rust-lang/rust · error · syn::Error

only allowed to match on `MACRO_FN_NAME`

Error message

only allowed to match on `MACRO_FN_NAME`

What it means

The scrutinee of the fn_extra match must be exactly the magic identifier `MACRO_FN_NAME`, which the macro rewrites per invocation (parse.rs:172-176). Matching on any other identifier is rejected because the dispatch would have no per-function meaning.

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/parse.rs:174

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);
    }

    let mut res = BTreeMap::new();

    for arm in arms {
        let Arm {
            attrs,
            pat,
            guard,
            fat_arrow_token: _,
            body,
            comma: _,
        } = arm;

        expect_empty_attrs(&attrs)?;

        let keys = match pat {

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Change the match scrutinee to exactly `MACRO_FN_NAME`.

Example fix

// before
fn_extra: match MACRO_FN_NAME_NORMALIZED {
    _ => |x| x,
},
// after
fn_extra: match MACRO_FN_NAME {
    _ => |x| x,
},

When it happens

Trigger: Writing `fn_extra: match foo { ... }`, `match MACRO_FN_NAME_NORMALIZED { ... }`, or any scrutinee other than MACRO_FN_NAME.

Common situations: Using a real variable as scrutinee; choosing the normalized magic token by mistake.

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/6faf3854258daa9c. Report an issue: GitHub.