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File 3581-features-Add-documentation-for-features-support.patch of Package erlang (Revision 6bc8bdb9448218a96809a06da74b3d6e)

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From f487df72f153263910c20d7ad4cc84167ff4336d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cons=20T=20=C3=85hs?= <cons@erlang.org>
Date: Tue, 3 May 2022 17:03:25 +0200
Subject: [PATCH] [features] Add documentation for features support

---
 erts/doc/src/erl_cmd.xml                    |  24 +++
 erts/doc/src/erlang.xml                     |   5 +
 erts/doc/src/erlc_cmd.xml                   |  42 +++-
 lib/compiler/doc/src/compile.xml            |  34 ++-
 lib/stdlib/doc/src/Makefile                 |   3 +-
 lib/stdlib/doc/src/erl_features.xml         | 101 +++++++++
 lib/stdlib/doc/src/ref_man.xml              |   3 +-
 lib/stdlib/doc/src/specs.xml                |   1 +
 lib/stdlib/src/Makefile                     |   1 +
 system/doc/reference_manual/expressions.xml |  12 +-
 system/doc/reference_manual/features.xml    | 218 ++++++++++++++++++++
 system/doc/reference_manual/macros.xml      |  16 +-
 system/doc/reference_manual/part.xml        |   1 +
 system/doc/reference_manual/xmlfiles.mk     |   1 +
 14 files changed, 452 insertions(+), 10 deletions(-)
 create mode 100644 lib/stdlib/doc/src/erl_features.xml
 create mode 100644 system/doc/reference_manual/features.xml

diff --git a/erts/doc/src/erl_cmd.xml b/erts/doc/src/erl_cmd.xml
index 98412fcf25..42901d91db 100644
--- a/erts/doc/src/erl_cmd.xml
+++ b/erts/doc/src/erl_cmd.xml
@@ -300,6 +300,18 @@ $ <input>erl \
           console. Useful for running daemons and backgrounds processes. Implies
           <c><![CDATA[-noinput]]></c>.</p>
       </item>
+      <tag>
+        <marker id="disable-feature"></marker>
+        <c><![CDATA[-disable-feature feature]]></c>
+      </tag>
+      <item>
+        <p>Disables the
+	<seeguide
+            marker="system/reference_manual:features#features">feature</seeguide> <c><![CDATA[feature]]></c>
+        in the runtime system.  The special feature <c>all</c> can be
+        used to disable all non permanent features.
+      </p>
+      </item>
       <tag><c><![CDATA[-emu_args]]></c></tag>
       <item>
         <p>Useful for debugging. Prints the arguments sent to the emulator.</p>
@@ -329,6 +341,18 @@ $ <input>erl \
           Use the <c>configure</c> option <c>--enable-lock-counter</c> to
           build the lock-counter emulator.)</p>
       </item>
+      <tag>
+        <marker id="enable-feature"></marker>
+        <c><![CDATA[-enable-feature feature]]></c>
+      </tag>
+      <item>
+        <p>Enables the
+	<seeguide
+            marker="system/reference_manual:features#features">feature</seeguide> <c><![CDATA[feature]]></c>
+        in the runtime system.  The special feature <c>all</c> can be
+        used to enable all features.
+      </p>
+      </item>
       <tag><c><![CDATA[-env Variable Value]]></c></tag>
       <item>
         <p>Sets the host OS environment variable <c><![CDATA[Variable]]></c> to
diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml
index 62f73cf747..c7dd44c42c 100644
--- a/erts/doc/src/erlang.xml
+++ b/erts/doc/src/erlang.xml
@@ -3823,6 +3823,11 @@ is_process_alive(P2Pid),
           previous current code for <c><anno>Module</anno></c> will remain
           until the <c>on_load</c> call has finished.
           </item>
+          <tag>not_allowed</tag>
+          <item>The code in <c><anno>Binary</anno></c> has been
+          compiled with features that are currently not enabled in the
+          runtime system.
+          </item>
         </taglist>
         <warning>
           <p>This BIF is intended for the code server (see
diff --git a/erts/doc/src/erlc_cmd.xml b/erts/doc/src/erlc_cmd.xml
index f6baefed7d..074626d9c0 100644
--- a/erts/doc/src/erlc_cmd.xml
+++ b/erts/doc/src/erlc_cmd.xml
@@ -4,7 +4,7 @@
 <comref>
   <header>
     <copyright>
-      <year>1997</year><year>2021</year>
+      <year>1997</year><year>2022</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -142,6 +142,46 @@
         <p>Use the
 	<seecom marker="#compile_server">compile server</seecom>.</p>
       </item>
+      <tag>
+        <marker id="enable-feature"/>
+        <c>-enable-feature &lt;Feature&gt;</c>
+      </tag>
+      <item>
+        <p>Enables the
+	<seeguide
+            marker="system/reference_manual:features#features">feature</seeguide> <c>feature</c>
+        during compilation.  The special feature <c>all</c> can be
+        used to enable all features.
