Improve printing of DisplayLists

Use box tree characters to make DisplayLists easier to scan when
printing them out.

components/util/lib.rs

@@ -61,6 +61,7 @@ pub mod mem;
 pub mod opts;
 pub mod persistent_list;
 pub mod prefs;
+pub mod print_tree;
 pub mod range;
 pub mod resource_files;
 pub mod str;

components/util/print_tree.rs

@@ -0,0 +1,59 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/// A struct that makes it easier to print out a pretty tree of data, which
+/// can be visually scanned more easily.
+pub struct PrintTree {
+    /// The current level of recursion.
+    level: u32,
+
+    /// An item which is queued up, so that we can determine if we need
+    /// a mid-tree prefix or a branch ending prefix.
+    queued_item: Option<String>,
+}
+
+impl PrintTree {
+    pub fn new(title: String) -> PrintTree {
+        println!("┌ {}", title);
+        PrintTree {
+            level: 1,
+            queued_item: None,
+        }
+    }
+
+    /// Descend one level in the tree with the given title.
+    pub fn new_level(&mut self, title: String) {
+        self.flush_queued_item("├─");
+
+        self.print_level_prefix();
+        println!("├─ {}", title);
+
+        self.level = self.level + 1;
+    }
+
+    /// Ascend one level in the tree.
+    pub fn end_level(&mut self) {
+        self.flush_queued_item("└─");
+        self.level = self.level - 1;
+    }
+
+    /// Add an item to the current level in the tree.
+    pub fn add_item(&mut self, text: String) {
+        self.flush_queued_item("├─");
+        self.queued_item = Some(text);
+    }
+
+    fn print_level_prefix(&self) {
+        for _ in 0..self.level {
+            print!("│  ");
+        }
+    }
+
+    fn flush_queued_item(&mut self, prefix: &str) {
+        if let Some(queued_item) = self.queued_item.take() {
+            self.print_level_prefix();
+            println!("{} {}", prefix, queued_item);
+        }
+    }
+}