feat(arr): report compiled subtitle engines

crates/arr-subs/src/lib.rs

@@ -32,6 +32,33 @@ pub type SearchFuture<'a> = Pin<Box<dyn Future<Output = Result<Vec<Candidate>>>
 /// One downloaded subtitle.
 pub type DownloadFuture<'a> = Pin<Box<dyn Future<Output = Result<Fetched>> + Send + 'a>>;
 
+/// Translation engine names DESIGN.md §15 recognises, whether or not this
+/// binary has the matching cargo feature compiled in.
+pub const ENGINES: &[&str] = &["openai", "deepl", "google", "command"];
+
+/// Which of [`ENGINES`] this binary actually compiled in.
+///
+/// A backend whose feature is not compiled in must not be selectable
+/// (DESIGN.md §15, issue #198): the settings API checks a requested engine
+/// against this list before it ever reaches the database.
+#[must_use]
+pub fn compiled_engines() -> Vec<&'static str> {
+    let mut engines = Vec::new();
+    if cfg!(feature = "translate-openai") {
+        engines.push("openai");
+    }
+    if cfg!(feature = "translate-deepl") {
+        engines.push("deepl");
+    }
+    if cfg!(feature = "translate-google") {
+        engines.push("google");
+    }
+    if cfg!(feature = "translate-command") {
+        engines.push("command");
+    }
+    engines
+}
+
 /// One subtitle source: OpenSubtitles.com, Podnapisi (DESIGN.md §15).
 ///
 /// Boxed futures rather than `async fn`, so that the trait stays object safe
@@ -158,6 +185,16 @@ mod tests {
         assert_eq!(candidates[0].language, Language::PortuguesePortugal);
     }
 
+    #[test]
+    fn no_engine_is_compiled_in_by_default() {
+        // The default feature set is empty (Cargo.toml), matching #198's
+        // scope: the backends themselves are later issues.
+        assert!(super::compiled_engines().is_empty());
+        for engine in super::compiled_engines() {
+            assert!(super::ENGINES.contains(&engine));
+        }
+    }
+
     #[tokio::test]
     async fn a_candidate_id_round_trips_to_its_download() {
         let provider = StubProvider;