Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

12 files changed, 3313 insertions, 0 deletions

diff --git a/xpcom/docs/cc-macros.rst b/xpcom/docs/cc-macros.rst
new file mode 100644
index 0000000000..7877b3aabe
--- /dev/null
+++ b/xpcom/docs/cc-macros.rst
@@ -0,0 +1,190 @@
+=======================================
+How to make a C++ class cycle collected
+=======================================
+
+Should my class be cycle collected?
+===================================
+
+First, you need to decide if your class should be cycle
+collected. There are three main criteria:
+
+* It can be part of a cycle of strong references, including
+  refcounted objects and JS. Usually, this happens when it can hold
+  alive and be held alive by cycle collected objects or JS.
+
+* It must be refcounted.
+
+* It must be single threaded. The cycle collector can only work with
+  objects that are used on a single thread. The main thread and DOM
+  worker and worklet threads each have their own cycle collectors.
+
+If your class meets the first criteria but not the second, then
+whatever class uniquely owns it should be cycle collected, assuming
+that is refcounted, and this class should be traversed and unlinked as
+part of that.
+
+The cycle collector supports both nsISupports and non-nsISupports
+(known as "native" in CC nomenclature) refcounting. However, we do not
+support native cycle collection in the presence of inheritance, if two
+classes related by inheritance need different CC
+implementations. (This is because we use QueryInterface to find the
+right CC implementation for an object.)
+
+Once you've decided to make a class cycle collected, there are a few
+things you need to add to your implementation:
+
+* Cycle collected refcounting. Special refcounting is needed so that
+  the CC can tell when an object is created, used, or destroyed, so
+  that it can determine if an object is potentially part of a garbage
+  cycle.
+
+* Traversal. Once the CC has decided an object **might** be garbage,
+  it needs to know what other cycle collected objects it holds strong
+  references to. This is done with a "traverse" method.
+
+* Unlinking. Once the CC has decided that an object **is** garbage, it
+  needs to break the cycles by clearing out all strong references to
+  other cycle collected objects. This is done with an "unlink"
+  method. This usually looks very similar to the traverse method.
+
+The traverse and unlink methods, along with other methods needed for
+cycle collection, are defined on a special inner class object, called a
+"participant", for performance reasons. The existence of the
+participant is mostly hidden behind macros so you shouldn't need
+to worry about it.
+
+Next, we'll go over what the declaration and definition of these parts
+look like. (Spoiler: there are lots of `ALL_CAPS` macros.) This will
+mostly cover the most common variants. If you need something slightly
+different, you should look at the location of the declaration of the
+macros we mention here and see if the variants already exist.
+
+
+Reference counting
+==================
+
+nsISupports
+-----------
+
+If your class inherits from nsISupports, you'll need to add
+`NS_DECL_CYCLE_COLLECTING_ISUPPORTS` to the class declaration. This
+will declare the QueryInterface (QI), AddRef and Release methods you
+need to implement nsISupports, as well as the actual refcount field.
+
+In the `.cpp` file for your class you'll have to define the
+QueryInterface, AddRef and Release methods. The ins and outs of
+defining the QI method is out-of-scope for this document, but you'll
+need to use the special cycle collection variants of the macros, like
+`NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION`. (This is because we use
+the nsISupports system to define a special interface used to
+dynamically find the correct CC participant for the object.)
+
+Finally, you'll have to actually define the AddRef and Release methods
+for your class. If your class is called `MyClass`, then you'd do this
+with the declarations `NS_IMPL_CYCLE_COLLECTING_ADDREF(MyClass)` and
+`NS_IMPL_CYCLE_COLLECTING_RELEASE(MyClass)`.
+
+non-nsISupports
+---------------
+
+If your class does **not** inherit from nsISupports, you'll need to
+add `NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING` to the class
+declaration. This will give inline definitions for the AddRef and
+Release methods, as well as the actual refcount field.
+
+Cycle collector participant
+===========================
+
+Next we need to declare and define the cycle collector
+participant. This is mostly boilerplate hidden behind macros, but you
+will need to specify which fields are to be traversed and unlinked
+because they are strong references to cycle collected objects.
+
+Declaration
+-----------
+
+First, we need to add a declaration for the participant. As before,
+let's say your class is `MyClass`.
+
+The basic way to declare this for an nsISupports class is
+`NS_DECL_CYCLE_COLLECTION_CLASS(MyClass)`.
+
+If your class inherits from multiple classes that inherit from
+nsISupports classes, say `Parent1` and `Parent2`, then you'll need to
+use `NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(MyClass, Parent1)` to
+tell the CC to cast to nsISupports via `Parent1`. You probably want to
+pick the first class it inherits from. (The cycle collector needs to be
+able to cast `MyClass*` to `nsISupports*`.)
+
+Another situation you might encounter is that your nsISupports class
+inherits from another cycle collected class `CycleCollectedParent`. In
+that case, your participant declaration will look like
+`NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(MyClass,
+CycleCollectedParent)`. (This is needed so that the CC can cast from
+nsISupports down to `MyClass`.) Note that we do not support inheritance
+for non-nsISupports classes.
+
+If your class is non-nsISupports, then you'll need to use the `NATIVE`
+family of macros, like
+`NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(MyClass)`.
+
+In addition to these modifiers, these different variations have
+further `SCRIPT_HOLDER` variations which are needed if your class
+holds alive JavaScript objects. This is because the tracing of JS
+objects held alive by this class must be declared separately from the
+tracing of C++ objects held alive by this class so that the garbage
+collector can also use the tracing. For example,
+`NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_AMBIGUOUS(MyClass,
+Parent1)`.
+
+There are also `WRAPPERCACHE` variants of the macros which you need to
+use if your class is wrapper cached. These are effectively a
+specialized form of `SCRIPT_HOLDER`, as a cached wrapper is a
+JS object held alive by the C++ object. For example,
+`NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS_AMBIGUOUS(MyClass,
+Parent1)`.
+
+There is yet another variant of these macros, `SKIPPABLE`. This
+document won't go into detail here about how this works, but the basic
+idea is that a class can tell the CC when it is definitely alive,
+which lets the CC skip it. This is a very important optimization for
+things like DOM elements in active documents, but a new class you are
+making cycle collected is likely not common enough to worry about.
+
+Implementation
+--------------
+
+Finally, you must write the actual implementation of the CC
+participant, in the .cpp file for your class. This will define the
+traverse and unlink methods, and some other random helper
+functions. In the simplest case, this can be done with a single macro
+like this: `NS_IMPL_CYCLE_COLLECTION(MyClass, mField1, mField2,
+mField3)`, where `mField1` and the rest are the names of the fields of
+your class that are strong references to cycle collected
+objects. There is some template magic that says how many common types
+like RefPtr, nsCOMPtr, and even some arrays, should be traversed and
+unlinked. There’s also a variant `NS_IMPL_CYCLE_COLLECTION_INHERITED`,
+which you should use when there’s a parent class that is also cycle
+collected, to ensure that fields of the parent class are traversed and
+unlinked. The name of that parent class is passed in as the second
+argument. If either of these work, then you are done. Your class is
+now cycle collected. Note that this does not work for fields that are
+JS objects.
+
+However, if that doesn’t work, you’ll have to get into the details a
+bit more. A good place to start is by copying the definition of
+`NS_IMPL_CYCLE_COLLECTION`.
+
+For a script holder method, you also need to define a trace method in
+addition to the traverse and unlink, using
+`NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN` and other similar
+macros. You'll need to include all of the JS fields that your class
+holds alive.  The trace method will be used by the GC as well as the
+CC, so if you miss something you can end up with use-after-free
+crashes. You'll also need to call `mozilla::HoldJSObjects(this);` in
+the ctor for your class, and `mozilla::DropJSObjects(this);` in the
+dtor. This will register (and unregister) each instance of your object
+with the JS runtime, to ensure that it gets traced properly. This
+does not apply if you have a wrapper cached class that does not have
+any additional JS fields, as nsWrapperCache deals with all of that
+for you.
diff --git a/xpcom/docs/collections.rst b/xpcom/docs/collections.rst
new file mode 100644
index 0000000000..8c9356e499
--- /dev/null
+++ b/xpcom/docs/collections.rst
@@ -0,0 +1,114 @@
+XPCOM Collections
+=================
+
+``nsTArray`` and ``AutoTArray``
+-------------------------------
+
+``nsTArray<T>`` is a typesafe array for holding various objects, similar to ``std::vector<T>``. (note that
+``nsTArray<T>`` is dynamically-sized, unlike ``std::array<T>``) Here's an
+incomplete list of mappings between the two:
+
+================== ==================================================
+std::vector<T>     nsTArray<T>
+================== ==================================================
+``size()``         ``Length()``
+``empty()``        ``IsEmpty()``
+``resize()``       ``SetLength()`` or ``SetLengthAndRetainStorage()``
+``capacity()``     ``Capacity()``
+``reserve()``      ``SetCapacity()``
+``push_back()``    ``AppendElement()``
+``insert()``       ``AppendElements()``
+``emplace_back()`` ``EmplaceBack()``
+``clear()``        ``Clear()`` or ``ClearAndRetainStorage()``
+``data()``         ``Elements()``
+``at()``           ``ElementAt()``
+``back()``         ``LastElement()``
+================== ==================================================
+
+Rust Bindings
+~~~~~~~~~~~~~
+
+When the ``thin_vec`` crate is built in Gecko, ``thin_vec::ThinVec<T>`` is
+guaranteed to have the same memory layout and allocation strategy as
+``nsTArray``, meaning that the two types may be used interchangeably across
+FFI boundaries. The type is **not** safe to pass by-value over FFI
+boundaries, due to Rust and C++ differing in when they run destructors.
+
+The element type ``T`` must be memory-compatible with both Rust and C++ code
+to use over FFI.
+
+``nsTHashMap`` and ``nsTHashSet``
+---------------------------------
+
+These types are the recommended interface for writing new XPCOM hashmaps and
+hashsets in XPCOM code.
+
+Supported Hash Keys
+~~~~~~~~~~~~~~~~~~~
+
+The following types are supported as the key parameter to ``nsTHashMap`` and
+``nsTHashSet``.
+
+========================== ======================
+Type                       Hash Key
+========================== ======================
+``T*``                     ``nsPtrHashKey<T>``
+``T*``                     ``nsPtrHashKey<T>``
+``nsCString``              ``nsCStringHashKey``
+``nsString``               ``nsStringHashKey``
+``uint32_t``               ``nsUint32HashKey``
+``uint64_t``               ``nsUint64HashKey``
+``intptr_t``               ``IntPtrHashKey``
+``nsCOMPtr<nsISupports>``  ``nsISupportsHashKey``
+``RefPtr<T>``              ``nsRefPtrHashKey<T>``
+``nsID``                   ``nsIDHashKey``
+========================== ======================
+
+Any key not in this list must inherit from the ``PLDHashEntryHdr`` class to
+implement manual hashing behaviour.
+
+Class Reference
+~~~~~~~~~~~~~~~
+
+.. note::
+
+    The ``nsTHashMap`` and ``nsTHashSet`` types are not declared exactly like
+    this in code. This is intended largely as a practical reference.
+
+.. cpp:class:: template<K, V> nsTHashMap<K, V>
+
+    The ``nsTHashMap<K, V>`` class is currently defined as a thin type alias
+    around ``nsBaseHashtable``. See the methods defined on that class for
+    more detailed documentation.
+
+    https://searchfox.org/mozilla-central/source/xpcom/ds/nsBaseHashtable.h
+
+    .. cpp:function:: uint32_t Count() const
+
+    .. cpp:function:: bool IsEmpty() const
+
+    .. cpp:function:: bool Get(KeyType aKey, V* aData) const
+
+        Get the value, returning a flag indicating the presence of the entry
+        in the table.
+
+    .. cpp:function:: V Get(KeyType aKey) const
+
+        Get the value, returning a default-initialized object if the entry is
+        not present in the table.
+
+    .. cpp:function:: Maybe<V> MaybeGet(KeyType aKey) const
+
+        Get the value, returning Nothing if the entry is not present in the table.
+
+    .. cpp:function:: V& LookupOrInsert(KeyType aKey, Args&&... aArgs) const
+
+    .. cpp:function:: V& LookupOrInsertWith(KeyType aKey, F&& aFunc) const
+
+.. cpp:class:: template<K> nsTHashSet<K>
+
+    The ``nsTHashSet<K>`` class is currently defined as a thin type alias
+    around ``nsTBaseHashSet``. See the methods defined on that class for
+    more detailed documentation.
+
+    https://searchfox.org/mozilla-central/source/xpcom/ds/nsTHashSet.h
diff --git a/xpcom/docs/hashtables.rst b/xpcom/docs/hashtables.rst
new file mode 100644
index 0000000000..8debd11eb7
--- /dev/null
+++ b/xpcom/docs/hashtables.rst
@@ -0,0 +1,141 @@
+XPCOM Hashtable Guide
+=====================
+
+.. note::
+
+   For a deep-dive into the underlying mechanisms that power our hashtables,
+   check out the :ref:`XPCOM Hashtable Technical Details`
+   document.
+
+What Is a Hashtable?
+--------------------
+
+A hashtable is a data construct that stores a set of **items**. Each
+item has a **key** that identifies the item. Items are found, added, and
+removed from the hashtable by using the key. Hashtables may seem like
+arrays, but there are important differences:
+
++-------------------------+----------------------+----------------------+
+|                         | Array                | Hashtable            |
++=========================+======================+======================+
+| **Keys**                | *integer:* arrays    | *any type:* almost   |
+|                         | are always keyed on  | any datatype can be  |
+|                         | integers and must    | used as key,         |
+|                         | be contiguous.       | including strings,   |
+|                         |                      | integers, XPCOM      |
+|                         |                      | interface pointers,  |
+|                         |                      | IIDs, and almost     |
+|                         |                      | anything else. Keys  |
+|                         |                      | can be disjunct      |
+|                         |                      | (i.e. you can store  |
+|                         |                      | entries with keys 1, |
+|                         |                      | 5, and 3000).        |
++-------------------------+----------------------+----------------------+
+| **Lookup Time**         | *O(1):* lookup time  | *O(1):* lookup time  |
+|                         | is a simple constant | is mostly-constant,  |
+|                         |                      | but the constant     |
+|                         |                      | time can be larger   |
+|                         |                      | than an array lookup |
++-------------------------+----------------------+----------------------+
+| **Sorting**             | *sorted:* stored     | *unsorted:* stored   |
+|                         | sorted; iterated     | unsorted; cannot be  |
+|                         | over in a sorted     | iterated over in a   |
+|                         | fashion.             | sorted manner.       |
++-------------------------+----------------------+----------------------+
+| **Inserting/Removing**  | *O(n):* adding and   | *O(1):* adding and   |
+|                         | removing items from  | removing items from  |
+|                         | a large array can be | hashtables is a      |
+|                         | time-consuming       | quick operation      |
++-------------------------+----------------------+----------------------+
+| **Wasted space**        | *none:* Arrays are   | *some:* hashtables   |
+|                         | packed structures,   | are not packed       |
+|                         | so there is no       | structures;          |
+|                         | wasted space.        | depending on the     |
+|                         |                      | implementation,      |
+|                         |                      | there may be         |
+|                         |                      | significant wasted   |
+|                         |                      | memory.              |
++-------------------------+----------------------+----------------------+
+
+In their implementation, hashtables take the key and apply a
+mathematical **hash function** to **randomize** the key and then use the
+hash to find the location in the hashtable. Good hashtable
+implementations will automatically resize the hashtable in memory if
+extra space is needed, or if too much space has been allocated.
+
+.. _When_Should_I_Use_a_Hashtable.3F:
+
+When Should I Use a Hashtable?
+------------------------------
+
+Hashtables are useful for
+
+-  sets of data that need swift **random access**
+-  with **non-integral keys** or **non-contiguous integral keys**
+-  or where **items will be frequently added or removed**
+
+Hashtables should *not* be used for
+
+-  Sets that need to be **sorted**
+-  Very small datasets (less than 12-16 items)
+-  Data that does not need random access
+
+In these situations, an array, a linked-list, or various tree data
+structures are more efficient.
+
+.. _Which_Hashtable_Should_I_Use.3F:
+
+Which Hashtable Should I Use?
+-----------------------------
+
+If there is **no** key type, you should use an ``nsTHashSet``.
+
+If there is a key type, you should use an ``nsTHashMap``.
+
+``nsTHashMap`` is a template with two parameters. The first is the hash key
+and the second is the data to be stored as the value in the map. Most of
+the time, you can simply pass the raw key type as the first parameter,
+so long as its supported by `nsTHashMap.h <https://searchfox.org/mozilla-central/source/xpcom/ds/nsTHashMap.h>`_.
+It is also possible to specify custom keys if necessary. See `nsHashKeys.h
+<https://searchfox.org/mozilla-central/source/xpcom/ds/nsHashKeys.h>`_ for examples.
+
+There are a number of more esoteric hashkey classes in nsHashKeys.h, and
+you can always roll your own if none of these fit your needs (make sure
+you're not duplicating an existing hashkey class though!)
+
+Once you've determined what hashtable and hashkey classes you need, you
+can put it all together. A few examples:
+
+-  A hashtable that maps UTF-8 origin names to a DOM Window -
+   ``nsTHashMap<nsCString, nsCOMPtr<nsIDOMWindow>>``
+-  A hashtable that maps 32-bit integers to floats -
+   ``nsTHashMap<uint32_t, float>``
+-  A hashtable that maps ``nsISupports`` pointers to reference counted
+   ``CacheEntry``\ s -
+   ``nsTHashMap<nsCOMPtr<nsISupports>, RefPtr<CacheEntry>>``
+-  A hashtable that maps ``JSContext`` pointers to a ``ContextInfo``
+   struct - ``nsTHashMap<JSContext*, UniquePtr<ContextInfo>>``
+-  A hashset of strings - ``nsTHashSet<nsString>``
+
+.. _nsBaseHashtable_and_friends:_nsDataHashtable.2C_nsInterfaceHashtable.2C_and_nsClassHashtable:
+
+Hashtable API
+-------------
+
+The hashtable classes all expose the same basic API. There are three
+key methods, ``Get``, ``InsertOrUpdate``, and ``Remove``, which retrieve entries from the
+hashtable, write entries into the hashtable, and remove entries from the
+hashtable respectively. See `nsBaseHashtable.h <https://searchfox.org/mozilla-central/source/xpcom/ds/nsBaseHashtable.h>`_
+for more details.
