`#[derive(Debug)]` is introducing potentially panicking code

It looks like in some cases #[derive(Debug)] is implemented via debug_struct_fields_finish() which contains the following assertion:

https://github.com/rust-lang/rust/blob/42ec52babac2cbf2bb2b9d794f980cbcb3ebe413/library/core/src/fmt/mod.rs#L2565

This breaks an otherwise panic-free code.

Update with MCVE

Cargo.toml:

[package]
name = "debug_struct"
version = "0.1.0"
edition = "2024"

[dependencies]

[profile.release]
lto = "thin"
debug = true
opt-level="z"
panic="abort"

main.rs

#![no_std]
#![no_main]
use core::fmt::Write;

#[repr(C)]
#[derive(Debug)]
struct Test {
    a0: u32,
    a1: u32,
    a2: u32,
    a3: u32,
    a4: u32,
    a5: [u32; 33],
}

impl Default for Test {
    fn default() -> Self {
        Self {
            a5: [0; 33],
            .. Default::default()
        }
    }
}

struct FakeWriter;
impl core::fmt::Write for FakeWriter {
    fn write_str(&mut self, s: &str) -> core::fmt::Result {
        for c in s.bytes() {
            unsafe {
                core::ptr::write_volatile(0x1000_0000 as *mut u8, c);
            }
        }
        Ok(())
    }
}
#[unsafe(no_mangle)]
extern "C" fn _start() {
    let test = Test::default();
    let _ = writeln!(FakeWriter, "{:#X?}", test);
}

#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
    unsafe extern "C" {
        #[link_name = "\n\nError: panic is possible on some code path\n\n"]
        unsafe fn never_panic() -> !;
    }
    unsafe { never_panic() }
}

Build with command:

cargo build --release --target=riscv32imc-unknown-none-elf

Will fail with linking error due to the panic handler referencing undefined function (credits to the no-panics-whatsoever crate authors.

Observations:

1. Will build with opt-level=3
2. Will build with Test::a5 shorter than 33 elements (curiously the same threshold as for #[derive(Default)]
3. Will build with number of fields 5 or less (likely because in that case debug_struct_fields_finish() is not being called, but debug_struct_fields_finishX is called instead

Start with the assertion in library/core/src/fmt/mod.rs around line 2565 and reproduce the MCVE using the provided Cargo project and riscv32imc-unknown-none-elf release build command. Compare the behavior across the reported field counts, array lengths, and optimization levels. Done means the reported panic path is addressed without breaking the affected Debug formatting behavior.