Change std::fs::File.set_len example

[jethrogb]

The File object needs to be writable for the set_len to succeed.

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[bluss]

Maybe it's best to even say in the doc that the file must be writable or have the appropriate permissions. Not sure how to word it so that it's correct for Windows.

[jethrogb]

MSDN is not quite clear on what permission exactly is needed, but I assume GENERIC_WRITE is ok, so we can just say 'writeable' or 'opened for writing'.

https://msdn.microsoft.com/en-us/library/windows/desktop/aa365539%28v=vs.85%29.aspx
https://msdn.microsoft.com/en-us/library/windows/desktop/aa364224%28v=vs.85%29.aspx

[alexcrichton]

Thanks @jethrogb! I agree with @bluss that it'd be nice to have a clause in the documentation which suggests that this works best if the file is opened for writing as well.

[jethrogb]

Done.

[alexcrichton]

@bors: r+ 9262d64

Thanks!

[bors]

⌛ Testing commit 9262d64 with merge 78547d2...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt