Add --side-by-side-fill-empty to fill up unmatched lines

This changes

│ 10 │ deleted line   │    │                │
│    │                │ 11 │ added line     │

to the following. The fill char and style can be configured.

│ 10 │ deleted line   │    │ ╱╱╱╱╱╱╱╱╱╱╱╱╱╱ │
│    │ ╱╱╱╱╱╱╱╱╱╱╱╱╱╱ │ 11 │ added line     │

Thanks to @guanghechen for the idea.

src/cli.rs

@@ -860,6 +860,14 @@ pub struct Opt {
     /// Display diffs in side-by-side layout.
     pub side_by_side: bool,
 
+    #[arg(long = "side-by-side-fill-empty", value_name = "CHAR_AND_STYLE")]
+    /// Fill empty side. Can be "true", or a fill character, or a character plus a style.
+    ///
+    /// On "true" the unmatched side will be filled with "/" in the style of
+    /// --line-numbers-zero-style, if a single character is given this is used instead.
+    /// If the character is followed by a style, that style is used.
+    pub side_by_side_fill_empty: Option<String>,
+
     #[arg(long = "syntax-theme", value_name = "SYNTAX_THEME")]
     /// The syntax-highlighting theme to use.
     ///

src/config.rs

@@ -128,6 +128,7 @@ pub struct Config {
     pub relative_paths: bool,
     pub show_themes: bool,
     pub side_by_side_data: side_by_side::SideBySideData,
+    pub side_by_side_fill_empty: Option<side_by_side::SideBySideFillEmpty>,
     pub side_by_side: bool,
     pub syntax_set: SyntaxSet,
     pub syntax_theme: Option<SyntaxTheme>,
@@ -248,6 +249,13 @@ impl From<cli::Opt> for Config {
             side_by_side_data,
         );
 
+        let side_by_side_fill_empty = side_by_side::SideBySideFillEmpty::from_str(
+            opt.side_by_side_fill_empty,
+            &opt.line_numbers_zero_style,
+            opt.computed.true_color,
+            opt.git_config.as_ref(),
+        );
+
         let navigate_regex = if (opt.navigate || opt.show_themes)
             && (opt.navigate_regex.is_none() || opt.navigate_regex == Some("".to_string()))
         {
@@ -427,6 +435,7 @@ impl From<cli::Opt> for Config {
             show_themes: opt.show_themes,
             side_by_side: opt.side_by_side && !handlers::hunk::is_word_diff(),
             side_by_side_data,
+            side_by_side_fill_empty,
             styles_map,
             syntax_set: opt.computed.syntax_set,
             syntax_theme: opt.computed.syntax_theme,

src/features/side_by_side.rs

@@ -4,6 +4,7 @@ use unicode_width::UnicodeWidthStr;
 
 use crate::ansi;
 use crate::cli;
+use crate::config::ensure_display_width_1;
 use crate::config::{self, delta_unreachable, Config};
 use crate::delta::DiffType;
 use crate::delta::State;
@@ -56,6 +57,59 @@ impl SideBySideData {
     }
 }
 
+#[derive(Clone, Debug)]
+pub struct SideBySideFillEmpty {
+    pub filler: smol_str::SmolStr,
+    pub style: Style,
+}
+
+impl SideBySideFillEmpty {
+    fn new(filler: smol_str::SmolStr, style: Style) -> Self {
+        ensure_display_width_1(
+            "first argument of side-by-side-fill-empty",
+            filler.to_string(),
+        );
+        Self { filler, style }
+    }
+
+    fn default_filler(style: Style) -> Self {
+        Self {
+            filler: "/".into(),
+            style,
+        }
+    }
+
+    pub fn from_str(
+        s: Option<String>,
+        default_style: &str,
+        true_color: bool,
+        git_config: Option<&crate::git_config::GitConfig>,
+    ) -> Option<Self> {
+        match s?.split(char::is_whitespace).collect::<Vec<_>>().as_slice() {
+            [] | [""] => None,
+            [single_arg] => match single_arg.to_lowercase().as_str() {
+                "false" => None,
+                "true" => Some(SideBySideFillEmpty::default_filler(Style::from_str(
+                    default_style,
+                    None,
+                    None,
+                    true_color,
+                    git_config,
+                ))),
+                _ => Some(SideBySideFillEmpty::new(
+                    single_arg.into(),
+                    Style::from_str(default_style, None, None, true_color, git_config),
+                )),
+            },
+            args => {
+                let style =
+                    Style::from_str(&args[1..].join(" "), None, None, true_color, git_config);
+                Some(SideBySideFillEmpty::new(args[0].into(), style))
+            }
+        }
+    }
+}
+
 pub fn available_line_width(
     config: &Config,
     data: &line_numbers::LineNumbersData,
@@ -518,18 +572,27 @@ fn pad_panel_line_to_width(
         config,
     );
 
-    match bg_fill_mode {
-        Some(BgFillMethod::TryAnsiSequence) => {
+    match (
+        bg_fill_mode,
+        config.side_by_side_fill_empty.as_ref(),
+        panel_line_is_empty,
+    ) {
+        (_, Some(fill), true) => panel_line.push_str(
+            &fill
+                .style
+                .paint(fill.filler.repeat(panel_width - text_width))
+                .to_string(),
+        ),
+        (Some(BgFillMethod::TryAnsiSequence), _, _) => {
             Painter::right_fill_background_color(panel_line, fill_style)
         }
-        Some(BgFillMethod::Spaces) if text_width >= panel_width => (),
-        Some(BgFillMethod::Spaces) => panel_line.push_str(
-            #[allow(clippy::unnecessary_to_owned)]
+        (Some(BgFillMethod::Spaces), _, _) if text_width >= panel_width => (),
+        (Some(BgFillMethod::Spaces), _, _) => panel_line.push_str(
             &fill_style
                 .paint(" ".repeat(panel_width - text_width))
                 .to_string(),
         ),
-        None => (),
+        (None, _, _) => (),
     }
 }
 

src/options/set.rs

@@ -218,6 +218,7 @@ pub fn set_options(
             show_colors,
             show_themes,
             side_by_side,
+            side_by_side_fill_empty,
             wrap_max_lines,
             wrap_right_prefix_symbol,
             wrap_right_percent,