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.

core/src/ui/painter.rs

@@ -2,7 +2,7 @@
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
     Axis, Holds, Len, RenderedText, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, Widget, WidgetId,
+    TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
         PrimitiveKind, TexturePrimitive,
@@ -111,16 +111,12 @@ impl<'a> Painter<'a> {
         self.widget_at(id, region)
     }
 
-    /// What a widget declares its lengths to be, which whoever draws it
+    /// What a widget's rules declare its lengths to be, which whoever draws
-    /// resolves into its box. `leftover` is not among them: a part of what is
+    /// it resolves into its box. Reading them depends on nothing -- the box
-    /// left over is only a length to the widget dividing one, so it passes
+    /// that comes of them is kept on the child, and `redraw` compares it
-    /// up in the size instead. Reading it depends on nothing -- the box that
+    /// there.
-    /// comes of it is kept on the child, and `redraw` compares it there.
     fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<Len>; 2] {
-        let Some(widget) = self.rsc.widgets().get_dyn(id.id()) else {
+        declared_lens(self.rsc.widgets(), id.id())
-            return [None; 2];
-        };
-        AXES.map(|axis| declared_len(widget, axis))
     }
 
     /// Takes back a child that was drawn only to find out how long it is.
@@ -201,11 +197,14 @@ impl<'a> Painter<'a> {
     /// What a child says its length is without being drawn, if it can say.
     /// Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
-        let hint = self
+        let widgets = self.rsc.widgets();
-            .rsc
+        // A rule is the answer where there is one: it wins over whatever the
-            .widgets()
+        // widget would draw, so it has to win over what the widget says too.
-            .get_dyn(id.id())
+        let hint = widgets.size_rules(id.id()).axis(axis).known().or_else(|| {
-            .and_then(|widget| widget.size_hint(axis));
+            widgets
+                .get_dyn(id.id())
+                .and_then(|widget| widget.size_hint(axis))
+        });
         #[cfg(feature = "layout-diagnostics")]
         diag::hint_read(id.id(), self.id, axis, hint);
         match hint {
@@ -439,8 +438,21 @@ impl PrimitiveLike for &TextureHandle {
 /// What a widget declares a length of its box to be. `leftover` is not one: a
 /// share of what is left over is only a length to the widget dividing one,
 /// so it passes up in the size instead.
-pub(crate) fn declared_len(widget: &dyn Widget, axis: Axis) -> Option<Len> {
+pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<Len>; 2] {
-    widget.size_hint(axis).filter(|len| len.leftover == 0.0)
+    let rules = widgets.size_rules(id);
+    let widget = widgets.get_dyn(id);
+    AXES.map(|axis| {
+        rules.axis(axis).declared().or_else(|| {
+            // A hint still narrows the box where no rule does, which is how a
+            // widget with a natural pixel size -- an image, a gap -- gets that
+            // size rather than the whole offer. That is the offer's business
+            // rather than a declaration's, and this falls away once a widget
+            // occupies its reported size inside the box it was offered.
+            widget
+                .and_then(|widget| widget.size_hint(axis))
+                .filter(|len| len.leftover == 0.0)
+        })
+    })
 }
 
 /// Takes a widget's declared lengths in the box `region` is given in, since a

core/src/ui/render_state.rs

@@ -1,6 +1,6 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
-use crate::ui::painter::declared_len;
+use crate::ui::painter::declared_lens;
 use crate::{
     ActiveData, Axis, DrawLayers, Holds, IdLike, Len, MaskIdx, MoveIdx, Moves, Painter,
     PixelRegion, Size, StrongWidget, UiRegion, UiRsc, UiScalar, UiSpan, WidgetId, Widgets,
@@ -256,6 +256,14 @@ impl UiRenderState {
             "'{}' ({id:?}) drew a size its size_hint disagrees with",
             rsc.widgets().label(id)
         );
+        // A rule wins on the axis it names, and the draw answers the rest.
+        // Applied here so it is one place rather than every widget that could
+        // carry one, and so the widget under a rule never learns of it.
+        let rules = rsc.widgets().size_rules(id);
+        let size = Size {
+            x: rules.x.apply(size.x),
+            y: rules.y.apply(size.y),
+        };
         let holds = [own[0].and(under[0]), own[1].and(under[1])];
         debug_assert!(
             holds[0].contains(px.x) && holds[1].contains(px.y),
@@ -739,11 +747,7 @@ impl UiRenderState {
         // whether to draw it at all, so a change to either is the parent's
         // to draw -- with the mark left on, so the parent draws it rather
         // than keeping it.
-        let declared_changed = rsc.widgets().get_dyn(id).is_some_and(|widget| {
+        let declared_changed = declared_lens(rsc.widgets(), id) != active.declared;
-            AXES.into_iter()
-                .zip(active.declared)
-                .any(|(axis, was)| declared_len(widget, axis) != was)
-        });
         if let Some(parent) = active.parent
             && (declared_changed || !active.drawn)
         {

