[cmake-developers] target_link_libraries: Allow use with targets in other directories

[Patrick Stotko]

Hi,

since my first try of relaxing the limitation that target_link_libraries 
may not be called in other directories was reverted due to a use case 
that was not fully covered, a more promising approach has been developed 
after a long but very insightful discussion which I wanted to present 
here to collect some feedback from you.

In order to allow linking from another directory, the actual directory 
where the RHS target was created (and should be found later via lookup) 
needs to be encoded. The proposed solution would add a new syntax for 
this (note the similarity to C++ lambda expressions):

[target_name]@{subdir}

This is necessary since it is not possible at all to use generator 
expressions. The syntax would be specific to values in the 
[INTERFACE_]LINK_LIBRARIES properties and not be meaningful elsewhere 
(besides target_link_libraries arguments used to populate the 
properties). Future extensions to this may include encoding of more 
properties:

[target_name]@{prop1:value1;prop2:value2;...}

Special characters such as : and = will be disallowed now. Furthermore, 
lists of targets may also be handled in some future extension:

[target1]@{"subdir"};[target2]@{"subdir"} => [target1;target2]@{"subdir"}

Note that these two extensions may or may not be included in some future 
version. Our primary focus here lies on the basic use case of encoding 
the directory property without limiting future generalizations. More 
details and an in-depth discussion about the problem can be found in 
this issue (https://gitlab.kitware.com/cmake/cmake/issues/17943). Please 
share your thoughts regarding the proposed syntax and let us know 
potential problems that should be tackled.

Best regards,
Patrick Stotko