Sync SyntaxSuggest

```
$ tool/sync_default_gems.rb syntax_suggest
```

1 files changed, 36 insertions, 0 deletions

diff --git a/lib/syntax_suggest/unvisited_lines.rb b/lib/syntax_suggest/unvisited_lines.rb
new file mode 100644
index 0000000000..32808db634
--- /dev/null
+++ b/lib/syntax_suggest/unvisited_lines.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module SyntaxSuggest
+  # Tracks which lines various code blocks have expanded to
+  # and which are still unexplored
+  class UnvisitedLines
+    def initialize(code_lines:)
+      @unvisited = code_lines.sort_by(&:indent_index)
+      @visited_lines = {}
+      @visited_lines.compare_by_identity
+    end
+
+    def empty?
+      @unvisited.empty?
+    end
+
+    def peek
+      @unvisited.last
+    end
+
+    def pop
+      @unvisited.pop
+    end
+
+    def visit_block(block)
+      block.lines.each do |line|
+        next if @visited_lines[line]
+        @visited_lines[line] = true
+      end
+
+      while @visited_lines[@unvisited.last]
+        @unvisited.pop
+      end
+    end
+  end
+end