+
+The hashtables that hold references to pointers (nsRefPtrHashtable and
+nsInterfaceHashtable) also have GetWeak methods that return non-AddRefed
+pointers.
+
+Note that ``nsRefPtrHashtable``, ``nsInterfaceHashtable`` and ``nsClassHashtable``
+are legacy hashtable types which have some extra methods, and don't have automatic
+key type handling.
+
+All of these hashtable classes can be iterated over via the ``Iterator``
+class, with normal C++11 iterators or using the ``Keys()`` / ``Values()`` ranges,
+and all can be cleared via the ``Clear`` method.
diff --git a/xpcom/docs/hashtables_detailed.rst b/xpcom/docs/hashtables_detailed.rst
new file mode 100644
index 0000000000..200c47490d
--- /dev/null
+++ b/xpcom/docs/hashtables_detailed.rst
@@ -0,0 +1,121 @@
+XPCOM Hashtable Technical Details
+=================================
+
+.. note::
+
+   This is a deep-dive into the underlying mechanisms that power the XPCOM
+   hashtables. Some of this information is quite old and may be out of date. If
+   you're looking for how to use XPCOM hashtables, you should consider reading
+   the :ref:`XPCOM Hashtable Guide` instead.
+
+Mozilla's Hashtable Implementations
+-----------------------------------
+
+Mozilla has several hashtable implementations, which have been tested
+and tuned, and hide the inner complexities of hashtable implementations:
+
+-  ``PLHashTable`` - low-level C API; entry class pointers are constant;
+   more efficient for large entry structures; often wastes memory making
+   many small heap allocations.
+-  ``nsTHashtable`` - low-level C++ wrapper around ``PLDHash``;
+   generates callback functions and handles most casting automagically.
+   Client writes their own entry class which can include complex key and
+   data types.
+-  ``nsTHashMap/nsInterfaceHashtable/nsClassHashtable`` -
+   simplifies the common usage pattern mapping a simple keytype to a
+   simple datatype; client does not need to declare or manage an entry class;
+   ``nsTHashMap`` datatype is a scalar such as ``uint64_t``;
+   ``nsInterfaceHashtable`` datatype is an XPCOM interface;
+   ``nsClassHashtable`` datatype is a class pointer owned by the
+   hashtable.
+
+.. _PLHashTable:
+
+PLHashTable
+~~~~~~~~~~~
+
+``PLHashTable`` is a part of NSPR. The header file can be found at `plhash.h
+<https://searchfox.org/mozilla-central/source/nsprpub/lib/ds/plhash.h>`_.
+
+There are two situations where ``PLHashTable`` may be preferable:
+
+-  You need entry-pointers to remain constant.
+-  The entries stored in the table are very large (larger than 12
+   words).
+
+.. _nsTHashtable:
+
+nsTHashtable
+~~~~~~~~~~~~
+
+To use ``nsTHashtable``, you must declare an entry-class. This
+entry class contains the key and the data that you are hashing. It also
+declares functions that manipulate the key. In most cases, the functions
+of this entry class can be entirely inline. For examples of entry classes,
+see the declarations at `nsHashKeys.h
+<https://searchfox.org/mozilla-central/source/xpcom/ds/nsHashKeys.h>`_.
+
+The template parameter is the entry class. After construction, use the
+functions ``PutEntry/GetEntry/RemoveEntry`` to alter the hashtable. The
+``Iterator`` class will do iteration, but beware that the iteration will
+occur in a seemingly-random order (no sorting).
+
+-  ``nsTHashtable``\ s can be allocated on the stack, as class members,
+   or on the heap.
+-  Entry pointers can and do change when items are added to or removed
+   from the hashtable. Do not keep long-lasting pointers to entries.
+-  because of this, ``nsTHashtable`` is not inherently thread-safe. If
+   you use a hashtable in a multi-thread environment, you must provide
+   locking as appropriate.
+
+Before using ``nsTHashtable``, see if ``nsBaseHashtable`` and relatives
+will work for you. They are much easier to use, because you do not have
+to declare an entry class. If you are hashing a simple key type to a
+simple data type, they are generally a better choice.
+
+.. _nsBaseHashtable_and_friends:nsTHashMap.2C_nsInterfaceHashtable.2C_and_nsClassHashtable:
+
+nsBaseHashtable and friends: nsTHashMap, nsInterfaceHashtable, and nsClassHashtable
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+These C++ templates provide a high-level interface for using hashtables
+that hides most of the complexities of the underlying implementation. They
+provide the following features:
+
+-  hashtable operations can be completed without using an entry class,
+   making code easier to read
+-  optional thread-safety: the hashtable can manage a read-write lock
+   around the table
+-  predefined key classes provide automatic cleanup of
+   strings/interfaces
+-  ``nsInterfaceHashtable`` and ``nsClassHashtable`` automatically
+   release/delete objects to avoid leaks.
+
+``nsBaseHashtable`` is not used directly; choose one of the three
+derivative classes based on the data type you want to store. The
+``KeyClass`` is taken from `nsHashKeys.h
+<https://searchfox.org/mozilla-central/source/xpcom/ds/nsHashKeys.h>`_ and is the same for all
+three classes:
+
+-  ``nsTHashMap<KeyClass, DataType>`` - ``DataType`` is a simple
+   type such as ``uint32_t`` or ``bool``.
+-  ``nsInterfaceHashtable<KeyClass, Interface>`` - ``Interface`` is an
+   XPCOM interface such as ``nsISupports`` or ``nsIDocShell``
+-  ``nsClassHashtable<KeyClass, T>`` - ``T`` is any C++ class. The
+   hashtable stores a pointer to the object, and deletes that object
+   when the entry is removed.
+
+The important files to read are
+`nsBaseHashtable.h <https://searchfox.org/mozilla-central/source/xpcom/ds/nsBaseHashtable.h>`_
+and
+`nsHashKeys.h <https://searchfox.org/mozilla-central/source/xpcom/ds/nsHashKeys.h>`_.
+These classes can be used on the stack, as a class member, or on the heap.
+
+.. _Using_nsTHashtable_as_a_hash-set:
+
+Using nsTHashtable as a hash-set
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A hash set only tracks the existence of keys: it does not associate data
+with the keys. This can be done using ``nsTHashtable<nsSomeHashKey>``.
+The appropriate entries are GetEntry and PutEntry.
diff --git a/xpcom/docs/huntingleaks.rst b/xpcom/docs/huntingleaks.rst
new file mode 100644
index 0000000000..9e0585da4e
--- /dev/null
+++ b/xpcom/docs/huntingleaks.rst
@@ -0,0 +1,22 @@
+Hunting Leaks
+=============
+
+.. contents:: Table of Contents
+    :local:
+    :depth: 2
+
+Different tools and techniques are used to hunt leaks:
+
+.. list-table::
+   :header-rows: 1
+
+   * - Tools
+     - Description
+   * - :ref:`Bloatview`
+     - BloatView is a tool that shows information about cumulative memory usage and leaks.
+   * - :ref:`Refcount Tracing and Balancing`
+     - Refcount tracing and balancing are advanced techniques for tracking down leak of refcounted objects found with BloatView.
+   * - `GC and CC logs </performance/memory/gc_and_cc_logs.html>`_
+     - Garbage collector (GC) and cycle collector (CC) logs give information about why various JS and C++ objects are alive in the heap.
+   * - :ref:`DMD Heap Scan Mode`
+     - Heap profiler within Firefox
diff --git a/xpcom/docs/index.rst b/xpcom/docs/index.rst
new file mode 100644
index 0000000000..bf4e3157c4
--- /dev/null
+++ b/xpcom/docs/index.rst
@@ -0,0 +1,19 @@
+XPCOM
+=====
+
+These pages contain documentation for Mozilla's Cross-Platform Component Object Model (XPCOM) module.  It abstracts core systems functionality for cross-platform use.  The component architecture follows the standard COM approach.
+
+.. toctree::
+   :maxdepth: 1
+
+   logging
+   stringguide
+   refptr
+   thread-safety
+   huntingleaks
+   collections
+   xpidl
+   writing-xpcom-interface
+   hashtables
+   hashtables_detailed
+   cc-macros
diff --git a/xpcom/docs/logging.rst b/xpcom/docs/logging.rst
new file mode 100644
index 0000000000..0105f018ad
--- /dev/null
+++ b/xpcom/docs/logging.rst
@@ -0,0 +1,488 @@
+Gecko Logging
+=============
+
+A minimal C++ logging framework is provided for use in core Gecko code. It is
+enabled for all builds and is thread-safe.
+
+This page covers enabling logging for particular logging module, configuring
+the logging output, and how to use the logging facilities in native code.
+
+Enabling and configuring logging
+++++++++++++++++++++++++++++++++
+
+Caveat: sandboxing when logging to a file
+-----------------------------------------
+
+A sandboxed content process cannot write to ``stderr`` or any file.  The easiest
+way to log these processes is to disable the content sandbox by setting the
+preference ``security.sandbox.content.level`` to ``0``, or setting the environment
+variable ``MOZ_DISABLE_CONTENT_SANDBOX`` to ``1``.
+
+On Windows, you can still see child process messages by using DOS (not the
+``MOZ_LOG_FILE`` variable defined below) to redirect output to a file.  For
+example: ``MOZ_LOG="CameraChild:5" mach run >& my_log_file.txt`` will include
+debug messages from the camera's child actor that lives in a (sandboxed) content
+process.
+
+Logging to the Firefox Profiler
+-------------------------------
+
+When a log statement is logged on a thread and the `Firefox Profiler
+<https://profiler.firefox.com>`_ is profiling that thread, the log statements is
+recorded as a profiler marker.
+
+This allows getting logs alongside profiler markers and lots of performance
+and contextual information, in a way that doesn't require disabling the
+sandbox, and works across all processes.
+
+The profile can be downloaded and shared e.g. via Bugzilla or email, or
+uploaded, and the logging statements will be visible either in the marker chart
+or the marker table.
+
+While it is possible to manually configure logging module and start the profiler
+with the right set of threads to profile, ``about:logging`` makes this task a lot
+simpler and error-proof.
+
+
+The ``MOZ_LOG`` syntax
+----------------------
+
+Logging is configured using a special but simple syntax: which module should be
+enabled, at which level, and what logging options should be enabled or disabled.
+
+The syntax is a list of terms, separated by commas. There are two types of
+terms:
+
+- A log module and its level, separated by a colon (``:``), such as
+  ``example_module:5`` to enable the module ``example_module`` at logging level
+  ``5`` (verbose). This `searchfox query
+  <https://searchfox.org/mozilla-central/search?q=LazyLogModule+.*%5C%28%22&path=&case=true&regexp=true>`_
+  returns the complete list of modules available.
+- A special string in the following table, to configure the logging behaviour.
+  Some configuration switch take an integer parameter, in which case it's
+  separated from the string by a colon (``:``). Most switches only apply in a
+  specific output context, noted in the **Context** column.
+
++----------------------+---------+-------------------------------------------------------------------------------------------+
+| Special module name  | Context | Action                                                                                    |
++======================+=========+===========================================================================================+
+| append               | File    | Append new logs to existing log file.                                                     |
++----------------------+---------+-------------------------------------------------------------------------------------------+
+| sync                 | File    | Print each log synchronously, this is useful to check behavior in real time or get logs   |
+|                      |         | immediately before crash.                                                                 |
++----------------------+---------+-------------------------------------------------------------------------------------------+
+| raw                  | File    | Print exactly what has been specified in the format string, without the                   |
+|                      |         | process/thread/timestamp, etc. prefix.                                                    |
++----------------------+---------+-------------------------------------------------------------------------------------------+
+| timestamp            | File    | Insert timestamp at start of each log line.                                               |
++----------------------+---------+-------------------------------------------------------------------------------------------+
+| rotate:**N**         | File    | | This limits the produced log files' size.  Only most recent **N megabytes** of log data |
+|                      |         | | is saved.  We rotate four log files with .0, .1, .2, .3 extensions.  Note: this option  |
+|                      |         | | disables 'append' and forces 'timestamp'.                                               |
++----------------------+---------+-------------------------------------------------------------------------------------------+
+| maxsize:**N**        | File    | Limit the log to **N** MB. Only work in append mode.                                      |
++----------------------+---------+-------------------------------------------------------------------------------------------+
+| prependheader        | File    | Prepend a simple header while distinguishing logging. Useful in append mode.              |
++----------------------+---------+-------------------------------------------------------------------------------------------+
+| profilerstacks       | Profiler| | When profiling with the Firefox Profiler and log modules are enabled, capture the call  |
+|                      |         | | stack for each log statement.                                                           |
++----------------------+---------+-------------------------------------------------------------------------------------------+
+
+This syntax is used for most methods of enabling logging, with the exception of
+settings preferences directly, see :ref:`this section <Enabling logging using preferences>` for directions.
+
+
+Enabling Logging
+----------------
+
+Enabling logging can be done in a variety of ways:
+
+- via environment variables
+- via command line switches
+- using ``about:config`` preferences
+- using ``about:logging``
+
+The first two allow logging from the start of the application and are also
+useful in case of a crash (when ``sync`` output is requested, this can also be
+done with ``about:config`` as well to a certain extent). The last two
+allow enabling and disabling logging at runtime and don't require using the
+command-line.
+
+By default all logging output is disabled.
+
+Enabling logging using ``about:logging``
+''''''''''''''''''''''''''''''''''''''''
+
+``about:logging`` allows enabling logging by entering a ``MOZ_LOG`` string in the
+text input, and validating.
+
+Options allow logging to a file or using the Firefox Profiler, that can be
+started and stopped right from the page.
+
+Logging presets for common scenarios are available in a drop-down. They can be
+associated with a profiler preset.
+
+It is possible, via URL parameters, to select a particular logging
+configuration, or to override certain parameters in a preset. This is useful to
+ask a user to gather logs efficiently without having to fiddle with prefs and/or
+environment variable.
+
+URL parameters are described in the following table:
+
++---------------------+---------------------------------------------------------------------------------------------+
+| Parameter           | Description                                                                                 |
++=====================+=============================================================================================+
+| ``preset``          |  a `logging preset <https://searchfox.org/mozilla-central/search?q=gLoggingPresets>`_       |
++---------------------+---------------------------------------------------------------------------------------------+
+| ``logging-preset``  |  alias for ``preset``                                                                       |
++---------------------+---------------------------------------------------------------------------------------------+
+| ``modules``         |  a string in ``MOZ_LOG`` syntax                                                             |
++---------------------+---------------------------------------------------------------------------------------------+
+| ``module``          |  alias for ``modules``                                                                      |
++---------------------+---------------------------------------------------------------------------------------------+
+| ``threads``         |  a list of threads to profile, overrides what a profiler preset would have picked           |
++---------------------+---------------------------------------------------------------------------------------------+
+| ``thread``          |  alias for ``threads``                                                                      |
++---------------------+---------------------------------------------------------------------------------------------+
+| ``output``          |  either ``profiler`` or ``file``                                                            |
++---------------------+---------------------------------------------------------------------------------------------+
+| ``output-type``     |  alias for ``output``                                                                       |
++---------------------+---------------------------------------------------------------------------------------------+
+| ``profiler-preset`` |  a `profiler preset <https://searchfox.org/mozilla-central/search?q=%40type+{Presets}>`_    |
++---------------------+---------------------------------------------------------------------------------------------+
+
+If a preset is selected, then ``threads`` or ``modules`` can be used to override the
+profiled threads or logging modules enabled, but keeping other aspects of the
+preset. If no preset is selected, then a generic profiling preset is used,
+``firefox-platform``. For example:
+
+::
+
+  about:logging?output=profiler&preset=media-playback&modules=cubeb:4,AudioSinkWrapper:4:AudioSink:4
+
+will profile the threads in the ``Media`` profiler preset, but will only log
+specific log modules (instead of the `long list
+<https://searchfox.org/mozilla-central/search?q="media-playback"&path=toolkit%2Fcontent%2FaboutLogging.js>`_
+in the ``media-playback`` preset). In addition, it disallows logging to a file.
+
+Enabling logging using environment variables
+''''''''''''''''''''''''''''''''''''''''''''
+
+On UNIX, setting and environment variable can be done in a variety of ways
+
+::
+
+  set MOZ_LOG="example_logger:3"
+  export MOZ_LOG="example_logger:3"
+  MOZ_LOG="example_logger:3" ./mach run
+
+In the Windows Command Prompt (``cmd.exe``), don't use quotes:
+
+::
+
+  set MOZ_LOG=example_logger:3
+
+If you want this on GeckoView example, use the following adb command to launch process:
+
+::
+
+  adb shell am start -n org.mozilla.geckoview_example/.GeckoViewActivity --es env0 "MOZ_LOG=example_logger:3"
+
+There are special module names to change logging behavior. You can specify one or more special module names without logging level.
+
+For example, if you want to specify ``sync``, ``timestamp`` and ``rotate``:
+
+::
+
+  set MOZ_LOG="example_logger:3,timestamp,sync,rotate:10"
+
+Enabling logging usually outputs the logging statements to the terminal. To
+have the logs written to a file instead (one file per process), the environment
+variable ``MOZ_LOG_FILE`` can be used. Logs will be written at this path
+(either relative or absolute), suffixed by a process type and its PID.
+``MOZ_LOG`` files are text files and have the extension ``.moz_log``.