+      </p>
+      </item>
+      <tag>
+        <marker id="disable-feature"/>
+        <c>-disable-feature &lt;feature&gt;</c>
+      </tag>
+      <item>
+        <p>Disables the
+	<seeguide
+            marker="system/reference_manual:features#features">feature</seeguide> <c>feature</c>
+        during compilation.  The special feature <c>all</c> can be
+        used to disable all non permanent features.
+      </p>
+      </item>
+      <tag><c>-list-features</c></tag>
+      <item>
+        <p><marker id="list-features"/>
+        List short descriptions of the current available
+	<seeguide
+            marker="system/reference_manual:features#features">features</seeguide>.
+      </p>
+      </item>
+      <tag><c>-describe-feature &lt;feature&gt;</c></tag>
+      <item>
+        <p><marker id="describe-feature"/>
+          Show long description and history of
+	<seeguide
+            marker="system/reference_manual:features#features">feature</seeguide> <c>feature</c>.
+      </p>
+      </item>
       <tag><c>-M</c></tag>
       <item>
         <p>Produces a Makefile rule to track header dependencies. The
diff --git a/lib/compiler/doc/src/compile.xml b/lib/compiler/doc/src/compile.xml
index 28fc79e671..f61aa1ee1c 100644
--- a/lib/compiler/doc/src/compile.xml
+++ b/lib/compiler/doc/src/compile.xml
@@ -4,7 +4,7 @@
 <erlref>
   <header>
     <copyright>
-      <year>1996</year><year>2021</year>
+      <year>1996</year><year>2022</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -227,6 +227,26 @@
 	    </p>
           </item>
 
+          <tag><c>{feature, Feature, enable | disable}</c></tag>
+          <item>
+            <p><marker id="feature-option"/>
+            Enable (disable) the <seeguide
+            marker="system/reference_manual:features#features">feature</seeguide>
+            <c>Feature</c> during compilation.  The special feature
+            <c>all</c> can be used to enable (disable) all features.
+	    </p>
+            <note>
+              <p><marker id="feature-directive"/>
+              A feature can also be enabled (disabled) using the
+              <c>-feature(Feature, enable | disable).</c> module attribute.
+              Note that this directive can only be present in a prefix
+              of the file, before exports and function definitions.
+              This is the preferred method of enabling and disabling
+              features, since it is a local property of a module.
+              </p>
+            </note>
+          </item>
+
           <tag><c>makedep</c></tag>
           <item>
             <p>Produces a Makefile rule to track headers dependencies.
@@ -655,6 +675,18 @@ module.beam: module.erl \
 	      emit warnings for such variables.</p>   
           </item>
 
+	  <tag><c>warn_keywords</c></tag>
+          <item>
+	    <p><marker id="warn-keywords"/>
+              Emits warnings when the code contains atoms that are
+            used as keywords in some <seeguide
+            marker="system/reference_manual:features#features">feature</seeguide>.
+            When the feature is enabled, any occurrences will lead to
+            a syntax error.  To prevent this, the atom has to
+            be renamed or quoted.
+            </p>
+          </item>
+
 	  <tag><c>nowarn_unused_function</c></tag>
           <item>
             <p>Turns off warnings for unused local functions. Default 
diff --git a/lib/stdlib/doc/src/Makefile b/lib/stdlib/doc/src/Makefile
index 8e60498fd6..8cd2ceb53c 100644
--- a/lib/stdlib/doc/src/Makefile
+++ b/lib/stdlib/doc/src/Makefile
@@ -1,7 +1,7 @@
 #
 # %CopyrightBegin%
 #
-# Copyright Ericsson AB 1997-2021. All Rights Reserved.
