CMP0058
-------

Ninja requires custom command byproducts to be explicit.

When an intermediate file generated during the build is consumed
by an expensive operation or a large tree of dependents, one may
reduce the work needed for an incremental rebuild by updating the
file timestamp only when its content changes.  With this approach
the generation rule must have a separate output file that is always
updated with a new timestamp that is newer than any dependencies of
the rule so that the build tool re-runs the rule only when the input
changes.  We refer to the separate output file as a rule's *witness*
and the generated file as a rule's *byproduct*.

Byproducts may not be listed as outputs because their timestamps are
allowed to be older than the inputs.  No build tools (like ``make``)
that existed when CMake was designed have a way to express byproducts.
Therefore CMake versions prior to 3.2 had no way to specify them.
Projects typically left byproducts undeclared in the rules that
generate them.  For example:

.. code-block:: cmake

  add_custom_command(
    OUTPUT witness.txt
    COMMAND ${CMAKE_COMMAND} -E copy_if_different
            ${CMAKE_CURRENT_SOURCE_DIR}/input.txt
            byproduct.txt # timestamp may not change
    COMMAND ${CMAKE_COMMAND} -E touch witness.txt
    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/input.txt
    )
  add_custom_target(Provider DEPENDS witness.txt)
  add_custom_command(
    OUTPUT generated.c
    COMMAND expensive-task -i byproduct.txt -o generated.c
    DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/byproduct.txt
    )
  add_library(Consumer generated.c)
  add_dependencies(Consumer Provider)

This works well for all generators except :generator:`Ninja`.
The Ninja build tool sees a rule listing ``byproduct.txt``
as a dependency and no rule listing it as an output.  Ninja then
complains that there is no way to satisfy the dependency and
stops building even though there are order-only dependencies
that ensure ``byproduct.txt`` will exist before its consumers
need it.  See discussion of this problem in `Ninja Issue 760`_
for further details on why Ninja works this way.

.. _`Ninja Issue 760`: https://github.com/martine/ninja/issues/760

Instead of leaving byproducts undeclared in the rules that generate
them, Ninja expects byproducts to be listed along with other outputs.
Such rules may be marked with a ``restat`` option that tells Ninja
to check the timestamps of outputs after the rules run.  This
prevents byproducts whose timestamps do not change from causing
their dependents to re-build unnecessarily.

Since the above approach does not tell CMake what custom command
generates ``byproduct.txt``, the Ninja generator does not have
enough information to add the byproduct as an output of any rule.
CMake 2.8.12 and above work around this problem and allow projects
using the above approach to build by generating ``phony`` build
rules to tell Ninja to tolerate such missing files.  However, this
workaround prevents Ninja from diagnosing a dependency that is
really missing.  It also works poorly in in-source builds where
every custom command dependency, even on source files, needs to
be treated this way because CMake does not have enough information
to know which files are generated as byproducts of custom commands.

CMake 3.2 introduced the ``BYPRODUCTS`` option to the
:command:`add_custom_command` and :command:`add_custom_target`
commands.  This option allows byproducts to be specified explicitly:

.. code-block:: cmake

  add_custom_command(
    OUTPUT witness.txt
    BYPRODUCTS byproduct.txt # explicit byproduct specification
    COMMAND ${CMAKE_COMMAND} -E copy_if_different
            ${CMAKE_CURRENT_SOURCE_DIR}/input.txt
            byproduct.txt # timestamp may not change
  ...

The ``BYPRODUCTS`` option is used by the :generator:`Ninja` generator
to list byproducts among the outputs of the custom commands that
generate them, and is ignored by other generators.

CMake 3.3 and above prefer to require projects to specify custom
command byproducts explicitly so that it can avoid using the
``phony`` rule workaround altogether.  Policy ``CMP0058`` was
introduced to provide compatibility with existing projects that
still need the workaround.

This policy has no effect on generators other than :generator:`Ninja`.
The ``OLD`` behavior for this policy is to generate Ninja ``phony``
rules for unknown dependencies in the build tree.  The ``NEW``
behavior for this policy is to not generate these and instead
require projects to specify custom command ``BYPRODUCTS`` explicitly.

This policy was introduced in CMake version 3.3.
CMake version |release| warns when it sees unknown dependencies in
out-of-source build trees if the policy is not set and then uses
``OLD`` behavior.  Use the :command:`cmake_policy` command to set
the policy to ``OLD`` or ``NEW`` explicitly.  The policy setting
must be in scope at the end of the top-level ``CMakeLists.txt``
file of the project and has global effect.
