Add links to how to disable the default prelude

[Nemo157]

Running grep -r no_implicit_prelude doc src/{libstd,libcore} resulted in only
the one hit in the reference. Added links from the documentation for
std::prelude and core::prelude::v1 to make it easier to find.

r? @steveklabnik

[rust-highfive]

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[Nemo157]

I think make docs has decided to do a full rebuild so it's probably gonna be a couple of hours till I can check that the relative references are actually correct... pretty sure they are though.

[Nemo157]

Finally finished building (under 2 hours 😄) and the links work fine.

[alexcrichton]

I... don't think that this was intended to be stable actually, it's probably a mistake that it's accepted on the stable compiler. We have an accepted RFC to rename this attribute but unfortunately the tracking issue has seen very little activity.

[petrochenkov]

Prelude mechanics will likely require some rethinking after the name resolution reform.

[Nemo157]

Heh, I did search the issues for no_implicit_prelude to see if there were any issues around it and couldn't find any. Looks like #20561 doesn't mention it by name at all.

[Nemo157]

@alexcrichton would no_prelude be stable if #20561 were implemented, or are there reasons to not want this stabilised yet?

[alexcrichton]

I think that no_prelude would likely have a period of instability after which it'd go through FCP for stabilization. (as most unstable features do)

[Nemo157]

Ok, I was thinking I could maybe look at implementing #20561 in the next few days to try and get it moving towards stabilisation. I guess no_implicit_prelude will need to be deprecated and no_prelude would be feature gated at first? If that were done would adding the documentation pointing to it prior to stabilisation be fine, or should it be left undocumented (or just documented in the nightly docs maybe?) until it's stable?

[alexcrichton]

Oh that'd be great to get that implemented! We'd probably want to hold off documentation in the prelude itself until its stable, but there are nightly sections of the docs it would fit well into.

[Nemo157]

Cool, I should hopefully get to it in the next week assuming it's not to hard. Thanks for the input.