Correctly handle fmt skip comments without internal spaces

CHANGES.md

@@ -10,6 +10,9 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Fix unstable formatting involving `# fmt: skip` comments without internal spaces
+  (#2970)
+
 ### Preview style
 
 <!-- Changes that affect Black's preview style -->

src/black/comments.py

@@ -214,8 +214,11 @@ def generate_ignored_nodes(
     container: Optional[LN] = container_of(leaf)
     if comment.value in FMT_SKIP:
         prev_sibling = leaf.prev_sibling
-        if comment.value in leaf.prefix and prev_sibling is not None:
-            leaf.prefix = leaf.prefix.replace(comment.value, "")
+        # 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, preview=preview)
+        if comments and comment.value == comments[0].value and prev_sibling is not None:
+            leaf.prefix = ""
             siblings = [prev_sibling]
             while (
                 "\n" not in prev_sibling.prefix

tests/data/fmtskip7.py

@@ -0,0 +1,11 @@
+a =     "this is some code"
+b =     5  #fmt:skip
+c = 9  #fmt: skip
+d = "thisisasuperlongstringthisisasuperlongstringthisisasuperlongstringthisisasuperlongstring"  #fmt:skip
+
+# output
+
+a = "this is some code"
+b =     5  # fmt:skip
+c = 9  # fmt: skip
+d = "thisisasuperlongstringthisisasuperlongstringthisisasuperlongstringthisisasuperlongstring"  # fmt:skip

tests/test_format.py

@@ -44,6 +44,7 @@
     "fmtskip4",
     "fmtskip5",
     "fmtskip6",
+    "fmtskip7",
     "fstring",
     "function",
     "function2",