Help: Overhaul and expand the Using Dependencies Guide

The guide previously only focused on the find_package() command,
with a bias towards libraries. FetchContent was not mentioned at all.
Reorganise and update the existing content. Add new sections to cover
providing dependencies with FetchContent and dependency providers.
Improve discoverability of the guide by mentioning it at the beginning
of the find_package(), FetchContent and dependency provider docs.

Help/command/cmake_language.rst

@@ -236,6 +236,9 @@ Dependency Providers
 
 .. versionadded:: 3.24
 
+.. note:: A high-level introduction to this feature can be found in the
+          :ref:`Using Dependencies Guide <dependency_providers_overview>`.
+
 .. code-block:: cmake
 
   cmake_language(SET_DEPENDENCY_PROVIDER <command>

Help/command/find_package.rst

@@ -11,6 +11,12 @@ find_package
 
    .. contents::
 
+.. note:: The :guide:`Using Dependencies Guide` provides a high-level
+  introduction to this general topic. It provides a broader overview of
+  where the ``find_package()`` command fits into the bigger picture,
+  including its relationship to the :module:`FetchContent` module.
+  The guide is recommended pre-reading before moving on to the details below.
+
 Find a package (usually provided by something external to the project),
 and load its package-specific details.  Calls to this command can also
 be intercepted by :ref:`dependency providers <dependency_providers>`.

