[flake8-comprehension] Mark autofix for C420 as unsafe if there's comments inside the dict comprehension (#18768)

crates/ruff_linter/resources/test/fixtures/flake8_comprehensions/C420.py

@@ -90,3 +90,14 @@ def func():
 
 def func():
     {(a, b): a + b for (a, b) in [(1, 2), (3, 4)]}  # OK
+
+# https://github.com/astral-sh/ruff/issues/18764
+{ # 1
+a # 2
+: # 3
+None # 4
+for # 5
+a # 6
+in # 7
+iterable # 8
+} # 9

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

@@ -1,4 +1,5 @@
 use ast::ExprName;
+use ruff_diagnostics::Applicability;
 use ruff_macros::{ViolationMetadata, derive_message_formats};
 use ruff_python_ast::comparable::ComparableExpr;
 use ruff_python_ast::helpers::any_over_expr;
@@ -31,6 +32,19 @@ use crate::{Edit, Fix, FixAvailability, Violation};
 /// dict.fromkeys(iterable, 1)
 /// ```
 ///
+/// ## Fix safety
+/// This rule's fix is marked as unsafe if there's comments inside the dict comprehension,
+/// as comments may be removed.
+///
+/// For example, the fix would be marked as unsafe in the following case:
+/// ```python
+/// {  # comment 1
+///     a:  # comment 2
+///     None  # comment 3
+///     for a in iterable  # comment 4
+/// }
+/// ```
+///
 /// ## References
 /// - [Python documentation: `dict.fromkeys`](https://docs.python.org/3/library/stdtypes.html#dict.fromkeys)
 #[derive(ViolationMetadata)]
@@ -121,7 +135,7 @@ pub(crate) fn unnecessary_dict_comprehension_for_iterable(
     );
 
     if checker.semantic().has_builtin_binding("dict") {
-        diagnostic.set_fix(Fix::safe_edit(Edit::range_replacement(
+        let edit = Edit::range_replacement(
             checker
                 .generator()
                 .expr(&fix_unnecessary_dict_comprehension(
@@ -129,7 +143,15 @@ pub(crate) fn unnecessary_dict_comprehension_for_iterable(
                     generator,
                 )),
             dict_comp.range(),
-        )));
+        );
+        diagnostic.set_fix(Fix::applicable_edit(
+            edit,
+            if checker.comment_ranges().intersects(dict_comp.range()) {
+                Applicability::Unsafe
+            } else {
+                Applicability::Safe
+            },
+        ));
     }
 }
 

crates/ruff_linter/src/rules/flake8_comprehensions/snapshots/ruff_linter__rules__flake8_comprehensions__tests__C420_C420.py.snap

@@ -202,3 +202,34 @@ C420.py:59:6: C420 [*] Unnecessary dict comprehension for iterable; use `dict.fr
 60 60 | 
 61 61 | 
 62 62 | # Non-violation cases: RUF025
+
+C420.py:95:1: C420 [*] Unnecessary dict comprehension for iterable; use `dict.fromkeys` instead
+    |
+ 94 |   # https://github.com/astral-sh/ruff/issues/18764
+ 95 | / { # 1
+ 96 | | a # 2
+ 97 | | : # 3
+ 98 | | None # 4
+ 99 | | for # 5
+100 | | a # 6
+101 | | in # 7
+102 | | iterable # 8
+103 | | } # 9
+    | |_^ C420
+    |
+    = help: Replace with `dict.fromkeys(iterable, value)`)
+
+ℹ Unsafe fix
+92 92 |     {(a, b): a + b for (a, b) in [(1, 2), (3, 4)]}  # OK
+93 93 | 
+94 94 | # https://github.com/astral-sh/ruff/issues/18764
+95    |-{ # 1
+96    |-a # 2
+97    |-: # 3
+98    |-None # 4
+99    |-for # 5
+100    |-a # 6
+101    |-in # 7
+102    |-iterable # 8
+103    |-} # 9
+   95 |+dict.fromkeys(iterable) # 9