+
+For example, setting:
+
+::
+
+  set MOZ_LOG_FILE="firefox-logs"
+
+can create a number of files like so:
+
+::
+
+  firefox-log-main.96353.moz_log
+  firefox-log-child.96354.moz_log
+
+respectively for a parent process of PID 96353 and a child process of PID
+96354.
+
+Enabling logging using command-line flags
+'''''''''''''''''''''''''''''''''''''''''
+
+The ``MOZ_LOG`` syntax can be used with the command line switch on the same
+name, and specifying a file with ``MOZ_LOG_FILE`` works in the same way:
+
+::
+
+  ./mach run -MOZ_LOG=timestamp,rotate:200,example_module:5 -MOZ_LOG_FILE=%TEMP%\firefox-logs
+
+will enable verbose (``5``) logging for the module ``example_module``, with
+timestamp prepended to each line, rotate the logs with 4 files of each 50MB
+(for a total of 200MB), and write the output to the temporary directory on
+Windows, with name starting with ``firefox-logs``.
+
+.. _Enabling logging using preferences:
+
+Enabling logging using preferences
+''''''''''''''''''''''''''''''''''
+
+To adjust the logging after Firefox has started, you can set prefs under the
+`logging.` prefix. For example, setting `logging.foo` to `3` will set the log
+module `foo` to start logging at level 3. A number of special prefs can be set,
+described in the table below:
+
++-------------------------------------+------------+-------------------------------+--------------------------------------------------------+
+|         Preference name             | Preference |   Preference value            |                  Description                           |
++=====================================+============+===============================+========================================================+
+| ``logging.config.clear_on_startup`` |    bool    | --                            | Whether to clear all prefs under ``logging.``          |
++-------------------------------------+------------+-------------------------------+--------------------------------------------------------+
+| ``logging.config.LOG_FILE``         |   string   | A path (relative or absolute) | The path to which the log files will be written.       |
++-------------------------------------+------------+-------------------------------+--------------------------------------------------------+
+| ``logging.config.add_timestamp``    |   bool     | --                            | Whether to prefix all lines by a timestamp.            |
++-------------------------------------+------------+-------------------------------+--------------------------------------------------------+
+| ``logging.config.sync``             |   bool     | --                            | Whether to flush the stream after each log statements. |
++-------------------------------------+------------+-------------------------------+--------------------------------------------------------+
+| ``logging.config.profilerstacks``   |   bool     | --                            | | When logging to the Firefox Profiler, whether to     |
+|                                     |            |                               | | include the call stack in each logging statement.    |
++-------------------------------------+------------+-------------------------------+--------------------------------------------------------+
+
+Enabling logging in Rust code
+-----------------------------
+
+We're gradually adding more Rust code to Gecko, and Rust crates typically use a
+different approach to logging. Many Rust libraries use the `log
+<https://docs.rs/log>`_ crate to log messages, which works together with
+`env_logger <https://docs.rs/env_logger>`_ at the application level to control
+what's actually printed via `RUST_LOG`.
+
+You can set an overall logging level, though it could be quite verbose:
+
+::
+
+  set RUST_LOG="debug"
+
+You can also target individual modules by path:
+
+::
+
+  set RUST_LOG="style::style_resolver=debug"
+
+.. note::
+  For Linux/MacOS users, you need to use `export` rather than `set`.
+
+.. note::
+  Sometimes it can be useful to only log child processes and ignore the parent
+   process. In Firefox 57 and later, you can use `RUST_LOG_CHILD` instead of
+   `RUST_LOG` to specify log settings that will only apply to child processes.
+
+The `log` crate lists the available `log levels <https://docs.rs/log/0.3.8/log/enum.LogLevel.html>`_:
+
++-----------+---------------------------------------------------------------------------------------------------------+
+| Log Level | Purpose                                                                                                 |
++===========+=========================================================================================================+
+| error     | Designates very serious errors.                                                                         |
++-----------+---------------------------------------------------------------------------------------------------------+
+| warn      | Designates hazardous situations.                                                                        |
++-----------+---------------------------------------------------------------------------------------------------------+
+| info      | Designates useful information.                                                                          |
++-----------+---------------------------------------------------------------------------------------------------------+
+| debug     | Designates lower priority information.                                                                  |
++-----------+---------------------------------------------------------------------------------------------------------+
+| trace     | Designates very low priority, often extremely verbose, information.                                     |
++-----------+---------------------------------------------------------------------------------------------------------+
+
+It is common for debug and trace to be disabled at compile time in release builds, so you may need a debug build if you want logs from those levels.
+
+Check the `env_logger <https://docs.rs/env_logger>`_ docs for more details on logging options.
+
+Additionally, a mapping from `RUST_LOG` is available. When using the `MOZ_LOG`
+syntax, it is possible to enable logging in rust crate using a similar syntax:
+
+::
+
+  MOZ_LOG=rust_crate_name::*:4
+
+will enable `debug` logging for all log statements in the crate
+``rust_crate_name``.
+
+`*` can be replaced by a series of modules if more specificity is needed:
+
+::
+
+  MOZ_LOG=rust_crate_name::module::submodule:4
+
+will enable `debug` logging for all log statements in the sub-module
+``submodule`` of the module ``module`` of the crate ``rust_crate_name``.
+
+
+A table mapping Rust log levels to `MOZ_LOG` log level is available below:
+
++----------------+---------------+-----------------+
+| Rust log level | MOZ_LOG level | Numerical value |
++================+===============+=================+
+|      off       |     Disabled  |        0        |
++----------------+---------------+-----------------+
+|      error     |     Error     |        1        |
++----------------+---------------+-----------------+
+|      warn      |     Warning   |        2        |
++----------------+---------------+-----------------+
+|      info      |     Info      |        3        |
++----------------+---------------+-----------------+
+|      debug     |     Debug     |        4        |
++----------------+---------------+-----------------+
+|      trace     |     Verbose   |        5        |
++----------------+---------------+-----------------+
+
+
+Enabling logging on Android, interleaved with system logs (``logcat``)
+----------------------------------------------------------------------
+
+While logging to the Firefox Profiler works it's sometimes useful to have
+system logs (``adb logcat``) interleaved with application logging. With a
+device (or emulator) that ``adb devices`` sees, it's possible to set
+environment variables like so, for e.g. ``GeckoView_example``:
+
+
+.. code-block:: sh
+
+  adb shell am start -n org.mozilla.geckoview_example/.GeckoViewActivity --es env0 MOZ_LOG=MediaDemuxer:4
+
+
+It is then possible to see the logging statements like so, to display all logs,
+including ``MOZ_LOG``:
+
+.. code-block:: sh
+
+  adb logcat
+
+and to only see ``MOZ_LOG`` like so:
+
+.. code-block:: sh
+
+  adb logcat Gecko:V '*:S'
+
+This expression means: print log module ``Gecko`` from log level ``Verbose``
+(lowest level, this means that all levels are printed), and filter out (``S``
+for silence) all other logging (``*``, be careful to quote it or escape it
+appropriately, it so that it's not expanded by the shell).
+
+While interactive with e.g. ``GeckoView`` code, it can be useful to specify
+more logging tags like so:
+
+.. code-block:: sh
+
+  adb logcat GeckoViewActivity:V Gecko:V '*:S'
+
+
+Enabling logging on Android, using the Firefox Profiler
+-------------------------------------------------------
+
+Set the logging modules using `about:config` (this requires a Nightly build)
+using the instructions outlined above, and start the profile using an
+appropriate profiling preset to profile the correct threads using the instructions
+written in Firefox Profiler documentation's `dedicated page
+<https://profiler.firefox.com/docs/#/./guide-profiling-android-directly-on-device>`_.
+
+`Bug 1803607 <https://bugzilla.mozilla.org/show_bug.cgi?id=1803607>`_ tracks
+improving the logging experience on mobile.
+
+Working with ``MOZ_LOG`` in the code
+++++++++++++++++++++++++++++++++++++
+
+Declaring a Log Module
+----------------------
+
+``LazyLogModule`` defers the creation the backing ``LogModule`` in a thread-safe manner and is the preferred method to declare a log module. Multiple ``LazyLogModules`` with the same name can be declared, all will share the same backing ``LogModule``. This makes it much simpler to share a log module across multiple translation units. ``LazyLogLodule`` provides a conversion operator to ``LogModule*`` and is suitable for passing into the logging macros detailed below.
+
+Note: Log module names can only contain specific characters. The first character must be a lowercase or uppercase ASCII char, underscore, dash, or dot. Subsequent characters may be any of those, or an ASCII digit.
+
+.. code-block:: cpp
+
+  #include "mozilla/Logging.h"
+
+  static mozilla::LazyLogModule sFooLog("foo");
+
+
+Logging interface
+-----------------
+
+A basic interface is provided in the form of 2 macros and an enum class.
+
++----------------------------------------+----------------------------------------------------------------------------+
+| MOZ_LOG(module, level, message)        | Outputs the given message if the module has the given log level enabled:   |
+|                                        |                                                                            |
+|                                        | *   module: The log module to use.                                         |
+|                                        | *   level: The log level of the message.                                   |
+|                                        | *   message: A printf-style message to output. Must be enclosed in         |
+|                                        |     parentheses.                                                           |
++----------------------------------------+----------------------------------------------------------------------------+
+| MOZ_LOG_TEST(module, level)            | Checks if the module has the given level enabled:                          |
+|                                        |                                                                            |
+|                                        | *    module: The log module to use.                                        |
+|                                        | *    level: The output level of the message.                               |
++----------------------------------------+----------------------------------------------------------------------------+
+
+
++-----------+---------------+-----------------------------------------------------------------------------------------+
+| Log Level | Numeric Value | Purpose                                                                                 |
++===========+===============+=========================================================================================+
+| Disabled  |      0        | Indicates logging is disabled. This should not be used directly in code.                |
++-----------+---------------+-----------------------------------------------------------------------------------------+
+| Error     |      1        | An error occurred, generally something you would consider asserting in a debug build.   |
++-----------+---------------+-----------------------------------------------------------------------------------------+
+| Warning   |      2        | A warning often indicates an unexpected state.                                          |
++-----------+---------------+-----------------------------------------------------------------------------------------+
+| Info      |      3        | An informational message, often indicates the current program state.                    |
++-----------+---------------+-----------------------------------------------------------------------------------------+
+| Debug     |      4        | A debug message, useful for debugging but too verbose to be turned on normally.         |
++-----------+---------------+-----------------------------------------------------------------------------------------+
+| Verbose   |      5        | A message that will be printed a lot, useful for debugging program flow and will        |
+|           |               | probably impact performance.                                                            |
++-----------+---------------+-----------------------------------------------------------------------------------------+
+
+Example Usage
+-------------
+
+.. code-block:: cpp
+
+  #include "mozilla/Logging.h"
+
+  using mozilla::LogLevel;
+
+  static mozilla::LazyLogModule sLogger("example_logger");
+
+  static void DoStuff()
+  {
+    MOZ_LOG(sLogger, LogLevel::Info, ("Doing stuff."));
+
+    int i = 0;
+    int start = Time::NowMS();
+    MOZ_LOG(sLogger, LogLevel::Debug, ("Starting loop."));
+    while (i++ &lt; 10) {
+      MOZ_LOG(sLogger, LogLevel::Verbose, ("i = %d", i));
+    }
+
+    // Only calculate the elapsed time if the Warning level is enabled.
+    if (MOZ_LOG_TEST(sLogger, LogLevel::Warning)) {
+      int elapsed = Time::NowMS() - start;
+      if (elapsed &gt; 1000) {
+        MOZ_LOG(sLogger, LogLevel::Warning, ("Loop took %dms!", elapsed));
+      }
+    }
+
+    if (i != 10) {
+      MOZ_LOG(sLogger, LogLevel::Error, ("i should be 10!"));
+    }
+  }
diff --git a/xpcom/docs/refptr.rst b/xpcom/docs/refptr.rst
new file mode 100644
index 0000000000..f71acc6828
--- /dev/null
+++ b/xpcom/docs/refptr.rst
@@ -0,0 +1,81 @@
+Reference Counting Helpers
+==========================
+
+RefPtr versus nsCOMPtr
+----------------------
+
+The general rule of thumb is to use ``nsCOMPtr<T>`` when ``T`` is an
+interface type which inherits from ``nsISupports``, and ``RefPtr<T>`` when
+``T`` is a concrete type.
+
+This basic rule derives from some ``nsCOMPtr<T>`` code being factored into
+the ``nsCOMPtr_base`` base class, which stores the pointer as a
+``nsISupports*``. This design was intended to save some space in the binary
+(though it is unclear if it still does). Since ``nsCOMPtr`` stores the
+pointer as ``nsISupports*``, it must be possible to unambiguously cast from
+``T*`` to ``nsISupports**``. Many concrete classes inherit from more than
+one XPCOM interface, meaning that they cannot be used with ``nsCOMPtr``,
+which leads to the suggestion to use ``RefPtr`` for these classes.
+
+``nsCOMPtr<T>`` also requires that the target type ``T`` be a valid target
+for ``QueryInterface`` so that it can assert that the stored pointer is a
+canonical ``T`` pointer (i.e. that ``mRawPtr->QueryInterface(T_IID) ==
+mRawPtr``).
+
+do_XXX() nsCOMPtr helpers
+-------------------------
+
+There are a number of ``do_XXX`` helper methods across the codebase which can
+be assigned into ``nsCOMPtr`` (and sometimes ``RefPtr``) to perform explicit
+operations based on the target pointer type.
+
+In general, when these operations succeed, they will initialize the smart
+pointer with a valid value, and otherwise they will silently initialize the
+smart pointer to ``nullptr``.
+
+``do_QueryInterface`` and ``do_QueryObject``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Attempts to cast the provided object to the target class using the XPCOM
+``QueryInterface`` mechanism. In general, use ``do_QueryInterface`` may only
+be used to cast between interface types in a ``nsCOMPtr<T>``, and
+``do_QueryObject`` in situations when downcasting to concrete types.
+
+
+``do_GetInterface``
+~~~~~~~~~~~~~~~~~~~
+
+Looks up an object implementing the requested interface using the
+``nsIInterfaceRequestor`` interface. If the target object doesn't implement
+``nsIInterfaceRequestor`` or doesn't provide the given interface, initializes
+the smart pointer with ``nullptr``.
+
+
+``do_GetService``
+~~~~~~~~~~~~~~~~~
+
+Looks up the component defined by the passed-in CID or ContractID string in
+the component manager, and returns a pointer to the service instance. This
+may start the service if it hasn't been started already. The resulting
+service will be cast to the target interface type using ``QueryInterface``.
+
+
+``do_CreateInstance``
+~~~~~~~~~~~~~~~~~~~~~
+
+Looks up the component defined by the passed-in CID or ContractID string in
+the component manager, creates and returns a new instance. The resulting
+object will be cast to the target interface type using ``QueryInterface``.
+
+
+``do_QueryReferent`` and ``do_GetWeakReference``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When passed a ``nsIWeakReference*`` (e.g. from a ``nsWeakPtr``),
+``do_QueryReferent`` attempts to re-acquire a strong reference to the held
+type, and cast it to the target type with ``QueryInterface``. Initializes the
+smart pointer with ``nullptr`` if either of these steps fail.
+
+In contrast ``do_GetWeakReference`` does the opposite, using
+``QueryInterface`` to cast the type to ``nsISupportsWeakReference*``, and
+acquire a ``nsIWeakReference*`` to the passed-in object.
diff --git a/xpcom/docs/stringguide.rst b/xpcom/docs/stringguide.rst
new file mode 100644
index 0000000000..a3266d9604
--- /dev/null
+++ b/xpcom/docs/stringguide.rst
@@ -0,0 +1,1094 @@
+String Guide
+============
+
+Most of the Mozilla code uses a C++ class hierarchy to pass string data,
+rather than using raw pointers. This guide documents the string classes which
+are visible to code within the Mozilla codebase (code which is linked into
+``libxul``).
+
+Introduction
+------------
+
+The string classes are a library of C++ classes which are used to manage
+buffers of wide (16-bit) and narrow (8-bit) character strings. The headers
+and implementation are in the `xpcom/string
+<https://searchfox.org/mozilla-central/source/xpcom/string>`_ directory. All
+strings are stored as a single contiguous buffer of characters.
+
+The 8-bit and 16-bit string classes have completely separate base classes,
+but share the same APIs. As a result, you cannot assign a 8-bit string to a
+16-bit string without some kind of conversion helper class or routine. For
+the purpose of this document, we will refer to the 16-bit string classes in
+class documentation. Every 16-bit class has an equivalent 8-bit class:
+
+===================== ======================
+Wide                  Narrow
+===================== ======================
+``nsAString``         ``nsACString``
+``nsString``          ``nsCString``
+``nsAutoString``      ``nsAutoCString``
+``nsDependentString`` ``nsDependentCString``
+===================== ======================
+
+The string classes distinguish, as part of the type hierarchy, between
+strings that must have a null-terminator at the end of their buffer
+(``ns[C]String``) and strings that are not required to have a null-terminator
+(``nsA[C]String``). nsA[C]String is the base of the string classes (since it
+imposes fewer requirements) and ``ns[C]String`` is a class derived from it.
+Functions taking strings as parameters should generally take one of these
+four types.
+
+In order to avoid unnecessary copying of string data (which can have
+significant performance cost), the string classes support different ownership
+models. All string classes support the following three ownership models
+dynamically:
+
+* reference counted, copy-on-write, buffers (the default)
+
+* adopted buffers (a buffer that the string class owns, but is not reference
+  counted, because it came from somewhere else)
+
+* dependent buffers, that is, an underlying buffer that the string class does
+  not own, but that the caller that constructed the string guarantees will
+  outlive the string instance
+
+Auto strings will prefer reference counting an existing reference-counted
+buffer over their stack buffer, but will otherwise use their stack buffer for
+anything that will fit in it.
+
+There are a number of additional string classes:
+
+
+* Classes which exist primarily as constructors for the other types,
+  particularly ``nsDependent[C]String`` and ``nsDependent[C]Substring``. These
+  types are really just convenient notation for constructing an
+  ``nsA[C]String`` with a non-default ownership mode; they should not be
+  thought of as different types.
