Help: Revise try_compile and try_run documentation (#15358)

Rewrite the documentation using better reStructuredText markup
constructs.  Clarify interaction of options like LINK_LIBRARIES and
CMAKE_FLAGS.

1 files changed, 83 insertions, 45 deletions

diff --git a/Help/command/try_run.rst b/Help/command/try_run.rst
index b8ea0fe..43ee219 100644
--- a/Help/command/try_run.rst
+++ b/Help/command/try_run.rst
@@ -1,59 +1,97 @@
 try_run
 -------
 
+.. only:: html
+
+   .. contents::
+
 Try compiling and then running some code.
 
+Try Compiling and Running Source Files
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 ::
 
   try_run(RUN_RESULT_VAR COMPILE_RESULT_VAR
-          bindir srcfile [CMAKE_FLAGS <flags>]
-          [COMPILE_DEFINITIONS <flags>]
-          [LINK_LIBRARIES <libs>]
-          [COMPILE_OUTPUT_VARIABLE comp]
-          [RUN_OUTPUT_VARIABLE run]
-          [OUTPUT_VARIABLE var]
-          [ARGS <arg1> <arg2>...])
-
-Try compiling a srcfile.  Return TRUE or FALSE for success or failure
-in COMPILE_RESULT_VAR.  Then if the compile succeeded, run the
-executable and return its exit code in RUN_RESULT_VAR.  If the
-executable was built, but failed to run, then RUN_RESULT_VAR will be
-set to FAILED_TO_RUN.  COMPILE_OUTPUT_VARIABLE specifies the variable
-where the output from the compile step goes.  RUN_OUTPUT_VARIABLE
-specifies the variable where the output from the running executable
-goes.
-
-The srcfile signature also accepts a LINK_LIBRARIES argument which may
-contain a list of libraries or IMPORTED targets which will be linked
-to in the generated project.  If LINK_LIBRARIES is specified as a
-parameter to try_run, then any LINK_LIBRARIES passed as
-CMAKE_FLAGS will be ignored.
-
-For compatibility reasons OUTPUT_VARIABLE is still supported, which
-gives you the output from the compile and run step combined.
-
-Cross compiling issues
+          bindir srcfile [CMAKE_FLAGS <flags>...]
+          [COMPILE_DEFINITIONS <defs>...]
+          [LINK_LIBRARIES <libs>...]
+          [COMPILE_OUTPUT_VARIABLE <var>]
+          [RUN_OUTPUT_VARIABLE <var>]
+          [OUTPUT_VARIABLE <var>]
+          [ARGS <args>...])
+
+Try compiling a ``<srcfile>``.  Returns ``TRUE`` or ``FALSE`` for success
+or failure in ``COMPILE_RESULT_VAR``.  If the compile succeeded, runs the
+executable and returns its exit code in ``RUN_RESULT_VAR``.  If the
+executable was built, but failed to run, then ``RUN_RESULT_VAR`` will be
+set to ``FAILED_TO_RUN``.  See the :command:`try_compile` command for
+information on how the test project is constructed to build the source file.
+
+The options are:
+
+``CMAKE_FLAGS <flags>...``
+  Specify flags of the form ``-DVAR:TYPE=VALUE`` to be passed to
+  the ``cmake`` command-line used to drive the test build.
+  The example in :command:`try_compile` shows how values for variables
+  ``INCLUDE_DIRECTORIES``, ``LINK_DIRECTORIES``, and ``LINK_LIBRARIES``
+  are used.
+
+``COMPILE_DEFINITIONS <defs>...``
+  Specify ``-Ddefinition`` arguments to pass to ``add_definitions``
+  in the generated test project.
+
+``COMPILE_OUTPUT_VARIABLE <var>``
+  Report the compile step build output in a given variable.
+
+``LINK_LIBRARIES <libs>...``
+  Specify libraries to be linked in the generated project.
+  The list of libraries may refer to system libraries and to
+  :ref:`Imported Targets <Imported Targets>` from the calling project.
+
+  If this option is specified, any ``-DLINK_LIBRARIES=...`` value
+  given to the ``CMAKE_FLAGS`` option will be ignored.
+
+``OUTPUT_VARIABLE <var>``
+  Report the compile build output and the output from running the executable
+  in the given variable.  This option exists for legacy reasons.  Prefer
+  ``COMPILE_OUTPUT_VARIABLE`` and ``RUN_OUTPUT_VARIABLE`` instead.
+
+``RUN_OUTPUT_VARIABLE <var>``
+  Report the output from running the executable in a given variable.
+
+Other Behavior Settings
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Set the :variable:`CMAKE_TRY_COMPILE_CONFIGURATION` variable to choose
+a build configuration.
+
+Behavior when Cross Compiling
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 When cross compiling, the executable compiled in the first step
-usually cannot be run on the build host.  try_run() checks the
-CMAKE_CROSSCOMPILING variable to detect whether CMake is in
-crosscompiling mode.  If that's the case, it will still try to compile
+usually cannot be run on the build host.  The ``try_run`` command checks
+the :variable:`CMAKE_CROSSCOMPILING` variable to detect whether CMake is in
+cross-compiling mode.  If that is the case, it will still try to compile
 the executable, but it will not try to run the executable.  Instead it
 will create cache variables which must be filled by the user or by
 presetting them in some CMake script file to the values the executable
 would have produced if it had been run on its actual target platform.
-These variables are RUN_RESULT_VAR (explanation see above) and if
-RUN_OUTPUT_VARIABLE (or OUTPUT_VARIABLE) was used, an additional cache
-variable RUN_RESULT_VAR__COMPILE_RESULT_VAR__TRYRUN_OUTPUT.This is
-intended to hold stdout and stderr from the executable.
-
-In order to make cross compiling your project easier, use try_run only
-if really required.  If you use try_run, use RUN_OUTPUT_VARIABLE (or
-OUTPUT_VARIABLE) only if really required.  Using them will require
-that when crosscompiling, the cache variables will have to be set
-manually to the output of the executable.  You can also "guard" the
-calls to try_run with if(CMAKE_CROSSCOMPILING) and provide an
-easy-to-preset alternative for this case.
-
-Set variable CMAKE_TRY_COMPILE_CONFIGURATION to choose a build
-configuration.
+These cache entries are:
+
+``<RUN_RESULT_VAR>``
+  Exit code if the executable were to be run on the target platform.
+
+``<RUN_RESULT_VAR>__TRYRUN_OUTPUT``
+  Output from stdout and stderr if the executable were to be run on
+  the target platform.  This is created only if the
+  ``RUN_OUTPUT_VARIABLE`` or ``OUTPUT_VARIABLE`` option was used.
+
+In order to make cross compiling your project easier, use ``try_run``
+only if really required.  If you use ``try_run``, use the
+``RUN_OUTPUT_VARIABLE`` or ``OUTPUT_VARIABLE`` options only if really
+required.  Using them will require that when cross-compiling, the cache
+variables will have to be set manually to the output of the executable.
+You can also "guard" the calls to ``try_run`` with an :command:`if`
+block checking the :variable:`CMAKE_CROSSCOMPILING` variable and
+provide an easy-to-preset alternative for this case.