--theme: return a list of all themes if the theme argument doesn't match

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Improve README documentation on pager options passed to less, see #3443 (@injust)
 
 - Use more robust approach to escaping in Bash completions, see #3448 (@akinomyoga)
+- `--theme` now provides a short list of all themes if the given theme does not exist, see #3456 (@monkeydom)
 
 ## Syntaxes
 

src/assets.rs

@@ -195,7 +195,11 @@ impl HighlightingAssets {
                     return self.get_theme("ansi");
                 }
                 if !theme.is_empty() {
-                    bat_warning!("Unknown theme '{theme}', using default.")
+                    let all_themes: Vec<&str> = self.get_theme_set().themes().collect();
+                    bat_warning!(
+                        "Unknown theme '{theme}', using default. (Available themes: {})",
+                        all_themes.join(", ")
+                    );
                 }
                 self.get_theme_set()
                     .get(

tests/integration_tests.rs

@@ -2762,6 +2762,22 @@ fn theme_env_overrides_config() {
         .stderr("");
 }
 
+#[test]
+fn theme_list_is_provided() {
+    bat()
+        .env("BAT_THEME_DARK", "Coldark-Dark")
+        .env("COLORTERM", "truecolor")
+        .arg("--theme")
+        .arg("some nonexistent theme")
+        .arg("--color=always")
+        .arg("--style=plain")
+        .write_stdin(" ")
+        .assert()
+        .success()
+        .stdout("\x1b[38;2;248;248;242m \x1b[0m")
+        .stderr(predicate::str::starts_with("\x1b[33m[bat warning]\x1b[0m: Unknown theme \'some nonexistent theme\', using default. (Available themes: "));
+}
+
 #[test]
 fn highlighting_is_skipped_on_long_lines() {
     let expected = "\u{1b}[38;5;231m{\u{1b}[0m\u{1b}[38;5;208m\"\u{1b}[0m\u{1b}[38;5;208mapi\u{1b}[0m\u{1b}[38;5;208m\"\u{1b}[0m\u{1b}[38;5;231m:\u{1b}[0m\n".to_owned() +