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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.
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/*
 * Copyright 2015 The Closure Compiler Authors.
 *
 * 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.google.javascript.jscomp;

import static com.google.common.base.Preconditions.checkArgument;

import com.google.common.base.Predicates;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import com.google.javascript.rhino.jstype.JSType;
import com.google.javascript.rhino.jstype.JSTypeRegistry;

/** Warn about types in JSDoc that are implicitly nullable. */
public final class ImplicitNullabilityCheck extends AbstractPostOrderCallback
    implements CompilerPass {

  public static final DiagnosticType IMPLICITLY_NULLABLE_JSDOC =
    DiagnosticType.disabled(
        "JSC_IMPLICITLY_NULLABLE_JSDOC",
        "Name {0} in JSDoc is implicitly nullable, and is discouraged by the style guide.\n"
        + "Please add a '!' to make it non-nullable, or a '?' to make it explicitly nullable.");

  public static final DiagnosticType IMPLICITLY_NONNULL_JSDOC =
      DiagnosticType.disabled(
          "JSC_IMPLICITLY_NONNULL_JSDOC",
          "Name {0} in JSDoc is implicitly non-null, and is discouraged by the style guide.\n"
              + "Please add a '!' to make it explicit.");

  private static final ImmutableSet NULLABILITY_OMITTED_TYPES =
      ImmutableSet.of(
          "*", //
          "?",
          "bigint",
          "boolean",
          "null",
          "number",
          "string",
          "symbol",
          "undefined",
          "void");

  private final AbstractCompiler compiler;

  public ImplicitNullabilityCheck(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseRoots(compiler, this, externs, root);
  }

  /**
   * Represents the types of implicit nullability errors caught by this pass: a) implicitly nonnull
   * (missing a "!"), and b) implicitly nullable (missing a "?").
   */
  public enum Nullability {
    NONNULL,
    NULLABLE;

    public boolean isNullable() {
      return this == Nullability.NULLABLE;
    }
  }

  /**
   * Information to represent a single "implicit nullability result", including the JSDoc string
   * node that needs a "!" or "?" to be explicit, as well as an enum indicating which of the two
   * nullability cases were found ("!" or "?").
   */
  public static class Result {
    final Node node;
    final Nullability nullability;

    private Result(Node node, Nullability nullability) {
      checkArgument(node.isStringLit());
      this.node = node;
      this.nullability = nullability;
    }

    static Result create(Node node, Nullability nullability) {
      return new Result(node, nullability);
    }

    public Nullability getNullability() {
      return nullability;
    }

    public Node getNode() {
      return node;
    }
  }

  /**
   * Finds and returns all the JSDoc nodes inside the given JSDoc object whose nullability is not
   * explict, using the NodeTraversal the necessary state (current scope, etc.)
   */
  public static ImmutableList findImplicitNullabilityResults(
      final JSDocInfo info, final NodeTraversal t) {
    if (info == null) {
      return ImmutableList.of();
    }

    final ImmutableList.Builder builder = ImmutableList.builder();
    for (Node typeRoot : info.getTypeNodes()) {
      NodeUtil.visitPreOrder(
          typeRoot,
          new NodeUtil.Visitor() {
            @Override
            public void visit(Node node) {
              if (!node.isStringLit()) {
                return;
              }
              Node parent = node.getParent();
              if (parent != null) {
                switch (parent.getToken()) {
                  case BANG:
                  case QMARK:
                  case THIS: // The names inside function(this:Foo) and
                  case NEW: // function(new:Bar) are already non-null.
                  case TYPEOF: // Names after 'typeof' don't have nullability.
                    return;
                  case PIPE:
                    { // Inside a union
                      Node gp = parent.getParent();
                      if (gp != null && gp.getToken() == Token.QMARK) {
                        return; // Inside an explicitly nullable union
                      }
                      for (Node child = parent.getFirstChild();
                          child != null;
                          child = child.getNext()) {
                        if ((child.isStringLit() && child.getString().equals("null"))
                            || child.getToken() == Token.QMARK) {
                          return; // Inside a union that contains null or nullable type
                        }
                      }
                      break;
                    }
                  default:
                    break;
                }
              }
              String typeName = node.getString();
              if (NULLABILITY_OMITTED_TYPES.contains(typeName)) {
                return;
              }
              JSTypeRegistry registry = t.getCompiler().getTypeRegistry();
              if (registry.getType(t.getScope(), typeName) == null) {
                return;
              }
              JSType type = registry.createTypeFromCommentNode(node);
              Nullability nullability =
                  type.isNullable() ? Nullability.NULLABLE : Nullability.NONNULL;
              builder.add(Result.create(node, nullability));
            }
          },
          Predicates.alwaysTrue());
    }
    return builder.build();
  }

  /** Crawls the JSDoc of the given node to find any names in JSDoc that are implicitly null. */
  @Override
  public void visit(final NodeTraversal t, final Node n, final Node p) {
    final JSDocInfo info = n.getJSDocInfo();
    for (Result r : findImplicitNullabilityResults(info, t)) {
      Node stringNode = r.getNode();
      DiagnosticType dt =
          r.getNullability().isNullable() ? IMPLICITLY_NULLABLE_JSDOC : IMPLICITLY_NONNULL_JSDOC;
      compiler.report(JSError.make(stringNode, dt, stringNode.getString()));
    }
  }
}