[primer] Rewrite standalone useless-suppression as removed message

When a useless-suppression appears without a paired locally-disabled
in the new messages, it means the suppressed checker no longer fires.
Parse the symbol from the message text and present it as a removed
message instead.

TODO: the useless_suppression test case may not accurately represent
the real primer scenario (astropy uses `# pylint: disable=W,C,R`).

Author: Pierre-Sassoulas

pylint/testutils/_primer/comparator.py

@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 import json
+import re
 from collections import defaultdict
 from collections.abc import Iterator
 from difflib import SequenceMatcher
@@ -16,6 +17,7 @@
 ChangedMessage = tuple[OldJsonExport, OldJsonExport]  # (old, new)
 
 _LOCATION_KEYS = {"line", "column", "endLine", "endColumn"}
+_USELESS_SUPPRESSION_RE = re.compile(r"Useless suppression of '(.+)'")
 
 
 def _position_key(msg: OldJsonExport) -> tuple[str, str, str]:
@@ -28,6 +30,45 @@ def _position_key(msg: OldJsonExport) -> tuple[str, str, str]:
     return (msg["symbol"], msg["path"], msg["obj"])
 
 
+def _rewrite_useless_suppressions(
+    new_messages: list[OldJsonExport],
+    missing_messages: list[OldJsonExport],
+) -> None:
+    """Convert standalone ``useless-suppression`` into removed messages.
+
+    A standalone ``useless-suppression`` (no paired ``locally-disabled`` at the
+    same location in *new_messages*) means the suppressed checker no longer
+    fires — i.e. the underlying message was effectively removed.  We parse the
+    symbol from the message text and move it to *missing_messages*.
+    """
+    locally_disabled_locations: set[tuple[str, int]] = {
+        (msg["path"], msg["line"])
+        for msg in new_messages
+        if msg["symbol"] == "locally-disabled"
+    }
+    to_remove: list[OldJsonExport] = []
+    for msg in new_messages:
+        if msg["symbol"] != "useless-suppression":
+            continue
+        if (msg["path"], msg["line"]) in locally_disabled_locations:
+            continue  # Paired with a new locally-disabled — keep both as-is.
+        match = _USELESS_SUPPRESSION_RE.match(msg["message"])
+        if not match:
+            continue
+        suppressed_symbol = match.group(1)
+        to_remove.append(msg)
+        missing_messages.append(
+            {
+                **msg,  # type: ignore[typeddict-item]
+                "symbol": suppressed_symbol,
+                "message": f"'{suppressed_symbol}' would have been emitted if it wasn't already user disabled",
+                "message-id": "",
+            }
+        )
+    for msg in to_remove:
+        new_messages.remove(msg)
+
+
 def _match_residuals(
     old_residuals: list[OldJsonExport], new_residuals: list[OldJsonExport]
 ) -> tuple[list[ChangedMessage], list[OldJsonExport], list[OldJsonExport]]:
@@ -156,6 +197,7 @@ def __init__(self, main_data: PackageMessages, pr_data: PackageMessages) -> None
                 residual_old, pr_data[package]["messages"]
             )
 
+            _rewrite_useless_suppressions(final_new, final_missing)
             self.missing_messages[package] = final_missing
             self.new_messages[package] = final_new
             self.changed_messages[package] = paired

tests/testutils/_primer/cases/useless_suppression/expected.txt

@@ -3,15 +3,12 @@
 
 **Effect on [astropy](https://github.com/astropy/astropy):**
 
-The following messages are now emitted:
+The following messages are no longer emitted:
 
 <details>
 
-1) locally-disabled:
-*Locally disabling looping-through-iterator (W4801)*
-https://github.com/astropy/astropy/blob/1fb40bc1f22f176254ef583065aa155f53f3b414/astropy/coordinates/angles/angle_parsetab.py#L14
-2) useless-suppression:
-*Useless suppression of 'looping-through-iterator'*
+1) looping-through-iterator:
+*'looping-through-iterator' would have been emitted if it wasn't already user disabled*
 https://github.com/astropy/astropy/blob/1fb40bc1f22f176254ef583065aa155f53f3b414/astropy/coordinates/angles/angle_parsetab.py#L14
 </details>
 

tests/testutils/_primer/cases/useless_suppression/expected_comparator.json

@@ -1 +1 @@
-[{ "package": "astropy", "missing": 0, "new": 2, "changed": 0 }]
+[{ "package": "astropy", "missing": 1, "new": 0, "changed": 0 }]

tests/testutils/_primer/cases/useless_suppression/main.json

@@ -1,6 +1,20 @@
 {
   "astropy": {
     "commit": "1fb40bc1f22f176254ef583065aa155f53f3b414",
-    "messages": []
+    "messages": [
+      {
+        "type": "info",
+        "module": "astropy.coordinates.angles.angle_parsetab",
+        "obj": "",
+        "line": 14,
+        "column": 0,
+        "endLine": null,
+        "endColumn": null,
+        "path": "tests/.pylint_primer_tests/astropy/astropy/astropy/coordinates/angles/angle_parsetab.py",
+        "symbol": "locally-disabled",
+        "message": "Locally disabling looping-through-iterator (W4801)",
+        "message-id": "I0011"
+      }
+    ]
   }
 }