Introduce EagerStringFormatting check

This new check flag code that can be simplified and/or optimized by
deferring certain string formatting operations.

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

@@ -0,0 +1,292 @@
+package tech.picnic.errorprone.bugpatterns;
+
+import static com.google.errorprone.BugPattern.LinkType.CUSTOM;
+import static com.google.errorprone.BugPattern.SeverityLevel.WARNING;
+import static com.google.errorprone.BugPattern.StandardTags.PERFORMANCE;
+import static com.google.errorprone.BugPattern.StandardTags.SIMPLIFICATION;
+import static com.google.errorprone.matchers.Matchers.anyOf;
+import static com.google.errorprone.matchers.Matchers.isSubtypeOf;
+import static com.google.errorprone.matchers.Matchers.staticMethod;
+import static com.google.errorprone.matchers.method.MethodMatchers.instanceMethod;
+import static java.util.stream.Collectors.joining;
+import static tech.picnic.errorprone.utils.Documentation.BUG_PATTERNS_BASE_URL;
+
+import com.google.auto.service.AutoService;
+import com.google.auto.value.AutoValue;
+import com.google.common.base.Preconditions;
+import com.google.common.base.Verify;
+import com.google.common.collect.ImmutableList;
+import com.google.errorprone.BugPattern;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.annotations.Var;
+import com.google.errorprone.bugpatterns.BugChecker;
+import com.google.errorprone.bugpatterns.BugChecker.MethodInvocationTreeMatcher;
+import com.google.errorprone.fixes.SuggestedFix;
+import com.google.errorprone.matchers.Description;
+import com.google.errorprone.matchers.Matcher;
+import com.google.errorprone.util.ASTHelpers;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.MethodInvocationTree;
+import com.sun.source.tree.Tree;
+import com.sun.tools.javac.code.Symbol;
+import com.sun.tools.javac.code.Symbol.MethodSymbol;
+import com.sun.tools.javac.code.Symbol.VarSymbol;
+import java.util.Collections;
+import java.util.Formattable;
+import java.util.List;
+import java.util.Locale;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.stream.Stream;
+import tech.picnic.errorprone.utils.SourceCode;
+
+/**
+ * A {@link BugChecker} that flags {@link String#format(String, Object...)}, {@link
+ * String#format(Locale, String, Object...)} and {@link String#formatted(Object...)} invocations
+ * that can be omitted by delegating to another format method.
+ */
+// XXX: The special-casing of Throwable applies only to SLF4J 1.6.0+; see
+// https://www.slf4j.org/faq.html#paramException. That should be documented.
+// XXX: Some of the `Matcher`s defined here are also declared by the `Slf4jLogStatement` and
+// `RedundantStringConversion` checks. Look into deduplicating them.
+// XXX: Should we also simplify e.g. `LOG.error(String.join("sep", arg1, arg2), throwable)`? Perhaps
+// that's too obscure.
+@AutoService(BugChecker.class)
+@BugPattern(
+    summary = "String formatting can be deferred",
+    link = BUG_PATTERNS_BASE_URL + "EagerStringFormatting",
+    linkType = CUSTOM,
+    severity = WARNING,
+    tags = {PERFORMANCE, SIMPLIFICATION})
+public final class EagerStringFormatting extends BugChecker implements MethodInvocationTreeMatcher {
+  private static final long serialVersionUID = 1L;
+  private static final Matcher<ExpressionTree> FORMATTABLE = isSubtypeOf(Formattable.class);
+  private static final Matcher<ExpressionTree> LOCALE = isSubtypeOf(Locale.class);
+  private static final Matcher<ExpressionTree> SLF4J_MARKER = isSubtypeOf("org.slf4j.Marker");
+  private static final Matcher<ExpressionTree> THROWABLE = isSubtypeOf(Throwable.class);
+  private static final Matcher<ExpressionTree> REQUIRE_NOT_NULL_INVOCATION =
+      staticMethod().onClass(Objects.class.getCanonicalName()).named("requireNonNull");
+  private static final Matcher<ExpressionTree> GUAVA_GUARD_INVOCATION =
+      anyOf(
+          staticMethod()
+              .onClass(Preconditions.class.getCanonicalName())
+              .namedAnyOf("checkArgument", "checkNotNull", "checkState"),
+          staticMethod()
+              .onClass(Verify.class.getCanonicalName())
+              .namedAnyOf("verify", "verifyNotNull"));
+  private static final Matcher<ExpressionTree> SLF4J_LOGGER_INVOCATION =
+      instanceMethod()
+          .onDescendantOf("org.slf4j.Logger")
+          .namedAnyOf("trace", "debug", "info", "warn", "error");
+  private static final Matcher<ExpressionTree> STATIC_FORMAT_STRING =
+      staticMethod().onClass(String.class.getCanonicalName()).named("format");
