What causes "Multiple stm32xx Cargo features enabled" in embassy-rs/embassy?

Keep exactly one stm32xx chip feature enabled per build (fix Cargo.toml features array)

Multiple stm32xx Cargo features enabled

Error message

Multiple stm32xx Cargo features enabled

What it means

embassy-stm32's build script requires exactly one chip target: it collects env vars starting with CARGO_FEATURE_STM32 (excluding CARGO_FEATURE_STM32_HRTIM) and asserts a single one. Multiple chip features cannot be unified into one generated codebase, so it panics.

Solutions

Keep exactly one stm32xx chip feature enabled per build (fix Cargo.toml features array)
Inspect `cargo tree -f '{p} {f}'` to find which crate pulls in the second chip feature and remove it
Split multi-board projects into separate packages or use a Cargo workspace where each package has its own single chip feature

Example fix

# before
features = ["stm32f103c8", "stm32f411ce"]
# after
features = ["stm32f103c8"]

Defensive patterns

Strategy: validation

Validate before calling

# ensure a single chip feature per build
cargo tree -f '{p} {f}' | grep -oE 'stm32[a-z0-9]+' | sort -u | wc -l  # expect 1

Prevention

One chip feature per package
Separate packages per board in a workspace
CI check for duplicated chip features via cargo tree

When it happens

Trigger: Cargo feature unification enables two chip features (e.g. stm32f103c8 + stm32f411ce) at once — usually because two crates in the same build each request a different chip, or a Cargo.toml lists two chip features.

Common situations: Same package used by binaries for different boards in one workspace; listing two chip features 'to be safe'; a dependency enabling a second chip feature.

Related errors

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/224ace08ebfd44ef. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/build.rs:59

fn main() {
    let mut cfgs = common::CfgSet::new();
    common::set_target_cfgs(&mut cfgs);

    if std::env::var("CARGO_FEATURE_RT").is_err()
        && std::env::var("CARGO_CFG_TARGET_OS") == Ok("none".to_string())
        && std::env::var("CARGO_CFG_TARGET_ARCH") == Ok("arm".to_string())
    {
        println!("cargo::warning=Building for bare-metal ARM without `rt` feature: interrupts will loop forever.");
    }

    let chip_name = match env::vars()
        .map(|(a, _)| a)
        .filter(|x| x.starts_with("CARGO_FEATURE_STM32") && x != "CARGO_FEATURE_STM32_HRTIM")
        .get_one()
    {
        Ok(x) => x,
        Err(GetOneError::None) => panic!("No stm32xx Cargo feature enabled"),
        Err(GetOneError::Multiple) => panic!("Multiple stm32xx Cargo features enabled"),
    }
    .strip_prefix("CARGO_FEATURE_")
    .unwrap()
    .to_ascii_lowercase();

    eprintln!("chip: {chip_name}");

    for p in METADATA.peripherals {
        if let Some(r) = &p.registers {
            // The AES driver enables the peripheral's clock, which the metadata does not
            // know for the AES of some chips (STM32L0, L1, F423): no driver there.
            if r.kind == "aes" && p.rcc.is_none() {
                continue;
            }
            cfgs.enable(r.kind);
            foreach_version_cfg(&mut cfgs, r.kind, r.version, |cfgs, cfg_name| {
                cfgs.enable(cfg_name);
            });

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