Measure container children without intermediate placement

tests/cases/retained.rs

@@ -914,3 +914,40 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
     }
 }
+
+#[test]
+fn a_span_does_not_place_its_measurement_before_assigning_the_childs_slot() {
+    struct MeasuredBox(Rc<Cell<usize>>);
+
+    impl Widget for MeasuredBox {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.px_size();
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::from((100, 50))
+        }
+    }
+
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = MeasuredBox(draws.clone()).add(&mut h.rsc);
+    h.set_root((leaf,).span(Dir::RIGHT).width(rel(1.0)).height(rel(1.0)));
+
+    assert_eq!(draws.get(), 3);
+    assert_corners!(h, leaf, (0, 75), (100, 125));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+    h.frame();
+    assert_eq!(draws.get(), 3);
+
+    h.resize((600, 300));
+    h.frame();
+    assert_eq!(draws.get(), 6);
+    assert_corners!(h, leaf, (0, 125), (100, 175));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+}

tests/layout_diagnostics.rs

@@ -5,11 +5,11 @@
 //!     cargo test --release --features layout-diagnostics \
 //!       --test layout_diagnostics -- --ignored --nocapture
 //!
-//! Uninstrumented hardware totals for one phase:
+//! Build the uninstrumented test with `cargo test --release --test
+//! layout_diagnostics --no-run`, then run the emitted executable directly:
 //!
-//!     IRIS_PHASE=resize IRIS_FRAMES=1000 perf stat \
-//!       -e cycles:u,instructions:u cargo test --release \
-//!       --test layout_diagnostics -- --ignored --nocapture
+//!     IRIS_PHASE=resize IRIS_FRAMES=10000 perf stat -r 7 \
+//!       -e cycles:u,instructions:u /path/to/layout_diagnostics --ignored --nocapture
 //!
 //! `IRIS_PHASE` is `cold`, `repaint`, `many`, `size`, `scroll`, `resize`, or
 //! `all`. `IRIS_SEED`, `IRIS_DEPTH`, and `IRIS_FRAMES` select the load, and
@@ -134,6 +134,9 @@ fn report(label: &str, mut elapsed: Vec<f64>, _harness: &Harness) {
     {
         let diagnostics = iris::core::layout_diagnostics::take();
         print!("{}", diagnostics.per_frame(frames));
+        for event in diagnostics.traces() {
+            println!("    {event:?}");
+        }
         for callsite in diagnostics.hot_text().iter().take(3) {
             let mut ancestry = Vec::new();
             let mut id = Some(callsite.id);