Emit JSON diagnostics from rss run

Author: olwang

README.md

@@ -406,8 +406,8 @@ rss review [--json] --diff <old.rss> <new.rss>
 rss review [--json] --map <file-or-directory>
 rss lower --rust <file.rss>
 rss lower --rust <file.rss> --out-dir <directory>
-rss run <file.rss>
-rss run <file.rss> --out-dir <directory>
+rss run [--json] <file.rss>
+rss run [--json] <file.rss> --out-dir <directory>
 rss remap-rustc [--json] <rsscript-source-map.json> <rustc-json-lines>
 rss verify-rust [--json] <file.rss>
 rss verify-rust [--json] <file.rss> --out-dir <directory>
@@ -419,7 +419,7 @@ rss verify-rust [--json] <file.rss> --out-dir <directory>
 
 `rss review --map` checks inputs before emitting a map. Files with frontend errors produce diagnostics instead of potentially misleading review classifications.
 
-If a lowered package contains `fn main() -> Unit` or `fn main() -> Result<Unit, E>`, `rss lower --rust --out-dir` also emits a Rust `src/main.rs` harness. `rss run <file.rss>` uses the same lowering path, writes a temporary Rust package, and delegates execution to `cargo run`. Use `rss run <file.rss> --out-dir <directory>` to keep the generated Rust package for inspection.
+If a lowered package contains `fn main() -> Unit` or `fn main() -> Result<Unit, E>`, `rss lower --rust --out-dir` also emits a Rust `src/main.rs` harness. `rss run <file.rss>` uses the same lowering path, writes a temporary Rust package, and delegates execution to `cargo run`. Use `rss run <file.rss> --out-dir <directory>` to keep the generated Rust package for inspection. Frontend and RSScript runtime diagnostics from `rss run` support `--json`; successful program output remains the program's own stdout.
 
 `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.
 

src/main.rs

@@ -387,7 +387,7 @@ fn run_generated_rust(args: &[String]) -> ExitCode {
     ) {
         Ok(package) => package,
         Err(diagnostics) => {
-            print_diagnostics(false, &diagnostics);
+            print_diagnostics(options.json, &diagnostics);
             return ExitCode::from(1);
         }
     };
@@ -441,7 +441,7 @@ fn run_generated_rust(args: &[String]) -> ExitCode {
     let stderr = String::from_utf8_lossy(&output.stderr);
     let diagnostics = parse_runtime_diagnostics(&stderr);
     if !diagnostics.is_empty() {
-        print_diagnostics(false, &diagnostics);
+        print_diagnostics(options.json, &diagnostics);
         return ExitCode::from(1);
     }
     if !stderr.trim().is_empty() {
@@ -563,6 +563,7 @@ struct LowerOptions<'a> {
 }
 
 struct RunOptions<'a> {
+    json: bool,
     path: Option<&'a str>,
     out_dir: Option<&'a str>,
 }
@@ -639,12 +640,15 @@ fn parse_lower_args(args: &[String]) -> LowerOptions<'_> {
 }
 
 fn parse_run_args(args: &[String]) -> RunOptions<'_> {
+    let mut json = false;
     let mut path = None;
     let mut out_dir = None;
     let mut index = 0;
 
     while let Some(arg) = args.get(index) {
-        if arg == "--out-dir" {
+        if arg == "--json" {
+            json = true;
+        } else if arg == "--out-dir" {
             index += 1;
             out_dir = args.get(index).map(String::as_str);
         } else if path.is_none() {
@@ -653,7 +657,11 @@ fn parse_run_args(args: &[String]) -> RunOptions<'_> {
         index += 1;
     }
 
