Rust 2018

[anti-social]

Need for #555

As we have to refactor crate imports anyway to be able to use async feature. So which one do you prefer?

An old way:

#[cfg(feature="containers")]
use crate::builder::context::Context;
#[cfg(feature="containers")]
use crate::builder::packages;
#[cfg(feature="containers")]
use crate::builder::commands::generic::{run_command_at_env, capture_command};
use build_step::{BuildStep, VersionError, StepError, Digest, Config, Guard};
use crate::file_util::copy;

or a compact one:

#[cfg(feature="containers")]
use crate::{
    builder::context::Context,
    builder::packages,
    builder::commands::generic::{run_command_at_env, capture_command},
};
use crate::{
    build_step::{BuildStep, VersionError, StepError, Digest, Config, Guard},
    file_util::copy,
};

[anti-social]

Possibly we can go further:

#[cfg(feature="containers")]
use crate::{
    builder::{
        context::Context,
        packages,
        commands::generic::{run_command_at_env, capture_command},
    },
};

[anti-social]

I like the second option.

[tailhook]

I think you can use indented block with cfg(feature as an exception. But for non optional blocks, please, use one use per line. It's easier to grep, copy and even read most of the time.

[tailhook]

Also feel free to convert to 2021 right away. It should be much easier change after 2018.