auto merge of #9863 : csainty/rust/issue-9755-once-fns-feature-directive, r=alexcrichton

Hello,

First time rust contributor here, please let me know if I need to sort out the contribution agreement for this.

I picked issue #9755 to dip my toe in the water, this pull request isn't quite complete though as I have not updated the documentation. The reason for this is that I haven't tracked down why this feature is gated so I don't feel I can write a justification of the same quality as the other features have been documented.
If someone would like to explain or point me at a mail thread I am happy to update with this change.

Hopefully I have understood the process of converting the old flag into a directive correctly.

Also just to call out what I am sure if a known quirk when adding feature directives, you can't build this code unless you have a snapshot of the compiler which knows about the feature directive. Chicken and the egg. I split the change into two commits, the first should be able to build a snapshot that can compile the second.

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