[feature] Setting `CMAKE_<LANG>_COMPILER` in a toolchain

Problem statement

Right now it is common to use a different compiler for conan install and CMake configure steps which result in an error during project configuration.

Potential solution

From my observation, it is typical to set CXX and CC environment variables in the Conan profile file. In case a user provides them and uses such a profile during conan install step it would be great if CMakeToolchain would set CMAKE_<LANG>_COMPILER cache variables based on those.

Thanks to that we will always end up with consistent builds and users will just have to call:

conan install .. -pr my_profile
cmake .. -toolchain conan_toolchain.cmake

• I’ve read the CONTRIBUTING guide.

[mpusz]

Please take into account that the paths set through the conf[] parameter should probably be set automatically by Conan as CC and CXX (or CMAKE_<LANG>_COMPILER) environment variables so conan install --build missing works correctly (such build does not include the toolchain file generated in this step). It would be unfortunate to force the user to set the same path twice in a profile file.

[jwillikers]

@mpusz You can do some more complicated stuff to configure the CMAKE_C_COMPILER and CMAKE_CXX_COMPILER in the generated conan_toolchain.cmake, or provide your own CMake toolchain file to Conan through the related [conf] variables. CMake toolchain files are really meant for that and additional configuration of the toolchain.

But, I completely agree that it’s a very common need, and I’d like it to be simpler. I’m not sure the [env] setting is the best approach, since Conan is trying to move away from the use of [env] variables. Configuring different names and locations for a compiler is useful for any generator and compiler, too. Perhaps a set of dedicated [conf] variables could be added to allow specifying a particular suffix and prefix for the various tools in the toolchain, i.e. gcc / g++ and/or an option to add additional compiler search paths may also be helpful. Or maybe it would be simplest to just provide a single option to configure the full path to the compiler. Whichever makes the most sense. This way any generator could make use of the configuration values to set the appropriate compiler.

Here’s a possible example using a made up configuration value, tools.c_compiler.path, that sets the path of the C compiler.

[settings]
compiler = gcc

[conf]
tools.c_compiler.path = /path/to/my/toolchain/custom-gcc

The CMakeToolchain generator could then use this value to set CMAKE_C_COMPILER in the conan_toolchain.cmake.

set(CMAKE_C_COMPILER /path/to/my/toolchain/custom-gcc)