+# Copyright Ericsson AB 1997-2022. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -48,6 +48,7 @@ XML_REF3_FILES = \
 	erl_error.xmlsrc \
 	erl_eval.xml \
 	erl_expand_records.xml \
+	erl_features.xml \
 	erl_id_trans.xml \
 	erl_internal.xml \
 	erl_lint.xml \
diff --git a/lib/stdlib/doc/src/erl_features.xml b/lib/stdlib/doc/src/erl_features.xml
new file mode 100644
index 0000000000..664a7858eb
--- /dev/null
+++ b/lib/stdlib/doc/src/erl_features.xml
@@ -0,0 +1,101 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE erlref SYSTEM "erlref.dtd">
+
+<erlref>
+  <header>
+    <copyright>
+      <year>2022</year><year>2022</year>
+      <holder>Ericsson AB. All Rights Reserved.</holder>
+    </copyright>
+    <legalnotice>
+      Licensed under the Apache License, Version 2.0 (the "License");
+      you may not use this file except in compliance with the License.
+      You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+      See the License for the specific language governing permissions and
+      limitations under the License.
+
+    </legalnotice>
+
+    <title>erl_features</title>
+    <prepared>Cons T &Aring;hs</prepared>
+    <responsible></responsible>
+    <docno>1</docno>
+    <approved></approved>
+    <checked></checked>
+    <date>2022-05-02</date>
+    <rev>B</rev>
+    <file>erl_features.xml</file>
+  </header>
+  <module since="OTP 25.0">erl_features</module>
+  <modulesummary>Feature support.</modulesummary>
+  <description>
+    <p>This module contains functions for supporting features that can
+    be enabled/disabled in Erlang.  It should be considered as mostly
+    for internal use, although there are some functions that might be
+    useful when writing tools.
+    <marker id="erl_features"/>
+    </p>
+  </description>
+
+  <datatypes>
+    <datatype>
+      <name name="feature"></name>
+    </datatype>
+    <datatype>
+      <name name="release"></name>
+    </datatype>
+    <datatype>
+      <name name="status"></name>
+    </datatype>
+    <datatype>
+      <name name="type"></name>
+    </datatype>
+  </datatypes>
+
+  <funcs>
+    <func>
+      <name name="all" arity="0" since="OTP 25.0"/>
+      <fsummary>Return all known features.</fsummary>
+      <desc>
+        <p>Return a list of all known features.</p>
+      </desc>
+    </func>
+
+    <func>
+      <name name="info" arity="1" since="OTP 25.0"/>
+      <fsummary>Return information about the given feature.</fsummary>
+      <desc>
+        <p>Return a map containing information about the given feature.</p>
+      </desc>
+    </func>
+
+    <func>
+      <name name="enabled" arity="0" since="OTP 25.0"/>
+      <fsummary>Return list of currently enabled features.</fsummary>
+      <desc>
+        <p>Return a list of the features that are currently enabled.
+        Note that the set of enabled is set during startup and can
+        then not be changed.</p>
+      </desc>
+    </func>
+
+    <func>
+      <name name="used" arity="1" since="OTP 25.0"/>
+      <fsummary>Return features used by a module or beam file.</fsummary>
+      <desc>
+        <p>Return the list of features enabled when compiling the
+        module.  The module need not be loaded, but is found if it
+        exists in the loadpath.  If not all features used by the
+        module are enabled in the runtime, loading the module is not
+        allowed.</p>
+      </desc>
+    </func>
+  </funcs>
+
+</erlref>
diff --git a/lib/stdlib/doc/src/ref_man.xml b/lib/stdlib/doc/src/ref_man.xml
index 042c080cf2..e63c455ec8 100644
--- a/lib/stdlib/doc/src/ref_man.xml
+++ b/lib/stdlib/doc/src/ref_man.xml
@@ -4,7 +4,7 @@
 <application xmlns:xi="http://www.w3.org/2001/XInclude">
   <header>
     <copyright>
-      <year>1996</year><year>2021</year>
+      <year>1996</year><year>2022</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -48,6 +48,7 @@
   <xi:include href="erl_error.xml"/>
   <xi:include href="erl_eval.xml"/>
   <xi:include href="erl_expand_records.xml"/>
+  <xi:include href="erl_features.xml"/>
   <xi:include href="erl_id_trans.xml"/>
   <xi:include href="erl_internal.xml"/>
   <xi:include href="erl_lint.xml"/>
diff --git a/lib/stdlib/doc/src/specs.xml b/lib/stdlib/doc/src/specs.xml
index 5f7e3555d8..9fdd0d1c21 100644
