Commits on Nov 20, 2024

1. Remove target_link_libraries in add_cxxbridge …

   Enforcing a link between the bridge and the crate has turned out
   to be problematic if the dependency between the C++ and Rust code is
   circular.
   
   Example:
   A reasonable way to build a CXX-enabled crate is to build 3 static
   libraries:
   1. my_crate_cpp - contains the C++ code, which may access Rust via CXX
   2. my_crate_rust - contains the Rust code, which may access C++ via CXX
     * Note here that corrosion creates an additional my_crate_rust-static
       target for the actual static library, my_crate_rust is an INTERFACE
       target that links to this -static target.
   3. my_crate_bridge - bridge code (generated via corrosion_add_cxxbridge)
   
   As the point of CXX is to freely communicate between C++ and Rust, these
   3 libraries sort off act as one in the end and the dependencies between
   them may be circular.
   
   In CMake 3.24+, cirular dependencies can be resolved via a LINK_GROUP.
   With corrosion, the right call would be:
   
   ```cmake
   target_link_libraries(my_crate_rust INTERFACE
       "$<LINK_GROUP:RESCAN,my_crate_cpp,my_crate_bridge,my_crate_rust-static>"
   )
   ```
   
   Which would allow C++ to call Rust feely and vice-versa.
   The result would be an interface target my_crate_rust, which corresponds
   to the Rust crate imported by corrosion and contains all 3 static
   libraries.
   
   However, this only works when removing the target_link_libraries call
   from the bridge target to the crate target.
   As otherwise, CMake complains that it cannot resolve the dependencies,
   as the bridge already depends on the _rust target, which also already
   depends on the `_rust-static` target, etc.
   We also cannot create a LINK_GROUP that includes the my_crate_rust
   target, as link groups are only allowed on STATIC library targets, and
   the my_crate_rust is an INTERFACE target.
   
   So this either needs to be left up to the user, or the
   target_link_libraries call must be changed to link to the `-static`
   target, instead of the INTERFACE target, as that is compatible with the
   LINK_GROUP call.

   

   LeonMatthesKDAB committed Nov 20, 2024
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