What causes "Failed to parse value of `{field}` as path." in tokio-rs/tokio?

Quote the path: `#[tokio::main(crate = "my_tokio")]`.

tokio-rs/tokio · error · syn::Error

Failed to parse value of `{field}` as path.

Error message

Failed to parse value of `{field}` as path.

What it means

Compile-time error from `parse_path`'s `_` arm: the value for the `crate` field is not a string literal at all. Distinct from error 22 (which fires when the string is present but unparseable).

Source

Thrown at tokio-macros/src/entry.rs:296

    }
}

fn parse_path(lit: syn::Lit, span: Span, field: &str) -> Result<Path, syn::Error> {
    match lit {
        syn::Lit::Str(s) => {
            let err = syn::Error::new(
                span,
                format!(
                    "Failed to parse value of `{}` as path: \"{}\"",
                    field,
                    s.value()
                ),
            );
            s.parse::<syn::Path>().map_err(|_| err.clone())
        }
        _ => Err(syn::Error::new(
            span,
            format!("Failed to parse value of `{field}` as path."),
        )),
    }
}

fn parse_bool(bool: syn::Lit, span: Span, field: &str) -> Result<bool, syn::Error> {
    match bool {
        syn::Lit::Bool(b) => Ok(b.value),
        _ => Err(syn::Error::new(
            span,
            format!("Failed to parse value of `{field}` as bool."),
        )),
    }
}

fn contains_impl_trait(ty: &syn::Type) -> bool {
    match ty {
        syn::Type::ImplTrait(_) => true,
        syn::Type::Array(t) => contains_impl_trait(&t.elem),

View on GitHub (pinned to 625954f365)

Solutions

Quote the path: `#[tokio::main(crate = "my_tokio")]`.
Drop the `crate` option if you are not repointing the macro to a renamed crate.

Example fix

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

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

Defensive patterns

Strategy: validation

Prevention

The `crate` field expects a quoted string literal, not an identifier or number.
Prefer omitting `crate` unless you are vendoring/renaming tokio.
Keep a single canonical macro snippet in docs.

When it happens

Trigger: `#[tokio::main(crate = 5)]`, `crate = true`, or `crate = 'a'`. Any non-string token for the `crate` field.

Common situations: Passing the crate as an identifier/path (`crate = my_tokio`) instead of a quoted string; passing a number/bool by mistake.

Understand the failure class

Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

AI-assisted analysis of tokio-rs/tokio@625954f365 (2026-08-11). Data as JSON: /api/errors/5eb200de83fad518. Report an issue: GitHub.