Move compare-to-empty-string to nursery (#5264)

## Summary

This rule has too many false positives. It has parity with the Pylint
version, but the Pylint version is part of an
[extension](https://pylint.readthedocs.io/en/stable/user_guide/messages/convention/compare-to-empty-string.html),
and so requires explicit opt-in.

I'm moving this rule to the nursery to require explicit opt-in, as with
Pylint.

Closes #4282.

crates/ruff/src/codes.rs

@@ -157,7 +157,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
 
         // pylint
         (Pylint, "C0414") => (RuleGroup::Unspecified, rules::pylint::rules::UselessImportAlias),
-        (Pylint, "C1901") => (RuleGroup::Unspecified, rules::pylint::rules::CompareToEmptyString),
+        (Pylint, "C1901") => (RuleGroup::Nursery, rules::pylint::rules::CompareToEmptyString),
         (Pylint, "C3002") => (RuleGroup::Unspecified, rules::pylint::rules::UnnecessaryDirectLambdaCall),
         (Pylint, "C0208") => (RuleGroup::Unspecified, rules::pylint::rules::IterationOverSet),
         (Pylint, "E0100") => (RuleGroup::Unspecified, rules::pylint::rules::YieldInInit),

crates/ruff/src/rules/pylint/rules/compare_to_empty_string.rs

@@ -7,49 +7,6 @@ use ruff_macros::{derive_message_formats, violation};
 
 use crate::checkers::ast::Checker;
 
-#[derive(Debug, PartialEq, Eq, Copy, Clone)]
-pub(crate) enum EmptyStringCmpOp {
-    Is,
-    IsNot,
-    Eq,
-    NotEq,
-}
-
-impl TryFrom<&CmpOp> for EmptyStringCmpOp {
-    type Error = anyhow::Error;
-
-    fn try_from(value: &CmpOp) -> Result<Self, Self::Error> {
-        match value {
-            CmpOp::Is => Ok(Self::Is),
-            CmpOp::IsNot => Ok(Self::IsNot),
-            CmpOp::Eq => Ok(Self::Eq),
-            CmpOp::NotEq => Ok(Self::NotEq),
-            _ => bail!("{value:?} cannot be converted to EmptyStringCmpOp"),
-        }
-    }
-}
-
-impl EmptyStringCmpOp {
-    pub(crate) fn into_unary(self) -> &'static str {
-        match self {
-            Self::Is | Self::Eq => "not ",
-            Self::IsNot | Self::NotEq => "",
-        }
-    }
-}
-
-impl std::fmt::Display for EmptyStringCmpOp {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let repr = match self {
-            Self::Is => "is",
-            Self::IsNot => "is not",
-            Self::Eq => "==",
-            Self::NotEq => "!=",
-        };
-        write!(f, "{repr}")
-    }
-}
-
 /// ## What it does
 /// Checks for comparisons to empty strings.
 ///
@@ -83,13 +40,15 @@ pub struct CompareToEmptyString {
 impl Violation for CompareToEmptyString {
     #[derive_message_formats]
     fn message(&self) -> String {
-        format!(
-            "`{}` can be simplified to `{}` as an empty string is falsey",
-            self.existing, self.replacement,
-        )
+        let CompareToEmptyString {
+            existing,
+            replacement,
+        } = self;
+        format!("`{existing}` can be simplified to `{replacement}` as an empty string is falsey",)
     }
 }
 
+/// PLC1901
 pub(crate) fn compare_to_empty_string(
     checker: &mut Checker,
     left: &Expr,
@@ -98,10 +57,12 @@ pub(crate) fn compare_to_empty_string(
 ) {
     // Omit string comparison rules within subscripts. This is mostly commonly used within
     // DataFrame and np.ndarray indexing.
-    for parent in checker.semantic().expr_ancestors() {
-        if matches!(parent, Expr::Subscript(_)) {
-            return;
-        }
+    if checker
+        .semantic()
+        .expr_ancestors()
+        .any(|parent| parent.is_subscript_expr())
+    {
+        return;
     }
 
     let mut first = true;
@@ -153,3 +114,46 @@ pub(crate) fn compare_to_empty_string(
         }
     }
 }
+
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+enum EmptyStringCmpOp {
+    Is,
+    IsNot,
+    Eq,
+    NotEq,
+}
+
+impl TryFrom<&CmpOp> for EmptyStringCmpOp {
+    type Error = anyhow::Error;
+
+    fn try_from(value: &CmpOp) -> Result<Self, Self::Error> {
+        match value {
+            CmpOp::Is => Ok(Self::Is),
+            CmpOp::IsNot => Ok(Self::IsNot),
+            CmpOp::Eq => Ok(Self::Eq),
+            CmpOp::NotEq => Ok(Self::NotEq),
+            _ => bail!("{value:?} cannot be converted to EmptyStringCmpOp"),
+        }
+    }
+}
+
+impl EmptyStringCmpOp {
+    fn into_unary(self) -> &'static str {
+        match self {
+            Self::Is | Self::Eq => "not ",
+            Self::IsNot | Self::NotEq => "",
+        }
+    }
+}
+
+impl std::fmt::Display for EmptyStringCmpOp {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let repr = match self {
+            Self::Is => "is",
+            Self::IsNot => "is not",
+            Self::Eq => "==",
+            Self::NotEq => "!=",
+        };
+        write!(f, "{repr}")
+    }
+}