perf(cli): avoid cloning JSON results into an intermediate tree

crates/cli/src/output.rs

@@ -67,14 +67,31 @@ pub fn print_error(mode: OutputMode, code: &str, message: &str) {
 }
 
 fn print_json_ok<T: Serialize>(data: &T) {
-    let value = serde_json::to_value(data).unwrap_or(serde_json::Value::Null);
-    let envelope = serde_json::json!({ "ok": true, "data": value });
-    match serde_json::to_string_pretty(&envelope) {
+    match json_ok(data) {
         Ok(json) => println!("{json}"),
         Err(err) => eprintln!("error: failed to serialize json output: {err}"),
     }
 }
 
+// Borrow the payload instead of cloning every field into a JSON value first.
+// Object key order follows the serializer rather than an intermediate map.
+// Consumers use the parsed keys; key ordering is not part of the CLI contract.
+#[derive(Serialize)]
+struct Success<'a, T: ?Sized> {
+    data: &'a T,
+    ok: bool,
+}
+
+pub(crate) fn json_ok<T: Serialize>(data: &T) -> serde_json::Result<String> {
+    serde_json::to_string_pretty(&Success { data, ok: true }).or_else(|_| {
+        // Preserve the previous fallback when a payload cannot be serialized.
+        serde_json::to_string_pretty(&Success {
+            data: &serde_json::Value::Null,
+            ok: true,
+        })
+    })
+}
+
 #[cfg(test)]
 mod tests {
     /// The success envelope is one shape for every command, so a script reads