core/src/widget/data.rs

@@ -1,9 +1,10 @@
-use crate::Widget;
+use crate::{SizeRules, Widget};
 
 pub struct WidgetData {
     pub widget: Box<dyn Widget>,
     pub label: String,
     pub(super) region_node: bool,
+    pub(super) size: SizeRules,
     /// dynamic borrow checking
     pub borrowed: bool,
 }
@@ -18,6 +19,7 @@ impl WidgetData {
             widget: Box::new(widget),
             label,
             region_node: false,
+            size: SizeRules::default(),
             borrowed: false,
         }
     }

core/src/widget/mod.rs

@@ -4,6 +4,7 @@ use std::any::Any;
 mod data;
 mod handle;
 mod like;
+mod size_rule;
 mod tag;
 mod view;
 mod widgets;
@@ -11,6 +12,7 @@ mod widgets;
 pub use data::*;
 pub use handle::*;
 pub use like::*;
+pub use size_rule::*;
 pub use tag::*;
 pub use view::*;
 pub use widgets::*;

core/src/widget/size_rule.rs

@@ -0,0 +1,86 @@
+use crate::{Axis, Len};
+
+/// What a widget's length on one axis is, as a rule its parent applies where
+/// it draws it rather than an answer the widget gives about itself.
+///
+/// A rule and a drawn size are not two opinions to reconcile: a rule wins on
+/// the axis it names, and the `Size` returned by `draw` answers only the axes
+/// with no rule. That is what lets a span divide its space around a length
+/// nobody has drawn yet, and it is why a rule lives beside the widget rather
+/// than inside it -- the widget under the rule never has to know about it.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub enum SizeRule {
+    /// Whatever the widget reports from drawing.
+    #[default]
+    Free,
+    /// This length, whatever the widget reports.
+    Exact(Len),
+}
+
+impl SizeRule {
+    /// The length this rule gives without the widget being drawn, if it can
+    /// give one. `leftover` is never among them: a share is a length only to
+    /// whoever divides one, so it passes up in the reported size instead and
+    /// is resolved there.
+    pub fn declared(&self) -> Option<Len> {
+        match self {
+            Self::Exact(len) if len.leftover == 0.0 => Some(*len),
+            _ => None,
+        }
+    }
+
+    /// The length this rule fixes, whether or not it can narrow a box. A
+    /// share is a length the widget's parent still has to divide, so it is
+    /// known here and resolved there -- unlike `declared`, which is only the
+    /// ones that give a box directly.
+    pub fn known(&self) -> Option<Len> {
+        match self {
+            Self::Free => None,
+            Self::Exact(len) => Some(*len),
+        }
+    }
+
+    /// The length a widget reporting `reported` ends up with.
+    pub fn apply(&self, reported: Len) -> Len {
+        match self {
+            Self::Free => reported,
+            Self::Exact(len) => *len,
+        }
+    }
+}
+
+impl From<Len> for SizeRule {
+    fn from(len: Len) -> Self {
+        Self::Exact(len)
+    }
+}
+
+impl From<Option<Len>> for SizeRule {
+    fn from(len: Option<Len>) -> Self {
+        len.map_or(Self::Free, Self::Exact)
+    }
+}
+
+/// One rule per axis, which is how a widget carries a length on one axis and
+/// leaves the other to whatever it draws.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct SizeRules {
+    pub x: SizeRule,
+    pub y: SizeRule,
+}
+
+impl SizeRules {
+    pub fn axis(&self, axis: Axis) -> SizeRule {
+        match axis {
+            Axis::X => self.x,
+            Axis::Y => self.y,
+        }
+    }
+
+    pub fn axis_mut(&mut self, axis: Axis) -> &mut SizeRule {
+        match axis {
+            Axis::X => &mut self.x,
+            Axis::Y => &mut self.y,
+        }
+    }
+}

