Implement missing shims for Windows

[RalfJung]

Here we track shims that Miri currently only implements for Linux and macOS targets, but not for Windows.

If you run into this, try cargo miri test --target x86_64-unknown-linux-gnu. This works even on a Windows host (without installing anything extra) and interprets programs on a well-supported Linux target.

• File system access (Windows file system shims are missing #3482)
• Threading
• std::thread::sleep
• std::sync
• Console functions (Windows console API shims are missing #2652)

[jmerdich]

Also add RNG to the list (rand's default initial seed comes from the OS via BCryptGenRandom).

[RalfJung]

We do test that getrandom works on Windows, why isn't rand using that? For this to work we implement SystemFunction036 which is the actual name of RtlGenRandom. I thought supporting getrandom would be enough to cover RNG initialization.

[RalfJung]

In fact we even test this through rand:

miri/test-cargo-miri/tests/test.rs

Line 28 in 2cdd174

let mut rng = SmallRng::from_entropy();

@jmerdich SmallRng::from_entropy() and rand::thread_rng() both work on Windows, so I cannot reproduce the problem you are describing.

[RalfJung]

Ah, looks like newer versions of getrandom changed which function they are using.

[RalfJung]

@jmerdich thanks for the notification, #1755 should fix this. :)

[jmerdich]

@RalfJung thanks for the amazingly prompt fix! I got set up to contribute something for this and was surprised to find it already done! :D

[RalfJung]

Ah, sorry for that.^^

Fixing std::thread::sleep should be rather easy, if you are looking for something small to get started with. :)

[beepster4096]

I'm trying to implement file IO right now
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[RalfJung]

That's great! I don't know a lot about the Windows File IO APIs, so I can't tell if it makes more sense to share code with Unix or to have a completely different implementation. But either way you can use the Unix version as a template. There's even a testcase already, tests/run-pass/fs.rs, that you just need to enable on Windows.

Yeah we don't have all the rustc tooling, but just leaving a comment is enough. :)