Upgrade to Rust 2018

[brson]

After TiKV itself is successfully upgraded (tikv/tikv#3896) we can bump prometheus to Rust 2018 as well. Do a major version bump.

[brson]

In thinking about backwards compatibility I'm inclined to explicitly break support for Rust 2015 when we do this upgrade - it's possible to be compatible with both (in some situations, I believe), but by doing so we have to be vigilant about breaking 2015 compatibility. I'm inclined to insert some boilerplate code into each of fail, prometheus, and grpc that is definitely 2018 only and breaks the build under 2015 so we don't have to worry about breaking 2015 compatibility in a minor version bump.

Let me know if anybody is opposed to that.

[Hoverbear]

@brson When you get there please investigate using const fn in our upgrade:

This is a common use case:

lazy_static! {
    static ref TIKV_REQUEST_DURATION_HISTOGRAM_VEC: HistogramVec = register_histogram_vec!(
        "tikv_request_duration_seconds",
        "Bucketed histogram of TiKV requests duration",
        &["type"]
    )
}

[brson]

I have a branch here that does the upgrade https://github.com/brson/rust-prometheus/tree/rust-2018

I have not looked into const fn yet but I will.

[brson]

I'm also trying to figure out how far back in the Rust stable toolchain one has to go before cargo starts just ignoring edition = "2018", since that's when it's theoretically possible to be compatible with both 2015 and 2018. I am hopefully though that cargo started rejecting edition = "2018 at the same time it started accepting crate in paths, making it very unlikely to have a single crate that's compatible with Rust 2018 and pre-2018 toolchains.

[brson]

The tikv toolchain is upgraded to a version that supports 2018 now, so I think once it's had a few days on master w/o being reverted it's safe to go ahead. I'll do so sometime next week.

[brson]

Oh, I haven't done anything with const fn in the posted patch yet.

[brson]

I filed another bug on const fn because I doubt I'm going to do that work in the upgrade PR.

[brson]

I don't see any CI here. Not sure if that's expected or not.

[Hoverbear]

@brson This is an issue, nothing to test. :)

[brson]

Lol 😆

[brson]

Reassigned to @nrc since he has a newer PR up

[breezewish]

Fixed in #242