// Copyright 2014 The Bazel Authors. 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,
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package com.google.devtools.build.lib.analysis;
/**
* Contains rolled-up data about the transitive closure of a configured target.
*
* For more information about how analysis works, see
* {@link com.google.devtools.build.lib.rules.RuleConfiguredTargetFactory}.
* TransitiveInfoProviders need to be serializable, and for that reason they must conform to
* the following restrictions:
*
* <ul>
* <li>The provider interface must directly extend {@code TransitiveInfoProvider}.
* <li>Every method must return immutable data.</li>
* <li>Every method must return the same object if called multiple times with the same
* arguments.</li>
* <li>Overloading a method name multiple times is forbidden.</li>
* <li>The return type of a method must satisfy one of the following conditions:
* <ul>
* <li>It must be from the set of {String, Integer, int, Boolean, bool, Label, PathFragment,
* Artifact}, OR</li>
* <li>it must be an ImmutableList/List/Collection/Iterable of T, where T is either
* one of the types above with a default serializer or T implements ValueSerializer, OR</li>
* <li>it must be serializable (TBD)</li>
* </ul>
* <li>If the method takes arguments, it must declare a custom serializer (TBD).</li>
* </ul>
*
* <p>Some typical uses of this interface are:
* <ul>
* <li>The set of Python source files in the transitive closure of this rule
* <li>The set of declared C++ header files in the transitive closure
* <li>The files that need to be built when the target is mentioned on the command line
* </ul>
*
* <p>Note that if implemented naively, this would result in the memory requirements
* being O(n^2): in a long dependency chain, if every target adds one single artifact, storing the
* transitive closures of every rule would take 1+2+3+...+n-1+n = O(n^2) memory.
*
* <p>In order to avoid this, we introduce the concept of nested sets, {@link
* com.google.devtools.build.lib.collect.nestedset.NestedSet}. A nested set is an immutable
* data structure that can contain direct members and other nested sets (recursively). Nested sets
* are iterable and can be flattened into ordered sets, where the order depends on which
* implementation of NestedSet you pick.
*
* @see TransitiveInfoCollection
* @see com.google.devtools.build.lib.rules.RuleConfiguredTargetFactory
*/
public interface TransitiveInfoProvider {
}