--- a/lib/stdlib/doc/src/specs.xml
+++ b/lib/stdlib/doc/src/specs.xml
@@ -15,6 +15,7 @@
   <xi:include href="../specs/specs_erl_error.xml"/>
   <xi:include href="../specs/specs_erl_eval.xml"/>
   <xi:include href="../specs/specs_erl_expand_records.xml"/>
+  <xi:include href="../specs/specs_erl_features.xml"/>
   <xi:include href="../specs/specs_erl_id_trans.xml"/>
   <xi:include href="../specs/specs_erl_internal.xml"/>
   <xi:include href="../specs/specs_erl_lint.xml"/>
diff --git a/lib/stdlib/src/Makefile b/lib/stdlib/src/Makefile
index b7b5ead962..369a448fe5 100644
--- a/lib/stdlib/src/Makefile
+++ b/lib/stdlib/src/Makefile
@@ -185,6 +185,7 @@ primary_bootstrap_compiler: \
   $(BOOTSTRAP_COMPILER)/ebin/epp.beam \
   $(BOOTSTRAP_COMPILER)/ebin/erl_anno.beam \
   $(BOOTSTRAP_COMPILER)/ebin/erl_error.beam \
+  $(BOOTSTRAP_COMPILER)/ebin/erl_features.beam \
   $(BOOTSTRAP_COMPILER)/ebin/erl_scan.beam \
   $(BOOTSTRAP_COMPILER)/ebin/erl_parse.beam \
   $(BOOTSTRAP_COMPILER)/ebin/erl_lint.beam \
diff --git a/system/doc/reference_manual/expressions.xml b/system/doc/reference_manual/expressions.xml
index b3259ec630..3a01066410 100644
--- a/system/doc/reference_manual/expressions.xml
+++ b/system/doc/reference_manual/expressions.xml
@@ -4,7 +4,7 @@
 <chapter>
   <header>
     <copyright>
-      <year>2003</year><year>2021</year>
+      <year>2003</year><year>2022</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -397,9 +397,13 @@ is_valid_signal(Signal) ->
   <section>
     <marker id="maybe"></marker>
     <title>Maybe</title>
-    <note><p><c>maybe</c> is an experimental new feature introduced in
-    OTP 25. By default, it is disabled. To enable <c>maybe</c>, use
-    compiler option <c>{enable_feature,maybe_expr}</c>.</p></note>
+    <note><p><c>maybe</c> is an experimental new <seeguide
+    marker="system/reference_manual:features#features">feature</seeguide>
+    introduced in OTP 25. By default, it is disabled. To enable
+    <c>maybe</c>, use compiler option
+    <c>{feature,maybe_expr,enable}</c>.  The feature must also be
+    enabled in the runtime using the <c>-enable-feature</c> option to
+    <c>erl</c>.</p></note>
 
     <code type="erl"><![CDATA[
 maybe
diff --git a/system/doc/reference_manual/features.xml b/system/doc/reference_manual/features.xml
new file mode 100644
index 0000000000..ed254db55a
--- /dev/null
+++ b/system/doc/reference_manual/features.xml
@@ -0,0 +1,218 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+  <header>
+    <copyright>
+      <year>2022</year><year>2022</year>
+      <holder>Ericsson AB. All Rights Reserved.</holder>
+    </copyright>
+    <legalnotice>
+      Licensed under the Apache License, Version 2.0 (the "License");
+      you may not use this file except in compliance with the License.
+      You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+      See the License for the specific language governing permissions and
+      limitations under the License.
+
+    </legalnotice>
+
+    <title>Features</title>
+    <prepared></prepared>
+    <docno></docno>
+    <date></date>
+    <rev></rev>
+    <file>features.xml</file>
+  </header>
+  <p>
+    <marker id="features"/>
+    Introduced in OTP 25, Erlang has the concept of selectable features.
+    A feature can change, add or remove behaviour of the language and/or
+    runtime system.  Examples can include
+  </p>
+  <list>
+    <item>Adding new syntactical constructs to the language</item>
+    <item>Change the semantics of an existing construct</item>
+    <item>Change the behaviour of some runtime aspect</item>
+  </list>
+  <p>
+    A feature will start out with a status of experimental part of OTP,
+    making it possible to try out for users and give feedback.  The
+    possibility to try out features is enabled by options to the
+    compiler, directives in a module and options to the runtime system.
+    Even when a feature is not experimental it will still be possible to
+    enable or disable it.  This makes it possible to adapt a code base
+    at a suitable pace instead of being forced when changing to a new
+    release.
+  </p>
+  <p>
+    The status of a feature will eventually end up as being either a
+    permanent part of OTP or rejected, being removed and no longer
+    selectable.
