Fix formatter abort on line-continuation before aligned ports Skip alignment for partitions where non-tree tokens precede a forced wrap (e.g. // comment then `\`), and stop CHECK-failing in align.h so formatting no longer aborts (issue #2539).

diff --git a/verible/common/formatting/align.cc b/verible/common/formatting/align.cc
index 9986fc7..318a76b 100644
--- a/verible/common/formatting/align.cc
+++ b/verible/common/formatting/align.cc

@@ -1001,6 +1001,18 @@
       auto &line = node.Value();
       auto ftokens = line.TokensRange();
 
+      // Leading non-tree tokens before a forced wrap must stay on their own
+      // line. Putting them in a kInline prolog cell would glue e.g. `//\` onto
+      // the following `input` (GitHub issue 2539). Preserve original spacing
+      // for the whole row instead.
+      if (align_actions.front().ftoken != ftokens.begin() &&
+          align_actions.front().ftoken->before.break_decision ==
+              SpacingOptions::kMustWrap) {
+        FormatUsingOriginalSpacing(TokenPartitionRange(*row, std::next(*row)));
+        ++row;
+        continue;
+      }
+
       line.SetPartitionPolicy(PartitionPolicyEnum::kAlreadyFormatted);
 
       verible::TokenPartitionTree *current_cell = nullptr;

diff --git a/verible/common/formatting/align.h b/verible/common/formatting/align.h
index d042ae0..5989a6f 100644
--- a/verible/common/formatting/align.h
+++ b/verible/common/formatting/align.h

@@ -410,19 +410,24 @@
     // Use next token as begining of trailing non-tree tokens
     trailing_tokens.set_begin(ftoken_it + 1);
 
-    // Breaking following condition leads to e.g. concatenation of EOL comment
-    // and code in a single line. To fix situation that lead to this, flatten
-    // token partitions that contain EOL comment subpartition just before a
-    // subpartition that starts with the same token as Origin(). Example of a
-    // partition that needs flattening:
+    // Leading non-tree tokens (e.g. // comments, line-continuation `\`) cannot
+    // be placed into alignment cells when the first syntax-tree token must
+    // start a new line (SpacingOptions::kMustWrap). Including them would glue
+    // the leading tokens onto the origin line via kInline cells.
     //
-    //   { (>>[...], (origin: "input bit second"))
-    //     { (>>[// comment] }
-    //     { (>>[input bit second], (origin: "input")) }
-    //   }
-    CHECK(leading_tokens.empty() || first_tree_token_it == ftokens.end() ||
-          first_tree_token_it->before.break_decision !=
-              SpacingOptions::kMustWrap);
+    // This shape often appears when a `//` comment is followed by `\` (line
+    // continuation) and is partitioned with the following port/declaration
+    // (GitHub issue 2539). Prefer leaving leading tokens out of alignment
+    // over CHECK-failing. Callers should also ignore such partitions (see
+    // PartitionHasLeadingTokensBeforeForcedWrap) so ApplyAlignment never
+    // builds a kInline prolog for them. A future improvement could split the
+    // comment into its own ignored row so the following declaration can still
+    // participate in alignment.
+    if (!leading_tokens.empty() && first_tree_token_it != ftokens.end() &&
+        first_tree_token_it->before.break_decision ==
+            SpacingOptions::kMustWrap) {
+      leading_tokens.set_end(leading_tokens.begin());
+    }
   } else {
     // All tokens are passed as leading
     leading_tokens.set_end(ftokens.end());

diff --git a/verible/verilog/formatting/align.cc b/verible/verilog/formatting/align.cc
index 1f7f472..e307047 100644
--- a/verible/verilog/formatting/align.cc
+++ b/verible/verilog/formatting/align.cc

@@ -147,6 +147,32 @@
          b == AlignmentGroupBoundary::kBlankLinesAndSeparatorComments;
 }
 
