tokio-rs/tokio · error · syn::Error

The `threaded_scheduler` runtime flavor has been renamed to

Error message

The `threaded_scheduler` runtime flavor has been renamed to `multi_thread`.

What it means

`RuntimeFlavor::from_str` (entry.rs:24) keeps the pre-1.0 name `threaded_scheduler` only to redirect users. The current multi-threaded flavor is `multi_thread`.

Source

Thrown at tokio-macros/src/entry.rs:24

// syn::AttributeArgs does not implement syn::Parse
type AttributeArgs = syn::punctuated::Punctuated<syn::Meta, syn::Token![,]>;

#[derive(Clone, Copy, PartialEq)]
enum RuntimeFlavor {
    CurrentThread,
    Threaded,
    Local,
}

impl RuntimeFlavor {
    fn from_str(s: &str) -> Result<RuntimeFlavor, String> {
        match s {
            "current_thread" => Ok(RuntimeFlavor::CurrentThread),
            "multi_thread" => Ok(RuntimeFlavor::Threaded),
            "local" => Ok(RuntimeFlavor::Local),
            "single_thread" => Err("The single threaded runtime flavor is called `current_thread`.".to_string()),
            "basic_scheduler" => Err("The `basic_scheduler` runtime flavor has been renamed to `current_thread`.".to_string()),
            "threaded_scheduler" => Err("The `threaded_scheduler` runtime flavor has been renamed to `multi_thread`.".to_string()),
            _ => Err(format!("No such runtime flavor `{s}`. The runtime flavors are `current_thread`, `local`, and `multi_thread`.")),
        }
    }
}

#[derive(Clone, Copy, PartialEq)]
enum UnhandledPanic {
    Ignore,
    ShutdownRuntime,
}

impl UnhandledPanic {
    fn from_str(s: &str) -> Result<UnhandledPanic, String> {
        match s {
            "ignore" => Ok(UnhandledPanic::Ignore),
            "shutdown_runtime" => Ok(UnhandledPanic::ShutdownRuntime),
            _ => Err(format!("No such unhandled panic behavior `{s}`. The unhandled panic behaviors are `ignore` and `shutdown_runtime`.")),
        }

View on GitHub (pinned to 625954f365)

Solutions

1. Replace `flavor = "threaded_scheduler"` with `flavor = "multi_thread"`.
2. Ensure the `rt-multi-thread` feature is enabled in `Cargo.toml` (required for the multi-thread flavor).

Example fix

// before (tokio 0.x)
#[tokio::main(threaded_scheduler)]
async fn main() {}

// after (tokio 1.x)
#[tokio::main(flavor = "multi_thread")]
async fn main() {}

When it happens

Trigger: Using `#[tokio::main(flavor = "threaded_scheduler")]` from old tokio code.

Common situations: Migrating from tokio 0.x; stale Stack Overflow answers / blog posts.

Related errors

• The `basic_scheduler` runtime flavor has been renamed to `cu
• The single threaded runtime flavor is called `current_thread
• `name` set multiple times.
• `flavor` set multiple times.
• No such runtime flavor `{s}`. The runtime flavors are `curre

AI-assisted analysis of tokio-rs/tokio@625954f365 (2026-08-11). Data as JSON: /api/errors/d7053367f48b7241. Report an issue: GitHub.