+
+* ``nsLiteral[C]String`` which should rarely be constructed explicitly but
+  usually through the ``""_ns`` and ``u""_ns`` user-defined string literals.
+  ``nsLiteral[C]String`` is trivially constructible and destructible, and
+  therefore does not emit construction/destruction code when stored in static,
+  as opposed to the other string classes.
+
+The Major String Classes
+------------------------
+
+The list below describes the main base classes. Once you are familiar with
+them, see the appendix describing What Class to Use When.
+
+
+* **nsAString**/**nsACString**: the abstract base class for all strings. It
+  provides an API for assignment, individual character access, basic
+  manipulation of characters in the string, and string comparison. This class
+  corresponds to the XPIDL ``AString`` or ``ACString`` parameter types.
+  ``nsA[C]String`` is not necessarily null-terminated.
+
+* **nsString**/**nsCString**: builds on ``nsA[C]String`` by guaranteeing a
+  null-terminated storage. This allows for a method (``.get()``) to access the
+  underlying character buffer.
+
+The remainder of the string classes inherit from either ``nsA[C]String`` or
+``ns[C]String``. Thus, every string class is compatible with ``nsA[C]String``.
+
+.. note::
+
+    In code which is generic over string width, ``nsA[C]String`` is sometimes
+    known as ``nsTSubstring<CharT>``. ``nsAString`` is a type alias for
+    ``nsTSubstring<char16_t>``, and ``nsACString`` is a type alias for
+    ``nsTSubstring<char>``.
+
+.. note::
+
+    The type ``nsLiteral[C]String`` technically does not inherit from
+    ``nsA[C]String``, but instead inherits from ``nsStringRepr<CharT>``. This
+    allows the type to not generate destructors when stored in static
+    storage.
+
+    It can be implicitly coerced to ``const ns[C]String&`` (though can never
+    be accessed mutably) and generally acts as-if it was a subclass of
+    ``ns[C]String`` in most cases.
+
+Since every string derives from ``nsAString`` (or ``nsACString``), they all
+share a simple API. Common read-only methods include:
+
+* ``.Length()`` - the number of code units (bytes for 8-bit string classes and ``char16_t`` for 16-bit string classes) in the string.
+* ``.IsEmpty()`` - the fastest way of determining if the string has any value. Use this instead of testing ``string.Length() == 0``
+* ``.Equals(string)`` - ``true`` if the given string has the same value as the current string. Approximately the same as ``operator==``.
+
+Common methods that modify the string:
+
+* ``.Assign(string)`` - Assigns a new value to the string. Approximately the same as ``operator=``.
+* ``.Append(string)`` - Appends a value to the string.
+* ``.Insert(string, position)`` - Inserts the given string before the code unit at position.
+* ``.Truncate(length)`` - shortens the string to the given length.
+
+More complete documentation can be found in the `Class Reference`_.
+
+As function parameters
+~~~~~~~~~~~~~~~~~~~~~~
+
+In general, use ``nsA[C]String`` references to pass strings across modules. For example:
+
+.. code-block:: cpp
+
+    // when passing a string to a method, use const nsAString&
+    nsFoo::PrintString(const nsAString& str);
+
+    // when getting a string from a method, use nsAString&
+    nsFoo::GetString(nsAString& result);
+
+The Concrete Classes - which classes to use when
+------------------------------------------------
+
+The concrete classes are for use in code that actually needs to store string
+data. The most common uses of the concrete classes are as local variables,
+and members in classes or structs.
+
+.. digraph:: concreteclasses
+
+    node [shape=rectangle]
+
+    "nsA[C]String" -> "ns[C]String";
+    "ns[C]String" -> "nsDependent[C]String";
+    "nsA[C]String" -> "nsDependent[C]Substring";
+    "nsA[C]String" -> "ns[C]SubstringTuple";
+    "ns[C]String" -> "nsAuto[C]StringN";
+    "ns[C]String" -> "nsLiteral[C]String" [style=dashed];
+    "nsAuto[C]StringN" -> "nsPromiseFlat[C]String";
+    "nsAuto[C]StringN" -> "nsPrintfCString";
+
+The following is a list of the most common concrete classes. Once you are
+familiar with them, see the appendix describing What Class to Use When.
+
+* ``ns[C]String`` - a null-terminated string whose buffer is allocated on the
+  heap. Destroys its buffer when the string object goes away.
+
+* ``nsAuto[C]String`` - derived from ``nsString``, a string which owns a 64
+  code unit buffer in the same storage space as the string itself. If a string
+  less than 64 code units is assigned to an ``nsAutoString``, then no extra
+  storage will be allocated. For larger strings, a new buffer is allocated on
+  the heap.
+
+  If you want a number other than 64, use the templated types ``nsAutoStringN``
+  / ``nsAutoCStringN``. (``nsAutoString`` and ``nsAutoCString`` are just
+  typedefs for ``nsAutoStringN<64>`` and ``nsAutoCStringN<64>``, respectively.)
+
+* ``nsDependent[C]String`` - derived from ``nsString``, this string does not
+  own its buffer. It is useful for converting a raw string pointer (``const
+  char16_t*`` or ``const char*``) into a class of type ``nsAString``. Note that
+  you must null-terminate buffers used by to ``nsDependentString``. If you
+  don't want to or can't null-terminate the buffer, use
+  ``nsDependentSubstring``.
+
+* ``nsPrintfCString`` - derived from ``nsCString``, this string behaves like an
+  ``nsAutoCString``. The constructor takes parameters which allows it to
+  construct a 8-bit string from a printf-style format string and parameter
+  list.
+
+There are also a number of concrete classes that are created as a side-effect
+of helper routines, etc. You should avoid direct use of these classes. Let
+the string library create the class for you.
+
+* ``ns[C]SubstringTuple`` - created via string concatenation
+* ``nsDependent[C]Substring`` - created through ``Substring()``
+* ``nsPromiseFlat[C]String`` - created through ``PromiseFlatString()``
+* ``nsLiteral[C]String`` - created through the ``""_ns`` and ``u""_ns`` user-defined literals
+
+Of course, there are times when it is necessary to reference these string
+classes in your code, but as a general rule they should be avoided.
+
+Iterators
+---------
+
+Because Mozilla strings are always a single buffer, iteration over the
+characters in the string is done using raw pointers:
+
+.. code-block:: cpp
+
+    /**
+     * Find whether there is a tab character in `data`
+     */
+    bool HasTab(const nsAString& data) {
+      const char16_t* cur = data.BeginReading();
+      const char16_t* end = data.EndReading();
+
+      for (; cur < end; ++cur) {
+        if (char16_t('\t') == *cur) {
+          return true;
+        }
+      }
+      return false;
+    }
+
+Note that ``end`` points to the character after the end of the string buffer.
+It should never be dereferenced.
+
+Writing to a mutable string is also simple:
+
+.. code-block:: cpp
+
+    /**
+    * Replace every tab character in `data` with a space.
+    */
+    void ReplaceTabs(nsAString& data) {
+      char16_t* cur = data.BeginWriting();
+      char16_t* end = data.EndWriting();
+
+      for (; cur < end; ++cur) {
+        if (char16_t('\t') == *cur) {
+          *cur = char16_t(' ');
+        }
+      }
+    }
+
+You may change the length of a string via ``SetLength()``. Note that
+Iterators become invalid after changing the length of a string. If a string
+buffer becomes smaller while writing it, use ``SetLength`` to inform the
+string class of the new size:
+
+.. code-block:: cpp
+
+    /**
+     * Remove every tab character from `data`
+     */
+    void RemoveTabs(nsAString& data) {
+      int len = data.Length();
+      char16_t* cur = data.BeginWriting();
+      char16_t* end = data.EndWriting();
+
+      while (cur < end) {
+        if (char16_t('\t') == *cur) {
+          len -= 1;
+          end -= 1;
+          if (cur < end)
+            memmove(cur, cur + 1, (end - cur) * sizeof(char16_t));
+        } else {
+          cur += 1;
+        }
+      }
+
+      data.SetLength(len);
+    }
+
+Note that using ``BeginWriting()`` to make a string longer is not OK.
+``BeginWriting()`` must not be used to write past the logical length of the
+string indicated by ``EndWriting()`` or ``Length()``. Calling
+``SetCapacity()`` before ``BeginWriting()`` does not affect what the previous
+sentence says. To make the string longer, call ``SetLength()`` before
+``BeginWriting()`` or use the ``BulkWrite()`` API described below.
+
+Bulk Write
+----------
+
+``BulkWrite()`` allows capacity-aware cache-friendly low-level writes to the
+string's buffer.
+
+Capacity-aware means that the caller is made aware of how the
+caller-requested buffer capacity was rounded up to mozjemalloc buckets. This
+is useful when initially requesting best-case buffer size without yet knowing
+the true size need. If the data that actually needs to be written is larger
+than the best-case estimate but still fits within the rounded-up capacity,
+there is no need to reallocate despite requesting the best-case capacity.
+
+Cache-friendly means that the zero terminator for C compatibility is written
+after the new content of the string has been written, so the result is a
+forward-only linear write access pattern instead of a non-linear
+back-and-forth sequence resulting from using ``SetLength()`` followed by
+``BeginWriting()``.
+
+Low-level means that writing via a raw pointer is possible as with
+``BeginWriting()``.
+
+``BulkWrite()`` takes three arguments: The new capacity (which may be rounded
+up), the number of code units at the beginning of the string to preserve
+(typically the old logical length), and a boolean indicating whether
+reallocating a smaller buffer is OK if the requested capacity would fit in a
+buffer that's smaller than current one. It returns a ``mozilla::Result`` which
+contains either a usable ``mozilla::BulkWriteHandle<T>`` (where ``T`` is the
+string's ``char_type``) or an ``nsresult`` explaining why none can be had
+(presumably OOM).
+
+The actual writes are performed through the returned
+``mozilla::BulkWriteHandle<T>``. You must not access the string except via this
+handle until you call ``Finish()`` on the handle in the success case or you let
+the handle go out of scope without calling ``Finish()`` in the failure case, in
+which case the destructor of the handle puts the string in a mostly harmless but
+consistent state (containing a single REPLACEMENT CHARACTER if a capacity
+greater than 0 was requested, or in the ``char`` case if the three-byte UTF-8
+representation of the REPLACEMENT CHARACTER doesn't fit, an ASCII SUBSTITUTE).
+
+``mozilla::BulkWriteHandle<T>`` autoconverts to a writable
+``mozilla::Span<T>`` and also provides explicit access to itself as ``Span``
+(``AsSpan()``) or via component accessors named consistently with those on
+``Span``: ``Elements()`` and ``Length()``. (The latter is not the logical
+length of the string but the writable length of the buffer.) The buffer
+exposed via these methods includes the prefix that you may have requested to
+be preserved. It's up to you to skip past it so as to not overwrite it.
+
+If there's a need to request a different capacity before you are ready to
+call ``Finish()``, you can call ``RestartBulkWrite()`` on the handle. It
+takes three arguments that match the first three arguments of
+``BulkWrite()``. It returns ``mozilla::Result<mozilla::Ok, nsresult>`` to
+indicate success or OOM. Calling ``RestartBulkWrite()`` invalidates
+previously-obtained span, raw pointer or length.
+
+Once you are done writing, call ``Finish()``. It takes two arguments: the new
+logical length of the string (which must not exceed the capacity returned by
+the ``Length()`` method of the handle) and a boolean indicating whether it's
+OK to attempt to reallocate a smaller buffer in case a smaller mozjemalloc
+bucket could accommodate the new logical length.
+
+Helper Classes and Functions
+----------------------------
+
+Converting Cocoa strings
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Use ``mozilla::CopyCocoaStringToXPCOMString()`` in
+``mozilla/MacStringHelpers.h`` to convert Cocoa strings to XPCOM strings.
+
+Searching strings - looking for substrings, characters, etc.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``nsReadableUtils.h`` header provides helper methods for searching in runnables.
+
+.. code-block:: cpp
+
+    bool FindInReadable(const nsAString& pattern,
+                        nsAString::const_iterator start, nsAString::const_iterator end,
+                        nsStringComparator& aComparator = nsDefaultStringComparator());
+
+To use this, ``start`` and ``end`` should point to the beginning and end of a
+string that you would like to search. If the search string is found,
+``start`` and ``end`` will be adjusted to point to the beginning and end of
+the found pattern. The return value is ``true`` or ``false``, indicating
+whether or not the string was found.
+
+An example:
+
+.. code-block:: cpp
+
+    const nsAString& str = GetSomeString();
+    nsAString::const_iterator start, end;
+
+    str.BeginReading(start);
+    str.EndReading(end);
+
+    constexpr auto valuePrefix = u"value="_ns;
+
+    if (FindInReadable(valuePrefix, start, end)) {
+        // end now points to the character after the pattern
+        valueStart = end;
+    }
+
+Checking for Memory Allocation failure
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Like other types in Gecko, the string classes use infallible memory
+allocation by default, so you do not need to check for success when
+allocating/resizing "normal" strings.
+
+Most functions that modify strings (``Assign()``, ``SetLength()``, etc.) also
+have an overload that takes a ``mozilla::fallible_t`` parameter. These
+overloads return ``false`` instead of aborting if allocation fails. Use them
+when creating/allocating strings which may be very large, and which the
+program could recover from if the allocation fails.
+
+Substrings (string fragments)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is very simple to refer to a substring of an existing string without
+actually allocating new space and copying the characters into that substring.
+``Substring()`` is the preferred method to create a reference to such a
+string.
+
+.. code-block:: cpp
+
+    void ProcessString(const nsAString& str) {
+        const nsAString& firstFive = Substring(str, 0, 5); // from index 0, length 5
+        // firstFive is now a string representing the first 5 characters
+    }
+
+Unicode Conversion
+------------------
+
+Strings can be stored in two basic formats: 8-bit code unit (byte/``char``)
+strings, or 16-bit code unit (``char16_t``) strings. Any string class with a
+capital "C" in the classname contains 8-bit bytes. These classes include
+``nsCString``, ``nsDependentCString``, and so forth. Any string class without
+the "C" contains 16-bit code units.
+
+A 8-bit string can be in one of many character encodings while a 16-bit
+string is always in potentially-invalid UTF-16. (You can make a 16-bit string
+guaranteed-valid UTF-16 by passing it to ``EnsureUTF16Validity()``.) The most
+common encodings are:
+
+
+* ASCII - 7-bit encoding for basic English-only strings. Each ASCII value
+  is stored in exactly one byte in the array with the most-significant 8th bit
+  set to zero.
+
+* `UCS2 <http://www.unicode.org/glossary/#UCS_2>`_ - 16-bit encoding for a
+  subset of Unicode, `BMP <http://www.unicode.org/glossary/#BMP>`_. The Unicode
+  value of a character stored in UCS2 is stored in exactly one 16-bit
+  ``char16_t`` in a string class.
+
+* `UTF-8 <http://www.faqs.org/rfcs/rfc3629.html>`_ - 8-bit encoding for
+  Unicode characters. Each Unicode characters is stored in up to 4 bytes in a
+  string class. UTF-8 is capable of representing the entire Unicode character
+  repertoire, and it efficiently maps to `UTF-32
+  <http://www.unicode.org/glossary/#UTF_32>`_. (Gtk and Rust natively use
+  UTF-8.)
+
+* `UTF-16 <http://www.unicode.org/glossary/#UTF_16>`_ - 16-bit encoding for
+  Unicode storage, backwards compatible with UCS2. The Unicode value of a
+  character stored in UTF-16 may require one or two 16-bit ``char16_t`` in a
+  string class. The contents of ``nsAString`` always has to be regarded as in
+  this encoding instead of UCS2. UTF-16 is capable of representing the entire
+  Unicode character repertoire, and it efficiently maps to UTF-32. (Win32 W
+  APIs and Mac OS X natively use UTF-16.)
+
+* Latin1 - 8-bit encoding for the first 256 Unicode code points. Used for
+  HTTP headers and for size-optimized storage in text node and SpiderMonkey
+  strings. Latin1 converts to UTF-16 by zero-extending each byte to a 16-bit
+  code unit. Note that this kind of "Latin1" is not available for encoding
+  HTML, CSS, JS, etc. Specifying ``charset=latin1`` means the same as
+  ``charset=windows-1252``. Windows-1252 is a similar but different encoding
+  used for interchange.
+
+In addition, there exist multiple other (legacy) encodings. The Web-relevant
+ones are defined in the `Encoding Standard <https://encoding.spec.whatwg.org/>`_.
+Conversions from these encodings to
+UTF-8 and UTF-16 are provided by `mozilla::Encoding
+<https://searchfox.org/mozilla-central/source/intl/Encoding.h#109>`_.
+Additionally, on Windows the are some rare cases (e.g. drag&drop) where it's
+necessary to call a system API with data encoded in the Windows
+locale-dependent legacy encoding instead of UTF-16. In those rare cases, use
+``MultiByteToWideChar``/``WideCharToMultiByte`` from kernel32.dll. Do not use
+``iconv`` on *nix. We only support UTF-8-encoded file paths on *nix, non-path
+Gtk strings are always UTF-8 and Cocoa and Java strings are always UTF-16.
+
+When working with existing code, it is important to examine the current usage
+of the strings that you are manipulating, to determine the correct conversion
+mechanism.
+
+When writing new code, it can be confusing to know which storage class and
+encoding is the most appropriate. There is no single answer to this question,
+but the important points are:
+
+
+* **Surprisingly many strings are very often just ASCII.** ASCII is a subset of
+  UTF-8 and is, therefore, efficient to represent as UTF-8. Representing ASCII
+  as UTF-16 bad both for memory usage and cache locality.
+
+* **Rust strongly prefers UTF-8.** If your C++ code is interacting with Rust
+  code, using UTF-8 in ``nsACString`` and merely validating it when converting
+  to Rust strings is more efficient than using ``nsAString`` on the C++ side.
