clap_complete: add unit test for fish env completer

Related to: #6196

This PR adds a unit test showing the current behavior of the the Fish
shell env completer, that quotes paths incorrectly when there are spaces
in the path. The issue should be demonstrated in this PR, and a
follow-up PR could have a fix for the issue.

Author: devjgm

clap_complete/src/env/shells.rs

@@ -446,3 +446,55 @@ impl Zsh {
         string.replace('\\', "\\\\")
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use snapbox::assert_data_eq;
+
+    // This test verifies that fish shell path quoting works with or without spaces in the path.
+    #[test]
+    #[cfg(all(unix, feature = "unstable-dynamic"))]
+    #[cfg(feature = "unstable-shell-tests")]
+    fn fish_env_completer_path_quoting_works() {
+        // Returns the dynamic registration line for the fish shell, for example:
+        // complete --keep-order --exclusive --command my-bin --arguments "(COMPLETE=fish /path/to/my-bin ... )"
+        let get_fish_registration = |completer_bin: &str| {
+            let mut buf = Vec::new();
+            let fish = Fish;
+            fish.write_registration(
+                "IGNORED_VAR",
+                "ignored-name",
+                "/ignored/bin",
+                completer_bin,
+                &mut buf,
+            )
+            .expect("write_registration failed");
+            return String::from_utf8(buf).expect("Invalid UTF-8");
+        };
+
+        let script = get_fish_registration("completer");
+        assert_data_eq!(
+            script.trim(),
+            snapbox::str![r#"complete [..] "([..] "'completer'"[..])""#]
+        );
+
+        let script = get_fish_registration("/path/completer");
+        assert_data_eq!(
+            script.trim(),
+            snapbox::str![r#"complete [..] "([..] "'/path/completer'"[..])""#]
+        );
+
+        // This case demonstrates the existing bug when handling paths with spaces as described in
+        // https://github.com/clap-rs/clap/issues/6196
+        // The problem shown here is:
+        // * The double quote started at `"(` is closed by the double quote before the path
+        // * Then we have an empty pair of single quotes
+        // * Then we have the _unquoted_ path with a space, which is problematic
+        let script = get_fish_registration("/path with a space/completer");
+        assert_data_eq!(
+            script.trim(),
+            snapbox::str![r#"complete [..] "([..] "''/path with a space/completer''"[..])""#]
+        );
+    }
+}