core/src/widget/widgets.rs

@@ -1,7 +1,7 @@
 use std::sync::mpsc::{Receiver, Sender, channel};
 
 use crate::{
-    IdLike, StrongWidget, WeakWidget, Widget, WidgetData, WidgetId,
+    Axis, IdLike, SizeRule, SizeRules, StrongWidget, WeakWidget, Widget, WidgetData, WidgetId,
     util::{DynBorrower, HashSet, SlotVec, forget_mut, to_mut},
 };
 
@@ -118,6 +118,36 @@ impl Widgets {
         self.needs_redraw.insert(id);
     }
 
+    /// The length rules whoever draws this widget applies to its box.
+    pub fn size_rules(&self, id: impl IdLike) -> SizeRules {
+        self.data(id).unwrap().size
+    }
+
+    /// Sets one axis's rule. The widget is marked rather than its parent
+    /// because the parent is not known here; `redraw` escalates a changed
+    /// declared length to whoever resolves it.
+    pub fn set_size_rule(&mut self, id: impl IdLike, axis: Axis, rule: SizeRule) {
+        let id = id.id();
+        let data = self.data_mut(id).unwrap();
+        if *data.size.axis_mut(axis) == rule {
+            return;
+        }
+        *data.size.axis_mut(axis) = rule;
+        self.needs_redraw.insert(id);
+    }
+
+    /// Both axes at once, for a caller holding a pair.
+    pub fn set_size_rules(
+        &mut self,
+        id: impl IdLike,
+        x: impl Into<SizeRule>,
+        y: impl Into<SizeRule>,
+    ) {
+        let id = id.id();
+        self.set_size_rule(id, Axis::X, x.into());
+        self.set_size_rule(id, Axis::Y, y.into());
+    }
+
     pub fn data_mut(&mut self, id: impl IdLike) -> Option<&mut WidgetData> {
         self.vec.get_mut(id.id())
     }

src/harness.rs

@@ -158,6 +158,13 @@ impl Harness {
         self.render.resize(size);
     }
 
+    /// Changes a length rule after the fact, the way `.width()` sets one.
+    pub fn set_len(&mut self, id: impl IdLike, axis: Axis, len: impl Into<Len>) {
+        self.rsc
+            .widgets_mut()
+            .set_size_rule(id, axis, SizeRule::Exact(len.into()));
+    }
+
     /// Sets the root and lays it out, so a pointer event has something to hit.
     pub fn set_root<T>(&mut self, widget: impl WidgetLike<DefaultRsc<HarnessState>, T>) {
         widget.set_root(&mut self.rsc, &mut self.state);

src/random.rs

@@ -8,14 +8,14 @@
 use crate::prelude::*;
 use std::collections::HashMap;
 
-/// The declared lengths of one `SetSize`, by axis.
+/// The declared lengths of one widget carrying a size rule, by axis.
 pub type Lens = [Option<Len>; 2];
 
 /// What a test changes between two trees grown from the same seed, so the
 /// warm one can be mutated and the cold one grown that way to begin with.
 #[derive(Default)]
 pub struct Edits {
-    /// Declared sizes, by the order the `SetSize` wrappers were made.
+    /// Declared sizes, by the order the rules were put on.
     pub sizes: HashMap<usize, Lens>,
     /// Which children a span has, by the order the spans were made.
     pub spans: HashMap<usize, SpanEdit>,
@@ -75,7 +75,7 @@ const WORDS: &str = "Wrapping shapes one source into as many lines as the box \
 #[derive(Default)]
 pub struct Tree {
     pub ids: Vec<WidgetId>,
-    pub sized: Vec<WeakWidget<SetSize>>,
+    pub sized: Vec<WidgetId>,
     pub spans: Vec<Spanned>,
     pub scrolls: Vec<WeakWidget<Scroll>>,
     /// Children a `SpanEdit` took out, held so that dropping the last share
@@ -201,15 +201,13 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
         let idx = self.tree.sized.len();
         let lens = [self.len(), self.len()];
         let lens = self.edits.sizes.get(&idx).copied().unwrap_or(lens);
-        let id = SetSize {
+        let id = inner.id();
-            inner,
+        self.rsc
-            x: lens[0],
+            .ui_mut()
-            y: lens[1],
+            .widgets
-        }
+            .set_size_rules(id, lens[0], lens[1]);
-        .add(self.rsc);
         self.tree.sized.push(id);
-        self.tree.ids.push(id.id());
+        inner
-        id.add_strong(self.rsc)
     }
 
