rust-lang/rust · error · syn::Error

unknown query modifier

Error message

unknown query modifier

What it means

Emitted by parse_query_modifiers in the rustc_queries! macro when an identifier in the modifier block doesn't match any of the ten known modifier keys: arena_cache, cache_on_disk, depth_limit, desc, eval_always, feedable, handle_cycle_error, no_force, no_hash, separate_provide_extern. The else branch of the if-else chain (query.rs:204) produces this error at the unknown identifier's span.

Solutions

  1. Check the canonical list in query.rs:178-202 or rustc_middle::query::modifiers for the exact spelling of each modifier.
  2. Correct the typo — note that all modifiers use snake_case.
  3. If the modifier exists in a different rustc version, update your source tree to match the version you're compiling.

Example fix

// before
query foo(key: DefId) -> Ty<'tcx> {
    desc { "foo for `{}`", key }
    eval_alwyas
}
// after: fix the typo
query foo(key: DefId) -> Ty<'tcx> {
    desc { "foo for `{}`", key }
    eval_always
}
Defensive patterns

Strategy: validation

Validate before calling

// Validate modifier names against the known set before committing.
const VALID_MODIFIERS: &[&str] = &[
    "arena_cache", "cache_on_disk", "depth_limit", "desc",
    "eval_always", "feedable", "handle_cycle_error",
    "no_force", "no_hash", "separate_provide_extern",
];

fn is_valid_modifier(name: &str) -> bool {
    VALID_MODIFIERS.contains(&name)
}

Try / catch

// Compile-time error — cannot be caught at runtime.
// Fix the typo or remove the unknown modifier.

Prevention

When it happens

Trigger: Typing a modifier name incorrectly (e.g., "eval_alwyas", "cach_on_disk") or using a modifier that doesn't exist in this version of the compiler. The modifier block is the { } body of a query declaration.

Common situations: Typo in modifier name; using a modifier from a newer/older rustc version; guessing a modifier name instead of checking the documented list; copy-paste from documentation that uses a different name.

Related errors

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/e1583a7f7f4901aa. Report an issue: GitHub.

Appendix: 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

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