[vcpkg/scripts] move toolchain into triplet?

[Neumann-A]

The question is if we should move:

vcpkg/scripts/cmake/vcpkg_configure_cmake.cmake

Lines 196 to 214 in 2ac2b5f

	if(NOT VCPKG_CHAINLOAD_TOOLCHAIN_FILE)
	if(NOT DEFINED VCPKG_CMAKE_SYSTEM_NAME OR _TARGETTING_UWP)
	set(VCPKG_CHAINLOAD_TOOLCHAIN_FILE "${SCRIPTS}/toolchains/windows.cmake")
	elseif(VCPKG_CMAKE_SYSTEM_NAME STREQUAL "Linux")
	set(VCPKG_CHAINLOAD_TOOLCHAIN_FILE "${SCRIPTS}/toolchains/linux.cmake")
	elseif(VCPKG_CMAKE_SYSTEM_NAME STREQUAL "Android")
	set(VCPKG_CHAINLOAD_TOOLCHAIN_FILE "${SCRIPTS}/toolchains/android.cmake")
	elseif(VCPKG_CMAKE_SYSTEM_NAME STREQUAL "Darwin")
	set(VCPKG_CHAINLOAD_TOOLCHAIN_FILE "${SCRIPTS}/toolchains/osx.cmake")
	elseif(VCPKG_CMAKE_SYSTEM_NAME STREQUAL "iOS")
	set(VCPKG_CHAINLOAD_TOOLCHAIN_FILE "${SCRIPTS}/toolchains/ios.cmake")
	elseif(VCPKG_CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
	set(VCPKG_CHAINLOAD_TOOLCHAIN_FILE "${SCRIPTS}/toolchains/freebsd.cmake")
	elseif(VCPKG_CMAKE_SYSTEM_NAME STREQUAL "MinGW")
	set(VCPKG_CHAINLOAD_TOOLCHAIN_FILE "${SCRIPTS}/toolchains/mingw.cmake")
	endif()
	endif()

into the triplet instead an just define VCPKG_CHAINLOAD_TOOLCHAIN_FILE there. (would also require to set VCPKG_LOAD_VCVARS_ENV there so that VCPKG loads the environment for windows)
This also applies to the other configure scripts. This would make the use of the used toolchains more transparent for the outside and shows how to use a custom toolchain better.
We then should probably also move toolchains out of scripts into the vcpkg_root or maybe even into triplets?

[BillyONeal]

Taking off my maintainer hat and speaking personally for a moment: I think being explicit about this in the triplet file would be an improvement because it removes a 'spooky action at a distance' point where the tool is figuring this out for you behind the scenes.

On the other hand this is likely to cause users to edit the toolchain scripts which are not intended as an extension / modification point, and we wouldn't be able to support user modifications like that.

Putting my maintainer hat back on again: While I would personally love a change like this I don't think it overcomes the usual "every feature starts out with -1000 points" test, so I don't think we would want to go in this direction.

That said, we should certainly improve error messages and documentation, for example

vcpkg/toolsrc/src/vcpkg/build.cpp

Line 640 in 85247c3

"Unable to determine toolchain to use for triplet %s with CMAKE_SYSTEM_NAME %s",

to point users at VCPKG_CHAINLOAD_TOOLCHAIN_FILE.

[strega-nil-ms]

I don't think it would be possible to do this as of right now; we could deprecate and do this on our triplets, but users may be relying on this behavior in their own custom triplets.