+  private static final Matcher<ExpressionTree> INSTANCE_FORMAT_STRING =
+      instanceMethod().onDescendantOf(String.class.getCanonicalName()).named("formatted");
+
+  /** Instantiates a new {@link EagerStringFormatting} instance. */
+  public EagerStringFormatting() {}
+
+  @Override
+  public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState state) {
+    Tree parent = state.getPath().getParentPath().getLeaf();
+    if (!(parent instanceof MethodInvocationTree methodInvocation)) {
+      /*
+       * Fast path: this isn't a method invocation whose result is an argument to another method
+       * invocation. (We assume that this expression isn't redundantly wrapped in parentheses.)
+       */
+      return Description.NO_MATCH;
+    }
+
+    return StringFormatExpression.tryCreate(tree, state)
+        .map(expr -> analyzeFormatStringContext(expr, methodInvocation, state))
+        .orElse(Description.NO_MATCH);
+  }
+
+  private Description analyzeFormatStringContext(
+      StringFormatExpression stringFormat, MethodInvocationTree context, VisitorState state) {
+    List<? extends ExpressionTree> arguments = context.getArguments();
+
+    if (REQUIRE_NOT_NULL_INVOCATION.matches(context, state)) {
+      if (arguments.size() != 2) {
+        /* Vacuous validation that string formatting doesn't yield `null`. */
+        // XXX: Maybe flag this, while we're here?
+        return Description.NO_MATCH;
+      }
+
+      if (stringFormat.arguments().stream()
+          .anyMatch(EagerStringFormatting::isNonFinalLocalVariable)) {
+        /*
+         * The format operation depends on a variable that isn't final or effectively final; moving
+         * it into a lambda expression would cause a compilation error.
+         */
+        // XXX: Define a custom message for this case?
+        return describeMatch(context);
+      }
+
+      /*
+       * Suggest that the string formatting is deferred. Note that moving the format operation into
+       * a lambda expression will cause a compilation in case any of its arguments aren't
+       * effectively final.
+       */
+      return describeMatch(context, SuggestedFix.prefixWith(stringFormat.expression(), "() -> "));
+    }
+
+    if (GUAVA_GUARD_INVOCATION.matches(context, state)) {
+      if (arguments.size() < 2) {
+        /* Vacuous validation that string formatting doesn't yield `null`. */
+        // XXX: Maybe flag this, while we're here?
+        return Description.NO_MATCH;
+      }
+
+      if (stringFormat.simplifiableFormatString().isEmpty()) {
+        /* We can't simplify this case; only flag it. */
+        // XXX: Use a custom message.
+        return describeMatch(context);
+      }
+
+      // XXX: Collapse into previous statement? (Depends on the extent to which we'll customize
+      // messages.)
+      if (arguments.size() > 2) {
+        /*
+         * The format string produces a format string itself, or its result is the input to another
+         * format operation. This is a complex case that we'll only flag.
+         */
+        // XXX: Use a custom message.
+        return describeMatch(context);
+      }
+
+      return describeMatch(context, stringFormat.suggestFlattening("%s", state));
+    }
+
+    if (SLF4J_LOGGER_INVOCATION.matches(context, state)) {
+      if (stringFormat.simplifiableFormatString().isEmpty()) {
+        /* We can't simplify this case; only flag it. */
+        // XXX: Use a custom message.
+        return describeMatch(context);
+      }
+
+      int leftOffset = SLF4J_MARKER.matches(arguments.get(0), state) ? 1 : 0;
+      int rightOffset = THROWABLE.matches(arguments.get(arguments.size() - 1), state) ? 1 : 0;
+      if (arguments.size() != leftOffset + 1 + rightOffset) {
+        /*
+         * The format string produces a format string itself, or its result is the input to another
+         * format operation. This is a complex case that we'll only flag.
+         */
+        // XXX: Use a custom message.
+        return describeMatch(context);
+      }
+
+      return describeMatch(context, stringFormat.suggestFlattening("{}", state));
+    }
+
+    /*
+     * The string formatting operation does not appear to happen in a context that admits of
+     * simplification or optimization.
+     */
+    return Description.NO_MATCH;
+  }
+
+  private static boolean isNonFinalLocalVariable(Tree tree) {
+    Symbol symbol = ASTHelpers.getSymbol(tree);