Help/guide/using-dependencies/index.rst

@@ -8,193 +8,408 @@ Using Dependencies Guide
 Introduction
 ============
 
-For developers wishing to use CMake to consume a third
-party binary package, there are multiple possibilities
-regarding how to optimally do so, depending on how
-CMake-aware the third-party library is.
-
-CMake files provided with a software package contain
-instructions for finding each build dependency.  Some
-build dependencies are optional in that the build may
-succeed with a different feature set if the dependency
-is missing, and some dependencies are required.  CMake
-searches well-known locations for each dependency, and
-the provided software may supply additional hints or
-locations to CMake to find each dependency.
-
-If a required dependency is not found by
-:manual:`cmake(1)`, the cache is populated with an entry
-which contains a ``NOTFOUND`` value.  This value can be
-replaced by specifying it on the command line, or in
-the :manual:`ccmake(1)` or :manual:`cmake-gui(1)` tool.
-See the :guide:`User Interaction Guide` for
-more about setting cache entries.
-
-Libraries providing Config-file packages
-========================================
-
-The most convenient way for a third-party to provide library
-binaries for use with CMake is to provide
-:ref:`Config File Packages`.  These packages are text files
-shipped with the library which instruct CMake how to use the
-library binaries and associated headers, helper tools and
-CMake macros provided by the library.
-
-The config files can usually be found in a directory whose
-name matches the pattern ``lib/cmake/<PackageName>``, though
-they may be in other locations instead.  The
-``<PackageName>`` corresponds to use in CMake code with the
-:command:`find_package` command such as
-``find_package(PackageName REQUIRED)``.
-
-The ``lib/cmake/<PackageName>`` directory will contain a
-file which is either named ``<PackageName>Config.cmake``
-or ``<PackageName>-config.cmake``.  This is the entry point
+Projects will frequently depend on other projects, assets, and artifacts.
+CMake provides a number of ways to incorporate such things into the build.
+Projects and users have the flexibility to choose between methods that
+best suit their needs.
+
+The primary methods of bringing dependencies into the build are the
+:command:`find_package` command and the :module:`FetchContent` module.
+The :module:`FindPkgConfig` module is also sometimes used, although it
+lacks some of the integration of the other two and is not discussed any
+further in this guide.
+
+Dependencies can also be made available by a custom
+:ref:`dependency provider <dependency_providers>`.
+This might be a third party package manager, or it might be custom code
+implemented by the developer.  Dependency providers co-operate with the
+primary methods mentioned above to extend their flexibility.
+
+.. _prebuilt_find_package:
+
+Using Pre-built Packages With ``find_package()``
+================================================
+
+A package needed by the project may already be built and available at some
+location on the user's system.  That package might have also been built by
+CMake, or it could have used a different build system entirely.  It might
+even just be a collection of files that didn't need to be built at all.
+CMake provides the :command:`find_package` command for these scenarios.
+It searches well-known locations, along with additional hints and paths
+provided by the project or user.  It also supports package components and
+packages being optional.  Result variables are provided to allow the project
+to customize its own behavior according to whether the package or specific
+components were found.
+
+In most cases, projects should generally use the :ref:`basic signature`.
+Most of the time, this will involve just the package name, maybe a version
+constraint, and the ``REQUIRED`` keyword if the dependency is not optional.
+A set of package components may also be specified.
+
+.. code-block:: cmake
+  :caption: Examples of ``find_package()`` basic signature
+
+  find_package(Catch2)
+  find_package(GTest REQUIRED)
+  find_package(Boost 1.79 COMPONENTS date_time)
+
+The :command:`find_package` command supports two main methods for carrying
+out the search:
+
+**Config mode**
+  With this method, the command looks for files that are typically provided
+  by the package itself.  This is the more reliable method of the two, since
+  the package details should always be in sync with the package.
+
+**Module mode**
+  Not all packages are CMake-aware. Many don't provide the files needed to
+  support config mode.  For such cases, a Find module file can be provided
+  separately, either by the project or by CMake.  A Find module is typically
+  a heuristic implementation which knows what the package normally provides
+  and how to present that package to the project.  Since Find modules are
+  usually distributed separately from the package, they are not as reliable.
+  They are typically maintained separately, and they are likely to follow
+  different release schedules, so they can easily become out-of-date.
+
+Depending on the arguments used, :command:`find_package` may use one or both
+of the above methods.  By restricting the options to just the basic signature,
+both config mode and module mode can be used to satisfy the dependency.
+The presence of other options may restrict the call to using only one of the