+// True when non-tree tokens (e.g. // comments, line-continuation `\`) precede
+// the origin, and the first origin token must start a new line. Aligning such
+// partitions would glue the leading tokens onto the origin line via kInline
+// cells (GitHub issue 2539). Leave them out of alignment instead.
+static bool PartitionHasLeadingTokensBeforeForcedWrap(
+    const TokenPartitionTree &partition) {
+  const auto &uwline = partition.Value();
+  const verible::Symbol *origin = uwline.Origin();
+  if (origin == nullptr) return false;
+
+  const auto ftokens = uwline.TokensRange();
+  if (ftokens.empty()) return false;
+
+  const verible::SyntaxTreeLeaf *first_leaf = verible::GetLeftmostLeaf(*origin);
+  if (first_leaf == nullptr) return false;
+
+  const verible::TokenInfo &first_tree_token = first_leaf->get();
+  auto ftoken_it = ftokens.begin();
+  while (ftoken_it != ftokens.end() &&
+         *(ftoken_it->token) != first_tree_token) {
+    ++ftoken_it;
+  }
+  if (ftoken_it == ftokens.begin() || ftoken_it == ftokens.end()) return false;
+  return ftoken_it->before.break_decision == verible::SpacingOptions::kMustWrap;
+}
+
 static bool IgnoreCommentsAndPreprocessingDirectives(
     const TokenPartitionTree &partition) {
   const auto &uwline = partition.Value();
@@ -159,6 +185,8 @@
   // ignore lines containing only comments
   if (TokensAreAllCommentsOrAttributes(token_range)) return true;
 
+  if (PartitionHasLeadingTokensBeforeForcedWrap(partition)) return true;
+
   // ignore partitions belonging to preprocessing directives
   return IsPreprocessorKeyword(
       verilog_tokentype(token_range.front().TokenEnum()));
@@ -199,6 +227,8 @@
     return true;
   }
 
+  if (PartitionHasLeadingTokensBeforeForcedWrap(partition)) return true;
+
   // ignore nested structs/unions
   if (verible::FindFirstSubtree(
           partition.Value().Origin(), [](const Symbol &symbol) {

diff --git a/verible/verilog/formatting/formatter_test.cc b/verible/verilog/formatting/formatter_test.cc
index e885d74..25a733d 100644
--- a/verible/verilog/formatting/formatter_test.cc
+++ b/verible/verilog/formatting/formatter_test.cc

@@ -19216,6 +19216,32 @@
   }
 }
 
+// Regression for https://github.com/chipsalliance/verible/issues/2539:
+// A // comment followed by a line-continuation `\` before aligned ports must
+// not abort in align.h, and must keep the comment on its own line.
+TEST(FormatterEndToEndTest, PortListCommentWithLineContinuationDoesNotAbort) {
+  static constexpr std::string_view kInput =
+      "module m (\n"
+      "//\\\n"
+      "input a\n"
+      ",input b\n"
+      ");\n"
+      "endmodule\n";
+  FormatStyle style;
+  std::ostringstream stream;
+  const auto status = FormatVerilog(kInput, "<filename>", style, stream);
+  EXPECT_OK(status) << status.message();
+  const std::string out = stream.str();
+  // Must not glue the line-continuation onto the following port declaration.
+  EXPECT_THAT(out, testing::Not(testing::HasSubstr("//\\  input")));
+  EXPECT_THAT(out, testing::HasSubstr("//\\"));
+  EXPECT_THAT(out, testing::HasSubstr("input a"));
+  EXPECT_THAT(out, testing::HasSubstr("input b"));
+  EXPECT_THAT(out, testing::HasSubstr("endmodule"));
+  // Comment line and first port remain on separate lines.
+  EXPECT_THAT(out, testing::ContainsRegex(R"(//\\\n\s*input a)"));
+}
+
 }  // namespace
 }  // namespace formatter
 }  // namespace verilog