[vcpkg] Fix toolchain compatibility with cmake < 3.15

[dg0yt]

Since 876e67c, the toolchain used list(PREPEND ...) by default. But this cmake command was introduced in cmake 3.15, and the toolchain is meant to work with 3.1 [!].

• What does your PR fix?

  This PR restores compatibility with cmake < 3.15 for user projects, fixing:

  CMake Error at /home/dg0yt/Projekte/vcpkg/vcpkg/scripts/buildsystems/vcpkg.cmake:390 (list):
    list does not recognize sub-command PREPEND
  

  Tested with CMake 3.10 (Ubuntu 18.04).

• Which triplets are supported/not supported? Have you updated the CI baseline?

  all, no

• Does your PR follow the maintainer guide?

  yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  not needed

[dg0yt]

CI didn't rebuild a single port, despite touching vcpkg.cmake.
There is no automated test of building at least a dummy project with the vcpkg toolchain and a supported ancient version of cmake.

[dg0yt]

CC @autoantwort This amends #17336.

[cenit]

CI didn't rebuild a single port, despite touching vcpkg.cmake.

There is no automated test of building at least a dummy project with the vcpkg toolchain and a supported ancient version of cmake.

a long standing very serious issue
(it has already let slip some bad bugs into master...)

@strega-nil

[autoantwort]

a long standing very serious issue
(it has already let slip some bad bugs into master...)

@strega-nil

There is already microsoft/vcpkg-tool#80

[dg0yt]

There is already microsoft/vcpkg-tool#80

Not sure if you really want to invalidate binary artifacts for each change, even documentation typos.

And this won't add testing for cmake versions which are claimed to be supported (cf. https://github.com/microsoft/vcpkg/pull/18319/files#r652632414).
(Nobody noticed the current issue until now. Is cmake 3.1 support really needed?)

---

FTR, you can't work around the current bug with VCPKG_PREFER_SYSTEM_LIBS=ON because this is not passed from the user project to cmake ports built by vcpkg, including detect_compiler and get_cmake_vars.

[autoantwort]

(Nobody noticed the current issue until now. Is cmake 3.1 support really needed?)

I thought the same, I searched for an issue complaining about it but couldn't find one.

[strega-nil]

One could probably do:

function(z_vcpkg_path_list_op out_var lst paths)
    if(VCPKG_PREFER_SYSTEM_LIBS)
        set("${out_var}" "${lst};${paths}" PARENT_SCOPE)
    else()
        set("${out_var}" "${paths};${lst}" PARENT_SCOPE)
    endif()
endfunction()

[dg0yt]

Does "one could probably do" mean I should update the PR? I would prefer to stick to plain cmake. If there shall be a function I would prefer a name which catures the specific operation "add" or "merge" instead of the generic "op", e.g. z_vcpkg_add_to_system_libs in analogy to VCPKG_PREFER_SYSTEM_LIBS.

[strega-nil]

I would prefer to switch to this; I don't like how this is currently working. Doing this weird INSERT bodging feels bad.

[dg0yt]

set("${out_var}" "${paths};${lst}" PARENT_SCOPE) doesn't work correctly for empty lists.
And list(INSERT ...) is unable to append.
😕

[Neumann-A]

Doing this weird INSERT bodging feels bad.

@strega-nil INSERT is also used by VTK to insert into the beginning of CMAKE_MODULE_PATH. It is the correct way to PREPEND.

And list(INSERT ...) is unable to append.

@dg0yt: have you tried using -1 as an index? Otherwise that is the reason why list(APPEND) existed from the beginning while PREPEND was added much later (since INSERT 0 just worked for this instead)

To add: From the 3.0 docs:

If is -1 or lesser, it is indexed from the end of the list, with -1 representing the last list element.

[dg0yt]

have you tried using -1 as an index? Otherwise that is the reason why list(APPEND) existed from the beginning while PREPEND was added much later (since INSERT 0 just worked for this instead)

To add: From the 3.0 docs:

If is -1 or lesser, it is indexed from the end of the list, with -1 representing the last list element.

Yes, I tried in a separate cmake script. INSERT inserts before the given list element, and INSERT -1 inserts before the last list element.
CMake lists aren't C++ containers...

[strega-nil]

You can do:

if(VCPKG_PREFER_SYSTEM_LIBS)
    list(APPEND lst "${paths}")
else()
    list(INSERT lst 0 "${paths}")
endif()
set("${out_var}" "${lst}" PARENT_SCOPE)

[dg0yt]

You can do ...

That's what I already did.

[strega-nil-ms]

oh, sorry, I clearly didn't reread the PR.

[vicroms]

Will be bundled in next roll-up to trigger world rebuild

[strega-nil-ms]

Closed for rollup