add_custom_command: Add DEPFILE option for Ninja

Provide a way for custom commands to inform the ninja build tool about
their implicit dependencies.  For now simply make use of the option an
error on other generators.

Closes: #15479

2 files changed, 13 insertions, 0 deletions

diff --git a/Help/command/add_custom_command.rst b/Help/command/add_custom_command.rst
index d421364..4ab4298 100644
--- a/Help/command/add_custom_command.rst
+++ b/Help/command/add_custom_command.rst
@@ -20,6 +20,7 @@ The first signature is for adding a custom command to produce an output::
                                       [<lang2> depend2] ...]
                      [WORKING_DIRECTORY dir]
                      [COMMENT comment]
+                     [DEPFILE depfile]
                      [VERBATIM] [APPEND] [USES_TERMINAL])
 
 This defines a command to generate specified ``OUTPUT`` file(s).
@@ -170,6 +171,12 @@ The options are:
   If it is a relative path it will be interpreted relative to the
   build tree directory corresponding to the current source directory.
 
+``DEPFILE``
+  Specify a ``.d`` depfile for the :generator:`Ninja` generator.
+  A ``.d`` file holds dependencies usually emitted by the custom
+  command itself.
+  Using ``DEPFILE`` with other generators than Ninja is an error.
+
 Build Events
 ^^^^^^^^^^^^
 
diff --git a/Help/release/dev/ninja-add_custom_command-depfile.rst b/Help/release/dev/ninja-add_custom_command-depfile.rst
new file mode 100644
index 0000000..c8099fe
--- /dev/null
+++ b/Help/release/dev/ninja-add_custom_command-depfile.rst
@@ -0,0 +1,6 @@
+ninja-add_custom_command-depfile
+--------------------------------
+
+* The :command:`add_custom_command` command gained a new ``DEPFILE``
+  option that works with the :generator:`Ninja` generator to provide
+  implicit dependency information to the build tool.