Prelude injection should not use gated features

[brson]

Right now the prelude magically uses both #[feature(phase)] and #[feature(globs)] without declaring so. Something as critical as the prelude should not require the use of features that aren't actually part of the stable language. Nominating.

[eddyb]

To remove the dependency on #[phase], we might be able to call syntax::ext::expand::load_extern_macros from rustc, but that looks like a hack to me.

If globs are removed, resolve could still special-case prelude(s), without having complicated semi-cycle-aware glob resolution logic (std::prelude would be disallowed from having a prelude).

[klutzy]

I think glob issue can be solved by managing prelude list at rustc::front::std_inject. POC code here.

[huonw]

Having the list in std is nice because it means it can be documented, and having this outside the compiler is generally nice too. I'm not sure if either of these two factors are strong enough to override the need to avoid feature gating (in that manner, at least).

[alexcrichton]

I was under the impression that use foo::bar::*; will be un-gated once pub use foo::bar::*; is removed, and #[phase(syntax)] will be ungated for loading macro_rules! macros. Dropping #[phase(syntax, link)] sounds a little dubious (what if you don't want the macros?), however.

[pnkfelix]

we can bless the use of certain implementation strategies in the prelude itself, such as in this case using #[feature(phase)] and #[feature(globs)], that are just being used to provide functionality that could be expressed by some other means. As long as we only allow the prelude to employ #[feature(...)] in cases like that, where we can still specify what provided functionality is, without referencing the features themselves that the prelude used.

[pnkfelix]

(we should avoid adding more feature cruft to the prelude, but if we need to do, it needs to basically adhere to the same constraints that I described above).

[pnkfelix]

Marking as P-low, not 1.0 milestone.

[alexcrichton]

Neither of these features are gated any more, so this is no longer an issue.