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Code style

smoltcp does not follow the rustfmt code style because whitequark (the original author of smoltcp) finds automated formatters annoying and impairing readability just as much as improving it in different cases.

In general, format the things like the existing code and it'll be alright. Here are a few things to watch out for, though:

Ordering use statements

Use statements should be separated into two sections, uses from other crates and uses from the current crate. The latter would ideally be sorted from most general to most specific, but it's not very important.

use core::cell::RefCell;

use crate::{Error, Result};
use crate::phy::{self, DeviceCapabilities, Device};

Wrapping function calls

Avoid rightwards drift. This is fine:

assert_eq!(iface.inner.process_ethernet(&mut socket_set, 0, frame.into_inner()),
           Ok(Packet::None));

This is also fine:

assert_eq!(iface.inner.lookup_hardware_addr(MockTxToken, 0,
    &IpAddress::Ipv4(Ipv4Address([0x7f, 0x00, 0x00, 0x01])),
    &IpAddress::Ipv4(remote_ip_addr)),
    Ok((remote_hw_addr, MockTxToken)));

This is not:

assert_eq!(iface.inner.lookup_hardware_addr(MockTxToken, 0,
                                            &IpAddress::Ipv4(Ipv4Address([0x7f, 0x00, 0x00, 0x01])),
                                            &IpAddress::Ipv4(remote_ip_addr)),
    Ok((remote_hw_addr, MockTxToken)));

Wrapping function prototypes

A function declaration might be wrapped...

right after ,,
right after >,
right after ),
right after ->,
right before and after where.

Here's an artificial example, wrapped at 50 columns:

fn dispatch_ethernet<Tx, F>
                    (&mut self, tx_token: Tx,
                     timestamp: u64, f: F) ->
                    Result<()>
    where Tx: TxToken,
          F: FnOnce(EthernetFrame<&mut [u8]>)
{
    // ...
}

Visually aligning tokens

This is fine:

struct State {
    rng_seed:    u32,
    refilled_at: u64,
    tx_bucket:   u64,
    rx_bucket:   u64,
}