Warn instead of error when removed rules are used in ignore (#14435)

Closes https://github.com/astral-sh/ruff/issues/13505

crates/ruff/tests/integration_test.rs

@@ -1250,6 +1250,68 @@ fn removed_indirect() {
     ");
 }
 
+#[test]
+fn removed_ignore_direct() {
+    let mut cmd = RuffCheck::default().args(["--ignore", "UP027"]).build();
+    assert_cmd_snapshot!(cmd, @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- stderr -----
+    warning: The following rules have been removed and ignoring them has no effect:
+        - UP027
+    ");
+}
+
+#[test]
+fn removed_ignore_multiple_direct() {
+    let mut cmd = RuffCheck::default()
+        .args(["--ignore", "UP027", "--ignore", "PLR1706"])
+        .build();
+    assert_cmd_snapshot!(cmd, @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- stderr -----
+    warning: The following rules have been removed and ignoring them has no effect:
+        - PLR1706
+        - UP027
+    ");
+}
+
+#[test]
+fn removed_ignore_remapped_direct() {
+    let mut cmd = RuffCheck::default().args(["--ignore", "PGH001"]).build();
+    assert_cmd_snapshot!(cmd, @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- stderr -----
+    warning: `PGH001` has been remapped to `S307`.
+    ");
+}
+
+#[test]
+fn removed_ignore_indirect() {
+    // `PLR170` includes removed rules but should not select or warn
+    // since it is not a "direct" selection
+    let mut cmd = RuffCheck::default().args(["--ignore", "PLR170"]).build();
+    assert_cmd_snapshot!(cmd, @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- stderr -----
+    ");
+}
+
 #[test]
 fn redirect_direct() {
     // Selection of a redirected rule directly should use the new rule and warn

crates/ruff_linter/src/codes.rs

@@ -513,6 +513,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pyupgrade, "024") => (RuleGroup::Stable, rules::pyupgrade::rules::OSErrorAlias),
         (Pyupgrade, "025") => (RuleGroup::Stable, rules::pyupgrade::rules::UnicodeKindPrefix),
         (Pyupgrade, "026") => (RuleGroup::Stable, rules::pyupgrade::rules::DeprecatedMockImport),
+        (Pyupgrade, "027") => (RuleGroup::Removed, rules::pyupgrade::rules::UnpackedListComprehension),
         (Pyupgrade, "028") => (RuleGroup::Stable, rules::pyupgrade::rules::YieldInForLoop),
         (Pyupgrade, "029") => (RuleGroup::Stable, rules::pyupgrade::rules::UnnecessaryBuiltinImport),
         (Pyupgrade, "030") => (RuleGroup::Stable, rules::pyupgrade::rules::FormatLiterals),

crates/ruff_linter/src/rules/pyupgrade/rules/mod.rs

@@ -31,6 +31,7 @@ pub(crate) use unnecessary_coding_comment::*;
 pub(crate) use unnecessary_default_type_args::*;
 pub(crate) use unnecessary_encode_utf8::*;
 pub(crate) use unnecessary_future_import::*;
+pub(crate) use unpacked_list_comprehension::*;
 pub(crate) use use_pep585_annotation::*;
 pub(crate) use use_pep604_annotation::*;
 pub(crate) use use_pep604_isinstance::*;
@@ -73,6 +74,7 @@ mod unnecessary_coding_comment;
 mod unnecessary_default_type_args;
 mod unnecessary_encode_utf8;
 mod unnecessary_future_import;
+mod unpacked_list_comprehension;
 mod use_pep585_annotation;
 mod use_pep604_annotation;
 mod use_pep604_isinstance;

crates/ruff_linter/src/rules/pyupgrade/rules/unpacked_list_comprehension.rs

@@ -0,0 +1,40 @@
+use ruff_diagnostics::Violation;
+use ruff_macros::violation;
+
+/// ## Removed
+/// There's no [evidence](https://github.com/astral-sh/ruff/issues/12754) that generators are
+/// meaningfully faster than list comprehensions when combined with unpacking.
+///
+/// ## What it does
+/// Checks for list comprehensions that are immediately unpacked.
+///
+/// ## Why is this bad?
+/// There is no reason to use a list comprehension if the result is immediately
+/// unpacked. Instead, use a generator expression, which avoids allocating
+/// an intermediary list.
+///
+/// ## Example
+/// ```python
+/// a, b, c = [foo(x) for x in items]
+/// ```
+///
+/// Use instead:
+/// ```python
+/// a, b, c = (foo(x) for x in items)
+/// ```
+///
+/// ## References
+/// - [Python documentation: Generator expressions](https://docs.python.org/3/reference/expressions.html#generator-expressions)
+/// - [Python documentation: List comprehensions](https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions)
+#[violation]
+pub struct UnpackedListComprehension;
+
+impl Violation for UnpackedListComprehension {
+    fn message(&self) -> String {
+        unreachable!("UP027 has been removed")
+    }
+
+    fn message_formats() -> &'static [&'static str] {
+        &["Replace unpacked list comprehension with a generator expression"]
+    }
+}

crates/ruff_workspace/src/configuration.rs

@@ -775,6 +775,7 @@ impl LintConfiguration {
         let mut redirects = FxHashMap::default();
         let mut deprecated_selectors = FxHashSet::default();
         let mut removed_selectors = FxHashSet::default();
+        let mut removed_ignored_rules = FxHashSet::default();
         let mut ignored_preview_selectors = FxHashSet::default();
 
         // Track which docstring rules are specifically enabled
@@ -926,9 +927,13 @@ impl LintConfiguration {
                 // Removed rules
                 if selector.is_exact() {
                     if selector.all_rules().all(|rule| rule.is_removed()) {
+                        if kind.is_disable() {
+                            removed_ignored_rules.insert(selector);
+                        } else {
                             removed_selectors.insert(selector);
                         }
                     }
+                }
 
                 // Redirected rules
                 if let RuleSelector::Prefix {
@@ -968,6 +973,20 @@ impl LintConfiguration {
             }
         }
 
+        if !removed_ignored_rules.is_empty() {
+            let mut rules = String::new();
+            for selection in removed_ignored_rules.iter().sorted() {
+                let (prefix, code) = selection.prefix_and_code();
+                rules.push_str("\n    - ");
+                rules.push_str(prefix);
+                rules.push_str(code);
+            }
+            rules.push('\n');
+            warn_user_once_by_message!(
+                "The following rules have been removed and ignoring them has no effect:{rules}"
+            );
+        }
+
         for (from, target) in redirects.iter().sorted_by_key(|item| item.0) {
             // TODO(martin): This belongs into the ruff crate.
             warn_user_once_by_id!(