rust-lang/rust · error · syn::Error

missing expected field `{name}`

Error message

missing expected field `{name}`

What it means

The only mandatory top-level field of for_each_function! is `callback`, the name of the macro to invoke once per function (parse.rs:76, 226-236). expect_field() returns this error (via `?`) when `callback` is absent; all other fields are optional (.ok()).

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/parse.rs:229

    Ok(res)
}

fn expect_empty_attrs(attrs: &[Attribute]) -> syn::Result<()> {
    if attrs.is_empty() {
        return Ok(());
    }

    let e = syn::Error::new(
        attrs.first().unwrap().span(),
        "no attributes allowed in this position",
    );
    Err(e)
}

/// Extract a named field from a map, raising an error if it doesn't exist.
fn expect_field(v: &mut Vec<Mapping>, name: &str) -> syn::Result<Expr> {
    let pos = v.iter().position(|v| v.name == name).ok_or_else(|| {
        syn::Error::new(
            Span::call_site(),
            format!("missing expected field `{name}`"),
        )
    })?;

    Ok(v.remove(pos).expr)
}

/// Coerce an expression into a simple identifier.
fn expect_ident(expr: Expr) -> syn::Result<Ident> {
    syn::parse2(expr.into_token_stream())
}

/// Coerce an expression into a simple keyword.
fn expect_litbool(expr: Expr) -> syn::Result<LitBool> {
    syn::parse2(expr.into_token_stream())
}

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Add a `callback: your_macro_name` field to the invocation.

Example fix

// before
libm_macros::for_each_function! {
    emit_types: all,
}
// after
libm_macros::for_each_function! {
    callback: my_callback,
    emit_types: all,
}

When it happens

Trigger: Omitting `callback: ...` from the invocation entirely.

Common situations: Starting to write a macro invocation and forgetting the callback macro reference; deleting the line accidentally.

Related errors

• unrecognized type identifier `{ty_name}`
• if `all` is specified, no other type identifiers may be give
• `fn_extra`: no default `_` pattern specified and the followi
• unrecognized meta expression `{s}`
• unexpected fields {map:?}

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/fadfba762c422a0a. Report an issue: GitHub.