Minimum supported Rust version

[francesca64]

I've noticed that we don't seem to pin to a specific stable Rust version on CI. Now that 1.26 changed match behavior, it's very likely someone's accidentally going to make winit unusable for anyone using an older version.

So, how far back do we want to support?

[elinorbgr]

wayland-client requires rust >= 1.20.0, and this is tested so unlikely to change unless absolutely needed.

So, I guess this puts a lower-bound onto what winit can require.

[b-r-u]

parking_lot requires Rust 1.24.0 and with some small changes winit also compiles with 1.24.0.
Edit: @francesca64 already mentioned these changes.

[francesca64]

1.20.0 is what I was hoping for, since I have some ideas for using associated constants.

I didn't realize that parking_lot requires 1.24.0, or I would've raised this issue sooner. I'd say "well, it's probably fine, since no one's complained about it", but that doesn't mean anything, since almost no one's used winit 0.14.0 in the first place.

Though, I don't know how important it really is that we support older versions; I think the main argument is for people who install Rust from a package manager instead of rustup?

[b-r-u]

Keeping a minimum supported Rust version is also nice because it ensures that CI doesn't fail for dependent crates that check a specific Rust version, although this is usually only the case for more low-level crates.

As parking_lot is unlikely to support versions older than 1.24 (Amanieu/parking_lot@bf0fa86), I tried replacing it with std::sync::Mutex and discovered that winit also requires 1.24 because std::mem::size_of was not const before.

So maybe this is not worth the trouble and winit should embrace Rust 1.24?

[francesca64]

I'm fine with 1.24, though I'm not sure how to specify that for CI other than Travis.