Include a page about 2018-only changes

[Centril]

Include a dedicated page that explains exactly what parts that are only available on 2018 + all the breakage.
This should be a tiny list.

[steveklabnik]

@Centril do you still plan on doing this this week? If not, it's going to miss the milestone.

[Centril]

@steveklabnik Yeah, I should; this feels important to do for RC1.

[steveklabnik]

This has to land tonight, RC1 is shipping in the morning.

[Centril]

@steveklabnik I'll get to working on it today; was very much asleep when you wrote your comment =P

[steveklabnik]

This did not land, de-milestoning.

[sdht0]

Is it still scheduled? This page would be really helpful.

[ehuss]

If I wrote a page for this, would you consider adding it? AFAIK, here is an exhaustive list of the stable 2018-specific changes:

• dyn is now a full keyword.
• async, await, try are now reserved keywords.
• Added ? kleene operator to macros-by-example.
• NLL
• tyvar_behind_raw_pointer is now a hard error.
• Overflowing literals are now rejected.
• Anonymous trait function parameters are not allowed.
  • All irrefutable patterns are allowed with a body.
• Paths:
  • Paths starting with :: can only reference crates.
  • use declarations can reference crates in the extern prelude.
  • use declarations cannot reference the crate root.
    • :: leading prefix required for name clashes (uniform_paths future-proofing).
  • Visibility paths must be absolute.

Additionally, I think it wouldn't hurt to include the 2018 idioms:

• bare_trait_objects: dyn Trait is recommended over Trait
• unused_extern_crates: extern crate should not be used
• ellipsis_inclusive_range_patterns: ... ranges are deprecated, should use ..=
• elided_lifetimes_in_paths: Lifetimes should not be elided in some cases.
• explicit_outlives_requirements: Lifetime bounds should be elided in some cases.

[Centril]

@ehuss Yeah I was supposed to fix it but never got the time; if you can do it, that'd be great.

[steveklabnik]

Additionally, I think it wouldn't hurt to include the 2018 idioms:

The release team has been very not interested in describing the idiom lints, as they're not ready yet. I would leave this out.