rust-lang/rust · error · syn::Error

`fn_extra` expects a match expression

Error message

`fn_extra` expects a match expression

What it means

The `fn_extra` field must be a `match` expression (parsed as syn::ExprMatch) so the macro can extract arms mapping function names to expressions (parse.rs:156-160). extract_fn_extra_field() rejects any non-match expression form.

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/parse.rs:158

            callback: expect_ident(cb_expr)?,
            emit_types,
            skip,
            skip_f16_f128,
            skip_builtins,
            only,
            only_span,
            attributes,
            extra,
            fn_extra,
            fn_extra_span,
            emit_types_span,
        })
    }
}

fn extract_fn_extra_field(expr: Expr) -> syn::Result<BTreeMap<Ident, Expr>> {
    let Expr::Match(mexpr) = expr else {
        let e = syn::Error::new(expr.span(), "`fn_extra` expects a match expression");
        return Err(e);
    };

    let ExprMatch {
        attrs,
        match_token: _,
        expr,
        brace_token: _,
        arms,
    } = mexpr;

    expect_empty_attrs(&attrs)?;

    let match_on = expect_ident(*expr)?;
    if match_on != "MACRO_FN_NAME" {
        let e = syn::Error::new(match_on.span(), "only allowed to match on `MACRO_FN_NAME`");
        return Err(e);
    }

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Wrap the value in `fn_extra: match MACRO_FN_NAME { _ => <value> }`.
2. If a single value applies to all functions, use the default arm form above.

Example fix

// before
fn_extra: my_default(),
// after
fn_extra: match MACRO_FN_NAME {
    _ => my_default(),
},

When it happens

Trigger: Writing `fn_extra: some_func()`, `fn_extra: 42`, or `fn_extra: my_value` where the value is not a `match` expression.

Common situations: Forgetting the match MACRO_FN_NAME { ... } syntax; passing a function pointer or literal instead.

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/cb4ffa6c4af5e598. Report an issue: GitHub.