Update futures to 0.3.1

[martell]

SpawnExt should take &self as their base traits
rust-lang/futures-rs#1959

Motivation

It seems that we might be cutting a new alpha soon ™️ #1708
We should probably use futures 0.3.1 because various libs that have been moving to the futures 0.3 like hyper, actix-net etc are on 0.3.1.
Building 2 versions of futures to include tokio when the alpha is cut would not be ideal

Solution

Update from futures 0.3.0 -> 0.3.1

[carllerche]

Could you explain why the change is necessary? Tokio does not use the Spawn trait and cargo will happily bump users to 0.3.1.

[martell]

Hey @carllerche, maybe I am misunderstanding how semver works with cargo deps?

Assumption :
0.3.0 will only build as 0.3.0
^0.3.0 is >=0.3.0 <0.4.0 // but we probably don't want to do this incase of breaking changes?
0.3.1 will only build as 0.3.1

If another crate uses futures @ 0.3.1 at the same time as using tokio (currently from master) will it build both futures 0.3.0 and futures 0.3.1 ?

If so I presume that is very undesirable for build time and maybe undesirable for stripping at link time?

[carllerche]

without a sigil, Cargo defaults to any "semver compatible" version, so that includes 0.3.1.

Cheers,

[martell]

Ahh Thank You. Sorry for the noise.
I eventually found it noted somewhere, in the rust book 🤦‍♂

For anyone else who runs into this in the future 😄
https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#specifying-dependencies-from-cratesio

Specifically the passage that says

The string "0.1.12" is a semver version requirement. Since this string does not have any operators in it, it is interpreted the same way as if we had specified "^0.1.12", which is called a caret requirement.