rust-lang/rust · error · syn::Error

`fn_extra`: no default `_` pattern specified and the followi

Error message

`fn_extra`: no default `_` pattern specified and the following patterns are not covered: {fns_not_covered:#?}

What it means

The `fn_extra` field maps each function to a per-invocation expression via a `match MACRO_FN_NAME { ... }`. If no `_ =>` default arm is supplied, validate() requires every function in the expansion set to have an explicit arm (lib.rs:294-316); otherwise expansion would unwrap a missing map entry. The error lists the uncovered functions.

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/lib.rs:307

            let ident = Ident::new(ty, Span::call_site());
            input.emit_types.push(ident);
        }
    }

    if let Some(map) = &input.fn_extra
        && !map.keys().any(|key| key == "_")
    {
        // No default provided; make sure every expected function is covered
        let mut fns_not_covered = Vec::new();
        for func in &fn_list {
            if !map.keys().any(|key| key == func.name) {
                // `name` was not mentioned in the `match` statement
                fns_not_covered.push(func);
            }
        }

        if !fns_not_covered.is_empty() {
            let e = syn::Error::new(
                input.fn_extra_span.unwrap(),
                format!(
                    "`fn_extra`: no default `_` pattern specified and the following \
                     patterns are not covered: {fns_not_covered:#?}"
                ),
            );
            return Err(e);
        }
    };

    Ok(fn_list)
}

/// Expand our structured macro input into invocations of the callback macro.
fn expand(input: StructuredInput, fn_list: &[&MathOpInfo]) -> syn::Result<pm2::TokenStream> {
    let mut out = pm2::TokenStream::new();
    let default_ident = Ident::new("_", Span::call_site());
    let callback = input.callback;

View on GitHub (pinned to 7088e4b63a)

Solutions

Add a `_ => <expr>` default arm to the fn_extra match.
Add explicit arms for every function named in the error's fns_not_covered list.
Narrow the expansion with skip/only so only the functions you have arms for are emitted.

Example fix

// before
fn_extra: match MACRO_FN_NAME {
    sin => |x| x.sin(),
},
// after
fn_extra: match MACRO_FN_NAME {
    sin => |x| x.sin(),
    _ => |x| x,
},

Defensive patterns

Strategy: validation

Prevention

Always include a `_ =>` default arm unless you intentionally enumerate every function.
When changing skip/only, re-check that fn_extra arms still cover the expansion set.

When it happens

Trigger: Providing `fn_extra: match MACRO_FN_NAME { sin => ... }` with no `_ =>` arm while the macro expands for functions beyond `sin` (i.e. they are not all skipped or excluded via only).

Common situations: Editing skip/only so the expanded function set changes and the explicit arms no longer cover it; forgetting the default arm; renaming a function.

Related errors

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/109185259b80688c. Report an issue: GitHub.