+  </p>
+
+  <section>
+    <title>Life cycle of features</title>
+    <p>A feature is in one of four possible states:</p>
+    <taglist>
+      <tag>Experimental</tag>
+      <item>The initial state, is meant for trying out and collecting
+      feedback.  The feature can be enabled but is disabled by
+      default.</item>
+      <tag>Approved</tag>
+      <item>The feature has been finalised and is now part of OTP.  By
+      default it is enabled, but can be disabled.</item>
+      <tag>Permanent</tag>
+      <item>The feature is now a permanent part of OTP.  It can no
+      longer be disabled.</item>
+      <tag>Rejected</tag>
+      <item>The feature never reached the approved state and will not
+      be part of OTP.  It cannot be enabled.</item>
+    </taglist>
+    <p>
+      After leaving the experimental state, a feature can enter any of
+      the other three states, and if the next state is approved, the
+      feature will eventually end up in the permanent state.  A feature
+      can change state only in connection with a release.
+    </p>
+    <p>
+      A feature may be in the approved state for several releases.
+    </p>
+    <table>
+      <row>
+        <cell>State</cell>
+        <cell>Default</cell>
+        <cell>Configurable</cell>
+        <cell>Available</cell>
+      </row>
+      <row>
+        <cell>Experimental</cell>
+        <cell>disabled</cell>
+        <cell>yes</cell>
+        <cell>yes</cell>
+      </row>
+      <row>
+        <cell>Approved</cell>
+        <cell>enabled</cell>
+        <cell>yes</cell>
+        <cell>yes</cell>
+      </row>
+      <row>
+        <cell>Permanent</cell>
+        <cell>enabled</cell>
+        <cell>no</cell>
+        <cell>yes</cell>
+      </row>
+      <row>
+        <cell>Rejected</cell>
+        <cell>disabled</cell>
+        <cell>no</cell>
+        <cell>no</cell>
+      </row>
+      <tcaption>Feature States</tcaption>
+    </table>
+    <list>
+      <item>Being configurable means the possibility to enable or
+      disable the feature by means of compiler options and directives
+      in the file being compiled.</item>
+      <item>Being available can be seen using the
+      <c>FEATURE_AVAILABLE</c> macro.</item>
+    </list>
+  </section>
+
+  <section>
+    <title>Enabling and Disabling Features</title>
+    <p>To use a feature that is in the experimental state, it has to
+    be enabled during compilation.  This can be done in a number of
+    different ways:
+    </p>
+    <taglist>
+      <tag>Options to <c>erlc</c></tag>
+      <item>Options <seecom
+      marker="erts:erlc#enable-feature"><c>-enable-feature</c></seecom>
+      and <seecom
+      marker="erts:erlc#disable-feature"><c>-disable-feature</c></seecom>
+      can be used to enable or disable individal features.</item>
+      <tag>Compiler options</tag>
+      <item>The compiler option <seeerl
+      marker="compiler:compile#feature-option"><c>{feature,
+      &lt;feature&gt;, enable|disable}</c></seeerl> can be used either
+      as a <c>+&lt;term&gt;</c> option to <c>erlc</c>, in the options
+      argument to function in the <c>compile</c> module, or in the
+      <c>-compile(..)</c> attribute in a file.</item>
+      <tag>The feature directive</tag>
+      <item>Inside a prefix of a module, one can use a <seeerl
+      marker="compiler:compile#feature-directive"><c>-feature(&lt;feature&gt;,
+      enable|disable)</c></seeerl> directive.  This is the preferred
+      method of enabling and disabling features.</item>
+    </taglist>
+    <p>
+      Note that to load a module compiled with features enabled, the
+      corresponding features must be enabled in the runtime.  This
+      is done using options <seecom
+      marker="erts:erl#enable-feature"><c>-enable-feature</c></seecom>
+      and <seecom
+      marker="erts:erl#disable-feature"><c>-disable-feature</c></seecom>
+      to <c>erl</c>.  This is to allow the possibility to prevent
+      the use of experimental features in, e.g., production.  This
+      will catch experimental features used in both own and third
+      party components.  An active choice to use experimental
+      features must be done.
+    </p>
+  </section>
+
+  <section>
+    <title>Preprocessor Additions</title>
+    <p>
+      To allow for conditional compilation during transitioning of a
+      code base and/or trying out experimental features <seeguide
+      marker="system/reference_manual:macros#predefined-macros">feature</seeguide>
+      <c>predefined macros</c> <c>?FEATURE_AVAILABLE(Feature)</c> and
+      <c>?FEATURE_ENABLED(Feature)</c> are available.
