Complete the std::time documentation to warn about the inconsistencies between OS

[poliorcetics]

Fixes #48980.

I put the new documentation in src/libstd/time.rs at the module-level because it affects all types, even the one that are not directly system dependents if they are used with affected types, but there may be a better place for it.

[rust-highfive]

r? @shepmaster

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[jhpratt]

This isn't universally true. Duration, for example, is represented as Duration { secs: u64, nanos: u32 }. That doesn't differ between operating systems.

Without checking, I'm fairly certain Instant already documents these potential inconsistencies, so I don't think this is necessary.

[rust-highfive]

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Ensuring the YAML anchors in the GitHub Actions config were expanded
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tidy error: /checkout/src/libstd/time.rs:18: unexplained "```ignore" doctest; try one:
* make the test actually pass, by adding necessary imports and declarations, or
* make the test actually pass, by adding necessary imports and declarations, or
* use "```text", if the code is not Rust code, or
* use "```compile_fail,Ennnn", if the code is expected to fail at compile time, or
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* use "```no_run", if the code should type-check but not necessary linkable/runnable, or
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command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo"


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== clock drift check ==
  local time: Sun May 31 21:15:21 UTC 2020
  network time: Sun, 31 May 2020 21:15:21 GMT
== end clock drift check ==

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[poliorcetics]

This isn't universally true. Duration, for example, is represented as Duration { secs: u64, nanos: u32 }. That doesn't differ between operating systems.

Without checking, I'm fairly certain Instant already documents these potential inconsistencies, so I don't think this is necessary.

The issue raised the case that Duration and Instant interact together and so Duration is affected too. I could clarify the added doc to something like:

//! Some of those types are wrapper around system-specific types and as such they
 //! may behave differently depending on the underlying operating system,
//! even when used with other, OS-independent types.

What do you think about it ?

[jhpratt]

Perhaps what I don't understand is why this need to be on the module. Clarifying the OS inconsistencies in any given struct would, of course, make sense.

[poliorcetics]

I put it at the module level because it can affect even "safe" types like Duration, but I can of course move it, maybe to Instant since SystemTime and SystemTimeError are already quite clear from their name ?

[jhpratt]

But you're not affecting the OS-independent types. It's only when interacting with Instant & SystemTime that varying behavior can occur.

[poliorcetics]

I moved the warning to Instant following you recommendations.

[poliorcetics]

@shepmaster I think this is done, could you have a look and confirm it ?

[shepmaster]

Sorry for the slow response!

[poliorcetics]

No problem !

[shepmaster]

And one last thing — might as well squash all these commits into one. Not much value in the intermediate steps. ❤️

[poliorcetics]

Will do in the following minutes !

[poliorcetics]

It should be done now, let's hope I didn't break something for CI while squashing.

[shepmaster]

@bors r+ rollup

[bors]

📌 Commit 9e51008 has been approved by shepmaster