Compile on stable Rust 2018

[LukasKalbertodt]

It would be neat to be able to use this on stable (once use_extern_macros is stabilized). Currently the following features are activated:

• #![feature(in_band_lifetimes)]: tracking issue, expected to be stabilized for Rust 2018. If not, we can easily remove the feature by changing one line of code. => probably won't be stabilized, so this has been removed from this crate for now.
• #![feature(proc_macro_span)]: used only for one Span::join() call. I think it's easy to remove join() with something we can use on stable. => guarded by nightly feature (see Guard unstable proc_macro things behind nightly feature #26)
• #![feature(proc_macro_diagnostic)]: this is the big part! I don't think the Diagnostics API will be stabilized in near future. So in order to compile on stable, we need to replace all nice diagnostics with simple panics. This shouldn't be too hard. => guarded by nightly feature (see Guard unstable proc_macro things behind nightly feature #26)

We still use Cargo's cargo-features = ["edition"] (EDIT: it's stabilized on nightly and beta RC1 now) with edition ="2018" which entails a couple of features. But all of these should be stabilized for Rust 2018 (EDIT: they are now).

• Implicit extern crates (see (Modules) Tracking issue for (use) crate_name:: paths without extern crate rust-lang/rust#53128)
• #![feature(extern_prelude)] (see (Modules) Tracking issue for (use) crate_name:: paths without extern crate rust-lang/rust#53128)
• #![feature(crate_visibility_modifier)] (see (Modules) Tracking issue for crate as a visibility modifier rust-lang/rust#53120) (TODO: this might not make it into Rust 2018, maybe switch to pub(crate) for now?) -> replaced by pub(crate)
• #![feature(crate_in_paths)] (see (Modules) Tracking issue for Picking a Module Path System variant rust-lang/rust#53130)

[Mark-Simulacrum]

replace all nice diagnostics with simple panics.

I would instead use the compile_error macro -- that'll give you slightly nicer error messages and it is already stable.

[LukasKalbertodt]

@Mark-Simulacrum Awesome idea! I'll certainly try that!

[LukasKalbertodt]

I think we should release a new version before the official Rust 2018 release. So I removed the v0.3 milestone.

[LukasKalbertodt]

With #31 this crate compiles on beta RC1. So we're on track for having this compile on stable Rust 2018. I guess we'll keep this issue open until the real Rust 2018 is released and we can be sure that all is fine.

EDIT: Also compiles with rustc 1.31.0-beta.1 (2824a67b0 2018-10-29)
EDIT2: Also compiles with rustc 1.31.0-beta.19 (42053f9f0 2018-11-26)

[LukasKalbertodt]

Seems to work with Rust 2018 (1.31), so I'm gonna close this :)

Happy Rust2018!