I am getting a double fault error that causes the MCU to lock up:


[at91samd21g17d.cpu] clearing lockup after double fault

Program received signal SIGINT, Interrupt.
0xfffffffe in ?? ()


The lockup occurs while executing the following line of code:

bare_metal::CriticalSection::new () at src/lib.rs:53


This fault occurs in the following point in my stack:

#0  cortex_m::interrupt::free<atsamd_hal::thumbv6m::usb::bus::buffer::{closure_env#0}, core::option::Option<&mut [u8; 2048]>> (f=...)
    at /home/fret/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-0.7.7/src/interrupt.rs:64
#1  0x00003a28 in atsamd_hal::thumbv6m::usb::bus::buffer () at src/thumbv6m/usb/bus.rs:157
>#3  0x0000045c in atsamd_hal::thumbv6m::usb::bus::UsbBus::new<atsamd_hal::gpio::pin::Pin<atsamd_hal::gpio::pin::PA24, atsamd_hal::gpio::pin::Alternate<atsamd_hal::gpio:
:pin::G>>, atsamd_hal::gpio::pin::Pin<atsamd_hal::gpio::pin::PA25, atsamd_hal::gpio::pin::Alternate<atsamd_hal::gpio::pin::G>>> (_clock=0x20003134, pm=0x20003580, 
    dm_pad=..., dp_pad=..., _usb=...) at /home/fret/.cargo/registry/src/github.com-1ecc6299db9ec823/atsamd-hal-0.15.1/src/thumbv6m/usb/bus.rs:590
#4  0x000008a6 in weart_est::bsp::usb_allocator<atsamd_hal::gpio::pin::Pin<atsamd_hal::gpio::pin::PA24, atsamd_hal::gpio::pin::Disabled<atsamd_hal::gpio::pin::Floating>
, atsamd_hal::gpio::pin::Pin<atsamd_hal::gpio::pin::PA25, atsamd_hal::gpio::pin::Disabled<atsamd_hal::gpio::pin::Floating>>> (usb=..., clocks=0x200035a8, 
    pm=0x20003580, dm=..., dp=...) at src/bsp.rs:293
#5  0x00000cd8 in weart_est::__cortex_m_rt_main () at src/main.rs:55
#6  0x00000c0c in weart_est::__cortex_m_rt_main_trampoline () at src/main.rs:24


I anticipate it's a memory allocation problem or similar but am struggling to identify the source as I can't catch the error in any handler or get a backtrace! Very challenging.