+two methods, potentially reducing the command's ability to find the dependency.
+See the :command:`find_package` documentation for full details about this
+complex topic.
+
+For both search methods, the user can also set cache variables on the
+:manual:`cmake(1)` command line or in the :manual:`ccmake(1)` or
+:manual:`cmake-gui(1)` UI tools to influence and override where to find
+packages. See the :ref:`User Interaction Guide <Setting Build Variables>`
+for more on how to set cache variables.
+
+.. _Libraries providing Config-file packages:
+
+Config-file packages
+--------------------
+
+The preferred way for a third party to provide executables, libraries,
+headers, and other files for use with CMake is to provide
+:ref:`config files <Config File Packages>`.  These are text files shipped
+with the package, which define CMake targets, variables, commands, and so on.
+The config file is an ordinary CMake script, which is read in by the
+:command:`find_package` command.
+
+The config files can usually be found in a directory whose name matches the
+pattern ``lib/cmake/<PackageName>``, although they may be in other locations
+instead (see :ref:`search procedure`).  The ``<PackageName>`` is usually the
+first argument to the :command:`find_package` command, and it may even be the
+only argument.  Alternative names can also be specified with the ``NAMES``
+option:
+
+.. code-block:: cmake
+  :caption: Providing alternative names when finding a package
+
+  find_package(SomeThing
+    NAMES
+      SameThingOtherName   # Another name for the package
+      SomeThing            # Also still look for its canonical name
+  )
+
+The config file must be named either ``<PackageName>Config.cmake`` or
+``<LowercasePackageName>-config.cmake`` (the former is used for the remainder
+of this guide, but both are supported).  This file is the entry point
 to the package for CMake.  A separate optional file named
-``<PackageName>ConfigVersion.cmake`` may also exist in the
-directory.  This file is used by CMake to determine whether
-the version of the third party package satisfies uses of the
-:command:`find_package` command which specify version
-constraints.  It is optional to specify a version when using
-:command:`find_package`, even if a ``ConfigVersion`` file is
-present.
-
-If the ``Config.cmake`` file is found and the
-optionally-specified version is satisfied, then the CMake
-:command:`find_package` command considers the package to be
-found and the entire library package is assumed to be
-complete as designed.
-
-There may be additional files providing CMake macros or
-:ref:`imported targets` for you to use.  CMake does not
-enforce any naming convention for these
-files.  They are related to the primary ``Config`` file by
-use of the CMake :command:`include` command.
-
-:guide:`Invoking CMake <User Interaction Guide>` with the
-intent of using a package of third party binaries requires
-that cmake :command:`find_package` commands succeed in finding
-the package.  If the location of the package is in a directory
-known to CMake, the :command:`find_package` call should
-succeed.  The directories known to cmake are platform-specific.
-For example, packages installed on Linux with a standard
-system package manager will be found in the ``/usr`` prefix
-automatically.  Packages installed in ``Program Files`` on
-Windows will similarly be found automatically.
-
-Packages which are not found automatically are in locations
-not predictable to CMake such as ``/opt/mylib`` or
-``$HOME/dev/prefix``.  This is a normal situation and CMake
-provides several ways for users to specify where to find
-such libraries.
+``<PackageName>ConfigVersion.cmake`` or
+``<LowercasePackageName>-config-version.cmake`` may also exist in the same
+directory.  This file is used by CMake to determine whether the version of
+the package satisfies any version constraint included in the call to
+:command:`find_package`.  It is optional to specify a version when calling
+:command:`find_package`, even if a ``<PackageName>ConfigVersion.cmake``
+file is present.
+
+If the ``<PackageName>Config.cmake`` file is found and any version constraint
+is satisfied, the :command:`find_package` command considers the package to be
+found, and the entire package is assumed to be complete as designed.
+
+There may be additional files providing CMake commands or
+:ref:`imported targets` for you to use.  CMake does not enforce any naming
+convention for these files.  They are related to the primary
+``<PackageName>Config.cmake`` file by use of the CMake :command:`include`
+command.  The ``<PackageName>Config.cmake`` file would typically include
+these for you, so they won't usually require any additional step other than
+the call to :command:`find_package`.
+
+If the location of the package is in a
+:ref:`directory known to CMake <search procedure>`, the
+:command:`find_package` call should succeed.  The directories known to CMake
+are platform-specific.  For example, packages installed on Linux with a
+standard system package manager will be found in the ``/usr`` prefix
+automatically.  Packages installed in ``Program Files`` on Windows will
+similarly be found automatically.
+
+Packages will not be found automatically without help if they are in
+locations not known to CMake, such as ``/opt/mylib`` or ``$HOME/dev/prefix``.
+This is a normal situation, and CMake provides several ways for users to
+specify where to find such libraries.
 
