rust-lang/rust · error · syn::Error

unrecognized function name `{mentioned}`

Error message

unrecognized function name `{mentioned}`

What it means

Thrown by the `for_each_function!` macro's `validate` (lib.rs:198) when a function name mentioned in `skip`, `only`, `attributes`, or `fn_extra` does not match any entry in the global `ALL_OPERATIONS` list. Every referenced name must be a real libm operation.

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/lib.rs:198

        .flat_map(|map_list| map_list.iter())
        .flat_map(|attr_map| attr_map.names.iter());
    let only_mentions = input.only.iter().flat_map(|only_list| only_list.iter());
    let fn_extra_mentions = input
        .fn_extra
        .iter()
        .flat_map(|v| v.keys())
        .filter(|name| *name != "_");
    let all_mentioned_fns = input
        .skip
        .iter()
        .chain(only_mentions)
        .chain(attr_mentions)
        .chain(fn_extra_mentions);

    // Make sure that every function mentioned is a real function
    for mentioned in all_mentioned_fns {
        if !ALL_OPERATIONS.iter().any(|func| mentioned == func.name) {
            let e = syn::Error::new(
                mentioned.span(),
                format!("unrecognized function name `{mentioned}`"),
            );
            return Err(e);
        }
    }

    if !input.skip.is_empty() && input.only.is_some() {
        let e = syn::Error::new(
            input.only_span.unwrap(),
            "only one of `skip` or `only` may be specified",
        );
        return Err(e);
    }

    // Construct a list of what we intend to expand
    let mut fn_list = Vec::new();
    for func in ALL_OPERATIONS.iter() {

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Check the name against the current `ALL_OPERATIONS` list (or the crate's exported symbols).
2. Remove or correct the misspelled name.
3. Use the magic matchers (`ALL_F32`, `ALL_F64`, etc.) instead of listing names to avoid typos.

Example fix

// before
libm_macros::for_each_function! {
    callback: cb,
    skip: [nonexistent_fn],
}

// after
libm_macros::for_each_function! {
    callback: cb,
    skip: [sin],
}

// Before adding a name to skip/only/attributes/fn_extra, confirm it is a real operation.
// (The authoritative list is api_list_common::ALL_OPERATIONS; cross-check the symbol export list.)
fn is_known(name: &str) -> bool {
    ALL_OPERATIONS.iter().any(|op| op.name == name)
}

When it happens

Trigger: Typing a non-existent function in `skip: [xyz]`, `only: [foo]`, an `attributes` block, or an `fn_extra` match arm — any name not present in `ALL_OPERATIONS`.

Common situations: Renaming/removing a libm symbol upstream; typos; referencing a function by a C name that differs from its registered operation name.

Related errors

• only one of `skip` or `only` may be specified
• expected one attribute
• expected an identifier
• unexpected token after identifier
• no attributes expected

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/8678b49bf07d6273. Report an issue: GitHub.