`use` imports are wrapped unless double-nested imports are present

rustfmt gives this:

use {
    fidl_fuchsia_hardware_ethernet_ext::EthernetQueueFlags, futures::TryStreamExt, std::fs::File,
    structopt::StructOpt,
};

which is, IMO, pretty gross as it mixes imports from many different crates all together onto the same line.

Adding a group after _ext:: and adding Bar to it gives this:

use {
    fidl_fuchsia_hardware_ethernet_ext::{Bar, EthernetQueueFlags},
    futures::TryStreamExt,
    std::fs::File,
    structopt::StructOpt,
};

which is more like what I would expect. What's going on here? Is there a setting that can be enabled to always get the bottom layout, even when no nested groups are present?

Start by reproducing the two import examples from the issue and inspect rustfmt's import-formatting behavior and available configuration options. Determine whether an existing setting can produce the grouped layout, or define the behavior needed for a new option; done means imports from separate crates remain on separate lines without requiring a nested group.