 The :variable:`CMAKE_PREFIX_PATH` variable may be
 :ref:`set when invoking CMake <Setting Build Variables>`.
-It is treated as a list of paths to search for
-:ref:`Config File Packages`.  A package installed in
-``/opt/somepackage`` will typically install config files
-such as
+It is treated as a list of base paths in which to search for
+:ref:`config files <Config File Packages>`.  A package installed in
+``/opt/somepackage`` will typically install config files such as
 ``/opt/somepackage/lib/cmake/somePackage/SomePackageConfig.cmake``.
 In that case, ``/opt/somepackage`` should be added to
 :variable:`CMAKE_PREFIX_PATH`.
 
-The environment variable ``CMAKE_PREFIX_PATH`` may also be
-populated with prefixes to search for packages.  Like the
-``PATH`` environment variable, this is a list and needs to use
-the platform-specific environment variable list item separator
-(``:`` on Unix and ``;`` on Windows).
-
-The :variable:`CMAKE_PREFIX_PATH` variable provides convenience
-in cases where multiple prefixes need to be specified, or when
-multiple different package binaries are available in the same
-prefix.  Paths to packages may also be specified by setting
-variables matching ``<PackageName>_DIR``, such as
-``SomePackage_DIR``.  Note that this is not a prefix but should
-be a full path to a directory containing a config-style package
-file, such as ``/opt/somepackage/lib/cmake/SomePackage/`` in
-the above example.
-
-Imported Targets from Packages
-==============================
-
-A third-party package which provides config-file packages may
-also provide :ref:`Imported targets`. These will be
-specified in files containing configuration-specific file
-paths relevant to the package, such as debug and release
-versions of libraries.
-
-Often the third-party package documentation will point out the
-names of imported targets available after a successful
-``find_package`` for a library.  Those imported target names
-can be used with the :command:`target_link_libraries` command.
-
-A complete example which makes a simple use of a third party
-library might look like:
+The environment variable ``CMAKE_PREFIX_PATH`` may also be populated with
+prefixes to search for packages.  Like the ``PATH`` environment variable,
+this is a list, but it needs to use the platform-specific environment variable
+list item separator (``:`` on Unix and ``;`` on Windows).
+
+The :variable:`CMAKE_PREFIX_PATH` variable provides convenience in cases
+where multiple prefixes need to be specified, or when multiple packages
+are available under the same prefix.  Paths to packages may also be
+specified by setting variables matching ``<PackageName>_DIR``, such as
+``SomePackage_DIR``.  Note that this is not a prefix, but should be a full
+path to a directory containing a config-style package file, such as
+``/opt/somepackage/lib/cmake/SomePackage`` in the above example.
+See the :command:`find_package` documentation for other CMake variables and
+environment variables that can affect the search.
+
+.. _Libraries not Providing Config-file Packages:
+
+Find Module Files
+-----------------
+
+Packages which do not provide config files can still be found with the
+:command:`find_package` command, if a ``FindSomePackage.cmake`` file is
+available.  These Find module files are different to config files in that:
+
+#. Find module files should not be provided by the package itself.
+#. The availability of a ``Find<PackageName>.cmake`` file does not indicate
+   the availability of the package, or any particular part of the package.
+#. CMake does not search the locations specified in the
+   :variable:`CMAKE_PREFIX_PATH` variable for ``Find<PackageName>.cmake``
+   files.  Instead, CMake searches for such files in the locations given
+   by the :variable:`CMAKE_MODULE_PATH` variable.  It is common for users to
+   set the :variable:`CMAKE_MODULE_PATH` when running CMake, and it is common
+   for CMake projects to append to :variable:`CMAKE_MODULE_PATH` to allow use
+   of local Find module files.
+#. CMake ships ``Find<PackageName>.cmake`` files for some
+   :manual:`third party packages <cmake-modules(7)>`.  These files are a
+   maintenance burden for CMake, and it is not unusual for these to fall
+   behind the latest releases of the packages they are associated with.
+   In general, new Find modules are not added to CMake any more.  Projects
+   should encourage the upstream packages to provide a config file where
+   possible.  If that is unsuccessful, the project should provide its own
+   Find module for the package.
+
+See :ref:`Find Modules` for a detailed discussion of how to write a
+Find module file.
+
+.. _Imported Targets from Packages:
+
+Imported Targets
+----------------
+
+Both config files and Find module files can define :ref:`Imported targets`.
+These will typically have names of the form ``SomePrefix::ThingName``.
+Where these are available, the project should prefer to use them instead of
+any CMake variables that may also be provided.  Such targets typically carry
+usage requirements and apply things like header search paths, compiler
+definitions, etc. automatically to other targets that link to them (e.g. using
+:command:`target_link_libraries`).  This is both more robust and more
+convenient than trying to apply the same things manually using variables.
+Check the documentation for the package or Find module to see what imported
+targets it defines, if any.
+
+Imported targets should also encapsulate any configuration-specific paths.
+This includes the location of binaries (libraries, executables), compiler
+flags, and any other configuration-dependent quantities.  Find modules may
+be less reliable in providing these details than config files.
+
+A complete example which finds a third party package and uses a library
+from it might look like the following:
 
