What causes "failed to serialize credential provider error" in rust-lang/cargo?

Update or fix the credential provider so its errors serialize cleanly to JSON.

failed to serialize credential provider error

Error message

failed to serialize credential provider error

What it means

This is a PANIC, not a returned error. In `cargo_credential::main` (credential/cargo-credential/src/lib.rs:237), when a credential provider's `doit()` fails, cargo tries to serialize the error as JSON to stdout for the parent cargo process. If `serde_json::to_writer` fails (extremely unlikely — only if the `Error` type is non-serializable or stdout itself is broken), `.expect("failed to serialize credential provider error")` panics, aborting the credential helper subprocess.

Source

Thrown at credential/cargo-credential/src/lib.rs:237

/// in the `CredentialHello` message. Cargo will then choose which protocol to use,
/// or it will error if there are no common protocol versions available.
pub const PROTOCOL_VERSION_1: u32 = 1;
pub trait Credential {
    /// Retrieves a token for the given registry.
    fn perform(
        &self,
        registry: &RegistryInfo<'_>,
        action: &Action<'_>,
        args: &[&str],
    ) -> Result<CredentialResponse, Error>;
}

/// Runs the credential interaction
pub fn main(credential: impl Credential) {
    let result = doit(credential).map_err(|e| Error::Other(e));
    if result.is_err() {
        serde_json::to_writer(std::io::stdout(), &result)
            .expect("failed to serialize credential provider error");
        println!();
    }
}

fn doit(
    credential: impl Credential,
) -> Result<(), Box<dyn std::error::Error + Send + Sync + 'static>> {
    let hello = CredentialHello {
        v: vec![PROTOCOL_VERSION_1],
    };
    serde_json::to_writer(std::io::stdout(), &hello)?;
    println!();

    loop {
        let mut buffer = String::new();
        let len = std::io::stdin().read_line(&mut buffer)?;
        if len == 0 {
            return Ok(());

View on GitHub (pinned to 0e07a15537)

Solutions

Update or fix the credential provider so its errors serialize cleanly to JSON.
Ensure the parent cargo process is not killed prematurely (which can close the stdout pipe).
Run the credential helper standalone to reproduce and inspect the failing error value.

Example fix

// before — provider returns a non-serializable error
fn perform(...) -> Result<_, Error> { Err(Error::Other(Box::new(my_struct))) }
// after — return a serializable error type
fn perform(...) -> Result<_, Error> { Err(Error::Message("token not found".into())) }

Defensive patterns

Strategy: validation

Validate before calling

// In a custom credential provider, ensure errors serialize before returning them.
fn err_serializes(e: &cargo_credential::Error) -> bool {
    serde_json::to_string(e).is_ok()
}
// assert err_serializes(&err) before returning from perform()

Prevention

Return only serializable error types from credential providers (use Error::Message / well-formed variants).
Test the provider's error path in isolation.
Avoid stdout interference that could break the JSON pipe to cargo.

When it happens

Trigger: A credential provider process encounters an internal error whose `Error` value cannot be serialized to JSON by serde_json, OR stdout is closed/broken at the moment of writing. The panic message is the expect string.

Common situations: A custom credential provider returning a non-serializable error variant; stdout pipe closed early by the parent cargo process being killed; a bug in a third-party credential helper producing an error type that breaks serde's Serialize impl.

Related errors

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/4871db1764b71dd0.json. Report an issue: GitHub.