[ICE]: Trying to flash the builtin LED of the Arduino Uno WIFI R2 board

Code

#![no_std]
#![no_main]
#![feature(asm_experimental_arch)]

use avr_device::atmega4809;
use core::panic::PanicInfo;

#[no_mangle]
pub extern "C" fn main() -> ! {
  let dp = atmega4809::Peripherals::take().unwrap();

  dp.PORTE.dirset.write(|w| unsafe { w.bits(1 << 2) });

  loop {
    dp.PORTE.outtgl.write(|w| unsafe { w.bits(1 << 2) });

    for _ in 0..30000 {
      unsafe { core::arch::asm!("nop") };
    }
  }
}

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
  loop {}
}

Meta

rustc --version --verbose:

rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: x86_64-unknown-linux-gnu
release: 1.92.0
LLVM version: 21.1.3

Error output

Compiling core v0.0.0 (/home/uapaxyeya/.asdf/installs/rust/1.92.0/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core)
   Compiling compiler_builtins v0.1.160 (/home/uapaxyeya/.asdf/installs/rust/1.92.0/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins)

thread 'rustc' (255327) panicked at /rustc-dev/38c0de8dcb14d42290042521be9958d37f3fa390/compiler/rustc_codegen_ssa/src/back/metadata.rs:376:17:
/rustc-dev/38c0de8dcb14d42290042521be9958d37f3fa390/compiler/rustc_codegen_ssa/src/back/metadata.rs:376:17: AVR CPU not explicitly specified
stack backtrace:
   0: std::panicking::begin_panic::<alloc::string::String>
   1: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   2: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   3: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   4: rustc_middle::util::bug::bug_fmt
   5: rustc_codegen_ssa::back::metadata::elf_e_flags.cold
   6: rustc_codegen_ssa::back::metadata::create_object_file
   7: rustc_codegen_ssa::back::metadata::create_wrapper_file
   8: rustc_codegen_ssa::back::link::link_rlib
   9: rustc_codegen_ssa::back::link::link_binary
  10: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::link
  11: <rustc_interface::queries::Linker>::link
  12: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: the compiler unexpectedly panicked. This is a bug

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/home/uapaxyeya/.asdf/installs/rust/1.92.0/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/rustc-ice-2026-03-02T02_04_09-255325.txt` to your bug report

note: rustc 1.96.0-nightly (38c0de8dc 2026-02-28) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C opt-level=3 -C embed-bitcode=no -Z unstable-options -C strip=debuginfo -Z force-unstable-if-unmarked

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `core` (lib)

Caused by:
  process didn't exit successfully: `/home/uapaxyeya/.asdf/installs/rust/1.92.0/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name core --edition=2024 /home/uapaxyeya/.asdf/installs/rust/1.92.0/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=208 --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no --warn=unexpected_cfgs --check-cfg 'cfg(no_fp_fmt_parse)' --check-cfg 'cfg(feature, values(any()))' --check-cfg 'cfg(target_has_reliable_f16)' --check-cfg 'cfg(target_has_reliable_f16_math)' --check-cfg 'cfg(target_has_reliable_f128)' --check-cfg 'cfg(target_has_reliable_f128_math)' --check-cfg 'cfg(llvm_enzyme)' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("debug_refcell", "llvm_enzyme", "optimize_for_size", "panic_immediate_abort"))' -C metadata=0871f1efbe247110 -C extra-filename=-296e39fb0880f35a --out-dir /usr/local/src/gittorrent/crates/microcontrollers/arduino_torrent/target/avr-atmega4809/release/deps --target /usr/local/src/gittorrent/crates/microcontrollers/arduino_torrent/avr-atmega4809.json -Zunstable-options -C strip=debuginfo -Z force-unstable-if-unmarked -L dependency=/usr/local/src/gittorrent/crates/microcontrollers/arduino_torrent/target/avr-atmega4809/release/deps -L dependency=/usr/local/src/gittorrent/crates/microcontrollers/arduino_torrent/target/release/deps --cap-lints allow` (exit status: 101)
warning: build failed, waiting for other jobs to finish...

thread 'rustc' (255362) panicked at /rustc-dev/38c0de8dcb14d42290042521be9958d37f3fa390/compiler/rustc_codegen_ssa/src/back/metadata.rs:376:17:
/rustc-dev/38c0de8dcb14d42290042521be9958d37f3fa390/compiler/rustc_codegen_ssa/src/back/metadata.rs:376:17: AVR CPU not explicitly specified
stack backtrace:
   0: std::panicking::begin_panic::<alloc::string::String>
   1: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   2: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   3: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   4: rustc_middle::util::bug::bug_fmt
   5: rustc_codegen_ssa::back::metadata::elf_e_flags.cold
   6: rustc_codegen_ssa::back::metadata::create_object_file
   7: rustc_codegen_ssa::back::metadata::create_wrapper_file
   8: rustc_codegen_ssa::back::link::link_rlib
   9: rustc_codegen_ssa::back::link::link_binary
  10: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::link
  11: <rustc_interface::queries::Linker>::link
  12: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: the compiler unexpectedly panicked. This is a bug

note: using internal features is not supported and expected to cause internal compiler errors when used incorrectly

note: please attach the file at `/home/uapaxyeya/.asdf/installs/rust/1.92.0/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins/rustc-ice-2026-03-02T02_04_16-255359.txt` to your bug report

note: rustc 1.96.0-nightly (38c0de8dc 2026-02-28) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C opt-level=3 -C embed-bitcode=no -Z unstable-options -C strip=debuginfo -Z force-unstable-if-unmarked