+    </p>
+  </section>
+
+  <section>
+    <title>Information about Existing Features</title>
+    <p>
+    The module <c>erl_features</c> <seeerl
+    marker="stdlib:erl_features"><c>erl_features</c></seeerl> exports
+    a number of functions that can be used to obtain information about
+    current features as well as the features used when compiling a
+    module.
+    </p>
+    <p>One can also use the <c>erlc</c> options <seecom
+    marker="erts:erlc#list-features"><c>-list-features</c></seecom>
+    and <seecom
+    marker="erts:erlc#describe-feature"><c>-describe-feature
+    &lt;feature&gt;</c></seecom> to get information about existing
+    features.
+    </p>
+    <p>
+      Additionally, there is the compiler option
+      <seeerl
+      marker="compiler:compile#warn-keywords"><c>warn_keywords</c></seeerl>
+      that can be used to find atoms in the code base that might
+      collide with keywords in features not yet enabled.
+    </p>
+  </section>
+
+  <section>
+    <title>Existing Features</title>
+    <p>
+      The following configurable features exist:
+    </p>
+    <taglist>
+      <tag><c>maybe_expr</c> (experimental)</tag>
+      <item>
+        Implementation of the <seeguide
+        marker="expressions#maybe"><c>maybe</c></seeguide> expression
+        proposed in <url href="https://www.erlang.org/eeps/eep-0049">EEP 49</url>.</item>
+    </taglist>
+  </section>
+</chapter>
diff --git a/system/doc/reference_manual/macros.xml b/system/doc/reference_manual/macros.xml
index 0943dbd56d..ffa5373613 100644
--- a/system/doc/reference_manual/macros.xml
+++ b/system/doc/reference_manual/macros.xml
@@ -4,7 +4,7 @@
 <chapter>
   <header>
     <copyright>
-      <year>2003</year><year>2021</year>
+      <year>2003</year><year>2022</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -134,7 +134,8 @@ bar(X) ->
 
   <section>
     <title>Predefined Macros</title>
-    <p>The following macros are predefined:</p>
+    <p>
+       The following macros are predefined:</p>
     <taglist>
       <tag><c>?MODULE</c></tag>
       <item>The name of the current module.</item>
@@ -155,6 +156,17 @@ bar(X) ->
         application is part of, as an integer. For details, see
         <seemfa marker="erts:erlang#system_info/1"><c>erlang:system_info(otp_release)</c></seemfa>.
         This macro was introduced in OTP release 21.</item>
+        <tag><c>?FEATURE_AVAILABLE(Feature)</c></tag>
+        <item>Expands to <c>true</c> if the <seeguide
+        marker="system/reference_manual:features#features">feature</seeguide>
+        <c>Feature</c> is available.  The feature might or might not
+        be enabled.  This macro was introduced with OTP release
+        25.</item>
+        <tag><c>?FEATURE_ENABLED(Feature)</c></tag>
+        <item>Expands to <c>true</c> if the <seeguide
+        marker="system/reference_manual:features#features">feature</seeguide>
+        <c>Feature</c> is enabled.  This macro was introduced with OTP
+        release 25.</item>
     </taglist>
   </section>
 
diff --git a/system/doc/reference_manual/part.xml b/system/doc/reference_manual/part.xml
index 6076a87bc3..ec2e3e0306 100644
--- a/system/doc/reference_manual/part.xml
+++ b/system/doc/reference_manual/part.xml
@@ -40,6 +40,7 @@
   <xi:include href="macros.xml"/>
   <xi:include href="records.xml"/>
   <xi:include href="errors.xml"/>
+  <xi:include href="features.xml"/>
   <xi:include href="processes.xml"/>
   <xi:include href="distributed.xml"/>
   <xi:include href="code_loading.xml"/>
diff --git a/system/doc/reference_manual/xmlfiles.mk b/system/doc/reference_manual/xmlfiles.mk
index af1c763923..8e2af09699 100644
--- a/system/doc/reference_manual/xmlfiles.mk
+++ b/system/doc/reference_manual/xmlfiles.mk
@@ -27,6 +27,7 @@ REF_MAN_CHAPTER_FILES = \
 	macros.xml \
 	records.xml \
 	errors.xml \
+	features.xml \
 	processes.xml \
 	distributed.xml \
 	code_loading.xml \
-- 
2.35.3

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File 3581-features-Add-documentation-for-features-support.patch of Package erlang (Revision 6bc8bdb9448218a96809a06da74b3d6e)

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