Document VCPKG_USE_HOST_TOOLS

[vicroms]

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[BillyONeal]

It's not very clear to me that VCPKG_USE_HOST_TOOLS goes in the user's CMakeLists.txt from this text but I think this is better than the status quo.

[dg0yt]

IMO this option shouldn't exist. If vcpkg is a using a different host triplet, it must use that triplet for CMAKE_PROGRAM_PATH.
microsoft/vcpkg#25529

[Neumann-A]

IMO this option shouldn't exist. If vcpkg is a using a different host triplet, it must use that triplet for CMAKE_PROGRAM_PATH.

Isn't that how it works? If the host triplet is set VCPKG_USE_HOST_TOOLS should be true.

[dg0yt]

Isn't that how it works? If the host triplet is set VCPKG_USE_HOST_TOOLS should be true.

Yes. "IF THE HOST TRIPLET IS SET".

The problem is that if the user only sets VCPKG_TARGET_TRIPLET, the vcpkg tool makes independent (and valid) decisions about the host triplet. But the user project isn't informed about them.

microsoft/vcpkg#25529 solved the missing link for CMake.
https://github.com/microsoft/vcpkg/pull/25529/files#r932552438 requested the same for msbuild. But nobody contributed a proposal.

[Neumann-A]

The problem is that if the user only sets VCPKG_TARGET_TRIPLET, the vcpkg tool makes independent (and valid) decisions about the host triplet. But the user project isn't informed about them.

That was discussed when the variables where introduce. The maintainers wanted to keep the old behavior if VCPKG_HOST_TRIPLET is not set. In my opinion vcpkg.cmake should be changed so that VCPKG_HOST_TRIPLET is the main variable and VCPKG_TTARGET_TRIPLET is the optional one.

[dg0yt]

In my opinion vcpkg.cmake should be changed so that VCPKG_HOST_TRIPLET is the main variable and VCPKG_TTARGET_TRIPLET is the optional one.

I don't agree.
It is expected that a build system can sufficiently discover the host system (VCPKG_HOST_TRIPLET).
It is expected that a build system must be told to build for a different target (VCPKG_TARGET_TRIPLET).
It is unexpected that a buildsystem needs redundant information about target and host (user project vs. integrated vcpkg)

[Neumann-A]

It is expected that a build system can sufficiently discover the host system (VCPKG_HOST_TRIPLET).

I would agree with that if there would be one correct one and not N. There have been M issues about why vcpkg builds stuff twice if it is considered a native build. The only triplet env variable I have set is VCPKG_DEFAULT_HOST_TRIPLET. If it is not I would set --host-triplet instead of --triplet.
The root problem by setting VCPKG_TARGET_TRIPLET is that the tool does not know if it is a cross build or not and the result thus can be both. Maybe a new variable like VCPKG_TRIPLET is needed to tell that it is a native build with that triplet and everything else is considered a cross build.

[dg0yt]

Let's move the focus back to VPCKG_USE_HOST_TOOLS. The key point is that this doesn't need to be documented or set by the user. The vcpkg toolchain must ensure a match between toolchain configuration and vcpkg tool decisions.

[learn-build-service-prod]

Learn Build status updates of commit a13a2a7:

✅ Validation status: passed

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[learn-build-service-prod]

Learn Build status updates of commit 91822a3:

✅ Validation status: passed

File	Status	Preview URL	Details
vcpkg/users/host-dependencies.md	✅Succeeded	View

For more details, please refer to the build report.

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