Fix removed comments in stub files (#3745)

CHANGES.md

@@ -15,6 +15,8 @@
 
 - Fix a bug where an illegal trailing comma was added to return type annotations using
   PEP 604 unions (#3735)
+- Fix several bugs and crashes where comments in stub files were removed or mishandled
+  under some circumstances. (#3745)
 - Fix a bug where multi-line open parenthesis magic comment like `type: ignore` were not
   correctly parsed (#3740)
 

src/black/nodes.py

@@ -714,6 +714,11 @@ def is_multiline_string(leaf: Leaf) -> bool:
 
 def is_stub_suite(node: Node) -> bool:
     """Return True if `node` is a suite with a stub body."""
+
+    # If there is a comment, we want to keep it.
+    if node.prefix.strip():
+        return False
+
     if (
         len(node.children) != 4
         or node.children[0].type != token.NEWLINE
@@ -722,6 +727,9 @@ def is_stub_suite(node: Node) -> bool:
     ):
         return False
 
+    if node.children[3].prefix.strip():
+        return False
+
     return is_stub_body(node.children[2])
 
 
@@ -735,7 +743,8 @@ def is_stub_body(node: LN) -> bool:
 
     child = node.children[0]
     return (
-        child.type == syms.atom
+        not child.prefix.strip()
+        and child.type == syms.atom
         and len(child.children) == 3
         and all(leaf == Leaf(token.DOT, ".") for leaf in child.children)
     )

tests/data/simple_cases/ignore_pyi.py

@@ -0,0 +1,41 @@
+def f():  # type: ignore
+    ...
+
+class x:  # some comment
+    ...
+
+class y:
+    ...  # comment
+
+# whitespace doesn't matter (note the next line has a trailing space and tab)
+class z:        
+    ...
+
+def g():
+    # hi
+    ...
+
+def h():
+    ...
+    # bye
+
+# output
+
+def f():  # type: ignore
+    ...
+
+class x:  # some comment
+    ...
+
+class y: ...  # comment
+
+# whitespace doesn't matter (note the next line has a trailing space and tab)
+class z: ...
+
+def g():
+    # hi
+    ...
+
+def h():
+    ...
+    # bye

tests/test_format.py

@@ -33,9 +33,10 @@ def check_file(
 @pytest.mark.parametrize("filename", all_data_cases("simple_cases"))
 def test_simple_format(filename: str) -> None:
     magic_trailing_comma = filename != "skip_magic_trailing_comma"
-    check_file(
-        "simple_cases", filename, black.Mode(magic_trailing_comma=magic_trailing_comma)
+    mode = black.Mode(
+        magic_trailing_comma=magic_trailing_comma, is_pyi=filename.endswith("_pyi")
     )
+    check_file("simple_cases", filename, mode)
 
 
 @pytest.mark.parametrize("filename", all_data_cases("preview"))