[cmake-developers] Proposed Feature: Selective search path re-rooting

[Chuck Atkins]

Following the topic branch to separately maintain each type of search path,
I'm working on adding a feature for cross-compiling to separately control
how each type of search path gets re-rooted. Currently, the options
CMAKE_FIND_ROOT_PATH_MODE_{INCLUDE_LIBRARY_PROGRAM} can be set to ONLY,
NEVER, or BOTH.  This takes effect for all search paths, both CMake
determined and user-specifies (through HINTS and PATHS options).  Before
putting the time and effort into implementing the feature this way, I
wanted to get some feedback on the design first:

My approach to control the path types independently is to allow the
variable to be specified as either a single mode, applying to all paths, or
a named list of the form "PATH_TYPE1 MODE1 PATH_TYPE2_MODE2..." much in the
way that properties are specified.  The following search path groups can be
specified:

   - CMAKE_PATH
   - CMAKE_ENVIRONMENT_PATH
   - USER_HINTS_PATH
   - SYSTEM_ENVIRONMENT_PATH
   - CMAKE_SYSTEM_PATH
   - USER_GUESS_PATH

In addition to each search path type, there will also be defined path
groups:

   - ALL - All search paths
   - DEFAULT
      - CMAKE_PATH
      - CMAKE_ENVIRONMENT_PATH
      - SYSTEM_ENVIRONMENT_PATH
      - CMAKE_SYSTEM_PATH
   - USER - User hints and user guess paths

If one had:

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ALL ONLY HINTS NEVER)

Then the following modes would be set:

   - find_program: All search paths set to never
   - find_library: user hints would never be re-rooted and all others would
   operate in ONLY mode.
   - find_file: All search paths set to BOTH mode since that's the default
   behavior.


- Chuck
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