feat: add pixi run completion for fish shell (#1680)

Add one more line to the fish script for code completion to support
listing tasks in `pixi run`.

src/cli/completion.rs

@@ -66,6 +66,7 @@ pub(crate) fn execute(args: Args) -> miette::Result<()> {
     let script = match args.shell {
         Shell::Bash => replace_bash_completion(&script),
         Shell::Zsh => replace_zsh_completion(&script),
+        Shell::Fish => replace_fish_completion(&script),
         Shell::Nushell => replace_nushell_completion(&script),
         _ => Cow::Owned(script),
     };
@@ -126,6 +127,17 @@ $2::task"#;
     re.replace(script, replacement)
 }
 
+fn replace_fish_completion(script: &str) -> Cow<str> {
+    // Adds tab completion to the pixi run command.
+    let addition = "complete -c pixi -n \"__fish_seen_subcommand_from run\" -f -a \"(string split ' ' (pixi task list --machine-readable  2> /dev/null))\"";
+    let new_script = format!("{}{}\n", script, addition);
+    let pattern = r#"-n "__fish_seen_subcommand_from run""#;
+    let replacement = r#"-n "__fish_seen_subcommand_from run; or __fish_seen_subcommand_from r""#;
+    let re = Regex::new(pattern).unwrap();
+    let result = re.replace_all(&new_script, replacement);
+    Cow::Owned(result.into_owned())
+}
+
 /// Replace the parts of the nushell completion script that need different functionality.
 fn replace_nushell_completion(script: &str) -> Cow<str> {
     // Adds tab completion to the pixi run command.
@@ -279,6 +291,15 @@ _arguments "${_arguments_options[@]}" \
         assert_ne!(replace_zsh_completion(&script), script);
     }
 
+    #[test]
+    pub fn test_fish_completion_working_regex() {
+        // Generate the original completion script.
+        let script = get_completion_script(Shell::Fish);
+        let replaced_script = replace_fish_completion(&script);
+        // Test if there was a replacement done on the clap generated completions
+        assert_ne!(replaced_script, script);
+    }
+
     #[test]
     pub fn test_nushell_completion_working_regex() {
         // Generate the original completion script.