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

Keep a single `crate = path` matching the re-exported tokio path.

tokio-rs/tokio · error · syn::Error

`crate` set multiple times.

Error message

`crate` set multiple times.

What it means

`set_crate_name` (entry.rs:156) records the first `crate = path` (an indirection used when tokio is re-exported under a different name) and rejects a second `crate` key in the same attribute.

Source

Thrown at tokio-macros/src/entry.rs:158

            return Err(syn::Error::new(span, "`worker_threads` may not be 0."));
        }
        self.worker_threads = Some((worker_threads, span));
        Ok(())
    }

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

        let start_paused = parse_bool(start_paused, span, "start_paused")?;
        self.start_paused = Some((start_paused, span));
        Ok(())
    }

    fn set_crate_name(&mut self, name: syn::Lit, span: Span) -> Result<(), syn::Error> {
        if self.crate_name.is_some() {
            return Err(syn::Error::new(span, "`crate` set multiple times."));
        }
        let name_path = parse_path(name, span, "crate")?;
        self.crate_name = Some(name_path);
        Ok(())
    }

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

View on GitHub (pinned to 625954f365)

Solutions

Keep a single `crate = path` matching the re-exported tokio path.

Example fix

// before
#[tokio::main(crate = renamed_tokio, crate = tokio)]
async fn main() {}

// after
#[tokio::main(crate = renamed_tokio)]
async fn main() {}

Defensive patterns

Strategy: validation

Prevention

Set `crate = path` at most once, pointing at your tokio re-export.
Only needed when tokio is re-exported under a different crate name.
Remove the option entirely if you depend on tokio directly.

When it happens

Trigger: Writing `#[tokio::main(crate = my_tokio, crate = other)]`.

Common situations: Renaming the tokio dependency and leaving both the old and new `crate =` paths; merge conflicts in attributes.

Related errors

AI-assisted analysis of tokio-rs/tokio@625954f365 (2026-08-11). Data as JSON: /api/errors/4cd0a0d0d4fd6ef2. Report an issue: GitHub.