Update lifetimes to include anonymous lifetime syntax

[steveklabnik]

This issue tracks inclusion of the new '_ syntax into the Book. This will happen in the 2018 edition.

Tracking issue: rust-lang/rust#48469

[steveklabnik]

None of these features seem actually stable yet; de-milestoning.

[steveklabnik]

This is supposedly stabilized.

[carols10cents]

@steveklabnik I'm removing the Anonymous Lifetime section for now (and not sending it to nostarch), and thus reopening this issue, and here's the main reason why:

• The bit in the mini table of contents says "making elision more obvious"
• The example shows a function with the signature fn wrap<'a>(string: &'a str) -> StrWrap<'a> and says "look, we can write fewer lifetimes by using '_ instead" which... doesn't make elision more obvious? Because the previous example doesn't have elision, it's explicit?
• So then I looked at the edition guide, which shows that it's valid to write that function without ANY lifetime, so fn wrap(string: &str) -> StrWrap, which does use elision, and the point of this feature is to make it clearer that there is a lifetime involved without making you add an explicit lifetime
• Which IMO means what we actually need to be motivating in this section is why you would choose to add more of what some folks see as "sigil noise" with '_ if you can just leave it off

And a secondary reason is that the tracking issue for this feature has unchecked checkboxes and isn't closed, which says to me that there's more work to be done here, the most interesting of which is "Apply to rustc, experimenting with naming conventions (e.g., working through the affect on rustc)." This feature sounds like an emerging convention that isn't ready to be codified in the book yet.

If you want to put this back in, I think we should iterate on this a bit, but I'm not going to hold up the batch of updates to the print book we're working on right now, I think we can let this go until the next batch.

[steveklabnik]

I don’t disagree it can use some work, but the intent in my understanding is to lint against not including the anonymous lifetime there.

…

On Jan 3, 2019, at 6:01 PM, Carol (Nichols || Goulding) ***@***.***> wrote: Reopened #1273. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or mute the thread.

[steveklabnik]

(So, to be 100% clear, I’m fine with reverting for now)

…

On Jan 3, 2019, at 8:41 PM, Steve Klabnik ***@***.***> wrote: I don’t disagree it can use some work, but the intent in my understanding is to lint against not including the anonymous lifetime there. > On Jan 3, 2019, at 6:01 PM, Carol (Nichols || Goulding) ***@***.***> wrote: > > Reopened #1273. > > — > You are receiving this because you were mentioned. > Reply to this email directly, view it on GitHub, or mute the thread.

[carols10cents]

the intent in my understanding is to lint against not including the anonymous lifetime there.

Yeah 1.31 doesn't lint for that right now, which makes me feel better about not including it at the moment.