+
+* **Networking code prefers 8-bit strings.** Networking code tends to use 8-bit
+  strings: either with UTF-8 or Latin1 (byte value is the Unicode scalar value)
+  semantics.
+
+* **JS and DOM prefer UTF-16.** Most Gecko code uses UTF-16 for compatibility
+  with JS strings and DOM string which are potentially-invalid UTF-16. However,
+  both DOM text nodes and JS strings store strings that only contain code points
+  below U+0100 as Latin1 (byte value is the Unicode scalar value).
+
+* **Windows and Cocoa use UTF-16.** Windows system APIs take UTF-16. Cocoa
+  ``NSString`` is UTF-16.
+
+* **Gtk uses UTF-8.** Gtk APIs take UTF-8 for non-file paths. In the Gecko
+  case, we support only UTF-8 file paths outside Windows, so all Gtk strings
+  are UTF-8 for our purposes though file paths received from Gtk may not be
+  valid UTF-8.
+
+To assist with ASCII, Latin1, UTF-8, and UTF-16 conversions, there are some
+helper methods and classes. Some of these classes look like functions,
+because they are most often used as temporary objects on the stack.
+
+Short zero-terminated ASCII strings
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you have a short zero-terminated string that you are certain is always
+ASCII, use these special-case methods instead of the conversions described in
+the later sections.
+
+* If you are assigning an ASCII literal to an ``nsACString``, use
+  ``AssignLiteral()``.
+* If you are assigning a literal to an ``nsAString``, use ``AssignLiteral()``
+  and make the literal a ``u""`` literal. If the literal has to be a ``""``
+  literal (as opposed to ``u""``) and is ASCII, still use ``AppendLiteral()``,
+  but be aware that this involves a run-time inflation.
+* If you are assigning a zero-terminated ASCII string that's not a literal from
+  the compiler's point of view at the call site and you don't know the length
+  of the string either (e.g. because it was looked up from an array of literals
+  of varying lengths), use ``AssignASCII()``.
+
+UTF-8 / UTF-16 conversion
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. cpp:function:: NS_ConvertUTF8toUTF16(const nsACString&)
+
+    a ``nsAutoString`` subclass that converts a UTF-8 encoded ``nsACString``
+    or ``const char*`` to a 16-bit UTF-16 string. If you need a ``const
+    char16_t*`` buffer, you can use the ``.get()`` method. For example:
+
+    .. code-block:: cpp
+
+        /* signature: void HandleUnicodeString(const nsAString& str); */
+        object->HandleUnicodeString(NS_ConvertUTF8toUTF16(utf8String));
+
+        /* signature: void HandleUnicodeBuffer(const char16_t* str); */
+        object->HandleUnicodeBuffer(NS_ConvertUTF8toUTF16(utf8String).get());
+
+.. cpp:function:: NS_ConvertUTF16toUTF8(const nsAString&)
+
+    a ``nsAutoCString`` which converts a 16-bit UTF-16 string (``nsAString``)
+    to a UTF-8 encoded string. As above, you can use ``.get()`` to access a
+    ``const char*`` buffer.
+
+    .. code-block:: cpp
+
+        /* signature: void HandleUTF8String(const nsACString& str); */
+        object->HandleUTF8String(NS_ConvertUTF16toUTF8(utf16String));
+
+        /* signature: void HandleUTF8Buffer(const char* str); */
+        object->HandleUTF8Buffer(NS_ConvertUTF16toUTF8(utf16String).get());
+
+.. cpp:function:: CopyUTF8toUTF16(const nsACString&, nsAString&)
+
+    converts and copies:
+
+    .. code-block:: cpp
+
+        // return a UTF-16 value
+        void Foo::GetUnicodeValue(nsAString& result) {
+          CopyUTF8toUTF16(mLocalUTF8Value, result);
+        }
+
+.. cpp:function:: AppendUTF8toUTF16(const nsACString&, nsAString&)
+
+    converts and appends:
+
+    .. code-block:: cpp
+
+        // return a UTF-16 value
+        void Foo::GetUnicodeValue(nsAString& result) {
+          result.AssignLiteral("prefix:");
+          AppendUTF8toUTF16(mLocalUTF8Value, result);
+        }
+
+.. cpp:function:: CopyUTF16toUTF8(const nsAString&, nsACString&)
+
+    converts and copies:
+
+    .. code-block:: cpp
+
+        // return a UTF-8 value
+        void Foo::GetUTF8Value(nsACString& result) {
+          CopyUTF16toUTF8(mLocalUTF16Value, result);
+        }
+
+.. cpp:function:: AppendUTF16toUTF8(const nsAString&, nsACString&)
+
+    converts and appends:
+
+    .. code-block:: cpp
+
+        // return a UTF-8 value
+        void Foo::GetUnicodeValue(nsACString& result) {
+          result.AssignLiteral("prefix:");
+          AppendUTF16toUTF8(mLocalUTF16Value, result);
+        }
+
+
+Latin1 / UTF-16 Conversion
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following should only be used when you can guarantee that the original
+string is ASCII or Latin1 (in the sense that the byte value is the Unicode
+scalar value; not in the windows-1252 sense). These helpers are very similar
+to the UTF-8 / UTF-16 conversion helpers above.
+
+
+UTF-16 to Latin1 converters
+```````````````````````````
+
+These converters are **very dangerous** because they **lose information**
+during the conversion process. You should **avoid UTF-16 to Latin1
+conversions** unless your strings are guaranteed to be Latin1 or ASCII. (In
+the future, these conversions may start asserting in debug builds that their
+input is in the permissible range.) If the input is actually in the Latin1
+range, each 16-bit code unit in narrowed to an 8-bit byte by removing the
+high half. Unicode code points above U+00FF result in garbage whose nature
+must not be relied upon. (In the future the nature of the garbage will be CPU
+architecture-dependent.) If you want to ``printf()`` something and don't care
+what happens to non-ASCII, please convert to UTF-8 instead.
+
+
+.. cpp:function:: NS_LossyConvertUTF16toASCII(const nsAString&)
+
+    A ``nsAutoCString`` which holds a temporary buffer containing the Latin1
+    value of the string.
+
+.. cpp:function:: void LossyCopyUTF16toASCII(Span<const char16_t>, nsACString&)
+
+    Does an in-place conversion from UTF-16 into an Latin1 string object.
+
+.. cpp:function:: void LossyAppendUTF16toASCII(Span<const char16_t>, nsACString&)
+
+    Appends a UTF-16 string to a Latin1 string.
+
+Latin1 to UTF-16 converters
+```````````````````````````
+
+These converters are very dangerous because they will **produce wrong results
+for non-ASCII UTF-8 or windows-1252 input** into a meaningless UTF-16 string.
+You should **avoid ASCII to UTF-16 conversions** unless your strings are
+guaranteed to be ASCII or Latin1 in the sense of the byte value being the
+Unicode scalar value. Every byte is zero-extended into a 16-bit code unit.
+
+It is correct to use these on most HTTP header values, but **it's always
+wrong to use these on HTTP response bodies!** (Use ``mozilla::Encoding`` to
+deal with response bodies.)
+
+.. cpp:function:: NS_ConvertASCIItoUTF16(const nsACString&)
+
+    A ``nsAutoString`` which holds a temporary buffer containing the value of
+    the Latin1 to UTF-16 conversion.
+
+.. cpp:function:: void CopyASCIItoUTF16(Span<const char>, nsAString&)
+
+    does an in-place conversion from Latin1 to UTF-16.
+
+.. cpp:function:: void AppendASCIItoUTF16(Span<const char>, nsAString&)
+
+    appends a Latin1 string to a UTF-16 string.
+
+Comparing ns*Strings with C strings
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You can compare ``ns*Strings`` with C strings by converting the ``ns*String``
+to a C string, or by comparing directly against a C String.
+
+.. cpp:function:: bool nsAString::EqualsASCII(const char*)
+
+    Compares with an ASCII C string.
+
+.. cpp:function:: bool nsAString::EqualsLiteral(...)
+
+    Compares with a string literal.
+
+Common Patterns
+---------------
+
+Literal Strings
+~~~~~~~~~~~~~~~
+
+A literal string is a raw string value that is written in some C++ code. For
+example, in the statement ``printf("Hello World\n");`` the value ``"Hello
+World\n"`` is a literal string. It is often necessary to insert literal
+string values when an ``nsAString`` or ``nsACString`` is required. Two
+user-defined literals are provided that implicitly convert to ``const
+nsString&`` resp. ``const nsCString&``:
+
+* ``""_ns`` for 8-bit literals, converting implicitly to ``const nsCString&``
+* ``u""_ns`` for 16-bit literals, converting implicitly to ``const nsString&``
+
+The benefits of the user-defined literals may seem unclear, given that
+``nsDependentCString`` will also wrap a string value in an ``nsCString``. The
+advantage of the user-defined literals is twofold.
+
+* The length of these strings is calculated at compile time, so the string does
+  not need to be scanned at runtime to determine its length.
+
+* Literal strings live for the lifetime of the binary, and can be moved between
+  the ``ns[C]String`` classes without being copied or freed.
+
+Here are some examples of proper usage of the literals (both standard and
+user-defined):
+
+.. code-block:: cpp
+
+    // call Init(const nsLiteralString&) - enforces that it's only called with literals
+    Init(u"start value"_ns);
+
+    // call Init(const nsAString&)
+    Init(u"start value"_ns);
+
+    // call Init(const nsACString&)
+    Init("start value"_ns);
+
+In case a literal is defined via a macro, you can just convert it to
+``nsLiteralString`` or ``nsLiteralCString`` using their constructor. You
+could consider not using a macro at all but a named ``constexpr`` constant
+instead.
+
+In some cases, an 8-bit literal is defined via a macro, either within code or
+from the environment, but it can't be changed or is used both as an 8-bit and
+a 16-bit string. In these cases, you can use the
+``NS_LITERAL_STRING_FROM_CSTRING`` macro to construct a ``nsLiteralString``
+and do the conversion at compile-time.
+
+String Concatenation
+~~~~~~~~~~~~~~~~~~~~
+
+Strings can be concatenated together using the + operator. The resulting
+string is a ``const nsSubstringTuple`` object. The resulting object can be
+treated and referenced similarly to a ``nsAString`` object. Concatenation *does
+not copy the substrings*. The strings are only copied when the concatenation
+is assigned into another string object. The ``nsSubstringTuple`` object holds
+pointers to the original strings. Therefore, the ``nsSubstringTuple`` object is
+dependent on all of its substrings, meaning that their lifetime must be at
+least as long as the ``nsSubstringTuple`` object.
+
+For example, you can use the value of two strings and pass their
+concatenation on to another function which takes an ``const nsAString&``:
+
+.. code-block:: cpp
+
+    void HandleTwoStrings(const nsAString& one, const nsAString& two) {
+      // call HandleString(const nsAString&)
+      HandleString(one + two);
+    }
+
+NOTE: The two strings are implicitly combined into a temporary ``nsString``
+in this case, and the temporary string is passed into ``HandleString``. If
+``HandleString`` assigns its input into another ``nsString``, then the string
+buffer will be shared in this case negating the cost of the intermediate
+temporary. You can concatenate N strings and store the result in a temporary
+variable:
+
+.. code-block:: cpp
+
+    constexpr auto start = u"start "_ns;
+    constexpr auto middle = u"middle "_ns;
+    constexpr auto end = u"end"_ns;
+    // create a string with 3 dependent fragments - no copying involved!
+    nsString combinedString = start + middle + end;
+
+    // call void HandleString(const nsAString&);
+    HandleString(combinedString);
+
+It is safe to concatenate user-defined literals because the temporary
+``nsLiteral[C]String`` objects will live as long as the temporary
+concatenation object (of type ``nsSubstringTuple``).
+
+.. code-block:: cpp
+
+    // call HandlePage(const nsAString&);
+    // safe because the concatenated-string will live as long as its substrings
+    HandlePage(u"start "_ns + u"end"_ns);
+
+Local Variables
+~~~~~~~~~~~~~~~
+
+Local variables within a function are usually stored on the stack. The
+``nsAutoString``/``nsAutoCString`` classes are subclasses of the
+``nsString``/``nsCString`` classes. They own a 64-character buffer allocated
+in the same storage space as the string itself. If the ``nsAutoString`` is
+allocated on the stack, then it has at its disposal a 64-character stack
+buffer. This allows the implementation to avoid allocating extra memory when
+dealing with small strings. ``nsAutoStringN``/``nsAutoCStringN`` are more
+general alternatives that let you choose the number of characters in the
+inline buffer.
+
+.. code-block:: cpp
+
+    ...
+    nsAutoString value;
+    GetValue(value); // if the result is less than 64 code units,
+                    // then this just saved us an allocation
+    ...
+
+Member Variables
+~~~~~~~~~~~~~~~~
+
+In general, you should use the concrete classes ``nsString`` and
+``nsCString`` for member variables.
+
+.. code-block:: cpp
+
+    class Foo {
+      ...
+      // these store UTF-8 and UTF-16 values respectively
+      nsCString mLocalName;
+      nsString mTitle;
+    };
+
+A common incorrect pattern is to use ``nsAutoString``/``nsAutoCString``
+for member variables. As described in `Local Variables`_, these classes have
+a built in buffer that make them very large. This means that if you include
+them in a class, they bloat the class by 64 bytes (``nsAutoCString``) or 128
+bytes (``nsAutoString``).
+
+
+Raw Character Pointers
+~~~~~~~~~~~~~~~~~~~~~~
+
+``PromiseFlatString()`` and ``PromiseFlatCString()`` can be used to create a
+temporary buffer which holds a null-terminated buffer containing the same
+value as the source string. ``PromiseFlatString()`` will create a temporary
+buffer if necessary. This is most often used in order to pass an
+``nsAString`` to an API which requires a null-terminated string.
+
+In the following example, an ``nsAString`` is combined with a literal string,
+and the result is passed to an API which requires a simple character buffer.
+
+.. code-block:: cpp
+
+    // Modify the URL and pass to AddPage(const char16_t* url)
+    void AddModifiedPage(const nsAString& url) {
+      constexpr auto httpPrefix = u"http://"_ns;
+      const nsAString& modifiedURL = httpPrefix + url;
+
+      // creates a temporary buffer
+      AddPage(PromiseFlatString(modifiedURL).get());
+    }
+
+``PromiseFlatString()`` is smart when handed a string that is already
+null-terminated. It avoids creating the temporary buffer in such cases.
+
+.. code-block:: cpp
+
+    // Modify the URL and pass to AddPage(const char16_t* url)
+    void AddModifiedPage(const nsAString& url, PRBool addPrefix) {
+        if (addPrefix) {
+            // MUST create a temporary buffer - string is multi-fragmented
+            constexpr auto httpPrefix = u"http://"_ns;
+            AddPage(PromiseFlatString(httpPrefix + modifiedURL));
+        } else {
+            // MIGHT create a temporary buffer, does a runtime check
+            AddPage(PromiseFlatString(url).get());
+        }
+    }
+
+.. note::
+
+    It is **not** possible to efficiently transfer ownership of a string
+    class' internal buffer into an owned ``char*`` which can be safely
+    freed by other components due to the COW optimization.
+
+    If working with a legacy API which requires malloced ``char*`` buffers,
+    prefer using ``ToNewUnicode``, ``ToNewCString`` or ``ToNewUTF8String``
+    over ``strdup`` to create owned ``char*`` pointers.
+
+``printf`` and a UTF-16 string
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For debugging, it's useful to ``printf`` a UTF-16 string (``nsString``,
+``nsAutoString``, etc). To do this usually requires converting it to an 8-bit
+string, because that's what ``printf`` expects. Use:
+
+.. code-block:: cpp
+
+    printf("%s\n", NS_ConvertUTF16toUTF8(yourString).get());
+
+Sequence of appends without reallocating
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+``SetCapacity()`` allows you to give the string a hint of the future string
+length caused by a sequence of appends (excluding appends that convert
+between UTF-16 and UTF-8 in either direction) in order to avoid multiple
+allocations during the sequence of appends. However, the other
+allocation-avoidance features of XPCOM strings interact badly with
+``SetCapacity()`` making it something of a footgun.
+
+``SetCapacity()`` is appropriate to use before a sequence of multiple
+operations from the following list (without operations that are not on the
+list between the ``SetCapacity()`` call and operations from the list):
+
+* ``Append()``
+* ``AppendASCII()``
+* ``AppendLiteral()``
+* ``AppendPrintf()``
+* ``AppendInt()``
+* ``AppendFloat()``
+* ``LossyAppendUTF16toASCII()``
+* ``AppendASCIItoUTF16()``
+
+**DO NOT** call ``SetCapacity()`` if the subsequent operations on the string
+do not meet the criteria above. Operations that undo the benefits of
+``SetCapacity()`` include but are not limited to:
+
+* ``SetLength()``
+* ``Truncate()``
+* ``Assign()``
+* ``AssignLiteral()``
+* ``Adopt()``
+* ``CopyASCIItoUTF16()``
+* ``LossyCopyUTF16toASCII()``
+* ``AppendUTF16toUTF8()``
+* ``AppendUTF8toUTF16()``
+* ``CopyUTF16toUTF8()``
+* ``CopyUTF8toUTF16()``
+
+If your string is an ``nsAuto[C]String`` and you are calling
+``SetCapacity()`` with a constant ``N``, please instead declare the string as
+``nsAuto[C]StringN<N+1>`` without calling ``SetCapacity()`` (while being
+mindful of not using such a large ``N`` as to overflow the run-time stack).
+
+There is no need to include room for the null terminator: it is the job of
+the string class.
+
+Note: Calling ``SetCapacity()`` does not give you permission to use the
+pointer obtained from ``BeginWriting()`` to write past the current length (as
+returned by ``Length()``) of the string. Please use either ``BulkWrite()`` or
+``SetLength()`` instead.
+
+.. _stringguide.xpidl:
+
+XPIDL
+-----
+
+The string library is also available through IDL. By declaring attributes and
+methods using the specially defined IDL types, string classes are used as
+parameters to the corresponding methods.
