[flake8-comprehensions] Fix false positive for C420 with attribute, subscript, or slice assignment targets (#19513)

## Summary

Fixes #19511

crates/ruff_linter/src/rules/flake8_comprehensions/mod.rs

@@ -25,6 +25,7 @@ mod tests {
     #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420.py"))]
     #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420_1.py"))]
     #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420_2.py"))]
+    #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420_3.py"))]
     #[test_case(Rule::UnnecessaryDoubleCastOrProcess, Path::new("C414.py"))]
     #[test_case(Rule::UnnecessaryGeneratorDict, Path::new("C402.py"))]
     #[test_case(Rule::UnnecessaryGeneratorList, Path::new("C400.py"))]

crates/ruff_linter/src/rules/flake8_comprehensions/rules/unnecessary_dict_comprehension_for_iterable.rs

@@ -96,6 +96,12 @@ pub(crate) fn unnecessary_dict_comprehension_for_iterable(
         return;
     }
 
+    // Don't suggest `dict.fromkeys` if the target contains side-effecting expressions
+    // (attributes, subscripts, or slices).
+    if contains_side_effecting_sub_expression(&generator.target) {
+        return;
+    }
+
     // Don't suggest `dict.fromkeys` if the value is not a constant or constant-like.
     if !is_constant_like(dict_comp.value.as_ref()) {
         return;
@@ -217,3 +223,12 @@ fn fix_unnecessary_dict_comprehension(value: &Expr, generator: &Comprehension) -
         node_index: ruff_python_ast::AtomicNodeIndex::dummy(),
     })
 }
+
+fn contains_side_effecting_sub_expression(target: &Expr) -> bool {
+    any_over_expr(target, &|expr| {
+        matches!(
+            expr,
+            Expr::Attribute(_) | Expr::Subscript(_) | Expr::Slice(_)
+        )
+    })
+}

crates/ruff_linter/src/rules/flake8_comprehensions/snapshots/ruff_linter__rules__flake8_comprehensions__tests__C420_C420_3.py.snap

@@ -0,0 +1,4 @@
+---
+source: crates/ruff_linter/src/rules/flake8_comprehensions/mod.rs
+---
+