Fix `F841` false negative on assignment to multiple variables (#8489)

## Summary

Closes #8441 behind preview feature flag.

## Test Plan

`cargo test`

crates/ruff_linter/resources/test/fixtures/pyflakes/F841_1.py

@@ -1,5 +1,5 @@
 def f(tup):
-    x, y = tup  # this does NOT trigger F841
+    x, y = tup
 
 
 def f():
@@ -7,17 +7,17 @@ def f():
 
 
 def f():
-    (x, y) = coords = 1, 2  # this does NOT trigger F841
+    (x, y) = coords = 1, 2
     if x > 1:
         print(coords)
 
 
 def f():
-    (x, y) = coords = 1, 2  # this triggers F841 on coords
+    (x, y) = coords = 1, 2
 
 
 def f():
-    coords = (x, y) = 1, 2  # this triggers F841 on coords
+    coords = (x, y) = 1, 2
 
 
 def f():

crates/ruff_linter/resources/test/fixtures/pyflakes/F841_4.py

@@ -0,0 +1,32 @@
+"""Test fix for issue #8441.
+
+Ref: https://github.com/astral-sh/ruff/issues/8441
+"""
+
+
+def foo():
+    ...
+
+
+def bar():
+    a = foo()
+    b, c = foo()
+
+
+def baz():
+    d, _e = foo()
+    print(d)
+
+
+def qux():
+    f, _ = foo()
+    print(f)
+
+
+def quux():
+    g, h = foo()
+    print(g, h)
+
+
+def quuz():
+    _i, _j = foo()

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

@@ -26,6 +26,7 @@ mod tests {
     use crate::linter::{check_path, LinterResult};
     use crate::registry::{AsRule, Linter, Rule};
     use crate::rules::pyflakes;
+    use crate::settings::types::PreviewMode;
     use crate::settings::{flags, LinterSettings};
     use crate::source_kind::SourceKind;
     use crate::test::{test_path, test_snippet};
@@ -145,6 +146,7 @@ mod tests {
     #[test_case(Rule::UnusedVariable, Path::new("F841_1.py"))]
     #[test_case(Rule::UnusedVariable, Path::new("F841_2.py"))]
     #[test_case(Rule::UnusedVariable, Path::new("F841_3.py"))]
+    #[test_case(Rule::UnusedVariable, Path::new("F841_4.py"))]
     #[test_case(Rule::UnusedAnnotation, Path::new("F842.py"))]
     #[test_case(Rule::RaiseNotImplemented, Path::new("F901.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
@@ -157,6 +159,24 @@ mod tests {
         Ok(())
     }
 
+    #[test_case(Rule::UnusedVariable, Path::new("F841_4.py"))]
+    fn preview_rules(rule_code: Rule, path: &Path) -> Result<()> {
+        let snapshot = format!(
+            "preview__{}_{}",
+            rule_code.noqa_code(),
+            path.to_string_lossy()
+        );
+        let diagnostics = test_path(
+            Path::new("pyflakes").join(path).as_path(),
+            &LinterSettings {
+                preview: PreviewMode::Enabled,
+                ..LinterSettings::for_rule(rule_code)
+            },
+        )?;
+        assert_messages!(snapshot, diagnostics);
+        Ok(())
+    }
+
     #[test]
     fn f841_dummy_variable_rgx() -> Result<()> {
         let diagnostics = test_path(
@@ -1126,7 +1146,8 @@ mod tests {
 
     #[test]
     fn used_as_star_unpack() {
-        // Star names in unpack are used if RHS is not a tuple/list literal.
+        // In stable, starred names in unpack are used if RHS is not a tuple/list literal.
+        // In preview, these should be marked as unused.
         flakes(
             r#"
         def f():

crates/ruff_linter/src/rules/pyflakes/rules/unused_variable.rs

@@ -12,6 +12,7 @@ use ruff_text_size::{Ranged, TextRange, TextSize};
 
 use crate::checkers::ast::Checker;
 use crate::fix::edits::delete_stmt;
+use crate::settings::types::PreviewMode;
 
