Avoid returning first-match for rule prefixes

crates/ruff/src/registry.rs

@@ -19,14 +19,22 @@ impl Rule {
     pub fn from_code(code: &str) -> Result<Self, FromCodeError> {
         let (linter, code) = Linter::parse_code(code).ok_or(FromCodeError::Unknown)?;
         let prefix: RuleCodePrefix = RuleCodePrefix::parse(&linter, code)?;
-        Ok(prefix.rules().next().unwrap())
+        let rule = prefix.rules().next().unwrap();
+        // TODO(charlie): Add a method to return an individual code, rather than matching on the
+        // prefix.
+        if rule.noqa_code().to_string() != format!("{}{}", linter.common_prefix(), code) {
+            return Err(FromCodeError::Prefix);
+        }
+        Ok(rule)
     }
 }
 
 #[derive(thiserror::Error, Debug)]
 pub enum FromCodeError {
     #[error("unknown rule code")]
     Unknown,
+    #[error("expected a rule code (like `SIM101`), not a prefix (like `SIM` or `SIM1`)")]
+    Prefix,
 }
 
 #[derive(EnumIter, Debug, PartialEq, Eq, Clone, Hash, RuleNamespace)]

crates/ruff_cli/src/commands/rule.rs

@@ -31,7 +31,6 @@ pub(crate) fn rule(rule: Rule, format: HelpFormat) -> Result<()> {
             output.push('\n');
             output.push('\n');
 
-            let (linter, _) = Linter::parse_code(&rule.noqa_code().to_string()).unwrap();
             output.push_str(&format!("Derived from the **{}** linter.", linter.name()));
             output.push('\n');
             output.push('\n');