+
+XPIDL String types
+~~~~~~~~~~~~~~~~~~
+
+The C++ signatures follow the abstract-type convention described above, such
+that all method parameters are based on the abstract classes. The following
+table describes the purpose of each string type in IDL.
+
++-----------------+----------------+----------------------------------------------------------------------------------+
+| XPIDL Type      | C++ Type       | Purpose                                                                          |
++=================+================+==================================================================================+
+| ``string``      | ``char*``      | Raw character pointer to ASCII (7-bit) string, no string classes used.           |
+|                 |                |                                                                                  |
+|                 |                | High bit is not guaranteed across XPConnect boundaries.                          |
++-----------------+----------------+----------------------------------------------------------------------------------+
+| ``wstring``     | ``char16_t*``  | Raw character pointer to UTF-16 string, no string classes used.                  |
++-----------------+----------------+----------------------------------------------------------------------------------+
+| ``AString``     | ``nsAString``  | UTF-16 string.                                                                   |
++-----------------+----------------+----------------------------------------------------------------------------------+
+| ``ACString``    | ``nsACString`` | 8-bit string. All bits are preserved across XPConnect boundaries.                |
++-----------------+----------------+----------------------------------------------------------------------------------+
+| ``AUTF8String`` | ``nsACString`` | UTF-8 string.                                                                    |
+|                 |                |                                                                                  |
+|                 |                | Converted to UTF-16 as necessary when value is used across XPConnect boundaries. |
++-----------------+----------------+----------------------------------------------------------------------------------+
+
+Callers should prefer using the string classes ``AString``, ``ACString`` and
+``AUTF8String`` over the raw pointer types ``string`` and ``wstring`` in
+almost all situations.
+
+C++ Signatures
+~~~~~~~~~~~~~~
+
+In XPIDL, ``in`` parameters are read-only, and the C++ signatures for
+``*String`` parameters follows the above guidelines by using ``const
+nsAString&`` for these parameters. ``out`` and ``inout`` parameters are
+defined simply as ``nsAString&`` so that the callee can write to them.
+
+.. code-block:: cpp
+
+    interface nsIFoo : nsISupports {
+        attribute AString utf16String;
+        AUTF8String getValue(in ACString key);
+    };
+
+.. code-block:: cpp
+
+    class nsIFoo : public nsISupports {
+      NS_IMETHOD GetUtf16String(nsAString& aResult) = 0;
+      NS_IMETHOD SetUtf16String(const nsAString& aValue) = 0;
+      NS_IMETHOD GetValue(const nsACString& aKey, nsACString& aResult) = 0;
+    };
+
+In the above example, ``utf16String`` is treated as a UTF-16 string. The
+implementation of ``GetUtf16String()`` will use ``aResult.Assign`` to
+"return" the value. In ``SetUtf16String()`` the value of the string can be
+used through a variety of methods including `Iterators`_,
+``PromiseFlatString``, and assignment to other strings.
+
+In ``GetValue()``, the first parameter, ``aKey``, is treated as a raw
+sequence of 8-bit values. Any non-ASCII characters in ``aKey`` will be
+preserved when crossing XPConnect boundaries. The implementation of
+``GetValue()`` will assign a UTF-8 encoded 8-bit string into ``aResult``. If
+the this method is called across XPConnect boundaries, such as from a script,
+then the result will be decoded from UTF-8 into UTF-16 and used as a Unicode
+value.
+
+String Guidelines
+-----------------
+
+Follow these simple rules in your code to keep your fellow developers,
+reviewers, and users happy.
+
+* Use the most abstract string class that you can. Usually this is:
+  * ``nsAString`` for function parameters
+  * ``nsString`` for member variables
+  * ``nsAutoString`` for local (stack-based) variables
+* Use the ``""_ns`` and ``u""_ns`` user-defined literals to represent literal strings (e.g. ``"foo"_ns``) as nsAString-compatible objects.
+* Use string concatenation (i.e. the "+" operator) when combining strings.
+* Use ``nsDependentString`` when you have a raw character pointer that you need to convert to an nsAString-compatible string.
+* Use ``Substring()`` to extract fragments of existing strings.
+* Use `iterators`_ to parse and extract string fragments.
+
+Class Reference
+---------------
+
+.. cpp:class:: template<T> nsTSubstring<T>
+
+    .. note::
+
+        The ``nsTSubstring<char_type>`` class is usually written as
+        ``nsAString`` or ``nsACString``.
+
+    .. cpp:function:: size_type Length() const
+
+    .. cpp:function:: bool IsEmpty() const
+
+    .. cpp:function:: bool IsVoid() const
+
+    .. cpp:function:: const char_type* BeginReading() const
+
+    .. cpp:function:: const char_type* EndReading() const
+
+    .. cpp:function:: bool Equals(const self_type&, comparator_type = ...) const
+
+    .. cpp:function:: char_type First() const
+
+    .. cpp:function:: char_type Last() const
+
+    .. cpp:function:: size_type CountChar(char_type) const
+
+    .. cpp:function:: int32_t FindChar(char_type, index_type aOffset = 0) const
+
+    .. cpp:function:: void Assign(const self_type&)
+
+    .. cpp:function:: void Append(const self_type&)
+
+    .. cpp:function:: void Insert(const self_type&, index_type aPos)
+
+    .. cpp:function:: void Cut(index_type aCutStart, size_type aCutLength)
+
+    .. cpp:function:: void Replace(index_type aCutStart, size_type aCutLength, const self_type& aStr)
+
+    .. cpp:function:: void Truncate(size_type aLength)
+
+    .. cpp:function:: void SetIsVoid(bool)
+
+        Make it null. XPConnect and WebIDL will convert void nsAStrings to
+        JavaScript ``null``.
+
+    .. cpp:function:: char_type* BeginWriting()
+
+    .. cpp:function:: char_type* EndWriting()
+
+    .. cpp:function:: void SetCapacity(size_type)
+
+        Inform the string about buffer size need before a sequence of calls
+        to ``Append()`` or converting appends that convert between UTF-16 and
+        Latin1 in either direction. (Don't use if you use appends that
+        convert between UTF-16 and UTF-8 in either direction.) Calling this
+        method does not give you permission to use ``BeginWriting()`` to
+        write past the logical length of the string. Use ``SetLength()`` or
+        ``BulkWrite()`` as appropriate.
+
+    .. cpp:function:: void SetLength(size_type)
+
+    .. cpp:function:: Result<BulkWriteHandle<char_type>, nsresult> BulkWrite(size_type aCapacity, size_type aPrefixToPreserve, bool aAllowShrinking)
diff --git a/xpcom/docs/thread-safety.rst b/xpcom/docs/thread-safety.rst
new file mode 100644
index 0000000000..be2f156804
--- /dev/null
+++ b/xpcom/docs/thread-safety.rst
@@ -0,0 +1,354 @@
+**Thread safety analysis in Gecko**
+===================================
+
+Clang thread-safety analysis is supported in Gecko. This means
+builds will generate warnings when static analysis detects an issue with
+locking of mutex/monitor-protected members and data structures. Note
+that Chrome uses the same feature. An example warning: ::
+
+  warning: dom/media/AudioStream.cpp:504:22 [-Wthread-safety-analysis]
+  reading variable 'mDataSource' requires holding mutex 'mMonitor'
+
+If your patch causes warnings like this, you’ll need to resolve them;
+they will be errors on checkin.
+
+This analysis depends on thread-safety attributions in the source. These
+have been added to Mozilla’s Mutex and Monitor classes and subclasses,
+but in practice the analysis is largely dependent on additions to the
+code being checked, in particular adding MOZ_GUARDED_BY(mutex) attributions
+on the definitions of member variables. Like this: ::
+
+  mozilla::Mutex mLock;
+  bool mShutdown MOZ_GUARDED_BY(mLock);
+
+For background on Clang’s thread-safety support, see `their
+documentation <https://clang.llvm.org/docs/ThreadSafetyAnalysis.html>`__.
+
+Newly added Mutexes and Monitors **MUST** use thread-safety annotations,
+and we are enabling static checks to verify this. Legacy uses of Mutexes
+and Monitors are marked with MOZ_UNANNOTATED.
+
+If you’re modifying code that has been annotated with
+MOZ_GUARDED_BY()/MOZ_REQUIRES()/etc, you should **make sure that the annotations
+are updated properly**; e.g. if you change the thread-usage of a member
+variable or method you should mark it accordingly, comment, and resolve
+any warnings. Since the warnings will be errors in autoland/m-c, you
+won’t be able to land code with active warnings.
+
+**Annotating locking and usage requirements in class definitions**
+------------------------------------------------------------------
+
+Values that require a lock to access, or which are simply used from more
+than one thread, should always have documentation in the definition
+about the locking requirements and/or what threads it’s touched from: ::
+
+  // This array is accessed from both the direct video thread, and the graph
+  // thread. Protected by mMutex.
+
+  nsTArray<std::pair<ImageContainer::FrameID, VideoChunk>> mFrames
+  MOZ_GUARDED_BY(mMutex);
+
+  // Set on MainThread, deleted on either MainThread mainthread, used on
+  // MainThread or IO Thread in DoStopSession
+  nsCOMPtr<nsITimer> mReconnectDelayTimer MOZ_GUARDED_BY(mMutex);
+
+It’s strongly recommended to group values by access pattern, but it’s
+**critical** to make it clear what the requirements to access a value
+are. With values protected by Mutexes and Monitors, adding a
+MOZ_GUARDED_BY(mutex/monitor) should be sufficient, though you may want to
+also document what threads access it, and if they read or write to it.
+
+Values which have more complex access requirements (see single-writer
+and time-based-locking below) need clear documentation where they’re
+defined: ::
+
+  MutexSingleWriter mMutex;
+
+  // mResource should only be modified on the main thread with the lock.
+  // So it can be read without lock on the main thread or on other threads
+  // with the lock.
+  RefPtr<ChannelMediaResource> mResource MOZ_GUARDED_BY(mMutex);
+
+**WARNING:** thread-safety analysis is not magic; it depends on you telling
+it the requirements around access. If you don’t mark something as
+MOZ_GUARDED_BY() it won’t figure it out for you, and you can end up with a data
+race. When writing multithreaded code, you should always be thinking about
+which threads can access what and when, and document this.
+
+**How to annotate different locking patterns in Gecko**
+-------------------------------------------------------
+
+Gecko uses a number of different locking patterns. They include:
+
+-  **Always Lock** -
+   Multiple threads may read and write the value
+
+-  **Single Writer** -
+   One thread does all the writing, other threads
+   read the value, but code on the writing thread also reads it
+   without the lock
+
+-  **Out-of-band invariants** -
+   A value may be accessed from other threads,
+   but only after or before certain other events or in a certain state,
+   like when after a listener has been added or before a processing
+   thread has been shut down.
+
+The simplest and easiest to check with static analysis is **Always
+Lock**, and generally you should prefer this pattern. This is very
+simple; you add MOZ_GUARDED_BY(mutex/monitor), and must own the lock to
+access the value. This can be implemented by some combination of direct
+Lock/AutoLock calls in the method; an assertion that the lock is already
+held by the current thread, or annotating the method as requiring the
+lock (MOZ_REQUIRES(mutex)) in the method definition: ::
+
+  // Ensures mSize is initialized, if it can be.
+  // mLock must be held when this is called, and mInput must be non-null.
+  void EnsureSizeInitialized() MOZ_REQUIRES(mLock);
+  ...
+  // This lock protects mSeekable, mInput, mSize, and mSizeInitialized.
+  Mutex mLock;
+  int64_t mSize MOZ_GUARDED_BY(mLock);
+
+**Single Writer** is tricky for static analysis normally, since it
+doesn’t know what thread an access will occur on. In general, you should
+prefer using Always Lock in non-performance-sensitive code, especially
+since these mutexes are almost always uncontended and therefore cheap to
+lock.
+
+To support this fairly common pattern in Mozilla code, we’ve added
+MutexSingleWriter and MonitorSingleWriter subclasses. To use these, you
+need to subclass SingleWriterLockOwner on one object (typically the
+object containing the Mutex), implement ::OnWritingThread(), and pass
+the object to the constructor for MutexSingleWriter. In code that
+accesses the guarded value from the writing thread, you need to add
+mMutex.AssertIsOnWritingThread(), which both does a debug-only runtime
+assertion by calling OnWritingThread(), and also asserts to the static
+analyzer that the lock is held (which it isn’t).
+
+There is one case this causes problems with: when a method needs to
+access the value (without the lock), and then decides to write to the
+value from the same method, taking the lock. To the static analyzer,
+this looks like a double-lock. Either you will need to add
+MOZ_NO_THREAD_SAFETY_ANALYSIS to the method, move the write into another
+method you call, or locally disable the warning with
+MOZ_PUSH_IGNORE_THREAD_SAFETY and MOZ_POP_THREAD_SAFETY. We’re discussing with
+the clang static analysis developers how to better handle this.
+
+Note also that this provides no checking that the lock is taken to write
+to the value: ::
+
+  MutexSingleWriter mMutex;
+  // mResource should only be modified on the main thread with the lock.
+  // So it can be read without lock on the main thread or on other threads
+  // with the lock.
+  RefPtr<ChannelMediaResource> mResource MOZ_GUARDED_BY(mMutex);
+  ...
+  nsresult ChannelMediaResource::Listener::OnStartRequest(nsIRequest *aRequest) {
+    mMutex.AssertOnWritingThread();
+
+    // Read from the only writing thread; no lock needed
+    if (!mResource) {
+      return NS_OK;
+    }
+    return mResource->OnStartRequest(aRequest, mOffset);
+  }
+
+If you need to assert you’re on the writing thread, then later take a
+lock to modify a value, it will cause a warning: ”acquiring mutex
+'mMutex' that is already held”. You can resolve this by turning off
+thread-safety analysis for the lock: ::
+
+  mMutex.AssertOnWritingThread();
+  ...
+  {
+    MOZ_PUSH_IGNORE_THREAD_SAFETY
+    MutexSingleWriterAutoLock lock(mMutex);
+    MOZ_POP_THREAD_SAFETY
+
+**Out-of-band Invariants** is used in a number of places (and may be
+combined with either of the above patterns). It's using other knowledge
+about the execution pattern of the code to assert that it's safe to avoid
+taking certain locks.   A primary example is when a value can
+only be accessed from a single thread for part of its lifetime (this can
+also be referred to as "time-based locking").
+
+Note that thread-safety analysis always ignores constructors and destructors
+(which shouldn’t have races with other threads barring really odd usages).
+Since only a single thread can access during those time periods, locking is
+not required there.  However, if a method is called from a constructor,
+that method may generate warnings since the compiler doesn't know if it
+might be called from elsewhere: ::
+
+  ...
+  class nsFoo {
+  public:
+    nsFoo() {
+      mBar = true; // Ok since we're in the constructor, no warning
+      Init();
+    }
+    void Init() {  // we're only called from the constructor
+      // This causes a thread-safety warning, since the compiler
+      // can't prove that Init() is only called from the constructor
+      mQuerty = true;
+    }
+    ...
+    mMutex mMutex;
+    uint32_t mBar MOZ_GUARDED_BY(mMutex);
+    uint32_t mQuerty MOZ_GUARDED_BY(mMutex);
+  }
+
+Another example might be a value that’s used from other threads, but only
+if an observer has been installed. Thus code that always runs before the
+observer is installed, or after it’s removed, does not need to lock.
+
+These patterns are impossible to statically check in most cases. If all
+the periods where it’s accessed from one thread only are on the same
+thread, you could use the Single Writer pattern support to cover this
+case. You would add AssertIsOnWritingThread() calls to methods that meet
+the criteria that only a single thread can access the value (but only if
+that holds). Unlike regular uses of SingleWriter, however, there’s no way
+to check if you added such an assertion to code that runs on the “right”
+thread, but during a period where another thread might modify it.
+
+For this reason, we **strongly** suggest that you convert cases of
+Out-of-band-invariants/Time-based-locking to Always Lock if you’re
+refactoring the code or making major modifications. This is far less prone
+to error, and also to future changes breaking the assumptions about other
+threads accessing the value. In all but a few cases where code is on a very
+‘hot’ path, this will have no impact on performance - taking an uncontended
+lock is cheap.
+
+To quiet warnings where these patterns are in use, you'll need to either
+add locks (preferred), or suppress the warnings with MOZ_NO_THREAD_SAFETY_ANALYSIS or
+MOZ_PUSH_IGNORE_THREAD_SAFETY/MOZ_POP_THREAD_SAFETY.
+
+**This pattern especially needs good documentation in the code as to what
+threads will access what members under what conditions!**::
+
+  // Can't be accessed by multiple threads yet
+  nsresult nsAsyncStreamCopier::InitInternal(nsIInputStream* source,
+                                             nsIOutputStream* sink,
+					     nsIEventTarget* target,
+					     uint32_t chunkSize,
+					     bool closeSource,
+					     bool closeSink)
+	MOZ_NO_THREAD_SAFETY_ANALYSIS {
+
+and::
+
+  // We can't be accessed by another thread because this hasn't been
+  // added to the public list yet
+  MOZ_PUSH_IGNORE_THREAD_SAFETY
+  mRestrictedPortList.AppendElement(gBadPortList[i]);
+  MOZ_POP_THREAD_SAFETY
+
+and::
+
+  // This is called on entries in another entry's mCallback array, under the lock
+  // of that other entry. No other threads can access this entry at this time.
+  bool CacheEntry::Callback::DeferDoom(bool* aDoom) const {
+
+**Known limitations**
+---------------------
+
+**Static analysis can’t handle all reasonable patterns.** In particular,
+per their documentation, it can’t handle conditional locks, like: ::
+
+  if (OnMainThread()) {
+    mMutex.Lock();
+  }
+
+You should resolve this either via MOZ_NO_THREAD_SAFETY_ANALYSIS on the
+method, or MOZ_PUSH_IGNORE_THREAD_SAFETY/MOZ_POP_THREAD_SAFETY.
