Ninja: Transform DEPFILEs with policy CMP0116

Fixes: #21267

Help/command/add_custom_command.rst

@@ -239,6 +239,11 @@ The options are:
   command itself.
   Using ``DEPFILE`` with other generators than Ninja is an error.
 
+  If the ``DEPFILE`` argument is relative, it should be relative to
+  :variable:`CMAKE_CURRENT_BINARY_DIR`, and any relative paths inside the
+  ``DEPFILE`` should also be relative to :variable:`CMAKE_CURRENT_BINARY_DIR`
+  (see policy :policy:`CMP0116`.)
+
 Build Events
 ^^^^^^^^^^^^
 

Help/manual/cmake-policies.7.rst

@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.20
 .. toctree::
    :maxdepth: 1
 
+   CMP0116: Ninja generators transform DEPFILEs from add_custom_command(). </policy/CMP0116>
    CMP0115: Source file extensions must be explicit. </policy/CMP0115>
 
 Policies Introduced by CMake 3.19

Help/policy/CMP0116.rst

@@ -0,0 +1,38 @@
+CMP0116
+-------
+
+.. versionadded:: 3.20
+
+Ninja generators transform ``DEPFILE`` s from :command:`add_custom_command`.
+
+In CMake 3.19 and below, files given to the ``DEPFILE`` argument of
+:command:`add_custom_command` were passed directly to Ninja's ``depfile``
+variable without any path resolution. This meant that if
+:command:`add_custom_command` was called from a subdirectory (created by
+:command:`add_subdirectory`), the ``DEPFILE`` argument would have to be either
+an absolute path or a path relative to :variable:`CMAKE_BINARY_DIR`, rather
+than :variable:`CMAKE_CURRENT_BINARY_DIR`. In addition, no transformation was
+done on the file listed in ``DEPFILE``, which meant that the paths within the
+``DEPFILE`` had the same restrictions.
+
+Starting with CMake 3.20, the ``DEPFILE`` argument is relative to
+:variable:`CMAKE_CURRENT_BINARY_DIR` (unless it is absolute), and the paths in
+the ``DEPFILE`` are also relative to :variable:`CMAKE_CURRENT_BINARY_DIR`.
+CMake automatically transforms the paths in the ``DEPFILE`` (unless they are
+absolute) after the custom command is run. The file listed in ``DEPFILE`` is
+not modified in any way. Instead, CMake writes the transformation to its own
+internal file, and passes this internal file to Ninja's ``depfile`` variable.
+This transformation happens regardless of whether or not ``DEPFILE`` is
+relative, and regardless of whether or not :command:`add_custom_command` is
+called from a subdirectory.
+
+The ``OLD`` behavior for this policy is to pass the ``DEPFILE`` to Ninja
+unaltered. The ``NEW`` behavior for this policy is to transform the ``DEPFILE``
+after running the custom command.
+
+This policy was introduced in CMake version 3.20.  Unlike most policies,
+CMake version |release| does *not* warn by default when this policy is not set
+(unless ``DEPFILE`` is used in a subdirectory) and simply uses ``OLD``
+behavior.  See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0116 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.

Help/release/dev/ninja-depfile-transformation.rst

@@ -0,0 +1,5 @@
+ninja-depfile-transformation
+----------------------------
+
+* Ninja generators now transform ``DEPFILE`` s from
+  :command:`add_custom_command`. See policy :policy:`CMP0116` for details.

Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst

@@ -27,6 +27,8 @@ warn by default:
   policy :policy:`CMP0102`.
 * ``CMAKE_POLICY_WARNING_CMP0112`` controls the warning for
   policy :policy:`CMP0112`.
+* ``CMAKE_POLICY_WARNING_CMP0116`` controls the warning for
+  policy :policy:`CMP0116`.
 
 This variable should not be set by a project in CMake code.  Project
 developers running CMake may set this variable in their cache to