This repo is a close to minimal crate to reproduce a hard-fault that occurs on cortex-m4 (thumbv7em-none-eabihf and likely others) with any Rust nightly later than 2023-08-08 (tested on 2023-08-09 and 2023-12-10 and some others in between).
It appears possible that this is related specifically to the Debug derive on Option, and occurs only when attempting to format! an Option::None (or probably write! or similar routines).
In addition to the hard fault, it has been possible by adjusting the code (in the process of reducing this down) to cause a panic in rtt-target's print routine where the call to critical-section's borrow_ref_mut() says the RefCell is already borrowed, which should of course not be possible. I suspect this is a sign that the problem relates to memory corruption of some kind, possibly stomping on the RefCell's borrow counter.
I've been running on nRF52840 hardware, though there's nothing left in the code that is specific to the 52840.
cargo +nightly-2023-08-09-x86_64-unknown-linux-gnu build
rust-objcopy target/thumbv7em-none-eabihf/debug/none-fault -O ihex none-fault.hex
probe-rs download --chip nrf52840 --format hex none-fault.hex
probe-rs reset --chip nrf52840
RTT=$(grep _SEGGER_RTT$ build.map | cut -d' ' -f1); echo $RTT >.rttlast
rtthost --chip nrf52840 --scan-region 0x$RTT
Produces this output:
Attaching to RTT...
Found control block at 0x200008e4
----------------
fail now?
ExceptionFrame {
r0: 0x00000000,
r1: 0x000038ed,
r2: 0x200000e4,
r3: 0x200000e4,
r12: 0x000003f8,
lr: 0x00001169,
pc: 0x200000e4,
xpsr: 0x40000000,
}
Doing exactly the same but with nightly 2023-08-08 gives this output:
Attaching to RTT...
Found control block at 0x200008e4
----------------
fail now?
format!: None
works