+
+**Sometimes the analyzer can’t figure out that two objects are both the
+same Mutex**, and it will warn you. You may be able to resolve this by
+making sure you’re using the same pattern to access the mutex: ::
+
+   mChan->mMonitor->AssertCurrentThreadOwns();
+   ...
+   {
+ -    MonitorAutoUnlock guard(*monitor);
+ +    MonitorAutoUnlock guard(*(mChan->mMonitor.get())); // avoids mutex warning
+     ok = node->SendUserMessage(port, std::move(aMessage));
+   }
+
+**Maybe<MutexAutoLock>** doesn’t tell the static analyzer when the mutex
+is owned or freed; follow locking via the MayBe<> by
+**mutex->AssertCurrentThreadOwns();** (and ditto for Monitors): ::
+
+  Maybe<MonitorAutoLock> lock(std::in_place, *mMonitor);
+  mMonitor->AssertCurrentThreadOwns(); // for threadsafety analysis
+
+If you reset() the Maybe<>, you may need to surround it with
+MOZ_PUSH_IGNORE_THREAD_SAFETY and MOZ_POP_THREAD_SAFETY macros: ::
+
+  MOZ_PUSH_IGNORE_THREAD_SAFETY
+  aLock.reset();
+  MOZ_POP_THREAD_SAFETY
+
+**Passing a protected value by-reference** sometimes will confuse the
+analyzer. Use MOZ_PUSH_IGNORE_THREAD_SAFETY and MOZ_POP_THREAD_SAFETY macros to
+resolve this.
+
+**Classes which need thread-safety annotations**
+------------------------------------------------
+
+-  Mutex
+
+-  StaticMutex
+
+-  RecursiveMutex
+
+-  BaseProfilerMutex
+
+-  Monitor
+
+-  StaticMonitor
+
+-  ReentrantMonitor
+
+-  RWLock
+
+-  Anything that hides an internal Mutex/etc and presents a Mutex-like
+      API (::Lock(), etc).
+
+**Additional Notes**
+--------------------
+
+Some code passes **Proof-of-Lock** AutoLock parameters, as a poor form of
+static analysis. While it’s hard to make mistakes if you pass an AutoLock
+reference, it is possible to pass a lock to the wrong Mutex/Monitor.
+
+Proof-of-lock is basically redundant to MOZ_REQUIRES() and obsolete, and
+depends on the optimizer to remove it, and per above it can be misused,
+with effort.  With MOZ_REQUIRES(), any proof-of-lock parameters can be removed,
+though you don't have to do so immediately.
+
+In any method taking an aProofOfLock parameter, add a MOZ_REQUIRES(mutex) to
+the definition (and optionally remove the proof-of-lock), or add a
+mMutex.AssertCurrentThreadOwns() to the method: ::
+
+    nsresult DispatchLockHeld(already_AddRefed<WorkerRunnable> aRunnable,
+ -                            nsIEventTarget* aSyncLoopTarget,
+ -                            const MutexAutoLock& aProofOfLock);
+ +                            nsIEventTarget* aSyncLoopTarget) MOZ_REQUIRES(mMutex);
+
+or (if for some reason it's hard to specify the mutex in the header)::
+
+    nsresult DispatchLockHeld(already_AddRefed<WorkerRunnable> aRunnable,
+ -                            nsIEventTarget* aSyncLoopTarget,
+ -                            const MutexAutoLock& aProofOfLock);
+ +                            nsIEventTarget* aSyncLoopTarget) {
+ +  mMutex.AssertCurrentThreadOwns();
+
+In addition to MOZ_GUARDED_BY() there’s also MOZ_PT_GUARDED_BY(), which says
+that the pointer isn’t guarded, but the data pointed to by the pointer
+is.
+
+Classes that expose a Mutex-like interface can be annotated like Mutex;
+see some of the examples in the tree that use MOZ_CAPABILITY and
+MOZ_ACQUIRE()/MOZ_RELEASE().
+
+Shared locks are supported, though we don’t use them much. See RWLock.
diff --git a/xpcom/docs/writing-xpcom-interface.rst b/xpcom/docs/writing-xpcom-interface.rst
new file mode 100644
index 0000000000..9eeb1c72a2
--- /dev/null
+++ b/xpcom/docs/writing-xpcom-interface.rst
@@ -0,0 +1,287 @@
+.. _writing_xpcom_interface:
+
+Tutorial for Writing a New XPCOM Interface
+==========================================
+
+High Level Overview
+-------------------
+
+In order to write code that works in native code (C++, Rust), and JavaScript contexts, it's necessary to have a mechanism to do so. For chrome privileged contexts, this is the XPCOM Interface Class.
+
+This mechanism starts with an :ref:`XPIDL` file to define the shape of the interface. In the `build system`_, this file is processed, and `Rust`_ and `C++`_ code is automatically generated.
+
+.. _build system: https://searchfox.org/mozilla-central/source/xpcom/idl-parser/xpidl
+.. _Rust: https://searchfox.org/mozilla-central/source/__GENERATED__/dist/xpcrs/rt
+.. _C++: https://searchfox.org/mozilla-central/source/__GENERATED__/dist/include
+
+Next, the interface's methods and attributes must be implemented. This can be done through either a JSM module, or through a C++ interface class. Once these steps are done, the new files must be added to the appropriate :code:`moz.build` files to ensure the build system knows how to find them and process them.
+
+Often these XPCOM components are wired into the :code:`Services` JavaScript object to allow for ergonomic access to the interface. For example, open the `Browser Console`_ and type :code:`Services.` to interactively access these components.
+
+.. _Browser Console: https://developer.mozilla.org/en-US/docs/Tools/Browser_Console
+
+From C++, components can be accessed via :code:`mozilla::components::ComponentName::Create()` using the :code:`name` option in the :code:`components.conf`.
+
+While :code:`Services` and :code:`mozilla::components` are the preferred means of accessing components, many are accessed through the historical (and somewhat arcane) :code:`createInstance` mechanism. New usage of these mechanisms should be avoided if possible.
+
+.. code:: javascript
+
+    let component = Cc["@mozilla.org/component-name;1"].createInstance(
+      Ci.nsIComponentName
+    );
+
+.. code:: c++
+
+    nsCOMPtr<nsIComponentName> component = do_CreateInstance(
+      "@mozilla.org/component-name;1");
+
+Writing an XPIDL
+----------------
+
+First decide on a name. Conventionally the interfaces are prefixed with :code:`nsI` (historically Netscape) or :code:`mozI` as they are defined in the global namespace. While the interface is global, the implementation of an interface can be defined in a namespace with no prefix. Historically many component implementations still use the :code:`ns` prefixes (notice that the :code:`I` was dropped), but this convention is no longer needed.
+
+This tutorial assumes the component is located at :code:`path/to` with the name :code:`ComponentName`. The interface name will be :code:`nsIComponentName`, while the implementation will be :code:`mozilla::ComponentName`.
+
+To start, create an :ref:`XPIDL` file:
+
+.. code:: bash
+
+    touch path/to/nsIComponentName.idl
+
+And hook it up to the :code:`path/to/moz.build`
+
+.. code:: python
+
+    XPIDL_SOURCES += [
+        "nsIComponentName.idl",
+    ]
+
+Next write the initial :code:`.idl` file: :code:`path/to/nsIComponentName.idl`
+
+.. _contract_ids:
+.. code:: c++
+
+    /* This Source Code Form is subject to the terms of the Mozilla Public
+     * License, v. 2.0. If a copy of the MPL was not distributed with this
+     * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+    // This is the base include which defines nsISupports. This class defines
+    // the QueryInterface method.
+    #include "nsISupports.idl"
+
+    // `scriptable` designates that this object will be used with JavaScript
+    // `uuid`       The example below uses a UUID with all Xs. Replace the Xs with
+    //              your own UUID generated here:
+    //              http://mozilla.pettay.fi/cgi-bin/mozuuid.pl
+
+    /**
+     * Make sure to document your interface.
+     */
+    [scriptable, uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)]
+    interface nsIComponentName : nsISupports {
+
+      // Fill out your definition here. This example attribute only returns a bool.
+
+      /**
+       * Make sure to document your attributes.
+       */
+      readonly attribute bool isAlive;
+    };
+
+This definition only includes one attribute, :code:`isAlive`, which will demonstrate that we've done our work correctly at the end. For a more comprehensive guide for this syntax, see the :ref:`XPIDL` docs.
+
+Once :code:`./mach build` is run, the XPIDL parser will read this file, and give any warnings if the syntax is wrong. It will then auto-generate the C++ (or Rust) code for us. For this example the generated :code:`nsIComponentName` class will be located in:
+
+:code:`{obj-directory}/dist/include/nsIComponentName.h`
+
+It might be useful to check out what was automatically generated here, or see the existing `generated C++ header files on SearchFox <https://searchfox.org/mozilla-central/source/__GENERATED__/dist/>`_.
+
+Writing the C++ implementation
+------------------------------
+
+Now we have a definition for an interface, but no implementation. The interface could be backed by a JavaScript implementation using a JSM, but for this example we'll use a C++ implementation.
+
+Add the C++ sources to :code:`path/to/moz.build`
+
+.. code:: python
+
+    EXPORTS.mozilla += [
+        "ComponentName.h",
+    ]
+
+    UNIFIED_SOURCES += [
+        "ComponentName.cpp",
+    ]
+
+Now write the header: :code:`path/to/ComponentName.h`
+
+.. code:: c++
+
+    /* This Source Code Form is subject to the terms of the Mozilla Public
+     * License, v. 2.0. If a copy of the MPL was not distributed with this
+     * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+    #ifndef mozilla_nsComponentName_h__
+    #define mozilla_nsComponentName_h__
+
+    // This will pull in the header auto-generated by the .idl file:
+    // {obj-directory}/dist/include/nsIComponentName.h
+    #include "nsIComponentName.h"
+
+    // The implementation can be namespaced, while the XPCOM interface is globally namespaced.
+    namespace mozilla {
+
+    // Notice how the class name does not need to be prefixed, as it is defined in the
+    // `mozilla` namespace.
+    class ComponentName final : public nsIComponentName {
+      // This first macro includes the necessary information to use the base nsISupports.
+      // This includes the QueryInterface method.
+      NS_DECL_ISUPPORTS
+
+      // This second macro includes the declarations for the attributes. There is
+      // no need to duplicate these declarations.
+      //
+      // In our case it includes a declaration for the isAlive attribute:
+      //   GetIsAlive(bool *aIsAlive)
+      NS_DECL_NSICOMPONENTNAME
+
+     public:
+      ComponentName() = default;
+
+     private:
+      // A private destructor must be declared.
+      ~ComponentName() = default;
+    };
+
+    }  // namespace mozilla
+
+    #endif
+
+Now write the definitions: :code:`path/to/ComponentName.cpp`
+
+.. code:: c++
+
+    /* This Source Code Form is subject to the terms of the Mozilla Public
+     * License, v. 2.0. If a copy of the MPL was not distributed with this
+     * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+    #include "ComponentName.h"
+
+    namespace mozilla {
+
+    // Use the macro to inject all of the definitions for nsISupports.
+    NS_IMPL_ISUPPORTS(ComponentName, nsIComponentName)
+
+    // This is the actual implementation of the `isAlive` attribute. Note that the
+    // method name is somewhat different than the attribute. We specified "read-only"
+    // in the attribute, so only a getter, not a setter was defined for us. Here
+    // the name was adjusted to be `GetIsAlive`.
+    //
+    // Another common detail of implementing an XPIDL interface is that the return values
+    // are passed as out parameters. The methods are treated as fallible, and the return
+    // value is an `nsresult`. See the XPIDL documentation for the full nitty gritty
+    // details.
+    //
+    // A common way to know the exact function signature for a method implementation is
+    // to copy and paste from existing examples, or inspecting the generated file
+    // directly: {obj-directory}/dist/include/nsIComponentName.h
+    NS_IMETHODIMP
+    ComponentName::GetIsAlive(bool* aIsAlive) {
+      *aIsAlive = true;
+      return NS_OK;
+    }
+
+    } // namespace: mozilla
+
+Registering the component
+-------------------------
+
+At this point, the component should be correctly written, but it's not registered with the component system. In order to this, we'll need to create or modify the :code:`components.conf`.
+
+.. code:: bash
+
+    touch path/to/components.conf
+
+
+Now update the :code:`moz.build` to point to it.
+
+.. code:: python
+
+    XPCOM_MANIFESTS += [
+        "components.conf",
+    ]
+
+It is probably worth reading over :ref:`defining_xpcom_components`, but the following config will be sufficient to hook up our component to the :code:`Services` object.
+Services should also be added to ``tools/lint/eslint/eslint-plugin-mozilla/lib/services.json``.
+The easiest way to do that is to copy from ``<objdir>/xpcom/components/services.json``.
+
+.. code:: python
+
+    Classes = [
+        {
+            # This CID is the ID for component entries, and needs a separate UUID from
+            # the .idl file. Replace the Xs with a uuid from:
+            # http://mozilla.pettay.fi/cgi-bin/mozuuid.pl
+            'cid': '{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}',
+            'interfaces': ['nsIComponentName'],
+
+            # A contract ID is a human-readable identifier for an _implementation_ of
+            # an XPCOM interface.
+            #
+            # "@mozilla.org/process/environment;1"
+            #  ^^^^^^^^^^^^ ^^^^^^^ ^^^^^^^^^^^ ^
+            #  |            |       |           |
+            #  |            |       |           The version number, usually just 1.
+            #  |            |       Component name
+            #  |            Module
+            #  Domain
+            #
+            # This design goes back to a time when XPCOM was intended to be a generalized
+            # solution for the Gecko Runtime Environment (GRE). At this point most (if
+            # not all) of mozilla-central has an @mozilla domain.
+            'contract_ids': ['@mozilla.org/component-name;1'],
+
+            # This is the name of the C++ type that implements the interface.
+            'type': 'mozilla::ComponentName',
+
+            # The header file to pull in for the implementation of the interface.
+            'headers': ['path/to/ComponentName.h'],
+
+            # In order to hook up this interface to the `Services` object, we can
+            # provide the "js_name" parameter. This is an ergonomic way to access
+            # the component.
+            'js_name': 'componentName',
+        },
+    ]
+
+At this point the full :code:`moz.build` file should look like:
+
+.. code:: python
+
+    # -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+    # vim: set filetype=python:
+    # This Source Code Form is subject to the terms of the Mozilla Public
+    # License, v. 2.0. If a copy of the MPL was not distributed with this
+    # file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+    XPIDL_SOURCES += [
+        "nsIComponentName.idl",
+    ]
+
+    XPCOM_MANIFESTS += [
+        "components.conf",
+    ]
+
+    EXPORTS.mozilla += [
+        "ComponentName.h",
+    ]
+
+    UNIFIED_SOURCES += [
+        "ComponentName.cpp",
+    ]
+
+This completes the implementation of a basic XPCOM Interface using C++. The component should be available via the `Browser Console`_ or other chrome contexts.
+
+.. code:: javascript
+
+    console.log(Services.componentName.isAlive);
+    > true
diff --git a/xpcom/docs/xpidl.rst b/xpcom/docs/xpidl.rst
new file mode 100644
index 0000000000..41a039710b
--- /dev/null
+++ b/xpcom/docs/xpidl.rst
@@ -0,0 +1,402 @@
+XPIDL
+=====
+
+**XPIDL** is an Interface Description Language used to specify XPCOM interface
+classes.
+
+Interface Description Languages (IDL) are used to describe interfaces in a
+language- and machine-independent way. IDLs make it possible to define
+interfaces which can then be processed by tools to autogenerate
+language-dependent interface specifications.
+
+An xpidl file is essentially just a series of declarations. At the top level,
+we can define typedefs, native types, or interfaces. Interfaces may
+furthermore contain typedefs, natives, methods, constants, or attributes.
+Most declarations can have properties applied to them.
+
+Types
+-----
+
+There are three ways to make types: a typedef, a native, or an interface. In
+addition, there are a few built-in native types. The built-in native types
+are those listed under the type_spec production above. The following is the
+correspondence table:
+
+=========================== =============== =========================== ============================ ======================= =======================
+IDL Type                    Javascript Type C++ in parameter            C++ out parameter            Rust in parameter       Rust out parameter
+=========================== =============== =========================== ============================ ======================= =======================
+``boolean``                 boolean         ``bool``                    ``bool*``                    ``bool``                ``*mut bool``
+``char``                    string          ``char``                    ``char*``                    ``c_char``              ``*mut c_char``
+``double``                  number          ``double``                  ``double*``                  ``f64``                 ``*mut f64``
+``float``                   number          ``float``                   ``float*``                   ``f32``                 ``*mut f32``
+``long``                    number          ``int32_t``                 ``int32_t*``                 ``i32``                 ``*mut i32``
+``long long``               number          ``int64_t``                 ``int64_t*``                 ``i64``                 ``*mut i64``
+``octet``                   number          ``uint8_t``                 ``uint8_t*``                 ``u8``                  ``*mut u8``
+``short``                   number          ``uint16_t``                ``uint16_t*``                ``u16``                 ``*mut u16``
+``string`` [#strptr]_       string          ``const char*``             ``char**``                   ``*const c_char``       ``*mut *mut c_char``
+``unsigned long``           number          ``uint32_t``                ``uint32_t*``                ``u32``                 ``*mut u32``
+``unsigned long long``      number          ``uint64_t``                ``uint64_t*``                ``u64``                 ``*mut u64``
+``unsigned short``          number          ``uint16_t``                ``uint16_t*``                ``u16``                 ``*mut u16``
+``wchar``                   string          ``char16_t``                ``char16_t*``                ``i16``                 ``*mut i16``
+``wstring`` [#strptr]_      string          ``const char16_t*``         ``char16_t**``               ``*const i16``          ``*mut *mut i16``
+``MozExternalRefCountType`` number          ``MozExternalRefCountType`` ``MozExternalRefCountType*`` ``u32``                 ``*mut u32``
+``Array<T>`` [#array]_      array           ``const nsTArray<T>&``      ``nsTArray<T>&``             ``*const ThinVec<T>``   ``*mut ThinVec<T>``
+=========================== =============== =========================== ============================ ======================= =======================
+
+.. [#strptr]
+
+    Prefer using the string class types such as ``AString``, ``AUTF8String``
+    or ``ACString`` to this type. The behaviour of these types is documented
+    more in the :ref:`String Guide <stringguide.xpidl>`
+
+.. [#array]
+
+    The C++ or Rust exposed type ``T`` will be an owned variant. (e.g.
