[RFC] Add a minimal baremetal testsuite to Phobos

[wilzbach]

Similarly to #6640, this will allow to add tests to Phobos for functionality that doesn't depend on libc.
It currently contains a minimal Linux x86 runtime.

Future

We probably want to add a special UDA (e.g. @test-baremetal and @test-betterc) to Phobos and thus extract such tests automatically (instead of keeping them separate).

Another possible route is to combine this with the testsuite runner of DMD or at least have support for minimal things like TEST_OUTPUT.

CC @ZombineDev

[dlang-bot]

Thanks for your pull request, @wilzbach!

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + phobos#6641"

[wilzbach]

We probably want to add a special UDA

dlang/tools#357 (also supports just strings, e.g. @("betterc"))

[wilzbach]

Posted about how the UDA (e.g. @betterCTest and @baremetalTest) could be named on the NG: https://forum.dlang.org/post/irovmywdhafdromajrwc@forum.dlang.org

[JinShil]

I don't care for the term baremetal; I suggest freestanding instead. But, I also notice that this seems to still link in the C standard library.

Also, -betterC does not imply baremetal or freestanding; it just doen't link in the D runtime. To demonstrate D operating in a baremetal/freestanding environment, it should have a D runtime ported to that environment, so I suggest using a minimal runtime instead.

Ultimately, though, D should have a FreeStanding version identifier and the official D runtime code should have implementations for that environment that are expanded over time to include more and more features of D. There's no reason why even a GC couldn't be used in a baremetal/freestanding environment if the system has the resources and the use case allows it. I think we should strive for that instead.

Note also the freestanding-on-linux is not the same as freestanding-on-AMD64, freestanding-on-x86, freestanding-on-AArch64, etc. We'll need the D runtime to have implementations of all those scenarios, and preferably a CI that can actually exercise them (maybe in virtual environment like VirtualBox, QEMU, etc. or if possible on physical hardware).

[andralex]

What is the use case?

[wilzbach]

To test that some functions in Phobos don't depend on druntime at all which is important for people like @JinShil who would love to use parts of Phobos in their baremetal development.
Anyhow, this isn't very important to me and we managed to get @betterC, so I'm closing this for now ...