rust-lang/rust · error · syn::Error

unrecognized type identifier `{ty_name}`

Error message

unrecognized type identifier `{ty_name}`

What it means

The `for_each_function!` macro's `emit_types` field selects which type metadata to pass to the callback. Only identifiers in KNOWN_TYPES (CFn, CArgs, CRet, RustFn, RustArgs, RustRet, path) or the keyword `all` are accepted (lib.rs:13-15). During validate(), each entry is checked against that set; an entry matching none triggers this error.

Source

Thrown at library/compiler-builtins/crates/libm-macros/src/lib.rs:268

            continue;
        }

        // Run everything else
        fn_list.push(func);
    }

    // Types that the user would like us to provide in the macro
    let mut add_all_types = false;
    for ty in &input.emit_types {
        let ty_name = ty.to_string();
        if ty_name == "all" {
            add_all_types = true;
            continue;
        }

        // Check that all requested types are valid
        if !KNOWN_TYPES.contains(&ty_name.as_str()) {
            let e = syn::Error::new(
                ty_name.span(),
                format!("unrecognized type identifier `{ty_name}`"),
            );
            return Err(e);
        }
    }

    if add_all_types {
        // Ensure that if `all` was specified that nothing else was
        if input.emit_types.len() > 1 {
            let e = syn::Error::new(
                input.emit_types_span.unwrap(),
                "if `all` is specified, no other type identifiers may be given",
            );
            return Err(e);
        }

        // ...and then add all types

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Replace the invalid identifier with one of CFn, CArgs, CRet, RustFn, RustArgs, RustRet, or path.
2. If every type is needed, use emit_types: all instead of listing names.
3. Cross-check the spelling against the KNOWN_TYPES list in lib.rs:13.

Example fix

// before
libm_macros::for_each_function! {
    callback: cb,
    emit_types: [RustSignature],
}
// after
libm_macros::for_each_function! {
    callback: cb,
    emit_types: [RustFn, RustArgs, RustRet],
}

When it happens

Trigger: Supplying `emit_types: [RustSig]`, `emit_types: [FooType]`, or any list containing an identifier not in {CFn, CArgs, CRet, RustFn, RustArgs, RustRet, path, all}.

Common situations: Typoing a known type name (e.g. `RustFnction`), inventing a type identifier that does not exist, or migrating from an older version of the macro that used different names.

Related errors

• 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:?}
• `fn_extra` expects a match expression

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/7f6ba3fb572290ec. Report an issue: GitHub.