Format expr generator exp (#5804)

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/generator_exp.py

@@ -0,0 +1,27 @@
+(a for b in c)
+
+# parens around generator expression not required
+len(a for b in c)
+
+# parens around generator expression required
+sum((a for b in c), start=0)
+
+# black keeps these atm, but intends to remove them in the future:
+# https://github.com/psf/black/issues/2943
+f((1 for _ in a))
+
+# make sure source parenthesis detection isn't fooled by these
+f((1) for _ in (a))
+
+# combination of the two above
+f(((1) for _ in (a)))
+
+
+# black keeps these atm, but intends to remove them in the future:
+# https://github.com/psf/black/issues/2943
+len(
+    ( # leading
+    a for b in c
+     # trailing
+    )
+)