rust-lang/rust · error · syn::Error

unknown query modifier

Error message

unknown query modifier

What it means

Thrown by parse_query_modifiers (query.rs:204) in the rustc_queries proc-macro when a token inside a query's modifier braces does not match any of the ten recognized modifier names. The parser is an if/else-if chain (query.rs:178-205) ending in a catch-all Err, so any unrecognized identifier (or a keyword reached via a typo) is rejected with the span of that identifier.

Source

Thrown at compiler/rustc_macros/src/query.rs:204

            // `desc { "foo {}", tcx.item_path(key) }`
            let attr_content;
            braced!(attr_content in input);
            let expr_list = attr_content.parse_terminated(Expr::parse, Token![,])?;
            try_insert!(desc = Desc { modifier, expr_list });
        } else if modifier == "eval_always" {
            try_insert!(eval_always = modifier);
        } else if modifier == "feedable" {
            try_insert!(feedable = modifier);
        } else if modifier == "handle_cycle_error" {
            try_insert!(handle_cycle_error = modifier);
        } else if modifier == "no_force" {
            try_insert!(no_force = modifier);
        } else if modifier == "no_hash" {
            try_insert!(no_hash = modifier);
        } else if modifier == "separate_provide_extern" {
            try_insert!(separate_provide_extern = modifier);
        } else {
            return Err(Error::new(modifier.span(), "unknown query modifier"));
        }
    }
    let Some(desc) = desc else {
        return Err(input.error("no description provided"));
    };
    Ok(QueryModifiers {
        // tidy-alphabetical-start
        arena_cache,
        cache_on_disk,
        depth_limit,
        desc,
        eval_always,
        feedable,
        handle_cycle_error,
        no_force,
        no_hash,
        separate_provide_extern,
        // tidy-alphabetical-end

View on GitHub (pinned to 22057b88b0)

Solutions

1. Correct the modifier identifier to one of the supported names listed in query.rs:178-202 (and rustc_middle::query::modifiers).
2. If you genuinely need a new modifier, add it to QueryModifiers (query.rs:137-150), the parse chain, and make_modifiers_stream before using it.
3. Check for typos and trailing punctuation that may have produced an unexpected token.

Example fix

// before
query type_of(key: DefId) -> Ty<'tcx> {
    cycle_error
    desc { "type of `{}`", tcx.def_path_str(key) }
}

// after
query type_of(key: DefId) -> Ty<'tcx> {
    handle_cycle_error
    desc { "type of `{}`", tcx.def_path_str(key) }
}

const KNOWN: &[&str] = &["eval_always","dep_kind","anon","feedable","cycle_delay","no_fetch"];
fn is_known_modifier(m: &str) -> bool { KNOWN.contains(&m) }
// for m in used { assert!(is_known_modifier(m), "unknown query modifier {m}"); }

fn modifier_kind(m: &str) -> Option<&'static str> {
    match m {
        "eval_always" | "dep_kind" | "anon" | "feedable" => Some(m),
        _ => None,
    }
}

When it happens

Trigger: Writing a query whose braces block contains a modifier name not in the set {arena_cache, cache_on_disk, depth_limit, desc, eval_always, feedable, handle_cycle_error, no_force, no_hash, separate_provide_extern}; e.g. `query foo(k: K) -> V { cycle_error, desc {"x"} }` (should be handle_cycle_error) or placing a stray token/typo in the modifier list.

Common situations: Contributing a new query to rustc and guessing a modifier name; renaming a modifier without updating all call sites; using an older modifier name that was renamed across rustc versions; copying an example from outdated documentation.

Related errors

• duplicate modifier
• Expected a string literal
• attributes not supported on queries
• attributes must be outer attributes (`///`), not inner attri
• {:?} shouldn't exist here

AI-assisted analysis of rust-lang/rust@22057b88b0 (2026-08-03). Data as JSON: /data/errors/e1583a7f7f4901aa.json. Report an issue: GitHub.