astral-sh/ruff · error · syn::Error

this variant already has another prefix starting with the ch

Error message

this variant already has another prefix starting with the character '{c}'

What it means

A syn compile error from the RuleNamespace derive: two #[prefix] attributes on the same variant start with the same character. Rule codes are dispatched by first character, so duplicate leading characters within a variant would be ambiguous.

Source

Thrown at crates/ruff_macros/src/rule_namespace.rs:60

                    ..
                }) = &attr.meta
                else {
                    return Err(Error::new(
                        attr.span(),
                        r#"expected attribute to be in the form of [#prefix = "..."]"#,
                    ));
                };
                let str = lit.value();
                match str.chars().next() {
                    None => {
                        return Err(Error::new(
                            lit.span(),
                            "expected prefix string to be non-empty",
                        ));
                    }
                    Some(c) => {
                        if !first_chars.insert(c) {
                            return Err(Error::new(
                                lit.span(),
                                format!(
                                    "this variant already has another prefix \
                                    starting with the character '{c}'"
                                ),
                            ));
                        }
                    }
                }
                if !all_prefixes.insert(str.clone()) {
                    return Err(Error::new(
                        lit.span(),
                        "prefix has already been defined before",
                    ));
                }
                Ok(str)
            })
            .collect();

View on GitHub (pinned to 26f38c119c)

Solutions

1. Give the variant prefixes distinct first characters
2. Remove one of the conflicting prefixes

When it happens

Trigger: Thrown at crates/ruff_macros/src/rule_namespace.rs:60 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/6b7877c710a73079. Report an issue: GitHub.