What causes "was not filled at beginning of the function" in rust-lang/cargo?

If embedding cargo as a library, call load_credentials() once at startup before spawning threads.

was not filled at beginning of the function

Error message

was not filled at beginning of the function

What it means

This panic is at the end of GlobalContext::load_credentials(). The function guards re-entry by checking self.credential_values.filled() at the top (line 1785) and returning Ok(()) early if already loaded. At line 1822-1824 it calls self.credential_values.set(credential_values) and expects success. The OnceCell .set() fails only if already filled — so if something filled credential_values between the top check and this call, the expect panics.

Source

Thrown at src/context/mod.rs:1824

        let mut credential_values = HashMap::default();
        if let CV::Table(map, _) = value {
            let base_map = self.values()?;
            for (k, v) in map {
                let entry = match base_map.get(&k) {
                    Some(base_entry) => {
                        let mut entry = base_entry.clone();
                        entry.merge(v, true)?;
                        entry
                    }
                    None => v,
                };
                credential_values.insert(k, entry);
            }
        }
        self.credential_values
            .set(credential_values)
            .expect("was not filled at beginning of the function");
        Ok(())
    }

    /// Looks for a path for `tool` in an environment variable or the given config, and returns
    /// `None` if it's not present.
    fn maybe_get_tool(
        &self,
        tool: &str,
        from_config: &Option<ConfigRelativePath>,
    ) -> Option<PathBuf> {
        let var = tool.to_uppercase();

        match self.get_env_os(&var).as_ref().and_then(|s| s.to_str()) {
            Some(tool_path) => {
                let maybe_relative = tool_path.contains('/') || tool_path.contains('\\');
                let path = if maybe_relative {
                    self.cwd.join(tool_path)
                } else {

View on GitHub (pinned to 0e07a15537)

Solutions

If embedding cargo as a library, call load_credentials() once at startup before spawning threads.
Synchronize access to GlobalContext credential loading with a mutex or ensure single-threaded init.
Verify the ~/.cargo/credentials file is not corrupted or concurrently modified.

Defensive patterns

Strategy: validation

Validate before calling

// Call load_credentials once at startup before any concurrent access
ctx.load_credentials()?;
// Subsequent calls are no-ops due to the filled() guard

Prevention

Initialize credentials once at startup in single-threaded context before spawning workers.
Do not call load_credentials() from multiple threads without synchronization.
Ensure ~/.cargo/credentials is not concurrently modified by external processes.

When it happens

Trigger: A re-entrant or concurrent call to load_credentials() that fills the OnceCell between the filled() check on line 1785 and the set() on line 1824. Since load_credentials takes &self (not &mut self), concurrent calls are possible if the OnceCell is not otherwise synchronized.

Common situations: Embedding cargo as a library and calling load_credentials() from multiple threads simultaneously; a cargo internal code path that triggers credential loading recursively; corrupted credentials file causing a partial load that fills the cell before the full load completes.

Related errors

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/69d97a4fbca5d11c.json. Report an issue: GitHub.