 .. code-block:: cmake
 
-    cmake_minimum_required(VERSION 3.10)
-    project(MyExeProject VERSION 1.0.0)
-
-    find_package(SomePackage REQUIRED)
-    add_executable(MyExe main.cpp)
-    target_link_libraries(MyExe PRIVATE SomePrefix::LibName)
-
-See :manual:`cmake-buildsystem(7)` for further information
-about developing a CMake buildsystem.
-
-Libraries not Providing Config-file Packages
---------------------------------------------
-
-Third-party libraries which do not provide config-file packages
-can still be found with the :command:`find_package` command, if
-a ``FindSomePackage.cmake`` file is available.
-
-These module-file packages are different to config-file packages
-in that:
-
-#. They should not be provided by the third party, except
-   perhaps in the form of documentation
-#. The availability of a ``Find<PackageName>.cmake`` file does
-   not indicate the availability of the binaries themselves.
-#. CMake does not search the :variable:`CMAKE_PREFIX_PATH` for
-   ``Find<PackageName>.cmake`` files.  Instead CMake searches
-   for such files in the :variable:`CMAKE_MODULE_PATH`
-   variable. It is common for users to set the
-   :variable:`CMAKE_MODULE_PATH` when running CMake, and it is
-   common for CMake projects to append to
-   :variable:`CMAKE_MODULE_PATH` to allow use of local
-   module-file packages.
-#. CMake ships ``Find<PackageName>.cmake`` files for some
-   :manual:`third party packages <cmake-modules(7)>`
-   for convenience in cases where the third party does
-   not provide config-file packages directly.  These files are
-   a maintenance burden for CMake, so new Find modules are
-   generally not added to CMake anymore.  Third-parties should
-   provide config file packages instead of relying on a Find
-   module to be provided by CMake.
-
-Module-file packages may also provide :ref:`Imported targets`.
-A complete example which finds such a package might look
-like:
+  cmake_minimum_required(VERSION 3.10)
+  project(MyExeProject VERSION 1.0.0)
+
+  # Make project-provided Find modules available
+  list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+
+  find_package(SomePackage REQUIRED)
+  add_executable(MyExe main.cpp)
+  target_link_libraries(MyExe PRIVATE SomePrefix::LibName)
+
+Note that the above call to :command:`find_package` could be resolved by
+a config file or a Find module.  It uses only the basic arguments supported
+by the :ref:`basic signature`.  A ``FindSomePackage.cmake`` file in the
+``${CMAKE_CURRENT_SOURCE_DIR}/cmake`` directory would allow the
+:command:`find_package` command to succeed using module mode, for example.
+If no such module file is present, the system would be searched for a config
+file.
+
+
+Downloading And Building From Source With ``FetchContent``
+==========================================================
+
+Dependencies do not necessarily have to be pre-built in order to use them
+with CMake.  They can be built from sources as part of the main project.
+The :module:`FetchContent` module provides functionality to download
+content (typically sources, but can be anything) and add it to the main
+project if the dependency also uses CMake.  The dependency's sources will
+be built along with the rest of the project, just as though the sources were
+part of the project's own sources.
+
+The general pattern is that the project should first declare all the
+dependencies it wants to use, then ask for them to be made available.
+The following demonstrates the principle (see :ref:`fetch-content-examples`
+for more):
 
