Add colored underline support

This commit adds support for colored underline and refines the dynamic
extra storage. The extra storage now is using `Arc` making cloning it way
faster compared to `Box` approach which scales really well when it comes
to cloning in `Term::write_at_cursor`, since cloning `Arc` is constant
time.

Fixes #4142.

2 files changed, 47 insertions, 19 deletions

diff --git a/alacritty_terminal/src/term/cell.rs b/alacritty_terminal/src/term/cell.rs
index 2af1fbac..716bcf9b 100644
--- a/alacritty_terminal/src/term/cell.rs
+++ b/alacritty_terminal/src/term/cell.rs
@@ -1,4 +1,4 @@
-use std::boxed::Box;
+use std::sync::Arc;
 
 use bitflags::bitflags;
 use serde::{Deserialize, Serialize};
@@ -57,19 +57,20 @@ impl ResetDiscriminant<Color> for Cell {
 /// allocation required ahead of time for every cell, with some additional overhead when the extra
 /// storage is actually required.
 #[derive(Serialize, Deserialize, Default, Debug, Clone, Eq, PartialEq)]
-struct CellExtra {
+pub struct CellExtra {
     zerowidth: Vec<char>,
+
+    underline_color: Option<Color>,
 }
 
 /// Content and attributes of a single cell in the terminal grid.
-#[derive(Serialize, Deserialize, Debug, Clone, Eq, PartialEq)]
+#[derive(Serialize, Deserialize, Clone, Debug, Eq, PartialEq)]
 pub struct Cell {
     pub c: char,
     pub fg: Color,
     pub bg: Color,
     pub flags: Flags,
-    #[serde(default)]
-    extra: Option<Box<CellExtra>>,
+    pub extra: Option<Arc<CellExtra>>,
 }
 
 impl Default for Cell {
@@ -94,25 +95,39 @@ impl Cell {
 
     /// Write a new zerowidth character to this cell.
     #[inline]
-    pub fn push_zerowidth(&mut self, c: char) {
-        self.extra.get_or_insert_with(Default::default).zerowidth.push(c);
-    }
-
-    /// Free all dynamically allocated cell storage.
-    #[inline]
-    pub fn drop_extra(&mut self) {
-        if self.extra.is_some() {
-            self.extra = None;
-        }
+    pub fn push_zerowidth(&mut self, character: char) {
+        let extra = self.extra.get_or_insert(Default::default());
+        Arc::make_mut(extra).zerowidth.push(character);
     }
 
     /// Remove all wide char data from a cell.
     #[inline(never)]
     pub fn clear_wide(&mut self) {
         self.flags.remove(Flags::WIDE_CHAR);
-        self.drop_extra();
+        if let Some(extra) = self.extra.as_mut() {
+            Arc::make_mut(extra).zerowidth = Vec::new();
+        }
         self.c = ' ';
     }
+
+    /// Set underline color on the cell.
+    pub fn set_underline_color(&mut self, color: Option<Color>) {
+        // If we reset color and we don't have zerowidth we should drop extra storage.
+        if color.is_none() && self.extra.as_ref().map_or(true, |extra| !extra.zerowidth.is_empty())
+        {
+            self.extra = None;
+            return;
+        }
+
+        let extra = self.extra.get_or_insert(Default::default());
+        Arc::make_mut(extra).underline_color = color;
+    }
+
+    /// Underline color stored in this cell.
+    #[inline]
+    pub fn underline_color(&self) -> Option<Color> {
+        self.extra.as_ref()?.underline_color
+    }
 }
 
 impl GridCell for Cell {
@@ -184,12 +199,23 @@ impl LineLength for grid::Row<Cell> {
 
 #[cfg(test)]
 mod tests {
-    use super::{Cell, LineLength};
+    use super::*;
+
+    use std::mem;
 
     use crate::grid::Row;
     use crate::index::Column;
 
     #[test]
+    fn cell_size_is_below_cap() {
+        // Expected cell size on 64-bit architectures.
+        const EXPECTED_CELL_SIZE: usize = 24;
+
+        // Ensure that cell size isn't growning by accident.
+        assert!(mem::size_of::<Cell>() <= EXPECTED_CELL_SIZE);
+    }
+
+    #[test]
     fn line_length_works() {
         let mut row = Row::<Cell>::new(10);
         row[Column(5)].c = 'a';
diff --git a/alacritty_terminal/src/term/mod.rs b/alacritty_terminal/src/term/mod.rs
index 96f0d1e7..fa7d2b66 100644
--- a/alacritty_terminal/src/term/mod.rs
+++ b/alacritty_terminal/src/term/mod.rs
@@ -1047,6 +1047,7 @@ impl<T> Term<T> {
         let fg = self.grid.cursor.template.fg;
         let bg = self.grid.cursor.template.bg;
         let flags = self.grid.cursor.template.flags;
+        let extra = self.grid.cursor.template.extra.clone();
 
         let mut cursor_cell = self.grid.cursor_cell();
 
@@ -1070,12 +1071,11 @@ impl<T> Term<T> {
             cursor_cell = self.grid.cursor_cell();
         }
 
-        cursor_cell.drop_extra();
-
         cursor_cell.c = c;
         cursor_cell.fg = fg;
         cursor_cell.bg = bg;
         cursor_cell.flags = flags;
+        cursor_cell.extra = extra;
     }
 
     #[inline]
@@ -1826,10 +1826,12 @@ impl<T: EventListener> Handler for Term<T> {
         match attr {
             Attr::Foreground(color) => cursor.template.fg = color,
             Attr::Background(color) => cursor.template.bg = color,
+            Attr::UnderlineColor(color) => cursor.template.set_underline_color(color),
             Attr::Reset => {
                 cursor.template.fg = Color::Named(NamedColor::Foreground);
                 cursor.template.bg = Color::Named(NamedColor::Background);
                 cursor.template.flags = Flags::empty();
+                cursor.template.set_underline_color(None);
             },
             Attr::Reverse => cursor.template.flags.insert(Flags::INVERSE),
             Attr::CancelReverse => cursor.template.flags.remove(Flags::INVERSE),