Docs should mention that CMAKE_TOOLCHAIN_FILE does not have access to the cache

I am unsure if this is intended behavior.

When setting the CMAKE_SYSROOT variable in a Toolchain file, cache variables defined inside the Toolchain file are expanded in the --sysroots option passed to the compiler, but cache variables defined on the command line do not expand

Example:

In the Toolchain file: set(internal-dir xyz)

(set CMAKE_SYSROOT {internal-dir}/{external-dir}/example) On the command line:

cmake -Dexternal-dir=abc -DCMAKE_TOOLCHAIN_FILE=Toolchain.cmake

When trying to compile, --sysroot will be 'xyz//example', but the value of ${external-dir} is indeed set as abc confirmed by printing the value of ${external-dir}.