 .. code-block:: cmake
 
-    cmake_minimum_required(VERSION 3.10)
-    project(MyExeProject VERSION 1.0.0)
+  include(FetchContent)
+  FetchContent_Declare(
+    googletest
+    GIT_REPOSITORY https://github.com/google/googletest.git
+    GIT_TAG        703bd9caab50b139428cea1aaff9974ebee5742e # release-1.10.0
+  )
+  FetchContent_Declare(
+    Catch2
+    GIT_REPOSITORY https://github.com/catchorg/Catch2.git
+    GIT_TAG        de6fe184a9ac1a06895cdd1c9b437f0a0bdf14ad # v2.13.4
+  )
+  FetchContent_MakeAvailable(googletest Catch2)
+
+Various download methods are supported, including downloading and extracting
+archives from a URL (a range of archive formats are supported), and a number
+of repository formats including Git, Subversion, and Mercurial.
+Custom download, update, and patch commands can also be used to support
+arbitrary use cases.
+
+When a dependency is added to the project with :module:`FetchContent`, the
+project links to the dependency's targets just like any other target from the
+project.  If the dependency provides namespaced targets of the form
+``SomePrefix::ThingName``, the project should link to those rather than to
+any non-namespaced targets.  See the next section for why this is recommended.
+
+Not all dependencies can be brought into the project this way.  Some
+dependencies define targets whose names clash with other targets from the
+project or other dependencies.  Concrete executable and library targets
+created by :command:`add_executable` and :command:`add_library` are global,
+so each one must be unique across the whole build.  If a dependency would
+add a clashing target name, it cannot be brought directly into the build
+with this method.
+
+``FetchContent`` And ``find_package()`` Integration
+===================================================
+
+Some dependencies support being added by either :command:`find_package` or
+:module:`FetchContent`.  Such dependencies must ensure they define the same
+namespaced targets in both installed and built-from-source scenarios.
+A consuming project then links to those namespaced targets and can handle
+both scenarios transparently, as long as the project does not use anything
+else that isn't provided by both methods.
+
+The project can indicate it is happy to accept a dependency by either method
+using the ``FIND_PACKAGE_ARGS`` option to :command:`FetchContent_Declare`.
+This allows :command:`FetchContent_MakeAvailable` to try satisfying the
+dependency with a call to :command:`find_package` first, using the arguments
+after the ``FIND_PACKAGE_ARGS`` keyword, if any.  If that doesn't find the
+dependency, it is built from source as described previously instead.
 
-    find_package(PNG REQUIRED)
+.. code-block:: cmake
 
-    # Add path to a FindSomePackage.cmake file
-    list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")
-    find_package(SomePackage REQUIRED)
+  include(FetchContent)
+  FetchContent_Declare(
+    googletest
+    GIT_REPOSITORY https://github.com/google/googletest.git
+    GIT_TAG        703bd9caab50b139428cea1aaff9974ebee5742e # release-1.10.0
+    FIND_PACKAGE_ARGS NAMES GTest
+  )
+  FetchContent_MakeAvailable(googletest)
+
+  add_executable(ThingUnitTest thing_ut.cpp)
+  target_link_libraries(ThingUnitTest GTest::gtest_main)
+
+The above example calls
+:command:`find_package(googletest NAMES GTest) <find_package>` first.
+CMake provides a :module:`FindGTest` module, so if that finds a GTest package
+installed somewhere, it will make it available, and the dependency will not be
+built from source.  If no GTest package is found, it *will* be built from
+source.  In either case, the ``GTest::gtest_main`` target is expected to be
+defined, so we link our unit test executable to that target.
+
+High-level control is also available through the
+:variable:`FETCHCONTENT_TRY_FIND_PACKAGE_MODE` variable.  This can be set to
+``NEVER`` to disable all redirection to :command:`find_package`.  It can be
+set to ``ALWAYS`` to try :command:`find_package` even if ``FIND_PACKAGE_ARGS``
+was not specified (this should be used with caution).
+
+The project might also decide that a particular dependency must be built from
+source.  This might be needed if a patched or unreleased version of the
+dependency is required, or to satisfy some policy that requires all
+dependencies to be built from source.  The project can enforce this by adding
+the ``OVERRIDE_FIND_PACKAGE`` keyword to :command:`FetchContent_Declare`.
+A call to :command:`find_package` for that dependency will then be redirected
+to :command:`FetchContent_MakeAvailable` instead.
 
