Carry a length as a rule beside a widget, not a widget around it

`.width()` built a `SetSize` whose whole job was to answer `size_hint`, so
every declared length cost a widget, an `ActiveData` and a link of chain to
say one number. It is now a `SizeRule` per axis on `WidgetData`, beside
`region_node`, resolved by `Painter` where the widget is drawn. `SetSize`
and `MaxSize` are gone; `MaxSize` had no caller but its own builders.

That settles which of two answers is the size. A rule wins on the axis it
names and the `Size` returned by `draw` answers the rest, applied once in
`draw_inner` rather than by each widget that could carry one -- so the
widget under a rule never learns of it. `Painter::size_hint` reads the rule
first for the same reason: a rule that beats what a widget would draw has
to beat what it says about itself.

`declared_lens` still falls back to a non-leftover `size_hint`, which is
how an image or a gap gets its own pixel size rather than the whole offer.
That is the offer's business rather than a declaration's, and it falls away
when a widget occupies its reported size inside the box it was offered.

`known` and `declared` are separate because a share is a length to whoever
divides one and not to whoever composes a box: `.width(leftover(3))` is
known without drawing but cannot narrow anything.

Checked: fmt, clippy, 85 tests, and 100 generated seeds agreeing warm
against cold in 67.6 s. `minimal`, `text` and `view` render byte-identical
at 1920x1200; `tabs` differs only in the widget count it prints about
itself, which is two wrapper types smaller.

tests/drift.rs

@@ -10,7 +10,7 @@ use iris::prelude::*;
 
 /// A row of a fixed height under a bar, so changing the bar's height moves the
 /// row without changing the box it is given: the move path, repeatedly.
-fn plant(h: &mut Harness, bar_height: f32) -> (WeakWidget<SetSize>, WeakWidget<Rect>) {
+fn plant(h: &mut Harness, bar_height: f32) -> (WeakWidget<Rect>, WeakWidget<Rect>) {
     let bar = rect(Color::RED).height(bar_height).add(&mut h.rsc);
     let inner = rect(Color::BLUE).add(&mut h.rsc);
     let row = (inner, rect(Color::GREEN)).span(Dir::RIGHT).height(100);
@@ -33,7 +33,7 @@ fn a_subtree_moved_many_times_stays_where_a_cold_layout_puts_it() {
     let mut height = 40.0;
     for step in 0..MOVES {
         height = 40.0 + (step % 300) as f32 * 0.37;
-        warm.rsc[bar].y = Some(Len::px(height));
+        warm.set_len(bar, Axis::Y, height);
         warm.frame();
     }
 

tests/generated.rs

@@ -61,9 +61,9 @@ fn resize_one(h: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Lens {
         Some(Len::px(20.0 + rng.below(180) as f32)),
         Some(Len::px(20.0 + rng.below(180) as f32)),
     ];
-    let sized = &mut h.rsc[tree.sized[idx]];
-    sized.x = lens[0];
-    sized.y = lens[1];
+    h.rsc
+        .widgets_mut()
+        .set_size_rules(tree.sized[idx], lens[0], lens[1]);
     lens
 }
 
@@ -174,18 +174,25 @@ fn reshuffle(
 /// written out by hand. A fuzz failure is a lead; the fast test that replaces
 /// it has to be buildable from what the failure printed.
 fn describe(id: WidgetId, h: &Harness) -> String {
+    let rules = h.rsc.widgets().size_rules(id);
+    let rule = |r: SizeRule| match r.known() {
+        Some(len) => format!("{len}"),
+        None => "-".into(),
+    };
+    // A size rule is a property of whatever carries it, so it prints with
+    // that widget rather than as one of its own.
+    match (rules.x, rules.y) {
+        (SizeRule::Free, SizeRule::Free) => describe_widget(id, h),
+        (x, y) => format!("{}[x:{},y:{}]", describe_widget(id, h), rule(x), rule(y)),
+    }
+}
+
+fn describe_widget(id: WidgetId, h: &Harness) -> String {
     let label = h.rsc.widgets().label(id).to_string();
     let Some(widget) = h.rsc.widgets().get_dyn(id) else {
         return label;
     };
     let any: &dyn std::any::Any = widget;
-    let len = |l: &Option<Len>| match l {
-        Some(l) => format!("{l}"),
-        None => "-".into(),
-    };
-    if let Some(w) = any.downcast_ref::<SetSize>() {
-        return format!("SetSize{{x:{},y:{}}}", len(&w.x), len(&w.y));
-    }
     if let Some(w) = any.downcast_ref::<Span>() {
         let sign = if w.dir.sign == Sign::Neg { "-" } else { "+" };
         return format!(

