Remove = requirement and use cargo update --precise instead

[est31]

Adopting the changes suggested by @cuviper in #1099 (comment) .

Fixes #1169

[Thomasdezeeuw]

Thanks!

@carllerche this might require a new 0.6.x release.

[carllerche]

If we are going to make this change, we should increase the minimum supported rust version.

[est31]

@carllerche why? There is no need to increase it on the 0.6.x branch too, and it is useful to have a large MSRV range.

Most people probably won't, but anyone who wants to actually use Rust 1.18 can just runcargo update --precise themselves, or if they want to, they can add a = requirement for cfg-if to their own Cargo.toml as well. Whatever mio does, it's already the case that you can't run cargo update blindly if you need to support an older Rust version, but you have to update crates one by one. So people just do cargo update -p mio.

There are further alternatives like a clone of the entire registry that contains only crates supported by a specific Rust version, or custom resolvers entirely (there exist prototypes for both). Managing MSRVs via = requirements is not good though and causes problems for everyone. Nor is increasing MSRV just for this :).

[carllerche]

sounds good