What solution did you come up with?

Here's mine:

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use aux5::{entry, prelude::*, Delay, Leds};

#[entry]
fn main() -> ! {
    let (mut delay, mut leds): (Delay, Leds) = aux5::init();

    let ms = 50_u8;
    loop {
        for curr in 0..8 {
            let next = (curr + 1) % 8;

            leds[next].on();
            delay.delay_ms(ms);
            leds[curr].off();
            delay.delay_ms(ms);
        }
    }
}

One more thing! Check that your solution also works when compiled in "release" mode:

$ cargo build --target thumbv7em-none-eabihf --release

You can test it with this gdb command:

$ # or, you could simply call `cargo run --target thumbv7em-none-eabihf --release`
$ arm-none-eabi-gdb target/thumbv7em-none-eabihf/release/led-roulette
$ #                                              ~~~~~~~

Binary size is something we should always keep an eye on! How big is your solution? You can check that using the size command on the release binary:

$ # equivalent to size target/thumbv7em-none-eabihf/debug/led-roulette
$ cargo size --target thumbv7em-none-eabihf --bin led-roulette -- -A
led-roulette  :
section               size        addr
.vector_table          392   0x8000000
.text                16404   0x8000188
.rodata               2924   0x80041a0
.data                    0  0x20000000
.bss                     4  0x20000000
.debug_str          602185         0x0
.debug_abbrev        24134         0x0
.debug_info         553143         0x0
.debug_ranges       112744         0x0
.debug_macinfo          86         0x0
.debug_pubnames      56467         0x0
.debug_pubtypes      94866         0x0
.ARM.attributes         58         0x0
.debug_frame        174812         0x0
.debug_line         354866         0x0
.debug_loc             534         0x0
.comment                75         0x0
Total              1993694

$ cargo size --target thumbv7em-none-eabihf --bin led-roulette --release -- -A
led-roulette  :
section              size        addr
.vector_table         392   0x8000000
.text                1826   0x8000188
.rodata                84   0x80008ac
.data                   0  0x20000000
.bss                    4  0x20000000
.debug_str          23334         0x0
.debug_loc           6964         0x0
.debug_abbrev        1337         0x0
.debug_info         40582         0x0
.debug_ranges        2936         0x0
.debug_macinfo          1         0x0
.debug_pubnames      5470         0x0
.debug_pubtypes     10016         0x0
.ARM.attributes        58         0x0
.debug_frame          164         0x0
.debug_line          9081         0x0
.comment               18         0x0
Total              102267

NOTE The Cargo project is already configured to build the release binary using LTO.

Know how to read this output? The text section contains the program instructions. It's around 2KB in my case. On the other hand, the data and bss sections contain variables statically allocated in RAM (static variables). A static variable is being used in aux5::init; that's why it shows 4 bytes of bss.

One final thing! We have been running our programs from within GDB but our programs don't depend on GDB at all. You can confirm this be closing both GDB and OpenOCD and then resetting the board by pressing the black button on the board. The LED roulette application will run without intervention of GDB.