feat: --list-themes support

assets/theme_preview.rs

@@ -0,0 +1,5 @@
+  // Output the square of a number.
+  fn print_square(num: f64) {
+      let result = f64::powf(num, 2.0);
+      println!("The square of {:.2} is {:.2}.", num, result);
+  }

src/args.rs

@@ -19,6 +19,10 @@ pub struct Args {
     #[arg(long, group = "mode")]
     pub list_languages: bool,
 
+    /// Display all supported highlighting themes
+    #[arg(long, group = "mode")]
+    pub list_themes: bool,
+
     /// Run as if git was started in `PATH` instead of the current working directory.
     ///
     /// When multiple -C options are given, each subsequent

src/blame.rs

@@ -335,15 +335,15 @@ pub struct Highlighter<'a> {
 }
 
 impl<'a> Highlighter<'a> {
-    fn enabled(
+    pub fn enabled(
         syntax: &'a syntect::parsing::SyntaxReference,
         theme: &'a syntect::highlighting::Theme,
     ) -> Self {
         let highlighter = Some(syntect::easy::HighlightLines::new(syntax, theme));
         Self { highlighter, theme }
     }
 
-    fn disabled() -> Self {
+    pub fn disabled() -> Self {
         let highlighter = None;
         static THEME: syntect::highlighting::Theme = syntect::highlighting::Theme {
             name: None,
@@ -392,7 +392,7 @@ impl<'a> Highlighter<'a> {
         self.theme
     }
 
-    fn highlight_line(
+    pub fn highlight_line(
         &mut self,
         line: &str,
         syntax_set: &syntect::parsing::SyntaxSet,

src/main.rs

@@ -52,6 +52,8 @@ fn run() -> proc_exit::ExitResult {
         config::dump_config(output_path, &mut config)?;
     } else if args.list_languages {
         list_languages(&mut config, colored_stdout)?;
+    } else if args.list_themes {
+        list_themes(&mut config, colored_stdout)?;
     } else if let Some(file_path) = args.file.as_deref() {
         blame::blame(
             file_path,
@@ -121,3 +123,45 @@ fn list_languages(config: &mut Config, colored_stdout: bool) -> proc_exit::ExitR
 
     Ok(())
 }
+
+fn list_themes(config: &mut Config, colored_stdout: bool) -> proc_exit::ExitResult {
+    let pager = config.get(&crate::git2_config::PAGER);
+    let mut pager = Pager::stdout(&pager);
+    let mut pager = pager.start();
+    let pager = pager.as_writer().with_code(proc_exit::Code::FAILURE)?;
+
+    let theme_set = syntect::highlighting::ThemeSet::load_defaults();
+    if colored_stdout {
+        let syntax_set = syntect::parsing::SyntaxSet::load_defaults_newlines();
+        let syntax = syntax_set
+            .find_syntax_by_name("Rust")
+            .expect("always included");
+        for (name, theme) in theme_set.themes.iter() {
+            let mut highlighter = blame::Highlighter::enabled(syntax, theme);
+            let _ = writeln!(
+                pager,
+                "Theme: {}{}{}",
+                anstyle::Effects::BOLD.render(),
+                name,
+                anstyle::Reset.render()
+            );
+            let _ = writeln!(pager);
+            for line in THEME_PREVIEW_DATA.lines() {
+                let _ = writeln!(
+                    pager,
+                    "{}{}",
+                    highlighter.highlight_line(line, &syntax_set).unwrap(),
+                    anstyle::Reset.render()
+                );
+            }
+            let _ = writeln!(pager);
+        }
+    } else {
+        for name in theme_set.themes.keys() {
+            let _ = writeln!(pager, "{}", name);
+        }
+    }
+    Ok(())
+}
+
+const THEME_PREVIEW_DATA: &str = include_str!("../assets/theme_preview.rs");