-    add_executable(MyExe main.cpp)
-    target_link_libraries(MyExe PRIVATE
-        PNG::PNG
-        SomePrefix::LibName
-    )
+.. code-block:: cmake
 
-The :variable:`<PackageName>_ROOT` variable is also
-searched as a prefix for :command:`find_package` calls using
-module-file packages such as ``FindSomePackage``.
+  include(FetchContent)
+  FetchContent_Declare(
+    Catch2
+    URL https://intranet.mycomp.com/vendored/Catch2_2.13.4_patched.tgz
+    URL_HASH MD5=abc123...
+    OVERRIDE_FIND_PACKAGE
+  )
+
+  # The following is automatically redirected to FetchContent_MakeAvailable(Catch2)
+  find_package(Catch2)
+
+For more advanced use cases, see the
+:variable:`CMAKE_FIND_PACKAGE_REDIRECTS_DIR` variable.
+
+.. _dependency_providers_overview:
+
+Dependency Providers
+====================
+
+The preceding section discussed techniques that projects can use to specify
+their dependencies.  Ideally, the project shouldn't really care where a
+dependency comes from, as long as it provides the things it expects (often
+just some imported targets).  The project says what it needs and may also
+specify where to get it from, in the absence of any other details, so that it
+can still be built out-of-the-box.
+
+The developer, on the other hand, may be much more interested in controlling
+*how* a dependency is provided to the project.  You might want to use a
+particular version of a package that you built themself.  You might want
+to use a third party package manager.  You might want to redirect some
+requests to a different URL on a system you control for security or
+performance reasons.  CMake supports these sort of scenarios through
+:ref:`dependency_providers`.
+
+A dependency provider can be set to intercept :command:`find_package` and
+:command:`FetchContent_MakeAvailable` calls.  The provider is given an
+opportunity to satisfy such requests before falling back to the built-in
+implementation if the provider doesn't fulfill it.
+
+Only one dependency provider can be set, and it can only be set at a very
+specific point early in the CMake run.
+The :variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES` variable lists CMake files
+that will be read while processing the first :command:`project()` call (and
+only that call).  This is the only time a dependency provider may be set.
+At most, one single provider is expected to be used throughout the whole
+project.
+
+For some scenarios, the user wouldn't need to know the details of how the
+dependency provider is set.  A third party may provide a file that can be
+added to :variable:`CMAKE_PROJECT_TOP_LEVEL_INCLUDES`, which will set up
+the dependency provider on the user's behalf.  This is the recommended
+approach for package managers.  The developer can use such a file like so::
+
+  cmake -DCMAKE_PROJECT_TOP_LEVEL_INCLUDES=/path/to/package_manager/setup.cmake ...
+
+For details on how to implement your own custom dependency provider, see the
+:command:`cmake_language(SET_DEPENDENCY_PROVIDER)` command.

Help/variable/CMAKE_FIND_PACKAGE_REDIRECTS_DIR.rst

@@ -15,7 +15,8 @@ when it populates a dependency.  This allows subsequent calls to
 :command:`find_package` for the same dependency to re-use the populated
 contents instead of trying to satisfy the dependency from somewhere external
 to the build.  Projects may also want to write files into this directory in
-some situations (see :ref:`FetchContent-find_package-integration` for examples).
+some situations (see :ref:`FetchContent-find_package-integration-examples`
+for examples).
 
 The directory that ``CMAKE_FIND_PACKAGE_REDIRECTS_DIR`` points to will always
 be erased and recreated empty at the start of every CMake run.  Any files

Modules/FetchContent.cmake

@@ -11,6 +11,12 @@ FetchContent
 
   .. contents::
 
+.. note:: The :guide:`Using Dependencies Guide` provides a high-level
+  introduction to this general topic. It provides a broader overview of
+  where the ``FetchContent`` module fits into the bigger picture,
+  including its relationship to the :command:`find_package` command.
+  The guide is recommended pre-reading before moving on to the details below.
+
 Overview
 ^^^^^^^^
 
@@ -720,7 +726,7 @@ frameworks are available to the main build:
   # Catch2 will be available to the rest of the build
   FetchContent_MakeAvailable(googletest Catch2)
 
-.. _FetchContent-find_package-integration:
+.. _FetchContent-find_package-integration-examples:
 
 Integrating With find_package()
 """""""""""""""""""""""""""""""