This patch adds a checkpatch.pl warning to detect //-style comments
placed above private Rust items.

Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
---
Changes in v2:
- Don't trigger a warning when the block of comments above a private rust item
  contains at least one doc comment.
- Trigger warning on private macros.
- Link to v1: https://lore.kernel.org/all/20250419212458.105705-1-contact@arnaud-lcm.com/
---
 scripts/checkpatch.pl | 56 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 3d22bf863eec..e49857c23034 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -1987,6 +1987,41 @@ sub ctx_locate_comment {
 	chomp($current_comment);
 	return($current_comment);
 }
+
+sub ctx_get_comment_behind_rust_private_item {
+    my ($linenr) = @_;
+    my $start_line = $linenr - 1;
+    my $idx = $start_line;
+
+    # Skip if it's a public Rust item (starts with 'pub')
+    return unless $rawlines[$start_line] !~ /^\+?\s*pub(\s|\([^)]*\)\s)/;
+    
+    # Check if it's an empty line
+    return unless $rawlines[$start_line] !~ /^\s*\+?\s*$/;
+
+    # Check if it's a comment
+    return unless $rawlines[$start_line] !~ /^\+?\s*\/\/\/?\/?.*$/;
+
+    # Check if it is a private macro
+    if ($rawlines[$start_line] =~ /^\+?\s*macro_rules!\s+([a-zA-Z_][a-zA-Z0-9_]*|#?[a-zA-Z_][a-zA-Z0-9_]*)\s*(?:\{|$)/ && $idx > 0) {
+	return unless $rawlines[$idx -1] !~ /^\+?\s*#\s*\[\s*macro_export\s*\]\s*$/;
+    } else {
+	# Check if it's a Rust item
+	return unless $rawlines[$start_line] =~ /^\+?\s*\b(fn|struct|const|enum|trait|type|mod|impl|use)\b/;
+    }
+
+    # Walk backwards through non-empty lines
+    $idx--;
+    while ($idx >= 0 && $rawlines[$idx] !~ /^\s*\+?\s*$/) {
+	# Check if is a document comment
+	return unless $rawlines[$idx] !~ m@^\+?\s*///([^/].*)?$@;
+	$idx--;
+    }
+
+    return ($idx+1, $start_line);
+}
+
+
 sub ctx_has_comment {
 	my ($first_line, $end_line) = @_;
 	my $cmt = ctx_locate_comment($first_line, $end_line);
@@ -2950,6 +2985,27 @@ sub process {
 			}
 		}
 
+# check for incorrect use of `//` instead of `\\\` for doc comments in Rust
+		if ($perl_version_ok && $realfile =~ /\.rs$/) {
+			my ($start_l, $end_l) = ctx_get_comment_behind_rust_private_item($linenr);
+			if (defined($start_l) && defined($end_l)) {
+				for (my $i = $start_l; $i <= $end_l; $i++) {
+					my $comment_line = $rawlines[$i - 1];
+					next unless $comment_line =~ m@^\+//([^/].*)$@;
+					my $comment = $1;
+			
+					# Skip obvious non-doc comments
+					next if $comment =~ m@^\s*(?:TODO|FIXME|XXX|NOTE|HACK|SAFETY):?@i;
+					next if $comment =~ m@^\s*[[:punct:]]{2,}@;	
+					my $line_issue = $i - 3;
+					
+					WARN("POSSIBLE_MISSING_RUST_DOC",
+						"Consider using `///` for private item documentation (line $line_issue)\n" .
+					"$here\n$comment_line");
+				}
+			}
+		}
+
 # Check the patch for a From:
 		if (decode("MIME-Header", $line) =~ /^From:\s*(.*)/) {
 			$author = $1;
-- 
2.43.0