-    RunOptions { path, out_dir }
+    RunOptions {
+        json,
+        path,
+        out_dir,
+    }
 }
 
 fn parse_verify_args(args: &[String]) -> VerifyOptions<'_> {
@@ -929,8 +937,8 @@ fn print_usage() {
     eprintln!("  rsscript fmt <file.rss>");
     eprintln!("  rsscript lower --rust <file.rss>");
     eprintln!("  rsscript lower --rust <file.rss> --out-dir <directory>");
-    eprintln!("  rsscript run <file.rss>");
-    eprintln!("  rsscript run <file.rss> --out-dir <directory>");
+    eprintln!("  rsscript run [--json] <file.rss>");
+    eprintln!("  rsscript run [--json] <file.rss> --out-dir <directory>");
     eprintln!("  rsscript remap-rustc [--json] <rsscript-source-map.json> <rustc-json-lines>");
     eprintln!("  rsscript verify-rust [--json] <file.rss>");
     eprintln!("  rsscript verify-rust [--json] <file.rss> --out-dir <directory>");

tests/checker.rs

@@ -1108,25 +1108,7 @@ fn rss_run_maps_runtime_conflict_to_rsscript_diagnostic() {
     let temp_dir = unique_temp_dir("rsscript-runtime-diagnostic");
     fs::create_dir_all(&temp_dir).expect("temp dir should be created");
     let source_path = temp_dir.join("empty_pool.rss");
-    fs::write(
-        &source_path,
-        r#"features: local
-
-fn main() -> Unit {
-    local pool = ResourcePool<DbConnection>.new(
-        create: || DbConnection.open(url: read "db://local"),
-        max_size: 0,
-    )
-
-    with ResourcePool.borrow(pool: mut pool) as conn {
-        Log.write(message: read "unreachable")
-    }
-
-    return Unit
-}
-"#,
-    )
-    .expect("runtime diagnostic fixture should be written");
+    write_runtime_conflict_fixture(&source_path);
 
     let output = Command::new(env!("CARGO_BIN_EXE_rss"))
         .arg("run")
@@ -1152,6 +1134,42 @@ fn main() -> Unit {
     );
 }
 
+#[test]
+fn rss_run_json_maps_runtime_conflict_to_diagnostics_json() {
+    let temp_dir = unique_temp_dir("rsscript-runtime-diagnostic-json");
+    fs::create_dir_all(&temp_dir).expect("temp dir should be created");
+    let source_path = temp_dir.join("empty_pool.rss");
+    write_runtime_conflict_fixture(&source_path);
+
+    let output = Command::new(env!("CARGO_BIN_EXE_rss"))
+        .arg("run")
+        .arg("--json")
+        .arg(&source_path)
+        .current_dir(env!("CARGO_MANIFEST_DIR"))
+        .output()
+        .expect("rss run should execute");
+    let _ = fs::remove_dir_all(&temp_dir);
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    let json: Value = serde_json::from_str(&stdout).expect("stdout should be diagnostics JSON");
+
+    assert!(!output.status.success());
+    assert!(stderr.trim().is_empty(), "{stderr}");
+    assert_eq!(json[0]["code"], "RS1201");
+    assert_eq!(json[0]["severity"], "error");
+    assert_eq!(
+        json[0]["spans"][0]["file"],
+        source_path.display().to_string()
+    );
+    assert!(
+        json[0]["causes"]
+            .as_array()
+            .expect("causes should be an array")
+            .iter()
+            .any(|cause| cause == "runtime error kind: resource_pool_empty")
+    );
+}
+
 #[test]
 fn rust_lowering_targets_runtime_crate_hooks() {
     let source = r#"
@@ -2235,6 +2253,28 @@ fn unique_temp_dir(prefix: &str) -> PathBuf {
     std::env::temp_dir().join(format!("{prefix}-{}-{nanos}", std::process::id()))
 }
 
+fn write_runtime_conflict_fixture(path: &Path) {
+    fs::write(
+        path,
+        r#"features: local
+
+fn main() -> Unit {
+    local pool = ResourcePool<DbConnection>.new(
+        create: || DbConnection.open(url: read "db://local"),
+        max_size: 0,
+    )
+
+    with ResourcePool.borrow(pool: mut pool) as conn {
+        Log.write(message: read "unreachable")
+    }
+
+    return Unit
+}
+"#,
+    )
+    .expect("runtime diagnostic fixture should be written");
+}
+
 fn expected_codes(source: &str) -> Vec<String> {
     let first_line = source.lines().next().unwrap_or_default();
     let Some(codes) = first_line.strip_prefix("// expect:") else {