rust-lang/rust · error · syn::Error

attributes not supported on queries

Error message

attributes not supported on queries

What it means

Emitted by the check_attributes function inside the rustc_queries! proc macro (rustc_macros/src/query.rs). The macro validates every attribute on a query or non_query declaration and only permits #[doc] attributes (doc comments). If any other attribute is present (e.g., #[inline], #[cfg(...)], #[allow(...)]), this syn::Error is produced at that attribute's span. The error surfaces as a compile-time error in the crate invoking rustc_queries!.

Solutions

1. Remove the non-doc attribute from the query declaration — query modifiers (eval_always, arena_cache, etc.) go inside the { } modifier block, not as attributes.
2. If you need conditional compilation, apply it at a higher level (the module or the rustc_queries! invocation site), not on individual queries.
3. Move lint allows to the enclosing module with #[allow(...)] on the mod or use #![allow(...)] at crate level.

Example fix

// before
rustc_queries! {
    #[inline]
    /// Computes the type of an expression.
    query type_of(key: DefId) -> Ty<'tcx> { desc { "type of `{}`", key } }
}
// after: remove the non-doc attribute
rustc_queries! {
    /// Computes the type of an expression.
    query type_of(key: DefId) -> Ty<'tcx> { desc { "type of `{}`", key } }
}

// In a test or CI lint, verify that query declarations only have #[doc] attributes.
// This is a code-review/lint concern — the proc macro itself enforces it at compile time.
// You can grep your query declarations:
//   rg -A5 'query |non_query ' --type rust | grep -v 'doc\|//' | grep '#\['
// Any hit indicates a non-doc attribute on a query.

// This is a compile-time error from a proc macro. It cannot be caught.
// Fix by removing the offending attribute from the query declaration.
// The error points to the exact attribute span.

When it happens

Trigger: A compiler developer adds a query declaration inside a rustc_queries! { } block and annotates it with a non-doc attribute such as #[inline], #[allow(dead_code)], #[cfg(feature = "x")], or any other helper attribute.

Common situations: Adding a new query to rustc and trying to suppress a lint or add an optimization hint via attributes; copy-pasting from a non-query context where attributes are allowed.

Related errors

• attributes must be outer attributes (`///`), not inner…
• duplicate modifier
• Expected a string literal
• unknown query modifier
• CFG_RELEASE env var

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/0662efebdee5d35f. Report an issue: GitHub.

Appendix: source

Thrown at compiler/rustc_macros/src/query.rs:21

use quote::{quote, quote_spanned};
use syn::parse::{Parse, ParseStream, Result};
use syn::punctuated::Punctuated;
use syn::spanned::Spanned;
use syn::{
    AttrStyle, Attribute, Error, Expr, Ident, Pat, ReturnType, Token, Type, braced, parenthesized,
    parse_macro_input, token,
};

mod kw {
    syn::custom_keyword!(non_query);
    syn::custom_keyword!(query);
}

/// Ensures only doc comment attributes are used
fn check_attributes(attrs: Vec<Attribute>) -> Result<Vec<Attribute>> {
    let inner = |attr: Attribute| {
        if !attr.path().is_ident("doc") {
            Err(Error::new(attr.span(), "attributes not supported on queries"))
        } else if attr.style != AttrStyle::Outer {
            Err(Error::new(
                attr.span(),
                "attributes must be outer attributes (`///`), not inner attributes",
            ))
        } else {
            Ok(attr)
        }
    };
    attrs.into_iter().map(inner).collect()
}

/// Declaration of a compiler query.
///
/// ```ignore (illustrative)
/// /// Doc comment for `my_query`.
/// //  ^^^^^^^^^^^^^^^^^^^^^^^^^^^              doc_comments
/// query my_query(key: DefId) -> Value { anon }

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