Consistent no_prelude attribute

[Nemo157]

Implementation of rust-lang/rfcs#501 tracked in #20561.

Adds the no_prelude attribute that stops automatic injecting of the standard prelude in a single module behind a feature gate.
Deprecates the existing no_implicit_prelude.

[Nemo157]

I'm tempted to leave the deprecation until no_prelude is stabilised, these changes mean there's no way to stop injection of the prelude on a stable compiler without a warning. This could be ok though, according to #31900 (comment) no_implicit_prelude shouldn't have been stabilised in the first place.

Since the behaviour of no_prelude and no_implicit_prelude are different I was thinking maybe the deprecation warning should mention this. I haven't given much thought to the wording but that might result in a really long message if there's no easy way to explain the difference.

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[jseyfried]

Reviewed, LGTM except that I don't think we should deprecate no_implicit_prelude until we've stabilized no_prelude (which shouldn't take very long given its simplicity). @alexcrichton what do you think?

[alexcrichton]

How come there are two warnings emitted here?

[Nemo157]

The feature gate visitor is run twice, once before cfg expansion and once after. I could try and find some way to avoid the double up, but the existing simd attribute deprecation warning does the same.

[alexcrichton]

Hm ok, we probably want to figure out whether we're deprecating or not before going to great lengths to figure this out, and it's probably fine either way.

[alexcrichton]

Thanks @Nemo157! In terms of tests, can you also add a new test to ensure that #![no_prelude] is feature gated?

In terms of deprecation, we have a few possible routes we can take here:

• Deprecate immediately as this PR is doing. The downside is that there is no recourse for users leveraging this attribute on stable Rust.
• Remove entirely immediately. This has the downside of breaking users of the attribute on stable Rust and forcing them to move to nightly.
• Not deprecating until #![no_prelude] is stable. Downside here is that we lengthen the lifespan of #![no_implicit_prelude] by quite a bit.

To me the answer of what to do here kinda depends on if there's any widespread usage of #![no_implicit_prelude]. If there's not actually any usage then we can probably do whatever we want here, but if there is the situation is a little more tricky.

Doing a quick grep over the latest code on crates.io right now I see:

• nailgun 0.1.0
• gtk 0.0.6 - couldn't find in https://github.com/gtk-rs/gtk

So given that we probably can't remove immediately. We may also not be able to deprecate either without stabilizing #![no_prelude].

I'm personally always a fan of more aggressive deprecation, so I'd prefer to deprecate immediately, but others may feel differently!

cc @rust-lang/lang

[bors]

☔ The latest upstream changes (presumably #30884) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #31954) made this pull request unmergeable. Please resolve the merge conflicts.

[aturon]

See also #32131 (comment)

[alexcrichton]

Nominating for discussion at a lang team meeting (I think I applied the right tags?)

[bors]

☔ The latest upstream changes (presumably #30652) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

We discussed how to proceed in the @rust-lang/lang meeting. Our feeling was that:

1. We should not deprecate #[no_implicit_prelude] until a stable alternative is available.
2. It's probably not worth it to remove #[no_implicit_prelude]; the code maintenance burden is low. You can simply think of #[no_implicit_prelude] as removing the prelude but substituting a module with the primitive types.

[bors]

☔ The latest upstream changes (presumably #32875) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

Closing due to inactivity, but feel free to resubmit with a rebase!