnostic contains:",
+            "    return !false;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  /** A {@link BugChecker} that simply delegates to {@link IsLikelyTrivialComputation}. */
+  @BugPattern(
+      summary = "Flags return statement expressions matched by `IsLikelyTrivialComputation`",
+      severity = ERROR)
+  public static final class MatcherTestChecker extends AbstractMatcherTestChecker {
+    private static final long serialVersionUID = 1L;
+
+    // XXX: This is a false positive reported by Checkstyle. See
+    // https://github.com/checkstyle/checkstyle/issues/10161#issuecomment-1242732120.
+    @SuppressWarnings("RedundantModifier")
+    public MatcherTestChecker() {
+      super(
+          (expressionTree, state) ->
+              state.getPath().getParentPath().getLeaf() instanceof ReturnTree
+                  && new IsLikelyTrivialComputation().matches(expressionTree, state));
+    }
+  }
+}