+    ``ns[C]String``, ``RefPtr<T>``, or ``uint32_t``)
+
+    ``string``, ``wstring``, ``[ptr] native`` and ``[ref] native`` are
+    unsupported as element types.
+
+
+In addition to this list, nearly every IDL file includes ``nsrootidl.idl`` in
+some fashion, which also defines the following types:
+
+======================= ======================= ======================= ======================= ======================= =======================
+IDL Type                Javascript Type         C++ in parameter        C++ out parameter       Rust in parameter       Rust out parameter
+======================= ======================= ======================= ======================= ======================= =======================
+``PRTime``              number                  ``uint64_t``            ``uint64_t*``           ``u64``                 ``*mut u64``
+``nsresult``            number                  ``nsresult``            ``nsresult*``           ``u32`` [#rsresult]_    ``*mut u32``
+``size_t``              number                  ``uint32_t``            ``uint32_t*``           ``u32``                 ``*mut u32``
+``voidPtr``             N/A                     ``void*``               ``void**``              ``*mut c_void``         ``*mut *mut c_void``
+``charPtr``             N/A                     ``char*``               ``char**``              ``*mut c_char``         ``*mut *mut c_char``
+``unicharPtr``          N/A                     ``char16_t*``           ``char16_t**``          ``*mut i16``            ``*mut *mut i16``
+``nsIDRef``             ID object               ``const nsID&``         ``nsID*``               ``*const nsID``         ``*mut nsID``
+``nsIIDRef``            ID object               ``const nsIID&``        ``nsIID*``              ``*const nsIID``        ``*mut nsIID``
+``nsCIDRef``            ID object               ``const nsCID&``        ``nsCID*``              ``*const nsCID``        ``*mut nsCID``
+``nsIDPtr``             ID object               ``const nsID*``         ``nsID**``              ``*const nsID``         ``*mut *mut nsID``
+``nsIIDPtr``            ID object               ``const nsIID*``        ``nsIID**``             ``*const nsIID``        ``*mut *mut nsIID``
+``nsCIDPtr``            ID object               ``const nsCID*``        ``nsCID**``             ``*const nsCID``        ``*mut *mut nsCID``
+``nsID``                N/A                     ``nsID``                ``nsID*``               N/A                     N/A
+``nsIID``               N/A                     ``nsIID``               ``nsIID*``              N/A                     N/A
+``nsCID``               N/A                     ``nsCID``               ``nsCID*``              N/A                     N/A
+``nsQIResult``          object                  ``void*``               ``void**``              ``*mut c_void``         ``*mut *mut c_void``
+``AUTF8String`` [#str]_ string                  ``const nsACString&``   ``nsACString&``         ``*const nsACString``   ``*mut nsACString``
+``ACString`` [#str]_    string                  ``const nsACString&``   ``nsACString&``         ``*const nsACString``   ``*mut nsACString``
+``AString`` [#str]_     string                  ``const nsAString&``    ``nsAString&``          ``*const nsAString``    ``*mut nsAString``
+``jsval``               any                     ``HandleValue``         ``MutableHandleValue``  N/A                     N/A
+``jsid``                N/A                     ``jsid``                ``jsid*``               N/A                     N/A
+``Promise``             Promise object          ``dom::Promise*``       ``dom::Promise**``      N/A                     N/A
+======================= ======================= ======================= ======================= ======================= =======================
+
+.. [#rsresult]
+
+    A bare ``u32`` is only for bare ``nsresult`` in/outparams in XPIDL. The
+    result should be wrapped as the ``nserror::nsresult`` type.
+
+.. [#str]
+
+    The behaviour of these types is documented more in the :ref:`String Guide
+    <stringguide.xpidl>`
+
+Typedefs in IDL are basically as they are in C or C++: you define first the
+type that you want to refer to and then the name of the type. Types can of
+course be one of the fundamental types, or any other type declared via a
+typedef, interface, or a native type.
+
+Native types are types which correspond to a given C++ type. Most native
+types are not scriptable: if it is not present in the list above, then it is
+certainly not scriptable (some of the above, particularly jsid, are not
+scriptable).
+
+The contents of the parentheses of a native type declaration (although native
+declarations without parentheses are parsable, I do not trust that they are
+properly handled by the xpidl handlers) is a string equivalent to the C++
+type. XPIDL itself does not interpret this string, it just literally pastes
+it anywhere the native type is used. The interpretation of the type can be
+modified by using the ``[ptr]`` or ``[ref]`` attributes on the native
+declaration. Other attributes are only intended for use in ``nsrootidl.idl``.
+
+WebIDL Interfaces
+~~~~~~~~~~~~~~~~~
+
+WebIDL interfaces are also valid XPIDL types. To declare a WebIDL interface in
+XPIDL, write:
+
+.. code-block:: JavaScript
+
+    webidl InterfaceName;
+
+WebIDL types will be passed as ``mozilla::dom::InterfaceName*`` when used as
+in-parameters, as ``mozilla::dom::InterfaceName**`` when used as out or
+inout-parameters, and as ``RefPtr<mozilla::dom::InterfaceName>`` when used as
+an array element.
+
+.. note::
+
+    Other WebIDL types (e.g. dictionaries, enums, and unions) are not currently
+    supported.
+
+Constants and CEnums
+~~~~~~~~~~~~~~~~~~~~
+
+Constants must be attached to an interface. The only constants supported are
+those which become integer types when compiled to source code; string constants
+and floating point constants are currently not supported.
+
+Often constants are used to describe a set of enum values. In cases like this
+the ``cenum`` construct can be used to group constants together. Constants
+grouped in a ``cenum`` will be reflected as-if they were declared directly on
+the interface, in Rust and Javascript code.
+
+.. code-block:: JavaScript
+
+   cenum MyCEnum : 8 {
+     eSomeValue,  // starts at 0
+     eSomeOtherValue,
+   };
+
+The number after the enum name, like ``: 8`` in the example above, defines the
+width of enum values with the given type. The cenum's type may be referenced in
+xpidl as ``nsIInterfaceName_MyCEnum``.
+
+Interfaces
+----------
+
+Interfaces are basically a collection of constants, methods, and attributes.
+Interfaces can inherit from one-another, and every interface must eventually
+inherit from ``nsISupports``.
+
+Interface Attributes
+~~~~~~~~~~~~~~~~~~~~
+
+Interfaces may have the following attributes:
+
+``uuid``
+````````
+
+The internal unique identifier for the interface. it must be unique, and the
+uuid must be generated when creating the interface. After that, it doesn't need
+to be changed any more.
+
+Online tools such as http://mozilla.pettay.fi/cgi-bin/mozuuid.pl can help
+generate UUIDs for new interfaces.
+
+``builtinclass``
+````````````````
+
+JavaScript classes are forbidden from implementing this interface. All child
+interfaces must also be marked with this property.
+
+``function``
+````````````
+
+The JavaScript implementation of this interface may be a function that is
+invoked on property calls instead of an object with the given property
+
+``scriptable``
+``````````````
+
+This interface is usable by JavaScript classes. Must inherit from a
+``scriptable`` interface.
+
+``rust_sync``
+`````````````
+
+This interface is safe to use from multiple threads concurrently. All child
+interfaces must also be marked with this property. Interfaces marked this way
+must be either non-scriptable or ``builtinclass``, and must use threadsafe
+reference counting.
+
+Interfaces marked as ``rust_sync`` will implement the ``Sync`` trait in Rust.
+For more details on what that means, read the trait's documentation:
+https://doc.rust-lang.org/nightly/std/marker/trait.Sync.html.
+
+Methods and Attributes
+~~~~~~~~~~~~~~~~~~~~~~
+
+Interfaces declare a series of attributes and methods. Attributes in IDL are
+akin to JavaScript properties, in that they are a getter and (optionally) a
+setter pair. In JavaScript contexts, attributes are exposed as a regular
+property access, while native code sees attributes as a Get and possibly a Set
+method.
+
+Attributes can be declared readonly, in which case setting causes an error to
+be thrown in script contexts and native contexts lack the Set method, by using
+the ``readonly`` keyword.
+
+To native code, on attribute declared ``attribute type foo;`` is syntactic
+sugar for the declaration of two methods ``type getFoo();`` and ``void
+setFoo(in type foo);``. If ``foo`` were declared readonly, the latter method
+would not be present.  Attributes support all of the properties of methods with
+the exception of ``optional_argc``, as this does not make sense for attributes.
+
+There are some special rules for attribute naming. As a result of vtable
+munging by the MSVC++ compiler, an attribute with the name ``IID`` is
+forbidden.  Also like methods, if the first character of an attribute is
+lowercase in IDL, it is made uppercase in native code only.
+
+Methods define a return type and a series of in and out parameters. When called
+from a JavaScript context, they invocation looks as it is declared for the most
+part; some parameter properties can adjust what the code looks like. The calls
+are more mangled in native contexts.
+
+An important attribute for methods and attributes is scriptability. A method or
+attribute is scriptable if it is declared in a ``scriptable`` interface and it
+lacks a ``noscript`` or ``notxpcom`` property. Any method that is not
+scriptable can only be accessed by native code. However, ``scriptable`` methods
+must contain parameters and a return type that can be translated to script: any
+native type, save a few declared in ``nsrootidl.idl`` (see above), may not be
+used in a scriptable method or attribute. An exception to the above rule is if
+a ``nsQIResult`` parameter has the ``iid_is`` property (a special case for some
+QueryInterface-like operations).
+
+Methods and attributes are mangled on conversion to native code. If a method is
+declared ``notxpcom``, the mangling of the return type is prevented, so it is
+called mostly as it looks. Otherwise, the return type of the native method is
+``nsresult``, and the return type acts as a final outparameter if it is not
+``void``.  The name is translated so that the first character is
+unconditionally uppercase; subsequent characters are unaffected. However, the
+presence of the ``binaryname`` property allows the user to select another name
+to use in native code (to avoid conflicts with other functions). For example,
+the method ``[binaryname(foo)] void bar();`` becomes ``nsresult Foo()`` in
+native code (note that capitalization is still applied). However, the
+capitalization is not applied when using ``binaryname`` with attributes; i.e.,
+``[binaryname(foo)] readonly attribute Quux bar;`` becomes ``Getfoo(Quux**)``
+in native code.
+
+The ``implicit_jscontext`` and ``optional_argc`` parameters are properties
+which help native code implementations determine how the call was made from
+script. If ``implicit_jscontext`` is present on a method, then an additional
+``JSContext* cx`` parameter is added just after the regular list which receives
+the context of the caller. If ``optional_argc`` is present, then an additional
+``uint8_t _argc`` parameter is added at the end which receives the number of
+optional arguments that were actually used (obviously, you need to have an
+optional argument in the first place). Note that if both properties are set,
+the ``JSContext* cx`` is added first, followed by the ``uint8_t _argc``, and
+then ending with return value parameter. Finally, as an exception to everything
+already mentioned, for attribute getters and setters the ``JSContext *cx``
+comes before any other arguments.
+
+Another native-only property is ``nostdcall``. Normally, declarations are made
+in the stdcall ABI on Windows to be ABI-compatible with COM interfaces. Any
+non-scriptable method or attribute with ``nostdcall`` instead uses the
+``thiscall`` ABI convention. Methods without this property generally use
+``NS_IMETHOD`` in their declarations and ``NS_IMETHODIMP`` in their definitions
+to automatically add in the stdcall declaration specifier on requisite
+compilers; those that use this method may use a plain ``nsresult`` instead.
+
+Another property, ``infallible``, is attribute-only. When present, it causes an
+infallible C++ getter function definition to be generated for the attribute
+alongside the normal fallible C++ getter declaration. It should only be used if
+the fallible getter will be infallible in practice (i.e. always return
+``NS_OK``) for all possible implementations. This infallible getter contains
+code that calls the fallible getter, asserts success, and returns the gotten
+value directly. The point of using this property is to make C++ code nicer -- a
+call to the infallible getter is more concise and readable than a call to the
+fallible getter. This property can only be used for attributes having built-in
+or interface types, and within classes that are marked with ``builtinclass``.
+The latter restriction is because C++ implementations of fallible getters can
+be audited for infallibility, but JS implementations can always throw (e.g. due
+to OOM).
+
+The ``must_use`` property is useful if the result of a method call or an
+attribute get/set should always (or usually) be checked, which is frequently
+the case.  (e.g. a method that opens a file should almost certainly have its
+result checked.) This property will cause ``[[nodiscard]]`` to be added to the
+generated function declarations, which means certain compilers (e.g. clang and
+GCC) will reports errors if these results are not used.
+
+Method Parameters
+~~~~~~~~~~~~~~~~~
+
+Each method parameter can be specified in one of three modes: ``in``, ``out``,
+or ``inout``. An ``out`` parameter is essentially an auxiliary return value,
+although these are moderately cumbersome to use from script contexts and should
+therefore be avoided if reasonable. An ``inout`` parameter is an in parameter
+whose value may be changed as a result of the method; these parameters are
+rather annoying to use and should generally be avoided if at all possible.
+
+``out`` and ``inout`` parameters are reflected as objects having the ``.value``
+property which contains the real value of the parameter; the ``value``
+attribute is missing in the case of ``out`` parameters and is initialized to
+the passed-in-value for ``inout`` parameters. The script code needs to set this
+property to assign a value to the parameter. Regular ``in`` parameters are
+reflected more or less normally, with numeric types all representing numbers,
+booleans as ``true`` or ``false``, the various strings (including ``AString``
+etc.) as a JavaScript string, and ``nsID`` types as a ``Components.ID``
+instance. In addition, the ``jsval`` type is translated as the appropriate
+JavaScript value (since a ``jsval`` is the internal representation of all
+JavaScript values), and parameters with the ``nsIVeriant`` interface have their
+types automatically boxed and unboxed as appropriate.
+
+The equivalent representations of all IDL types in native code is given in the
+earlier tables; parameters of type ``inout`` follow their ``out`` form. Native
+code should pay particular attention to not passing in null values for out
+parameters (although some parts of the codebase are known to violate this, it
+is strictly enforced at the JS<->native barrier).
+
+Representations of types additionally depend on some of the many types of
+properties they may have. The ``array`` property turns the parameter into an array;
+the parameter must also have a corresponding ``size_is`` property whose argument is
+the parameter that has the size of the array. In native code, the type gains
+another pointer indirection, and JavaScript arrays are used in script code.
+Script code callers can ignore the value of array parameter, but implementers
+must still set the values appropriately.
+
+.. note::
+
+    Prefer using the ``Array<T>`` builtin over the ``[array]`` attribute for
+    new code. It is more ergonomic to use from both JS and C++. In the future,
+    ``[array]`` may be deprecated and removed.
+
+The ``const`` and ``shared`` properties are special to native code. As its name
+implies, the ``const`` property makes its corresponding argument ``const``. The
+``shared`` property is only meaningful for ``out`` or ``inout`` parameters and
+it means that the pointer value should not be freed by the caller. Only simple
+native pointer types like ``string``, ``wstring``, and ``octetPtr`` may be
+declared shared.  The shared property also makes its corresponding argument
+const.
+
+The ``retval`` property indicates that the parameter is actually acting as the
+return value, and it is only the need to assign properties to the parameter
+that is causing it to be specified as a parameter. It has no effect on native
+code, but script code uses it like a regular return value. Naturally, a method
+which contains a ``retval`` parameter must be declared ``void``, and the
+parameter itself must be an ``out`` parameter and the last parameter.
+
+Other properties are the ``optional`` and ``iid_is`` property. The ``optional``
+property indicates that script code may omit the property without problems; all
+subsequent parameters must either by optional themselves or the retval
+parameter. Note that optional out parameters still pass in a variable for the
+parameter, but its value will be ignored. The ``iid_is`` parameter indicates
+that the real IID of an ``nsQIResult`` parameter may be found in the
+corresponding parameter, to allow script code to automatically unbox the type.
+
+Not all type combinations are possible. Native types with the various string
+properties are all forbidden from being used as an ``inout`` parameter or as an
+``array`` parameter. In addition, native types with the ``nsid`` property but
+lacking either a ``ptr`` or ``ref`` property are forbidden unless the method is
+``notxpcom`` and it is used as an ``in`` parameter.
+
+Ownership Rules
+```````````````
+
+For types that reference heap-allocated data (strings, arrays, interface
+pointers, etc), you must follow the XPIDL data ownership conventions in order
+to avoid memory corruption and security vulnerabilities:
+
+* For ``in`` parameters, the caller allocates and deallocates all data. If the
+  callee needs to use the data after the call completes, it must make a private
+  copy of the data, or, in the case of interface pointers, ``AddRef`` it.
+* For ``out`` parameters, the callee creates the data, and transfers ownership
+  to the caller. For buffers, the callee allocates the buffer with ``malloc``,
+  and the caller frees the buffer with ``free``. For interface pointers, the
+  callee does the ``AddRef`` on behalf of the caller, and the caller must call
+  ``Release``. This manual reference/memory management should be performed
+  using the ``getter_AddRefs`` and ``getter_Transfers`` helpers in new code.
+* For ``inout`` parameters, the callee must clean up the old data if it chooses
+  to replace it. Buffers must be deallocated with ``free``, and interface
+  pointers must be ``Release``'d. Afterwards, the above rules for ``out``
+  apply.
+* ``shared`` out-parameters should not be freed, as they are intended to refer
+  to constant string literals.