tests/layout.rs

@@ -81,7 +81,7 @@ fn a_child_drawn_twice_moves_once() {
     h.set_root((left, centered).span(Dir::RIGHT));
     assert_corners!(h, inner, (100, 0), (300, 200));
 
-    h.rsc[left].x = Some(Len::px(150));
+    h.set_len(left, Axis::X, 150);
     h.frame();
 
     assert_corners!(h, inner, (150, 0), (350, 200));
@@ -131,7 +131,7 @@ fn a_fixed_box_is_drawn_again_rather_than_stretched() {
     h.set_root(stack.align(Align::TOP));
     assert_corners!(h, panel, (0, 0), (400, 100));
 
-    h.rsc[leaf].y = Some(Len::px(250));
+    h.set_len(leaf, Axis::Y, 250);
     h.frame();
 
     assert_corners!(h, panel, (0, 0), (400, 250));
@@ -146,7 +146,7 @@ fn a_moved_subtree_takes_its_children_with_it() {
     h.set_root((first, row).span(Dir::DOWN));
     assert_corners!(h, inner, (10, 50), (390, 70));
 
-    h.rsc[first].y = Some(Len::px(80));
+    h.set_len(first, Axis::Y, 80);
     h.frame();
 
     // The row opted into one movable region, so its descendants follow one
@@ -167,7 +167,7 @@ fn a_fixed_length_child_keeps_it_when_the_box_around_it_grows() {
     assert_corners!(h, fixed, (100, 0), (150, 200));
     assert_corners!(h, leftover, (150, 0), (400, 200));
 
-    h.rsc[bar].x = Some(Len::px(200));
+    h.set_len(bar, Axis::X, 200);
     h.frame();
 
     // The panel's box is 100 shorter, so the fixed child is the same 50 wide
@@ -195,7 +195,7 @@ fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
     h.set_root((bar, column).span(Dir::RIGHT));
     assert_corners!(h, inner, (110, 10), (390, 30));
 
-    h.rsc[bar].x = Some(Len::px(200));
+    h.set_len(bar, Axis::X, 200);
     h.frame();
 
     assert_corners!(h, inner, (210, 10), (390, 30));
@@ -282,24 +282,14 @@ fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
 }
 
 fn hairline(h: &mut Harness, marks: &mut Vec<WidgetId>) -> StrongWidget {
-    let inner = rect(Color::RED).add_strong(&mut h.rsc);
-    let mark = SetSize {
-        inner,
-        x: Some(Len::px(1.0)),
-        y: None,
-    }
-    .add_strong(&mut h.rsc);
+    let mark = rect(Color::RED).width(1).add_strong(&mut h.rsc);
     marks.push(mark.id());
     mark
 }
 
 fn share(h: &mut Harness, inner: StrongWidget, ratio: f32) -> StrongWidget {
-    SetSize {
-        inner,
-        x: Some(Len::leftover(ratio)),
-        y: None,
-    }
-    .add_strong(&mut h.rsc)
+    h.set_len(&inner, Axis::X, Len::leftover(ratio));
+    inner
 }
 
 /// Shares in weights no binary fraction lands on, a padding on one branch
@@ -402,18 +392,15 @@ fn only_a_pure_leftover_child_disappears_when_nothing_is_left() {
 
