[llvm] Add PR check to suggest alternatives to using undef (PR #118506)

[David Spickett via llvm-commits]

================
@@ -312,7 +313,84 @@ def format_run(self, changed_files: List[str], args: FormatArgs) -> Optional[str
             return None
 
 
-ALL_FORMATTERS = (DarkerFormatHelper(), ClangFormatHelper())
+class UndefGetFormatHelper(FormatHelper):
+    name = "undef deprecator"
+    friendly_name = "undef deprecator"
+
+    @property
+    def instructions(self) -> str:
+        return " ".join(f"'{c}'" for c in self.cmd)
+
+    def filter_changed_files(self, changed_files: List[str]) -> List[str]:
+        filtered_files = []
+        for path in changed_files:
+            _, ext = os.path.splitext(path)
+            if ext in (".cpp", ".c", ".h", ".hpp", ".hxx", ".cxx", ".inc", ".cppm", ".ll"):
+                filtered_files.append(path)
+        return filtered_files
+
+    def has_tool(self) -> bool:
+        return True
+
+    def format_run(self, changed_files: List[str], args: FormatArgs) -> Optional[str]:
+        files = self.filter_changed_files(changed_files)
+
+        regex = '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)'
+        cmd = ['git', 'diff', '-U0', '--pickaxe-regex', '-S', regex]
+
+        if args.start_rev and args.end_rev:
+            cmd.append(args.start_rev)
+            cmd.append(args.end_rev)
+
+        cmd += files
+        self.cmd = cmd
+
+        if args.verbose:
+            print(f"Running: {self.instructions}")
+        proc = subprocess.run(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+        sys.stdout.write(proc.stderr.decode("utf-8"))
+        stdout = proc.stdout.decode("utf-8")
+
+        files = []
+        for file in re.split('^diff --git ', stdout, 0, re.MULTILINE):
+            if re.search('^[+].*'+regex, file, re.MULTILINE):
+                files.append(re.match('a/([^ ]+)', file.splitlines()[0])[1])
+
+        if files:
+            files = '\n'.join(files)
+            report = f'''
+The following files introduce new uses of undef:
+{files}
+
+Undef is now deprecated and should only be used in the rare cases where no
----------------
DavidSpickett wrote:

Tip if you do convert this to Markdown is that these manual line breaks here will end up in the comment as well. So to make GitHub flow the text properly, put each paragraph on a single line.

I know it's going to make the line length very high but I think it's ok given the context.

https://github.com/llvm/llvm-project/pull/118506