+    return symbol instanceof VarSymbol
+        && symbol.owner instanceof MethodSymbol
+        && !ASTHelpers.isConsideredFinal(symbol);
+  }
+
+  /** Description of a string format expression. */
+  @AutoValue
+  abstract static class StringFormatExpression {
+    /** The full string format expression. */
+    abstract Tree expression();
+
+    /** The string format arguments to be plugged into its format string. */
+    abstract ImmutableList<ExpressionTree> arguments();
+
+    /**
+     * The constant format string, if it contains only {@code %s} placeholders, and the number of
+     * said placeholders matches the number of format arguments.
+     */
+    abstract Optional<String> simplifiableFormatString();
+
+    private SuggestedFix suggestFlattening(String newPlaceholder, VisitorState state) {
+      return SuggestedFix.replace(
+          expression(),
+          Stream.concat(
+                  Stream.of(deriveFormatStringExpression(newPlaceholder)),
+                  arguments().stream().map(arg -> SourceCode.treeToString(arg, state)))
+              .collect(joining(", ")));
+    }
+
+    final String deriveFormatStringExpression(String newPlaceholder) {
+      return SourceCode.toStringConstantExpression(
+          String.format(
+              simplifiableFormatString()
+                  .orElseThrow(
+                      () -> new IllegalStateException("Format string cannot be simplified")),
+              Collections.nCopies(arguments().size(), newPlaceholder).toArray()));
+    }
+
+    private static Optional<StringFormatExpression> tryCreate(
+        MethodInvocationTree tree, VisitorState state) {
+      if (INSTANCE_FORMAT_STRING.matches(tree, state)) {
+        return Optional.of(
+            create(
+                tree,
+                ASTHelpers.getReceiver(tree),
+                ImmutableList.copyOf(tree.getArguments()),
+                state));
+      }
+
+      if (STATIC_FORMAT_STRING.matches(tree, state)) {
+        List<? extends ExpressionTree> arguments = tree.getArguments();
+        int argOffset = LOCALE.matches(arguments.get(0), state) ? 1 : 0;
+        return Optional.of(
+            create(
+                tree,
+                arguments.get(argOffset),
+                ImmutableList.copyOf(arguments.subList(argOffset + 1, arguments.size())),
+                state));
+      }
+
+      return Optional.empty();
+    }
+
+    private static StringFormatExpression create(
+        Tree expression,
+        Tree formatString,
+        ImmutableList<ExpressionTree> arguments,
+        VisitorState state) {
+      return new AutoValue_EagerStringFormatting_StringFormatExpression(
+          expression,
+          arguments,
+          Optional.ofNullable(ASTHelpers.constValue(formatString, String.class))
+              .filter(template -> isSimplifiable(template, arguments, state)));
+    }
+
+    private static boolean isSimplifiable(
+        String formatString, ImmutableList<ExpressionTree> arguments, VisitorState state) {
+      if (arguments.stream().anyMatch(arg -> FORMATTABLE.matches(arg, state))) {
+        /* `Formattable` arguments can have arbitrary format semantics. */
+        return false;
+      }
+
+      @Var int placeholderCount = 0;
+      for (int p = formatString.indexOf('%'); p != -1; p = formatString.indexOf('%', p + 2)) {
+        if (p == formatString.length() - 1) {
+          /* Malformed format string with trailing `%`. */
+          return false;
+        }
+
+        char modifier = formatString.charAt(p + 1);
+        if (modifier == 's') {
+          placeholderCount++;
+        } else if (modifier != '%') {
+          /* Only `%s` and `%%` (a literal `%`) are supported. */
+          return false;
+        }
+      }
+
+      return placeholderCount == arguments.size();
+    }
+  }
+}

error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/Slf4jLogStatement.java

@@ -28,8 +28,6 @@
 /** A {@link BugChecker} that flags SLF4J usages that are likely to be in error. */
 // XXX: The special-casing of Throwable applies only to SLF4J 1.6.0+; see
 // https://www.slf4j.org/faq.html#paramException. That should be documented.
-// XXX: Also simplify `LOG.error(String.format("Something %s", arg), throwable)`.
-// XXX: Also simplify `LOG.error(String.join("sep", arg1, arg2), throwable)`? Perhaps too obscure.
 // XXX: Write a similar checker for Spring RestTemplates, String.format and friends, Guava
 // preconditions, ...
 @AutoService(BugChecker.class)