Allow user to specify tab replacement string

Fixes #522

src/cli.rs

@@ -470,10 +470,16 @@ pub struct Opt {
     #[structopt(short = "w", long = "width")]
     pub width: Option<String>,
 
-    /// The number of spaces to replace tab characters with. Use --tabs=0 to pass tab characters
-    /// through directly, but note that in that case delta will calculate line widths assuming tabs
-    /// occupy one character's width on the screen: if your terminal renders tabs as more than than
-    /// one character wide then delta's output will look incorrect.
+    /// A string to use to display tab characters with. By default, each tab character will be
+    /// replaced by space characters. (See `tabs`)
+    #[structopt(long = "tab-string")]
+    pub tab_string: Option<String>,
+
+    /// The number of spaces to replace tab characters with, if tab-string has not been
+    /// supplied. Use --tabs=0 to pass tab characters through directly, but note that in that case
+    /// delta will calculate line widths assuming tabs occupy one character's width on the screen:
+    /// if your terminal renders tabs as more than than one character wide then delta's output will
+    /// look incorrect.
     #[structopt(long = "tabs", default_value = "4")]
     pub tab_width: usize,
 

src/config.rs

@@ -73,7 +73,7 @@ pub struct Config {
     pub syntax_dummy_theme: SyntaxTheme,
     pub syntax_set: SyntaxSet,
     pub syntax_theme: Option<SyntaxTheme>,
-    pub tab_width: usize,
+    pub tab_string: String,
     pub tokenization_regex: Regex,
     pub true_color: bool,
     pub truncation_symbol: String,
@@ -218,7 +218,11 @@ impl From<cli::Opt> for Config {
             syntax_dummy_theme: SyntaxTheme::default(),
             syntax_set: opt.computed.syntax_set,
             syntax_theme: opt.computed.syntax_theme,
-            tab_width: opt.tab_width,
+            tab_string: if let Some(s) = opt.tab_string {
+                s
+            } else {
+                " ".repeat(opt.tab_width)
+            },
             tokenization_regex,
             true_color: opt.computed.true_color,
             truncation_symbol: "→".to_string(),

src/main.rs

@@ -258,7 +258,7 @@ fn show_config(config: &config::Config, writer: &mut dyn Write) -> std::io::Resu
     side-by-side                  = {side_by_side}
     syntax-theme                  = {syntax_theme}
     width                         = {width}
-    tabs                          = {tab_width}
+    tab_string                    = '{tab_string}'
     word-diff-regex               = {tokenization_regex}",
         max_line_distance = config.max_line_distance,
         max_line_length = config.max_line_length,
@@ -278,7 +278,7 @@ fn show_config(config: &config::Config, writer: &mut dyn Write) -> std::io::Resu
             cli::Width::Fixed(width) => width.to_string(),
             cli::Width::Variable => "variable".to_string(),
         },
-        tab_width = config.tab_width,
+        tab_string = config.tab_string,
         tokenization_regex = format_option_value(&config.tokenization_regex.to_string()),
     )?;
     Ok(())

src/options/set.rs

@@ -170,6 +170,7 @@ pub fn set_options(
             plus_non_emph_style,
             raw,
             side_by_side,
+            tab_string,
             tab_width,
             tokenization_regex,
             true_color,

src/paint.rs

@@ -114,10 +114,15 @@ impl<'a> Painter<'a> {
     where
         I: Iterator<Item = &'b str>,
     {
-        if self.config.tab_width > 0 {
-            let tab_replacement = " ".repeat(self.config.tab_width);
-            line.map(|s| if s == "\t" { &tab_replacement } else { s })
-                .collect::<String>()
+        if !self.config.tab_string.is_empty() {
+            line.map(|s| {
+                if s == "\t" {
+                    &self.config.tab_string
+                } else {
+                    s
+                }
+            })
+            .collect::<String>()
         } else {
             line.collect::<String>()
         }