     // An undrawn child remains a dependency of the span, so making room for
     // it draws it without rebuilding the tree.
-    h.rsc[fixed].x = Some(Len::px(60));
+    h.set_len(fixed, Axis::X, 60);
     h.frame();
     assert_corners!(h, leftover, (60, 0), (100, 20));
 
     let mut h = Harness::new((100, 20));
     let fixed = rect(Color::RED).width(100).add(&mut h.rsc);
-    let mixed = SetSize {
-        inner: rect(Color::BLUE).add_strong(&mut h.rsc),
-        x: Some(Len::px(20) + Len::LEFTOVER),
-        y: None,
-    }
-    .add(&mut h.rsc);
+    let mixed = rect(Color::BLUE)
+        .width(Len::px(20) + Len::LEFTOVER)
+        .add(&mut h.rsc);
     h.set_root((fixed, mixed).span(Dir::RIGHT));
 
     // Pixels and fractions still overflow; only a child whose entire length
@@ -432,7 +419,7 @@ fn leftover_children_disappear_at_the_exact_fixed_content_boundary() {
     assert!(h.region(&a).is_some());
     assert!(h.region(&b).is_some());
 
-    h.rsc[first].y = Some(Len::px(96.0));
+    h.set_len(first, Axis::Y, 96.0);
     h.frame();
 
     assert!(h.region(&a).is_none());

tests/layout_diagnostics.rs

@@ -216,7 +216,11 @@ fn layout_cost() {
         trace_selected(&tree);
         let sized = tree.sized[0];
         run("size", frames, &mut harness, move |harness, frame| {
-            harness.rsc[sized].x = Some(Len::px(100.0 + (frame % 2) as f32 * 40.0));
+            let len = Len::px(100.0 + (frame % 2) as f32 * 40.0);
+            harness
+                .rsc
+                .widgets_mut()
+                .set_size_rule(sized, Axis::X, SizeRule::Exact(len));
         });
     }
 

tests/replace_cost.rs

@@ -35,7 +35,7 @@ fn remapping_rows_every_frame() {
     }
     h.set_root(span);
     for i in 0..FRAMES {
-        h.rsc[first].y = Some(Len::px(40.0 + (i % 2) as f32));
+        h.set_len(first, Axis::Y, 40.0 + (i % 2) as f32);
         h.frame();
     }
 }

tests/retained.rs

@@ -236,7 +236,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     h.set_root(parent);
     assert_corners!(h, inner, (0, 0), (400, 80));
 
-    h.rsc[inner].y = Some(Len::px(120));
+    h.set_len(inner, Axis::Y, 120);
     h.frame();
 
     assert_corners!(h, inner, (0, 0), (400, 120));
@@ -484,7 +484,7 @@ fn stretching_a_subtree_carries_the_children_in_it() {
     let settled = draws.get();
     assert_corners!(h, inner, (0, 40), (400, 400));
 
-    h.rsc[first].y = Some(Len::px(80));
+    h.set_len(first, Axis::Y, 80);
     h.frame();
 
     assert_eq!(
@@ -508,7 +508,7 @@ fn a_widened_row_redraws_what_reads_its_length_and_nothing_else() {
     h.set_root((bar, row).span(Dir::RIGHT));
     let (settled_wrap, settled_back) = (wrap_draws.get(), back_draws.get());
 
-    h.rsc[bar].x = Some(Len::px(200));
+    h.set_len(bar, Axis::X, 200);
     h.frame();
 
     // The span reads every child's size, so redrawing one takes the span
@@ -534,7 +534,7 @@ fn a_declared_length_child_is_not_redrawn_when_the_box_around_it_grows() {
     h.set_root((bar, row).span(Dir::RIGHT));
     let settled = draws.get();
 
