unified: Switch the Swift front-end to swift-syntax

Flip the runtime Swift front-end from tree-sitter to swift-syntax. The
mapping rules were already ported, so the mapped AST is unchanged; this
makes the switch live.

- `language_spec` now builds a language-free desugarer
  (`ConcreteDesugarer::without_language`) and wires the swift-syntax
  parser (`swift_parse::parse`) as the front-end, dropping the
  tree-sitter language and node types. The desugarer supplies the output
  schema, so `node_types` is left empty.
- The `swift_adapter`/`swift_parse` modules are no longer
  `allow(dead_code)`: they are now reached from the live extraction
  path.
- `corpus_tests` skips (rather than fails) when the external
  `swift-syntax-parse` binary is unavailable, since it cannot run
  without the Swift-backed parser.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

unified/extractor/src/languages/mod.rs

@@ -4,23 +4,13 @@ use codeql_extractor::extractor::desugaring;
 mod swift;
 
 /// swift-syntax JSON -> `yeast::Ast` adapter for the Swift front-end.
-///
-/// Currently exercised by tests and the forthcoming runtime extraction path;
-/// `allow(dead_code)` because this is a binary crate, so its public API isn't
-/// counted as used until the binary itself calls it.
 #[path = "swift/adapter.rs"]
-#[allow(dead_code)]
 pub mod swift_adapter;
 
 /// Swift front-end parser: shells out to `swift-syntax-parse` and adapts its
-/// JSON output via [`swift_adapter`].
-///
-/// Dormant for now: the runtime Swift front-end is still tree-sitter, so
-/// nothing in the binary calls this yet. `allow(dead_code)` for the same
-/// binary-crate reason as [`swift_adapter`]; both allows are removed once the
-/// runtime switches the Swift front-end to swift-syntax.
+/// JSON output via [`swift_adapter`]. This is the live Swift front-end used by
+/// [`all_language_specs`].
 #[path = "swift/parse.rs"]
-#[allow(dead_code)]
 pub mod swift_parse;
 
 /// Shared YEAST output AST schema for all languages.

unified/extractor/src/languages/swift/swift.rs

@@ -1155,16 +1155,15 @@ fn translation_rules() -> Vec<Rule<SwiftContext>> {
 }
 
 pub fn language_spec(desugared_ast_schema: &'static str) -> desugaring::LanguageSpec {
-    let ts_language: tree_sitter::Language = tree_sitter_swift::LANGUAGE.into();
     let config = DesugaringConfig::<SwiftContext>::new()
         .add_phase("translate", PhaseKind::OneShot, translation_rules())
         .with_output_node_types_yaml(desugared_ast_schema);
-    let desugarer = ConcreteDesugarer::new(ts_language.clone(), config)
-        .expect("failed to build Swift desugarer");
+    let desugarer =
+        ConcreteDesugarer::without_language(config).expect("failed to build Swift desugarer");
     desugaring::LanguageSpec {
         prefix: "swift",
-        parser: Box::new(codeql_extractor::extractor::tree_sitter_parser(ts_language)),
-        node_types: tree_sitter_swift::NODE_TYPES,
+        parser: Box::new(super::swift_parse::parse),
+        node_types: "",
         file_globs: vec!["*.swift".into(), "*.swiftinterface".into()],
         desugarer: Box::new(desugarer),
     }

unified/extractor/tests/corpus_tests.rs

@@ -20,6 +20,14 @@ fn update_mode_enabled() -> bool {
         .unwrap_or(false)
 }
 
+/// Whether the external swift-syntax parser is available. When it is not (e.g.
+/// no Swift toolchain / the `swift-syntax-parse` binary is not on `PATH` and
+/// `CODEQL_EXTRACTOR_UNIFIED_SWIFT_SYNTAX_PARSE` is unset), the corpus test is
+/// skipped rather than failed — it cannot run without the Swift-backed parser.
+fn parser_available() -> bool {
+    languages::swift_parse::parse(b"").is_ok()
+}
+
 /// Parse a corpus `.output` file. The file holds a single test case made of
 /// three sections separated by `---` delimiter lines:
 ///
@@ -28,7 +36,7 @@ fn update_mode_enabled() -> bool {
 ///
 /// ---
 ///
-/// <raw tree-sitter parse tree>
+/// <raw swift-syntax AST (the yeast tree the adapter builds, pre-desugaring)>
 ///
 /// ---
 ///
@@ -98,6 +106,14 @@ fn collect_corpus_stems(dir: &Path, out: &mut Vec<std::path::PathBuf>) {
 
 #[test]
 fn test_corpus() {
+    if !parser_available() {
+        eprintln!(
+            "skipping test_corpus: the swift-syntax parser is unavailable \
+             (set CODEQL_EXTRACTOR_UNIFIED_SWIFT_SYNTAX_PARSE or put \
+             `swift-syntax-parse` on PATH)"
+        );
+        return;
+    }
     let update_mode = update_mode_enabled();
     let all_languages = languages::all_language_specs();
     let corpus_dir = Path::new("tests/corpus");