Fix/clang10 linux

[ignacionr]

Describe the pull request

• What does your PR fix?
  Fixes warnings treated as errors on Clang 10.0.0 Linux. Also one unused fn.

• Which triplets are supported/not supported? Have you updated the CI baseline?
  Does not apply (this is a fix for the tool).

• Does your PR follow the maintainer guide?
  Yes.

Additional notice (so to understand some of these changes)

• I like the idea of Span<> being a view into contiguous memory (it basically avoids going through abstracted iterators).

• When initializing them with vectors, we are making a claim that will not necessarily last: std::vector may need to re-allocate at some point, so it will all move elsewhere. But while within a fn called with a temporary object, it's fairly safe.

• Clang is wiser than other compilers and won't let us just go ahead and use vectors for that reason. So I changed it to the address of the first element, and the count (totally equivalent, and the potential risks made explicit).

Of course, I may have completely misunderstood something here, so comments and clarifications are very welcome.

Compiler Messages

clang version 10.0.0-4ubuntu1 
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin


/home/ignacio/src/tests/vcpkg/toolsrc/src/vcpkg/base/system.cpp:163:39: error: unused function 'get_xdg_config_home' [-Werror,-Wunused-function]
    static const ExpectedS<fs::path>& get_xdg_config_home() noexcept
                                      ^
1 error generated.
make[2]: *** [CMakeFiles/vcpkglib.dir/build.make:219: CMakeFiles/vcpkglib.dir/src/vcpkg/base/system.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:162: CMakeFiles/vcpkglib.dir/all] Error 2
make: *** [Makefile:95: all] Error 2
/home/ignacio/src/tests/vcpkg/toolsrc/src/vcpkg/build.cpp:303:75: error: unused parameter 'paths' [-Werror,-Wunused-parameter]
    const System::Environment& EnvCache::get_action_env(const VcpkgPaths& paths, const AbiInfo& abi_info)
                                                                          ^
/home/ignacio/src/tests/vcpkg/toolsrc/src/vcpkg/build.cpp:303:97: error: unused parameter 'abi_info' [-Werror,-Wunused-parameter]
    const System::Environment& EnvCache::get_action_env(const VcpkgPaths& paths, const AbiInfo& abi_info)
                                                                                                ^
2 errors generated.
make[2]: *** [CMakeFiles/vcpkglib.dir/build.make:323: CMakeFiles/vcpkglib.dir/src/vcpkg/build.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:162: CMakeFiles/vcpkglib.dir/all] Error 2
make: *** [Makefile:95: all] Error 2

[strega-nil]

@ignacionr this... seems like an unfortunate change. What warnings does clang throw after passing the vector as a span?

Fwiw, this path has been tread before, by the standards committee, so they agree we're doing the right thing here.

[ras0219]

I used docker[1] to get a copy of clang 10 and didn't see a motivation (i.e. compiles without warnings) for two of the changes; otherwise, looks great!

[1] https://solarianprogrammer.com/2017/12/14/clang-in-docker-container-cpp-17-development/

[ignacionr]

@ignacionr this... seems like an unfortunate change. What warnings does clang throw after passing the vector as a span?

Fwiw, this path has been tread before, by the standards committee, so they agree we're doing the right thing here.

I was able to reproduce this only for a minute on the Visual Studio Code messages. At some point, the messages just disappeared. It may have been related to whatever version of compiler/standard library the IDE was using before re-configuring. I will remove the changes (since they proved unnecessary), as I agree they don't look appealing to read.

They were as follows.

{
	"resource": "/mnt/hdd/src/vcpkg-clean/toolsrc/src/vcpkg/build.cpp",
	"owner": "C/C++",
	"severity": 8,
	"message": "no suitable user-defined conversion from \"const std::vector<std::string, std::allocator<std::string>>\" to \"vcpkg::View<std::string>\" exists",
	"startLineNumber": 170,
	"startColumn": 67,
	"endLineNumber": 170,
	"endColumn": 71
}

{
	"resource": "/mnt/hdd/src/vcpkg-clean/toolsrc/src/vcpkg/build.cpp",
	"owner": "C/C++",
	"severity": 8,
	"message": "no suitable user-defined conversion from \"std::vector<vcpkg::Build::AbiEntry, std::allocator<vcpkg::Build::AbiEntry>>\" to \"vcpkg::Span<const vcpkg::Build::AbiEntry>\" exists",
	"startLineNumber": 962,
	"startColumn": 74,
	"endLineNumber": 962,
	"endColumn": 89
}

But I will keep those that generate warnings (unused fn, unused params).

[ignacionr]

@strega-nil @ras0219 this seems ready now. Care to merge?

[strega-nil]

/azp run

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