 /// ## What it does
 /// Checks for the presence of unused variables in function scopes.
@@ -24,6 +25,9 @@ use crate::fix::edits::delete_stmt;
 /// prefixed with an underscore, or some other value that adheres to the
 /// [`dummy-variable-rgx`] pattern.
 ///
+/// Under [preview mode](https://docs.astral.sh/ruff/preview), this rule also
+/// triggers on unused unpacked assignments (for example, `x, y = foo()`).
+///
 /// ## Example
 /// ```python
 /// def foo():
@@ -318,7 +322,10 @@ pub(crate) fn unused_variable(checker: &Checker, scope: &Scope, diagnostics: &mu
         .bindings()
         .map(|(name, binding_id)| (name, checker.semantic().binding(binding_id)))
         .filter_map(|(name, binding)| {
-            if (binding.kind.is_assignment() || binding.kind.is_named_expr_assignment())
+            if (binding.kind.is_assignment()
+                || binding.kind.is_named_expr_assignment()
+                || (matches!(checker.settings.preview, PreviewMode::Enabled)
+                    && binding.kind.is_unpacked_assignment()))
                 && !binding.is_nonlocal()
                 && !binding.is_global()
                 && !binding.is_used()

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F841_F841_1.py.snap

@@ -20,7 +20,7 @@ F841_1.py:6:8: F841 Local variable `y` is assigned to but never used
 F841_1.py:16:14: F841 [*] Local variable `coords` is assigned to but never used
    |
 15 | def f():
-16 |     (x, y) = coords = 1, 2  # this triggers F841 on coords
+16 |     (x, y) = coords = 1, 2
    |              ^^^^^^ F841
    |
    = help: Remove assignment to unused variable `coords`
@@ -29,8 +29,8 @@ F841_1.py:16:14: F841 [*] Local variable `coords` is assigned to but never used
 13 13 | 
 14 14 | 
 15 15 | def f():
-16    |-    (x, y) = coords = 1, 2  # this triggers F841 on coords
+16    |-    (x, y) = coords = 1, 2
-   16 |+    (x, y) = 1, 2  # this triggers F841 on coords
+   16 |+    (x, y) = 1, 2
 17 17 | 
 18 18 | 
 19 19 | def f():
@@ -38,7 +38,7 @@ F841_1.py:16:14: F841 [*] Local variable `coords` is assigned to but never used
 F841_1.py:20:5: F841 [*] Local variable `coords` is assigned to but never used
    |
 19 | def f():
-20 |     coords = (x, y) = 1, 2  # this triggers F841 on coords
+20 |     coords = (x, y) = 1, 2
    |     ^^^^^^ F841
    |
    = help: Remove assignment to unused variable `coords`
@@ -47,8 +47,8 @@ F841_1.py:20:5: F841 [*] Local variable `coords` is assigned to but never used
 17 17 | 
 18 18 | 
 19 19 | def f():
-20    |-    coords = (x, y) = 1, 2  # this triggers F841 on coords
+20    |-    coords = (x, y) = 1, 2
-   20 |+    (x, y) = 1, 2  # this triggers F841 on coords
+   20 |+    (x, y) = 1, 2
 21 21 | 
 22 22 | 
 23 23 | def f():

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F841_F841_4.py.snap

@@ -0,0 +1,23 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+F841_4.py:12:5: F841 [*] Local variable `a` is assigned to but never used
+   |
+11 | def bar():
+12 |     a = foo()
+   |     ^ F841
+13 |     b, c = foo()
+   |
+   = help: Remove assignment to unused variable `a`
+
+ℹ Suggested fix
+9  9  | 
+10 10 | 
+11 11 | def bar():
+12    |-    a = foo()
+   12 |+    foo()
+13 13 |     b, c = foo()
+14 14 | 
+15 15 | 
+
+

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__preview__F841_F841_4.py.snap

@@ -0,0 +1,41 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+F841_4.py:12:5: F841 [*] Local variable `a` is assigned to but never used
+   |
+11 | def bar():
+12 |     a = foo()
+   |     ^ F841
+13 |     b, c = foo()
+   |
+   = help: Remove assignment to unused variable `a`
+
+ℹ Suggested fix
+9  9  | 
+10 10 | 
+11 11 | def bar():
+12    |-    a = foo()
+   12 |+    foo()
+13 13 |     b, c = foo()
+14 14 | 
+15 15 | 
+
+F841_4.py:13:5: F841 Local variable `b` is assigned to but never used
+   |
+11 | def bar():
+12 |     a = foo()
+13 |     b, c = foo()
+   |     ^ F841
+   |
+   = help: Remove assignment to unused variable `b`
+
+F841_4.py:13:8: F841 Local variable `c` is assigned to but never used
+   |
+11 | def bar():
+12 |     a = foo()
+13 |     b, c = foo()
+   |        ^ F841
+   |
+   = help: Remove assignment to unused variable `c`
+
+