refactor: Change RuleNamespace::prefixes to common_prefix

Previously Linter::parse_code("E401") returned
(Linter::Pycodestyle, "401") ... after this commit it returns
(Linter::Pycodestyle, "E401") instead, which is important
for the future implementation of the many-to-many mapping.
(The second value of the tuple isn't used currently.)

ruff_dev/src/generate_rules_table.rs

@@ -1,6 +1,7 @@
 //! Generate a Markdown-compatible table of supported lint rules.
 
 use anyhow::Result;
+use itertools::Itertools;
 use ruff::registry::{Linter, LinterCategory, Rule, RuleNamespace};
 use strum::IntoEnumIterator;
 
@@ -47,7 +48,15 @@ pub fn main(args: &Args) -> Result<()> {
     let mut table_out = String::new();
     let mut toc_out = String::new();
     for linter in Linter::iter() {
-        let codes_csv: String = linter.prefixes().join(", ");
+        let codes_csv: String = match linter.common_prefix() {
+            "" => linter
+                .categories()
+                .unwrap()
+                .iter()
+                .map(|LinterCategory(prefix, ..)| prefix)
+                .join(", "),
+            prefix => prefix.to_string(),
+        };
         table_out.push_str(&format!("### {} ({codes_csv})", linter.name()));
         table_out.push('\n');
         table_out.push('\n');

ruff_macros/src/rule_namespace.rs

@@ -15,13 +15,15 @@ pub fn derive_impl(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream>
 
     let mut parsed = Vec::new();
 
-    let mut prefix_match_arms = quote!();
+    let mut common_prefix_match_arms = quote!();
     let mut name_match_arms = quote!(Self::Ruff => "Ruff-specific rules",);
     let mut url_match_arms = quote!(Self::Ruff => None,);
+    let mut into_iter_match_arms = quote!();
 
     let mut all_prefixes = HashSet::new();
 
     for variant in variants {
+        let mut first_chars = HashSet::new();
         let prefixes: Result<Vec<_>, _> = variant
             .attrs
             .iter()
@@ -33,7 +35,9 @@ pub fn derive_impl(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream>
                 let str = lit.value();
                 match str.chars().next() {
                     None => return Err(Error::new(lit.span(), "expected prefix string to be non-empty")),
-                    Some(_) => {},
+                    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"));
@@ -63,31 +67,43 @@ pub fn derive_impl(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream>
         }
 
         for lit in &prefixes {
-            parsed.push((lit.clone(), variant_ident.clone()));
+            parsed.push((
+                lit.clone(),
+                variant_ident.clone(),
+                match prefixes.len() {
+                    1 => ParseStrategy::SinglePrefix,
+                    _ => ParseStrategy::MultiplePrefixes,
+                },
+            ));
         }
 
-        prefix_match_arms.extend(quote! {
-            Self::#variant_ident => &[#(#prefixes),*],
-        });
+        if let [prefix] = &prefixes[..] {
+            common_prefix_match_arms.extend(quote! { Self::#variant_ident => #prefix, });
+
+            let prefix_ident = Ident::new(prefix, Span::call_site());
+            into_iter_match_arms.extend(quote! {
+                #ident::#variant_ident => RuleCodePrefix::#prefix_ident.into_iter(),
+            });
+        } else {
+            // There is more than one prefix. We already previously asserted
+            // that prefixes of the same variant don't start with the same character
+            // so the common prefix for this variant is the empty string.
+            common_prefix_match_arms.extend(quote! { Self::#variant_ident => "", });
+        }
     }
 
-    parsed.sort_by_key(|(prefix, _)| Reverse(prefix.len()));
+    parsed.sort_by_key(|(prefix, ..)| Reverse(prefix.len()));
 
     let mut if_statements = quote!();
-    let mut into_iter_match_arms = quote!();
 
-    for (prefix, field) in parsed {
+    for (prefix, field, strategy) in parsed {
+        let ret_str = match strategy {
+            ParseStrategy::SinglePrefix => quote!(rest),
+            ParseStrategy::MultiplePrefixes => quote!(code),
+        };
         if_statements.extend(quote! {if let Some(rest) = code.strip_prefix(#prefix) {
-            return Some((#ident::#field, rest));
+            return Some((#ident::#field, #ret_str));
         }});
-
-        let prefix_ident = Ident::new(&prefix, Span::call_site());
-
-        if field != "Pycodestyle" {
-            into_iter_match_arms.extend(quote! {
-                #ident::#field => RuleCodePrefix::#prefix_ident.into_iter(),
-            });
-        }
     }
 
     into_iter_match_arms.extend(quote! {
@@ -104,9 +120,8 @@ pub fn derive_impl(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream>
                 None
             }
 
-
-            fn prefixes(&self) -> &'static [&'static str] {
-                match self { #prefix_match_arms }
+            fn common_prefix(&self) -> &'static str {
+                match self { #common_prefix_match_arms }
             }
 
             fn name(&self) -> &'static str {
@@ -152,3 +167,8 @@ fn parse_doc_attr(doc_attr: &Attribute) -> syn::Result<(String, String)> {
 fn parse_markdown_link(link: &str) -> Option<(&str, &str)> {
     link.strip_prefix('[')?.strip_suffix(')')?.split_once("](")
 }
+
+enum ParseStrategy {
+    SinglePrefix,
+    MultiplePrefixes,
+}

src/registry.rs

@@ -615,9 +615,16 @@ pub enum Linter {
 }
 
 pub trait RuleNamespace: Sized {
-    fn parse_code(code: &str) -> Option<(Self, &str)>;
+    /// Returns the prefix that every single code that ruff uses to identify
+    /// rules from this linter starts with.  In the case that multiple
+    /// `#[prefix]`es are configured for the variant in the `Linter` enum
+    /// definition this is the empty string.
+    fn common_prefix(&self) -> &'static str;
 
-    fn prefixes(&self) -> &'static [&'static str];
+    /// Attempts to parse the given rule code. If the prefix is recognized
+    /// returns the respective variant along with the code with the common
+    /// prefix stripped.
+    fn parse_code(code: &str) -> Option<(Self, &str)>;
 
     fn name(&self) -> &'static str;
 
@@ -766,10 +773,12 @@ mod tests {
     }
 
     #[test]
-    fn test_linter_prefixes() {
+    fn test_linter_parse_code() {
         for rule in Rule::iter() {
-            Linter::parse_code(rule.code())
-                .unwrap_or_else(|| panic!("couldn't parse {:?}", rule.code()));
+            let code = rule.code();
+            let (linter, rest) =
+                Linter::parse_code(code).unwrap_or_else(|| panic!("couldn't parse {:?}", code));
+            assert_eq!(code, format!("{}{rest}", linter.common_prefix()));
         }
     }
 }