astral-sh/ruff · error · syn::Error

A new type index cannot be generic.

Error message

A new type index cannot be generic.

What it means

Compile-time validation error from the newtype_index derive macro. A #[derive(newtype_index)] (or newtype_index proc macro) is meant to wrap a unit struct used as a salsa-style interned index; a generic type parameter would make the generated index trait impls ambiguous, so generate_newtype_index checks item.generics.params and fails the expansion with this error at the struct's span when any generic parameters are declared. It is a macro-authoring constraint violation, not a runtime error; remove the generic parameters from the struct definition.

Source

Thrown at crates/ruff_macros/src/newtype_index.rs:14

use quote::quote;
use syn::spanned::Spanned;
use syn::{Error, ItemStruct};

pub(super) fn generate_newtype_index(item: ItemStruct) -> syn::Result<proc_macro2::TokenStream> {
    if !item.fields.is_empty() {
        return Err(Error::new(
            item.span(),
            "A new type index cannot have any fields.",
        ));
    }

    if !item.generics.params.is_empty() {
        return Err(Error::new(
            item.span(),
            "A new type index cannot be generic.",
        ));
    }

    let ItemStruct {
        attrs,
        vis,
        struct_token,
        ident,
        generics: _,
        fields: _,
        semi_token,
    } = item;

    let debug_name = ident.to_string();

    let semi_token = semi_token.unwrap_or_default();

View on GitHub (pinned to 26f38c119c)

Solutions

1. Remove generic parameters from the struct
2. Generate a separate concrete newtype index per instantiation

When it happens

Trigger: Thrown at crates/ruff_macros/src/newtype_index.rs:14 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of astral-sh/ruff@26f38c119c (2026-09-05). Data as JSON: /api/errors/ebf1407d97cba544. Report an issue: GitHub.