-    h.rsc[bar].x = Some(Len::px(200));
+    h.set_len(bar, Axis::X, 200);
     h.frame();
 
     assert_eq!(draws.get(), settled, "its own length did not change");

tests/shrink.rs

@@ -159,12 +159,8 @@ impl Node {
             }
             Node::Sized(x, y, kid) => {
                 let inner = kid.build(h, out, spans);
-                SetSize {
-                    inner,
-                    x: *x,
-                    y: *y,
-                }
-                .add_strong(&mut h.rsc)
+                h.rsc.widgets_mut().set_size_rules(&inner, *x, *y);
+                inner
             }
             Node::Scroll(down, kid) => {
                 let inner = kid.build(h, out, spans);

tests/trace_unsettled.rs

@@ -10,12 +10,7 @@ use iris::prelude::*;
 fn plant(h: &mut Harness) -> Vec<WidgetId> {
     let plain = wtext("Wrapping").size(16).wrap(false).add(&mut h.rsc);
     let wrapped = wtext("Wrapping shapes").size(16).wrap(true).add(&mut h.rsc);
-    let sized = SetSize {
-        inner: wrapped.add_strong(&mut h.rsc),
-        x: Some(Len::px(76.0)),
-        y: None,
-    }
-    .add(&mut h.rsc);
+    let sized = wrapped.width(76).add(&mut h.rsc);
     let aligned = Aligned {
         inner: sized.add_strong(&mut h.rsc),
         align: Align {
@@ -108,12 +103,7 @@ fn plant_fixed(h: &mut Harness) -> Vec<WidgetId> {
     }
     .add(&mut h.rsc);
     let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
-    let sized = SetSize {
-        inner: inner.add_strong(&mut h.rsc),
-        x: Some(Len::px(189.0)),
-        y: Some(Len::px(176.0)),
-    }
-    .add(&mut h.rsc);
+    let sized = inner.sized((189, 176)).add(&mut h.rsc);
     let filler = rect(Color::RED).add(&mut h.rsc);
     let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
     h.state.root = Some(root.add_strong(&mut h.rsc));

tests/unsettled.rs

@@ -15,12 +15,7 @@ use iris::prelude::*;
 fn plant(h: &mut Harness) -> Vec<WidgetId> {
     let plain = wtext("Wrapping").size(16).wrap(false).add(&mut h.rsc);
     let wrapped = wtext("Wrapping shapes").size(16).wrap(true).add(&mut h.rsc);
-    let sized = SetSize {
-        inner: wrapped.add_strong(&mut h.rsc),
-        x: Some(Len::px(76.0)),
-        y: None,
-    }
-    .add(&mut h.rsc);
+    let sized = wrapped.width(76).add(&mut h.rsc);
     let aligned = Aligned {
         inner: sized.add_strong(&mut h.rsc),
         align: Align {
@@ -110,12 +105,7 @@ fn plant_fixed(h: &mut Harness) -> Vec<WidgetId> {
     }
     .add(&mut h.rsc);
     let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
-    let sized = SetSize {
-        inner: inner.add_strong(&mut h.rsc),
-        x: Some(Len::px(189.0)),
-        y: Some(Len::px(176.0)),
-    }
-    .add(&mut h.rsc);
+    let sized = inner.sized((189, 176)).add(&mut h.rsc);
     let filler = rect(Color::RED).add(&mut h.rsc);
     let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
     h.state.root = Some(root.add_strong(&mut h.rsc));
@@ -235,12 +225,7 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
     }
     .add(&mut h.rsc);
     let block = rect(Color::RED).add(&mut h.rsc);
-    let fixed = SetSize {
-        inner: block.add_strong(&mut h.rsc),
-        x: Some(Len::px(87.0)),
-        y: None,
-    }
-    .add(&mut h.rsc);
+    let fixed = block.width(87).add(&mut h.rsc);
     let mut outer_children: Vec<StrongWidget> =
         vec![fixed.add_strong(&mut h.rsc), inner.add_strong(&mut h.rsc)];
     if swapped {
@@ -253,12 +238,9 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
         ortho: OrthoSize::Children,
     }
     .add(&mut h.rsc);
-    let through = SetSize {
-        inner: outer.add_strong(&mut h.rsc),
-        x: None,
-        y: None,
-    }
-    .add(&mut h.rsc);
+    // Carried no rule even before rules were a property: it is here to be a
+    // widget between the span and the scroll, not to declare anything.
+    let through = (outer,).span(Dir::RIGHT).add(&mut h.rsc);
     let scroll = Scroll::new(through.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
     h.state.root = Some(scroll.add_strong(&mut h.rsc));
     (