CI: Pin a minimum supported stable Rust version

[b-r-u]

To avoid accidentally increasing the minimum supported Rust version, the CI scripts could include a build for a fixed stable Rust version.
I'm not sure what the minimum working version is right now (I tried to lower it with #31), because I couldn't test all platforms.

[b-r-u]

Ok, so I tested different Rust versions on a fork (https://github.com/b-r-u/directories-rs/commits/ci-rust-1.20) and it looks like everything works even with 1.13 (from 2016-11-10, which stabilized the ? operator) with a minor change (b-r-u@7466791).

I also conducted a small survey on the minimum supported Rust version of some popular crates:

Crate	Minimum Rust Version
serde	1.13
log	1.16
lazy_static	1.21
regex	1.20

https://github.com/rust-lang/regex#minimum-rust-version-policy

[soc]

Thanks @b-r-u, I'll look into this tomorrow!

[soc]

@b-r-u Sorry for the delay ... so what I'm wondering is whether there is some way to explicitly set minimal versions for Rust and crates in a separate CI run.

Apart from that, would you suggest lowering the crate versions mentioned in cargo.toml to the minimum you figured out here?

[b-r-u]

so what I'm wondering is whether there is some way to explicitly set minimal versions for Rust and crates in a separate CI run.

There is rust-lang/cargo#4100 for minimal crate versions but I think it's not yet stable.

Apart from that, would you suggest lowering the crate versions mentioned in cargo.toml to the minimum you figured out here?

Maybe there was a misunderstanding? I was looking at popular crates to get a feeling for the minimal Rust version they support. I don't think you should lower any dependency versions.

[soc]

Ah, alright, understood. Thanks!

[b-r-u]

I could have been clearer and updated the comment! #32 (comment)