Improve primer diff rendering and add test fixtures

- Show ^^ caret hints under changed spans in diff blocks
- Make symbol name a clickable link to the source location
- Use backtick-formatted spans for location changes
- Extract _details_section helper to deduplicate <details> wrapping
- Add mixed_changes and type_changed test fixtures

Author: Pierre-Sassoulas

pylint/testutils/_primer/comparator.py

@@ -7,6 +7,7 @@
 import json
 from collections import defaultdict
 from collections.abc import Iterator
+from difflib import SequenceMatcher
 from pathlib import Path
 
 from pylint.reporters.json_reporter import OldJsonExport
@@ -66,8 +67,8 @@ def format_span(msg: OldJsonExport) -> str:
     end_line = msg.get("endLine")
     end_col = msg.get("endColumn")
     if end_line is not None and end_col is not None:
-        return f"{start} to {end_line}:{end_col}"
-    return start
+        return f"from `{start}` to `{end_line}:{end_col}`"
+    return f"at `{start}`"
 
 
 def message_diff(old: OldJsonExport, new: OldJsonExport) -> str:
@@ -85,17 +86,43 @@ def message_diff(old: OldJsonExport, new: OldJsonExport) -> str:
     parts: list[str] = []
     # Location: combine line/column/endLine/endColumn into one sentence.
     if changed_keys & _LOCATION_KEYS:
-        parts.append(f"Was raised on {format_span(old)}, now on {format_span(new)}.")
+        parts.append(f"Was raised {format_span(old)}, now {format_span(new)}.")
 
     # Other fields (typically ``message``).
     for key in sorted(changed_keys - _LOCATION_KEYS):
         old_val = old[key]  # type: ignore[literal-required]
         new_val = new[key]  # type: ignore[literal-required]
         if isinstance(old_val, str) and isinstance(new_val, str):
-            parts.append(f"The {key} changed:\n```diff\n- {old_val}\n+ {new_val}\n```")
+            caret_line = _caret_hint(old_val, new_val)
+            diff_block = f"```diff\n- {old_val}\n+ {new_val}\n"
+            if caret_line:
+                diff_block += f"  {caret_line}\n"
+            diff_block += "```"
+            parts.append(diff_block)
         else:
             parts.append(f"{key}: {old_val!r} -> {new_val!r}")
-    return "\n".join(parts)
+    return "\n\n".join(parts)
+
+
+def _caret_hint(old: str, new: str) -> str:
+    """Build a ``^`` marker line highlighting changed spans in *new*.
+
+    Uses SequenceMatcher to find which parts of *new* differ from *old*
+    and places ``^`` characters under them (aligned with the ``+ `` prefix).
+    Returns an empty string when the whole line changed (carets wouldn't help).
+    """
+    matcher = SequenceMatcher(None, old, new)
+    carets = [" "] * len(new)
+    changed_count = 0
+    for tag, _i1, _i2, j1, j2 in matcher.get_opcodes():
+        if tag != "equal":
+            for j in range(j1, j2):
+                carets[j] = "^"
+            changed_count += j2 - j1
+    # If most of the string changed, carets are just noise.
+    if changed_count > len(new) * 0.6:
+        return ""
+    return "".join(carets).rstrip()
 
 
 class Comparator:

pylint/testutils/_primer/primer_compare_command.py

@@ -66,18 +66,23 @@ def _source_link(msg: OldJsonExport) -> str:
             filepath = str(PurePosixPath(msg["path"]).relative_to(clone_dir))
             return f"{url}/blob/{commit}/{filepath}#L{msg['line']}"
 
+        def _details_section(title: str, body: str) -> str:
+            # Two line breaks required after <details> for links to work.
+            return f"{title}\n\n<details>\n\n{body}</details>\n\n"
+
         comment = f"\n**Effect on [{package}]({url}):**\n\n"
 
         # -- Changed messages (same diagnostic, different details) -----------
         if changed_messages:
-            comment += "The following messages have been changed:\n\n<details>\n"
+            body = ""
             for count, (old, new) in enumerate(changed_messages, 1):
-                comment += (
-                    f"{count}) {new['symbol']}:\n"
+                body += (
+                    f"{count}) [{new['symbol']}]({_source_link(new)}):\n"
                     f"{message_diff(old, new)}\n"
-                    f"{_source_link(new)}\n"
                 )
-            comment += "</details>\n\n"
+            comment += _details_section(
+                "The following messages have been changed:", body
+            )
 
         # -- New messages ----------------------------------------------------
         astroid_errors = 0
@@ -101,21 +106,21 @@ def _source_link(msg: OldJsonExport) -> str:
                 "Please open the GitHub Actions log to see what failed or crashed.\n\n"
             )
         if new_non_astroid_messages:
-            comment += (
-                "The following messages are now emitted:\n\n<details>\n"
-                + new_non_astroid_messages
-                + "</details>\n\n"
+            comment += _details_section(
+                "The following messages are now emitted:", new_non_astroid_messages
             )
 
         # -- Missing messages ------------------------------------------------
         if missing_messages:
-            comment += "The following messages are no longer emitted:\n\n<details>\n"
+            body = ""
             for count, message in enumerate(missing_messages, 1):
-                comment += (
+                body += (
                     f"{count}) {message['symbol']}:\n*{message['message']}*\n"
                     f"{_source_link(message)}\n"
                 )
-            comment += "</details>\n\n"
+            comment += _details_section(
+                "The following messages are no longer emitted:", body
+            )
 
         return comment
 

…ls/_primer/fixtures/batched/expected.txt …utils/_primer/cases/batched/expected.txttests/testutils/_primer/fixtures/batched/expected.txt renamed to tests/testutils/_primer/cases/batched/expected.txt tests/testutils/_primer/fixtures/batched/expected.txt renamed to tests/testutils/_primer/cases/batched/expected.txt

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