Fix bug where # fmt: skip is not being respect with one-liner functions (#4535)

Author: Pedro-Muller29

Diff for: CHANGES.md

@@ -16,6 +16,9 @@
 - Fix crash when formatting `with` statements containing tuple generators/unpacking
   (#4538)
 
+- Fix a bug where one-liner functions marked with `# fmt: skip` would still be
+  formatted (#)
+
 ### Preview style
 
 <!-- Changes that affect Black's preview style -->

Diff for: src/black/comments.py

@@ -318,26 +318,55 @@ def _generate_ignored_nodes_from_fmt_skip(
     """Generate all leaves that should be ignored by the `# fmt: skip` from `leaf`."""
     prev_sibling = leaf.prev_sibling
     parent = leaf.parent
+    ignored_nodes: list[LN] = []
     # Need to properly format the leaf prefix to compare it to comment.value,
     # which is also formatted
     comments = list_comments(leaf.prefix, is_endmarker=False)
     if not comments or comment.value != comments[0].value:
         return
     if prev_sibling is not None:
+        # Generates the nodes to be ignored by `fmt: skip`.
+
+        # Nodes to ignore are those on the same line as the `# fmt: skip` comment,
+        # excluding the `# fmt: skip` node itself.
+
+        # Traversal process (starting at the `# fmt: skip` node):
+        # 1. Move to the `prev_sibling` of the current node.
+        # 2. If `prev_sibling` has children, go to its rightmost leaf.
+        # 3. If there’s no `prev_sibling`, move up to the parent node and repeat.
+        # 4. Continue until:
+        #    a. You encounter an `INDENT` or `NEWLINE` node (indicates
+        #       start of the line).
+        #    b. You reach the root node.
+
+        # Include all visited LEAVES in the ignored list, except INDENT
+        # or NEWLINE leaves.
+
         leaf.prefix = ""
-        siblings = [prev_sibling]
-        while "\n" not in prev_sibling.prefix and prev_sibling.prev_sibling is not None:
-            prev_sibling = prev_sibling.prev_sibling
-            siblings.insert(0, prev_sibling)
-        yield from siblings
+        current_node = prev_sibling
+        ignored_nodes = [current_node]
+        if current_node.prev_sibling is None and current_node.parent is not None:
+            current_node = current_node.parent
+        while "\n" not in current_node.prefix and current_node.prev_sibling is not None:
+            leaf_nodes = list(current_node.prev_sibling.leaves())
+            current_node = leaf_nodes[-1] if leaf_nodes else current_node
+
+            if current_node.type in (token.NEWLINE, token.INDENT):
+                current_node.prefix = ""
+                break
+
+            ignored_nodes.insert(0, current_node)
+
+            if current_node.prev_sibling is None and current_node.parent is not None:
+                current_node = current_node.parent
+        yield from ignored_nodes
     elif (
         parent is not None and parent.type == syms.suite and leaf.type == token.NEWLINE
     ):
         # The `# fmt: skip` is on the colon line of the if/while/def/class/...
         # statements. The ignored nodes should be previous siblings of the
         # parent suite node.
         leaf.prefix = ""
-        ignored_nodes: list[LN] = []
         parent_sibling = parent.prev_sibling
         while parent_sibling is not None and parent_sibling.type != syms.suite:
             ignored_nodes.insert(0, parent_sibling)

Diff for: tests/data/cases/fmtskip10.py

@@ -0,0 +1 @@
+def foo(): return "mock"  # fmt: skip