Constrain offered boxes with independent widget size bounds

tests/bounds_cost.rs

@@ -0,0 +1,60 @@
+//! CPU comparison of bounds attributes and the former wrapper, using the
+//! same builder calls and geometry. Run the release executable under perf;
+//! process totals include the cold frame. MODE=plain|exact|cap, REDRAW=0|1.
+use iris::{harness::Harness, prelude::*};
+
+#[test]
+#[ignore = "instruction-count measurement"]
+fn bounds_cost() {
+    let mode = std::env::var("MODE").unwrap_or_else(|_| "cap".into());
+    let redraw = std::env::var("REDRAW").is_ok_and(|value| value == "1");
+    let frames = std::env::var("FRAMES")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(2000);
+    let mut h = Harness::new((300, 512));
+    let mut column = Span::empty(Dir::DOWN);
+    let mut leaves = Vec::new();
+    for _ in 0..128 {
+        let first = match mode.as_str() {
+            "plain" => rect(Color::RED).add_strong(&mut h.rsc).any(),
+            "exact" => rect(Color::RED).width(40).add_strong(&mut h.rsc).any(),
+            "cap" => rect(Color::RED).max_width(80).add_strong(&mut h.rsc).any(),
+            _ => panic!("unknown MODE {mode}"),
+        };
+        leaves.push(first.id());
+        let second = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let row = Span {
+            children: vec![first, second],
+            dir: Dir::RIGHT,
+            gap: Px::ZERO,
+        };
+        column.push(row.height(4).add_strong(&mut h.rsc));
+    }
+    h.set_root(column);
+    let ids: Vec<_> = h.render.active.keys().copied().collect();
+    println!(
+        "mode={mode}, widgets={}, rule_bytes={}",
+        ids.len(),
+        std::mem::size_of::<SizeRule>()
+    );
+    for frame in 0..frames {
+        if redraw {
+            for &id in &ids {
+                h.rsc.widgets_mut().mark_for_redraw(id);
+            }
+        }
+        let width = if frame % 2 == 0 { 100 } else { 300 };
+        h.resize((width, 512));
+        h.frame();
+        let expected = match mode.as_str() {
+            "plain" => width / 2,
+            "exact" => 40,
+            "cap" => (width / 2).min(80),
+            _ => unreachable!(),
+        };
+        for id in &leaves {
+            assert_eq!(h.region(id).unwrap().size().x, Px::from_int(expected));
+        }
+    }
+}