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `compiler_builtins` (lib)

thread 'rustc' panicked at /rustc-dev/38c0de8dcb14d42290042521be9958d37f3fa390/compiler/rustc_codegen_ssa/src/back/metadata.rs:376:17:
/rustc-dev/38c0de8dcb14d42290042521be9958d37f3fa390/compiler/rustc_codegen_ssa/src/back/metadata.rs:376:17: AVR CPU not explicitly specified
stack backtrace:
   0:     0x7f5eda57d6bb - <std[228bdc9d2b636708]::backtrace::Backtrace>::create
   1:     0x7f5eda57d605 - <std[228bdc9d2b636708]::backtrace::Backtrace>::force_capture
   2:     0x7f5ed956f3c4 - std[228bdc9d2b636708]::panicking::update_hook::<alloc[da121800ca3a425d]::boxed::Box<rustc_driver_impl[b079875acc15f158]::install_ice_hook::{closure#1}>>::{closure#0}
   3:     0x7f5eda5901b2 - std[228bdc9d2b636708]::panicking::panic_with_hook
   4:     0x7f5ed9b80992 - std[228bdc9d2b636708]::panicking::begin_panic::<alloc[da121800ca3a425d]::string::String>::{closure#0}
   5:     0x7f5ed9b6c9c6 - std[228bdc9d2b636708]::sys::backtrace::__rust_end_short_backtrace::<std[228bdc9d2b636708]::panicking::begin_panic<alloc[da121800ca3a425d]::string::String>::{closure#0}, !>
   6:     0x7f5ed9b65133 - std[228bdc9d2b636708]::panicking::begin_panic::<alloc[da121800ca3a425d]::string::String>
   7:     0x7f5ed9b80d86 - rustc_middle[ed586653b79aa9c2]::util::bug::opt_span_bug_fmt::<rustc_span[9fc5ff705eb51a66]::span_encoding::Span>::{closure#0}
   8:     0x7f5ed9b80f02 - rustc_middle[ed586653b79aa9c2]::ty::context::tls::with_opt::<rustc_middle[ed586653b79aa9c2]::util::bug::opt_span_bug_fmt<rustc_span[9fc5ff705eb51a66]::span_encoding::Span>::{closure#0}, !>::{closure#0}
   9:     0x7f5ed9b71f8b - rustc_middle[ed586653b79aa9c2]::ty::context::tls::with_context_opt::<rustc_middle[ed586653b79aa9c2]::ty::context::tls::with_opt<rustc_middle[ed586653b79aa9c2]::util::bug::opt_span_bug_fmt<rustc_span[9fc5ff705eb51a66]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  10:     0x7f5ed7e88f14 - rustc_middle[ed586653b79aa9c2]::util::bug::bug_fmt
  11:     0x7f5edcca1444 - rustc_codegen_ssa[9f9f62c73c3e6b04]::back::metadata::elf_e_flags.cold
  12:     0x7f5edbf4fa62 - rustc_codegen_ssa[9f9f62c73c3e6b04]::back::metadata::create_object_file
  13:     0x7f5edbf4ffb6 - rustc_codegen_ssa[9f9f62c73c3e6b04]::back::metadata::create_wrapper_file
  14:     0x7f5edbc12789 - rustc_codegen_ssa[9f9f62c73c3e6b04]::back::link::link_rlib
  15:     0x7f5edbba26e3 - rustc_codegen_ssa[9f9f62c73c3e6b04]::back::link::link_binary
  16:     0x7f5edbba20e9 - <rustc_codegen_llvm[f83347ca346a9490]::LlvmCodegenBackend as rustc_codegen_ssa[9f9f62c73c3e6b04]::traits::backend::CodegenBackend>::link
  17:     0x7f5edbee9d7a - <rustc_interface[d301261baebd53ed]::queries::Linker>::link
  18:     0x7f5edbe6a19a - rustc_interface[d301261baebd53ed]::interface::run_compiler::<(), rustc_driver_impl[b079875acc15f158]::run_compiler::{closure#0}>::{closure#1}
  19:     0x7f5edbde4f7e - std[228bdc9d2b636708]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[d301261baebd53ed]::util::run_in_thread_with_globals<rustc_interface[d301261baebd53ed]::util::run_in_thread_pool_with_globals<rustc_interface[d301261baebd53ed]::interface::run_compiler<(), rustc_driver_impl[b079875acc15f158]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  20:     0x7f5edbde55e0 - <std[228bdc9d2b636708]::thread::lifecycle::spawn_unchecked<rustc_interface[d301261baebd53ed]::util::run_in_thread_with_globals<rustc_interface[d301261baebd53ed]::util::run_in_thread_pool_with_globals<rustc_interface[d301261baebd53ed]::interface::run_compiler<(), rustc_driver_impl[b079875acc15f158]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[ca6b92032f8209e6]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  21:     0x7f5edbde646c - <std[228bdc9d2b636708]::sys::thread::unix::Thread>::new::thread_start
  22:     0x7f5ed58a597a - <unknown>
  23:     0x7f5ed59292bc - <unknown>
  24:                0x0 - <unknown>


rustc version: 1.96.0-nightly (38c0de8dc 2026-02-28)

I'm hesitant to make this bug report, because I easily fixed this bug by changing '/.cargo/config.toml' from this:

[build]
target = "avr-atmega4809.json"

[unstable]
build-std        = ["core"]
json-target-spec = true

to this:

[build]
target = "avr-atmega4809.json"
rustflags = ["-C", "target-cpu=atmega4809"]

[unstable]
build-std        = ["core"]
json-target-spec = true

Reproduce the AVR build from the supplied no_std example, then inspect compiler/rustc_codegen_ssa/src/back/metadata.rs around elf_e_flags, create_object_file, and create_wrapper_file. Done means the reported AVR compilation no longer causes an internal compiler panic, with regression coverage for the failing invocation.