[ruff] Implemented used-dummy-variable (RUF052) (#14611)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_linter/resources/test/fixtures/ruff/RUF052.py

@@ -0,0 +1,106 @@
+# Correct
+
+for _ in range(5):
+    pass
+
+_valid_type = int
+
+_valid_var_1: _valid_type
+
+_valid_var_1 = 1
+
+_valid_var_2 = 2
+
+_valid_var_3 = _valid_var_1 + _valid_var_2
+
+def _valid_fun():
+    pass    
+
+_valid_fun()
+
+def fun(arg):
+    _valid_unused_var = arg
+    pass
+
+_x = "global"
+
+def fun():
+    global _x
+    return _x
+
+def fun():
+    __dunder__ = "dunder variable"
+    return __dunder__
+
+def fun():
+    global _x
+    _x = "reassigned global"
+    return _x  
+
+class _ValidClass:
+    pass
+
+_ValidClass()
+
+class ClassOk:
+    _valid_private_cls_attr = 1
+
+    print(_valid_private_cls_attr)
+
+    def __init__(self):
+        self._valid_private_ins_attr = 2
+        print(self._valid_private_ins_attr)
+
+    def _valid_method(self):
+        return self._valid_private_ins_attr
+
+    def method(arg):
+        _valid_unused_var = arg
+        return 
+
+def fun(x):
+    _ = 1
+    __ = 2
+    ___ = 3
+    if x == 1:
+        return _
+    if x == 2:
+        return __
+    if x == 3:
+        return ___
+    return x
+
+# Incorrect
+
+class Class_:
+    def fun(self):
+        _var = "method variable" # [RUF052]
+        return _var
+
+def fun(_var): # [RUF052]
+    return _var
+
+def fun():
+    _list = "built-in" # [RUF052]
+    return _list
+
+x = "global"
+
+def fun():
+    global x
+    _x = "shadows global" # [RUF052]
+    return _x
+
+def foo():
+  x = "outer"
+  def bar():
+    nonlocal x
+    _x = "shadows nonlocal" # [RUF052]
+    return _x
+  bar()
+  return x
+
+def fun():
+    x = "local"
+    _x = "shadows local" # [RUF052]
+    return _x