[flake8-comprehensions] Handle builtins at top of file correctly fo…

…r `unnecessary-dict-comprehension-for-iterable` (`C420`) (#15837)

Builtin bindings are given a range of `0..0`, which causes strange
behavior when range checks are made at the top of the file. In this
case, the logic of the rule demands that the value of the dict
comprehension is not self-referential (i.e. it does not contain
definitions for any of the variables used within it). This logic was
confused by builtins which looked like they were defined "in the
comprehension", if the comprehension appeared at the top of the file.

Closes #15830

crates/ruff_linter/resources/test/fixtures/flake8_comprehensions/C420_1.py

@@ -0,0 +1,7 @@
+{x: NotImplemented for x in "XY"}
+
+
+# Builtin bindings are placed at top of file, but should not count as
+# an "expression defined within the comprehension". So the above
+# should trigger C420
+# See https://github.com/astral-sh/ruff/issues/15830

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

@@ -22,6 +22,7 @@ mod tests {
     #[test_case(Rule::UnnecessaryComprehensionInCall, Path::new("C419.py"))]
     #[test_case(Rule::UnnecessaryComprehensionInCall, Path::new("C419_2.py"))]
     #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420.py"))]
+    #[test_case(Rule::UnnecessaryDictComprehensionForIterable, Path::new("C420_1.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

@@ -99,6 +99,14 @@ pub(crate) fn unnecessary_dict_comprehension_for_iterable(
 
         let binding = checker.semantic().binding(id);
 
+        // Builtin bindings have a range of 0..0, and are never
+        // defined within the comprehension, so we abort before
+        // checking the range overlap below. Note this only matters
+        // if the comprehension appears at the top of the file!
+        if binding.kind.is_builtin() {
+            return false;
+        }
+
         dict_comp.range().contains_range(binding.range())
     });
     if self_referential {

crates/ruff_linter/src/rules/flake8_comprehensions/snapshots/ruff_linter__rules__flake8_comprehensions__tests__C420_C420_1.py.snap

@@ -0,0 +1,16 @@
+---
+source: crates/ruff_linter/src/rules/flake8_comprehensions/mod.rs
+---
+C420_1.py:1:1: C420 [*] Unnecessary dict comprehension for iterable; use `dict.fromkeys` instead
+  |
+1 | {x: NotImplemented for x in "XY"}
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C420
+  |
+  = help: Replace with `dict.fromkeys(iterable)`)
+
+ℹ Safe fix
+1   |-{x: NotImplemented for x in "XY"}
+  1 |+dict.fromkeys("XY", NotImplemented)
+2 2 | 
+3 3 | 
+4 4 | # Builtin bindings are placed at top of file, but should not count as