Cover Option lowering path

Author: olwang

README.md

@@ -418,7 +418,7 @@ If a lowered package contains `fn main() -> Unit` or `fn main() -> Result<Unit,
 
 `rss verify-rust <file.rss> --out-dir <directory>` keeps the generated package used for backend checking, including `rsscript-source-map.json`, so unmappable rustc diagnostics can be inspected against the generated Rust.
 
-The first observable runtime boundaries are `Log.write(message: read String)` and `Assert.equal(left: read String, right: read String)`, which lower to explicit runtime hooks. Simple core operations such as `String.concat(left: read String, right: read String)` keep RSScript `.rssi` signatures for checking and review, but lower directly to Rust standard-library expressions. Built-in boolean literals, numeric operators, and comparison operators lower to the corresponding Rust literals and operators; user-defined operator overloading remains forbidden.
+The first observable runtime boundaries are `Log.write(message: read String)` and `Assert.equal(left: read String, right: read String)`, which lower to explicit runtime hooks. Simple core operations such as `String.concat(left: read String, right: read String)` keep RSScript `.rssi` signatures for checking and review, but lower directly to Rust standard-library expressions. Built-in boolean literals, numeric operators, comparison operators, and `Option<T>` constructors lower to the corresponding Rust literals, operators, and enum constructors; user-defined operator overloading remains forbidden.
 
 Smallest runnable example:
 

examples/option.rss

@@ -0,0 +1,12 @@
+pub fn maybe_value(flag: Bool) -> Option<Int> {
+    if flag {
+        return Some(42)
+    }
+    return None
+}
+
+fn main() -> Unit {
+    let _ = maybe_value(flag: true)
+    Log.write(message: read "option ran")
+    return Unit
+}

tests/checker.rs

@@ -265,6 +265,23 @@ pub fn unit_result() -> Result<Unit, BuildError> {
     assert!(rust.contains("return Ok(());"));
 }
 
+#[test]
+fn rust_lowering_maps_option_type_and_constructors_to_rust() {
+    let source = r#"
+pub fn maybe_value(flag: Bool) -> Option<Int> {
+    if flag {
+        return Some(42)
+    }
+    return None
+}
+"#;
+    let rust = lower_source_to_rust("option.rss", source).expect("source should lower");
+
+    assert!(rust.contains("pub fn maybe_value(flag: bool) -> Option<i64>"));
+    assert!(rust.contains("return Some(42);"));
+    assert!(rust.contains("return None;"));
+}
+
 #[test]
 fn rust_lowering_maps_log_write_to_runtime_output_hook() {
     let source = r#"