What causes "`name` set multiple times." in tokio-rs/tokio?

Remove the duplicate `name` argument so it appears exactly once.

tokio-rs/tokio · error · syn::Error

`name` set multiple times.

Error message

`name` set multiple times.

What it means

The `#[tokio::main]` / `#[tokio::test]` attribute accepts a `name = "..."` option that sets the runtime builder's name. The macro's config builder (`set_name`, entry.rs:104) rejects a second `name` occurrence in the same invocation with a `syn::Error`, surfaced as a compile error.

Source

Thrown at tokio-macros/src/entry.rs:106

        Configuration {
            name: None,
            rt_multi_thread_available: rt_multi_thread,
            default_flavor: match is_test {
                true => RuntimeFlavor::CurrentThread,
                false => RuntimeFlavor::Threaded,
            },
            flavor: None,
            worker_threads: None,
            start_paused: None,
            is_test,
            crate_name: None,
            unhandled_panic: None,
        }
    }

    fn set_name(&mut self, name: syn::Lit, span: Span) -> Result<(), syn::Error> {
        if self.name.is_some() {
            return Err(syn::Error::new(span, "`name` set multiple times."));
        }

        let runtime_name = parse_string(name, span, "name")?;
        self.name = Some(runtime_name);
        Ok(())
    }

    fn set_flavor(&mut self, runtime: syn::Lit, span: Span) -> Result<(), syn::Error> {
        if self.flavor.is_some() {
            return Err(syn::Error::new(span, "`flavor` set multiple times."));
        }

        let runtime_str = parse_string(runtime, span, "flavor")?;
        let runtime =
            RuntimeFlavor::from_str(&runtime_str).map_err(|err| syn::Error::new(span, err))?;
        self.flavor = Some(runtime);
        Ok(())
    }

View on GitHub (pinned to 625954f365)

Solutions

Remove the duplicate `name` argument so it appears exactly once.
If two names were intended for different runtimes, split the code into two annotated entry points.

Example fix

// before
#[tokio::main(name = "rt1", name = "rt2")]
async fn main() {}

// after
#[tokio::main(name = "rt1")]
async fn main() {}

Defensive patterns

Strategy: validation

Prevention

Each `#[tokio::main]`/`#[tokio::test]` option key must appear at most once — self-check your attribute before compiling.
When merging configs, replace rather than append option keys.
Keep a single canonical macro invocation per entry point.

When it happens

Trigger: Writing `#[tokio::main(name = "a", name = "b")]` (or the `tokio::test` equivalent) — two `name` keys in one attribute.

Common situations: Merging two attribute configs by hand; copy-paste of options; editing tools that append rather than replace.

Related errors

AI-assisted analysis of tokio-rs/tokio@625954f365 (2026-08-11). Data as JSON: /api/errors/0eb845fb3c457c98. Report an issue: GitHub.