ICE: AVR CPU not explicitly specified when building `core` for `avr-atmega328p` custom target

[vincentmuriithi]

Problem Description:

I am attempting to cross-compile a Rust project for an AVR ATmega328p microcontroller. I'm using a custom target JSON file and trying to build core and compiler_builtins from source using cargo build -Z build-std.

The compiler consistently panics with the error "AVR CPU not explicitly specified" during the compilation of core (and subsequently compiler_builtins).

What I've tried:

1. Updated Rustup and Nightly Toolchain: I have ensured my rustup and nightly toolchain are up to date.

   • rustup --version: rustup 1.28.2 (e4f3ad6f8 2025-04-28)
   • rustc version (from rustup update nightly output): rustc 1.90.0-nightly (0d9592026 2025-07-19)

2. Attempted different linker flavors:

   • Initially, the linker-flavor in my custom target JSON was gcc.
   • I changed linker-flavor to ld.lld in the target JSON to see if that resolved the issue, but the panic persists. (Note: rust-lld component was not explicitly added as rustup indicated it's not a separate component for this host target).

Expected Behavior:

The core and compiler_builtins libraries should compile successfully for the avr-atmega328p target, recognizing the cpu specified in the target JSON.

My Custom Target JSON (../targets/avr-atmega328p.json):

{
  "llvm-target": "avr-unknown-unknown",
  "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
  "target-endian": "little",
  "target-pointer-width": "16",
  "arch": "avr",
  "os": "none",
  "vendor": "unknown",
  "cpu": "atmega328p",
  "linker-flavor": "ld.lld",
  "relocation-model": "static",
  "panic-strategy": "abort",
  "disable-redzone": true,
  "executables": true,
  "max-atomic-width": 8
}

[jfrimmel]

Since a while you don't need to use a custom target anymore, instead you use avr-none. This is also the reason for the error message you're facing: you need to specify the CPU to be used (normally for the generic target) by passing -Ctarget-cpu=atmega328p, e.g. via the .cargo/config.toml:

[build]
rustflags = ["-C", "target-cpu=atmega328p"]

This is documented on the target support page for AVR.

I suggest to use the built-in avr-none-target, but the issue in question is resolved by supplying -Ctarget-cpu=....

[jfrimmel]

@rustbot label -I-ICE +O-AVR