{"id":"4871db1764b71dd0","repo":"rust-lang/cargo","slug":"failed-to-serialize-credential-provider-error","errorCode":null,"errorMessage":"failed to serialize credential provider error","messagePattern":"failed to serialize credential provider error","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"credential/cargo-credential/src/lib.rs","lineNumber":237,"sourceCode":"/// in the `CredentialHello` message. Cargo will then choose which protocol to use,\n/// or it will error if there are no common protocol versions available.\npub const PROTOCOL_VERSION_1: u32 = 1;\npub trait Credential {\n    /// Retrieves a token for the given registry.\n    fn perform(\n        &self,\n        registry: &RegistryInfo<'_>,\n        action: &Action<'_>,\n        args: &[&str],\n    ) -> Result<CredentialResponse, Error>;\n}\n\n/// Runs the credential interaction\npub fn main(credential: impl Credential) {\n    let result = doit(credential).map_err(|e| Error::Other(e));\n    if result.is_err() {\n        serde_json::to_writer(std::io::stdout(), &result)\n            .expect(\"failed to serialize credential provider error\");\n        println!();\n    }\n}\n\nfn doit(\n    credential: impl Credential,\n) -> Result<(), Box<dyn std::error::Error + Send + Sync + 'static>> {\n    let hello = CredentialHello {\n        v: vec![PROTOCOL_VERSION_1],\n    };\n    serde_json::to_writer(std::io::stdout(), &hello)?;\n    println!();\n\n    loop {\n        let mut buffer = String::new();\n        let len = std::io::stdin().read_line(&mut buffer)?;\n        if len == 0 {\n            return Ok(());","sourceCodeStart":219,"sourceCodeEnd":255,"githubUrl":"https://github.com/rust-lang/cargo/blob/0e07a155371a6ce88ae53a2c00df940280c09a67/credential/cargo-credential/src/lib.rs#L219-L255","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","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."],"exampleFix":"// before — provider returns a non-serializable error\nfn perform(...) -> Result<_, Error> { Err(Error::Other(Box::new(my_struct))) }\n// after — return a serializable error type\nfn perform(...) -> Result<_, Error> { Err(Error::Message(\"token not found\".into())) }","handlingStrategy":"validation","validationCode":"// In a custom credential provider, ensure errors serialize before returning them.\nfn err_serializes(e: &cargo_credential::Error) -> bool {\n    serde_json::to_string(e).is_ok()\n}\n// assert err_serializes(&err) before returning from perform()","typeGuard":null,"tryCatchPattern":null,"preventionTips":["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."],"tags":["cargo","credential","serde","panic","json"],"analyzedSha":"0e07a155371a6ce88ae53a2c00df940280c09a67","analyzedAt":"2026-08-06T01:46:58.334Z","schemaVersion":2}