Deprecate define violation (#3358)

* Add `#[violation]` proc macro as a replacement for `define_violation!`

* Switch all rules to #[violation]

crates/ruff_macros/src/define_violation.rs

@@ -1,7 +1,7 @@
 use proc_macro2::TokenStream;
 use quote::quote;
 use syn::parse::{Parse, ParseStream};
-use syn::{Attribute, Error, Ident, Lit, LitStr, Meta, Result, Token};
+use syn::{Attribute, Error, Ident, ItemStruct, Lit, LitStr, Meta, Result, Token};
 
 fn parse_attr<const LEN: usize>(path: [&'static str; LEN], attr: &Attribute) -> Option<LitStr> {
     if let Meta::NameValue(name_value) = attr.parse_meta().ok()? {
@@ -73,3 +73,45 @@ pub fn define_violation(input: &TokenStream, meta: LintMeta) -> TokenStream {
         }
     }
 }
+
+/// Collect all doc comment attributes into a string
+fn get_docs(attrs: &[Attribute]) -> Result<String> {
+    let mut explanation = String::new();
+    for attr in attrs {
+        if attr.path.is_ident("doc") {
+            if let Some(lit) = parse_attr(["doc"], attr) {
+                let value = lit.value();
+                // `/// ` adds
+                let line = value.strip_prefix(' ').unwrap_or(&value);
+                explanation.push_str(line);
+                explanation.push('\n');
+            } else {
+                return Err(Error::new_spanned(attr, "unimplemented doc comment style"));
+            }
+        }
+    }
+    Ok(explanation)
+}
+
+pub fn violation(violation: &ItemStruct) -> Result<TokenStream> {
+    let ident = &violation.ident;
+    let explanation = get_docs(&violation.attrs)?;
+    let violation = if explanation.trim().is_empty() {
+        quote! {
+            #[derive(Debug, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+            #violation
+        }
+    } else {
+        quote! {
+            #[derive(Debug, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+            #violation
+
+            impl #ident {
+                pub fn explanation() -> Option<&'static str> {
+                    Some(#explanation)
+                }
+            }
+        }
+    };
+    Ok(violation)
+}

crates/ruff_macros/src/lib.rs

@@ -2,7 +2,7 @@
 
 use crate::cache_key::derive_cache_key;
 use proc_macro::TokenStream;
-use syn::{parse_macro_input, DeriveInput, ItemFn};
+use syn::{parse_macro_input, DeriveInput, ItemFn, ItemStruct};
 
 mod cache_key;
 mod config;
@@ -45,6 +45,16 @@ pub fn define_violation(item: proc_macro::TokenStream) -> proc_macro::TokenStrea
     define_violation::define_violation(&item.into(), meta).into()
 }
 
+/// Adds an `explanation()` method from the doc comment and
+/// `#[derive(Debug, PartialEq, Eq, serde::Serialize, serde::Deserialize)]`
+#[proc_macro_attribute]
+pub fn violation(_attr: TokenStream, item: TokenStream) -> TokenStream {
+    let violation = parse_macro_input!(item as ItemStruct);
+    define_violation::violation(&violation)
+        .unwrap_or_else(syn::Error::into_compile_error)
+        .into()
+}
+
 #[proc_macro_derive(RuleNamespace, attributes(prefix))]
 pub fn derive_rule_namespace(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
     let input = parse_macro_input!(input as DeriveInput);