Factor out a constant for the default terminal width

src/display/side_by_side.rs

@@ -182,6 +182,10 @@ impl SourceDimensions {
         let lhs_line_nums_width = format_line_num(lhs_max_line).len();
         let rhs_line_nums_width = format_line_num(rhs_max_line).len();
 
+        assert!(
+            terminal_width > SPACER.len(),
+            "Terminal total width should not overflow"
+        );
         let lhs_total_width = (terminal_width - SPACER.len()) / 2;
 
         let lhs_content_width = if lhs_line_nums_width < lhs_total_width {
@@ -590,6 +594,7 @@ mod tests {
 
     use super::*;
     use crate::{
+        options::DEFAULT_DISPLAY_WIDTH,
         parse::guess_language::Language,
         syntax::{AtomKind, MatchKind, TokenKind},
     };
@@ -597,7 +602,7 @@ mod tests {
     #[test]
     fn test_width_calculations() {
         let line_nums = [(Some(1.into()), Some(10.into()))];
-        let source_dims = SourceDimensions::new(80, &line_nums);
+        let source_dims = SourceDimensions::new(DEFAULT_DISPLAY_WIDTH, &line_nums);
 
         assert_eq!(source_dims.lhs_line_nums_width, 2);
         assert_eq!(source_dims.rhs_line_nums_width, 3);
@@ -606,7 +611,7 @@ mod tests {
     #[test]
     fn test_format_missing_line_num() {
         let source_dims = SourceDimensions::new(
-            80,
+            DEFAULT_DISPLAY_WIDTH,
             &[
                 (Some(0.into()), Some(0.into())),
                 (Some(1.into()), Some(1.into())),

src/options.rs

@@ -50,6 +50,8 @@ pub(crate) struct DisplayOptions {
     pub(crate) sort_paths: bool,
 }
 
+pub(crate) const DEFAULT_DISPLAY_WIDTH: usize = 80;
+
 impl Default for DisplayOptions {
     fn default() -> Self {
         Self {
@@ -58,7 +60,7 @@ impl Default for DisplayOptions {
             display_mode: DisplayMode::SideBySide,
             print_unchanged: true,
             tab_width: 8,
-            display_width: 80,
+            display_width: DEFAULT_DISPLAY_WIDTH,
             num_context_lines: 3,
             syntax_highlight: true,
             sort_paths: false,
@@ -865,7 +867,7 @@ fn detect_display_width() -> usize {
         return columns.into();
     }
 
-    80
+    DEFAULT_DISPLAY_WIDTH
 }
 
 pub(crate) fn should_use_color(color_output: ColorOutput) -> bool {