CMake/Help/policy/CMP0168.rst

CMP0168
-------

.. versionadded:: 3.30

The :module:`FetchContent` module implements steps directly instead of through
a sub-build.

CMake 3.29 and below implement FetchContent as a separate sub-build.
This required configuring that separate project and using a build tool.
This approach can be very slow with some generators and operating systems.
CMake 3.30 and above prefer to implement the download, update, and patch
steps directly as part of the main project.

The ``NEW`` behavior has the following characteristics:

* No sub-build is used. All operations are implemented directly from the
  main project's CMake configure step. When running in CMake script mode,
  no build tool needs to be available.
* Generator expressions and GNU Make variables of the form ``$(SOMEVAR)`` are
  not supported. They should not be used in any argument to
  :command:`FetchContent_Declare` or :command:`FetchContent_Populate`.
* All ``LOG_...`` and ``USES_TERMINAL_...`` options, the ``QUIET`` option, and
  the :variable:`FETCHCONTENT_QUIET` variable are ignored.
  :module:`FetchContent` output is always part of the main project's configure
  output. This also means it now respects the message logging level (see
  :variable:`CMAKE_MESSAGE_LOG_LEVEL` and
  :option:`--log-level <cmake --log-level>`). The default message log level
  should be comparable to using ``QUIET`` with the ``OLD`` policy setting,
  except that warnings will now be shown.
* The ``PREFIX``, ``TMP_DIR``, ``STAMP_DIR``, ``LOG_DIR``, and ``DOWNLOAD_DIR``
  options and their associated directory properties are ignored. The
  :module:`FetchContent` module controls those locations internally.
* :option:`cmake --fresh` will remove the stamp and script files used for
  tracking and populating the dependency. This will force the dependency's
  download, update, and patch steps to be re-executed. The directory used for
  downloads is not affected by :option:`cmake --fresh`, so any previously
  downloaded files for the ``URL`` download method can still be re-used.

The ``OLD`` behavior has the following characteristics:

* A sub-build is always used to implement the download, update, and patch
  steps. A build tool must be available, even when using
  :command:`FetchContent_Populate` in CMake script mode.
* Generator expressions and GNU Make variables of the form ``$(SOMEVAR)`` can
  be used, although such use is almost always inappropriate. They are evaluated
  in the sub-build, so they do not see any information from the main build.
* All logging, terminal control, and directory options related to the download,
  update, or patch steps are supported.
* If the ``QUIET`` option is used, or the :variable:`FETCHCONTENT_QUIET`
  variable is set to true, warnings will not be shown in the output.
* :option:`cmake --fresh` has no effect on the dependency's stamp or script
  files. Previously executed steps will only re-run if details about the
  dependency have changed.

There's a reasonably good chance that users can set the
:variable:`CMAKE_POLICY_DEFAULT_CMP0168 <CMAKE_POLICY_DEFAULT_CMP<NNNN>>`
variable to ``NEW`` to globally switch to the ``NEW`` behavior while waiting
for the project and its dependencies to be updated use the ``NEW`` policy
setting by default. Projects don't typically make use of the features that the
``NEW`` behavior no longer supports, and even those projects that do will often
still work fine when those options are ignored. Before setting this behavior
globally, check whether any :command:`FetchContent_Declare` or
:command:`FetchContent_Populate` calls use the ignored options in a way that
would change observable behavior, other than putting temporary or
internally-generated files in different locations.

.. |INTRODUCED_IN_CMAKE_VERSION| replace:: 3.30
.. |WARNS_OR_DOES_NOT_WARN| replace:: does *not* warn
.. include:: STANDARD_ADVICE.txt

.. include:: DEPRECATED.txt