crates/cli/tests/json_cost.rs

@@ -0,0 +1,167 @@
+//! Synthetic output-only comparison. Run in release with --ignored --nocapture.
+//! This excludes IPC, DTO construction, stdout and the connected daemon.
+#![allow(dead_code)]
+
+#[path = "../src/output.rs"]
+mod output;
+
+use std::alloc::{GlobalAlloc, Layout, System};
+use std::cell::Cell;
+use std::hint::black_box;
+use std::time::Instant;
+
+use oxidezap_wire::dto::MessageDto;
+use serde::Serialize;
+
+thread_local! {
+    static ACTIVE: Cell<bool> = const { Cell::new(false) };
+    static LIVE: Cell<isize> = const { Cell::new(0) };
+    static PEAK: Cell<isize> = const { Cell::new(0) };
+    static CALLS: Cell<usize> = const { Cell::new(0) };
+}
+
+// Count only this thread during the allocation sample. Timing runs separately
+// with accounting disabled. Signed balances also tolerate cross-thread frees.
+struct Counting;
+fn add(bytes: usize) {
+    if !ACTIVE.try_with(Cell::get).unwrap_or(false) {
+        return;
+    }
+    let _ = LIVE.try_with(|v| {
+        v.set(v.get() + bytes as isize);
+        let _ = PEAK.try_with(|p| p.set(p.get().max(v.get())));
+    });
+    let _ = CALLS.try_with(|v| v.set(v.get() + 1));
+}
+fn subtract(bytes: usize) {
+    if !ACTIVE.try_with(Cell::get).unwrap_or(false) {
+        return;
+    }
+    let _ = LIVE.try_with(|v| v.set(v.get() - bytes as isize));
+}
+unsafe impl GlobalAlloc for Counting {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        // SAFETY: forward the caller's layout unchanged to System.
+        let ptr = unsafe { System.alloc(layout) };
+        if !ptr.is_null() {
+            add(layout.size());
+        }
+        ptr
+    }
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        subtract(layout.size());
+        // SAFETY: forward the pointer and its layout unchanged to System.
+        unsafe { System.dealloc(ptr, layout) }
+    }
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, size: usize) -> *mut u8 {
+        // SAFETY: forward the caller's allocation and requested size to System.
+        let next = unsafe { System.realloc(ptr, layout, size) };
+        if !next.is_null() {
+            subtract(layout.size());
+            add(size);
+        }
+        next
+    }
+}
+#[global_allocator]
+static ALLOCATOR: Counting = Counting;
+
+fn legacy<T: Serialize>(data: &T) -> String {
+    let value = serde_json::to_value(data).unwrap_or(serde_json::Value::Null);
+    let envelope = serde_json::json!({ "ok": true, "data": value });
+    serde_json::to_string_pretty(&envelope).unwrap()
+}
+fn messages(count: usize, text_len: usize) -> Vec<MessageDto> {
+    (0..count)
+        .map(|i| MessageDto {
+            id: format!("synthetic-{i}"),
+            chat_jid: "synthetic@example.invalid".into(),
+            sender_jid: "sender@example.invalid".into(),
+            text: Some("é\n\"\\".repeat(text_len / 5)),
+            kind: "text".into(),
+            status: "read".into(),
+            ..Default::default()
+        })
+        .collect()
+}
+
+#[test]
+fn borrowed_envelope_preserves_json_values() {
+    for data in [messages(0, 0), messages(2, 100)] {
+        let old: serde_json::Value = serde_json::from_str(&legacy(&data)).unwrap();
+        let new: serde_json::Value =
+            serde_json::from_str(&output::json_ok(&data).unwrap()).unwrap();
+        assert_eq!(old, new);
+    }
+    let map = serde_json::json!({ "nested": { "empty": [], "null": null, "number": 123 } });
+    assert_eq!(
+        serde_json::from_str::<serde_json::Value>(&legacy(&map)).unwrap(),
+        serde_json::from_str::<serde_json::Value>(&output::json_ok(&map).unwrap()).unwrap(),
+    );
+}
+
+#[test]
+fn failed_serialization_keeps_the_null_fallback() {
+    struct Failure;
+    impl Serialize for Failure {
+        fn serialize<S: serde::Serializer>(&self, _: S) -> Result<S::Ok, S::Error> {
+            Err(serde::ser::Error::custom("synthetic failure"))
+        }
+    }
+    let result: serde_json::Value =
+        serde_json::from_str(&output::json_ok(&Failure).unwrap()).unwrap();
+    assert_eq!(result["ok"], true);
+    assert!(result["data"].is_null());
+    assert_eq!(
+        serde_json::from_str::<serde_json::Value>(&legacy(&Failure)).unwrap(),
+        result,
+    );
+}
+
+// Native-only output measurement; the CLI deliberately has no wacore dependency.
+#[allow(clippy::disallowed_methods)]
+fn sample(mut f: impl FnMut() -> String) -> (u128, usize, usize, usize) {
+    ACTIVE.with(|v| v.set(true));
+    let live = LIVE.with(Cell::get);
+    PEAK.with(|p| p.set(live));
+    let calls = CALLS.with(Cell::get);
+    let measured = black_box(f());
+    let peak = (PEAK.with(Cell::get) - live) as usize;
+    let calls = CALLS.with(Cell::get) - calls;
+    drop(measured);
+    ACTIVE.with(|v| v.set(false));
+    let start = Instant::now();
+    let mut bytes = 0;
+    for _ in 0..10 {
+        bytes = black_box(f()).len();
+    }
+    let nanos = start.elapsed().as_nanos() / 10;
+    (nanos, peak, calls, bytes)
+}
+
+#[test]
+#[ignore = "release-only output cost measurement"]
+fn json_output_cost() {
+    for (count, text_len) in [(1, 100), (200, 100), (5000, 100), (200, 16000)] {
+        let data = messages(count, text_len);
+        // Warm both paths, then alternate their order in every paired sample.
+        black_box(legacy(&data));
+        black_box(output::json_ok(&data).unwrap());
+        for round in 0..12 {
+            let (base, head) = if round % 2 == 0 {
+                (
+                    sample(|| legacy(&data)),
+                    sample(|| output::json_ok(&data).unwrap()),
+                )
+            } else {
+                let head = sample(|| output::json_ok(&data).unwrap());
+                (sample(|| legacy(&data)), head)
+            };
+            assert_eq!(base.3, head.3);
+            println!(
+                "rows={count} text={text_len} round={round} base_ns={} head_ns={} base_peak={} head_peak={} base_calls={} head_calls={} bytes={}",
+                base.0, head.0, base.1, head.1, base.2, head.2, base.3
+            );
+        }
+    }
+}