     fn node(&mut self, depth: usize) -> StrongWidget {

src/widget/position/max_size.rs

@@ -1,25 +0,0 @@
-use crate::prelude::*;
-
-pub struct MaxSize {
-    pub inner: StrongWidget,
-    pub x: Option<Len>,
-    pub y: Option<Len>,
-}
-
-impl Widget for MaxSize {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        let child = painter.widget(&self.inner).size();
-        let own = painter.px_size();
-        Size {
-            x: capped(child.x, self.x, own.x),
-            y: capped(child.y, self.y, own.y),
-        }
-    }
-}
-
-fn capped(len: Len, max: Option<Len>, own: f32) -> Len {
-    match max {
-        Some(max) if len.apply_leftover().to_px(own) > max.apply_leftover().to_px(own) => max,
-        _ => len,
-    }
-}

src/widget/position/mod.rs

@@ -1,19 +1,15 @@
 mod align;
 mod layer;
-mod max_size;
 mod offset;
 mod pad;
 mod scroll;
-mod set_size;
 mod span;
 mod stack;
 
 pub use align::*;
 pub use layer::*;
-pub use max_size::*;
 pub use offset::*;
 pub use pad::*;
 pub use scroll::*;
-pub use set_size::*;
 pub use span::*;
 pub use stack::*;

src/widget/position/set_size.rs

@@ -1,30 +0,0 @@
-use crate::prelude::*;
-
-pub struct SetSize {
-    pub inner: StrongWidget,
-    pub x: Option<Len>,
-    pub y: Option<Len>,
-}
-
-impl Widget for SetSize {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
-        // Nothing to apply: a declared length is taken where this widget is
-        // drawn, so the box it has already is that length, and `leftover` is a
-        // share only whoever divides a length can work out. Both reach them
-        // through `size_hint`.
-        let child = painter.widget(&self.inner).size();
-        Size {
-            x: self.x.unwrap_or(child.x),
-            y: self.y.unwrap_or(child.y),
-        }
-    }
-
-    /// A declared axis is known without looking at the child, which is what
-    /// lets a span lay out around `.height(leftover(1))` without drawing it.
-    fn size_hint(&self, axis: Axis) -> Option<Len> {
-        match axis {
-            Axis::X => self.x,
-            Axis::Y => self.y,
-        }
-    }
-}

src/widget/trait_fns.rs

@@ -39,48 +39,38 @@ widget_trait! {
         }
     }
 
-    fn sized(self, size: impl Into<Size>) -> impl WidgetFn<Rsc, SetSize> {
+    fn sized(self, size: impl Into<Size>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         let size = size.into();
-        move |state| SetSize {
+        move |state| {
-            inner: self.add_strong(state),
+            let id = self.add(state);
-            x: Some(size.x),
+            let widgets = &mut state.ui_mut().widgets;
-            y: Some(size.y),
+            widgets.set_size_rule(id, Axis::X, SizeRule::Exact(size.x));
+            widgets.set_size_rule(id, Axis::Y, SizeRule::Exact(size.y));
+            id
         }
     }
 
-    fn max_width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, MaxSize> {
+    fn width(self, len: impl Into<Len>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         let len = len.into();
-        move |state| MaxSize {
+        move |state| {
-            inner: self.add_strong(state),
+            let id = self.add(state);
-            x: Some(len),
+            state
-            y: None,
+                .ui_mut()
+                .widgets
+                .set_size_rule(id, Axis::X, SizeRule::Exact(len));
+            id
         }
     }
 
-    fn max_height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, MaxSize> {
+    fn height(self, len: impl Into<Len>) -> impl WidgetIdFn<Rsc, WL::Widget> {
         let len = len.into();
-        move |state| MaxSize {
+        move |state| {
-            inner: self.add_strong(state),
+            let id = self.add(state);
-            x: None,
+            state
-            y: Some(len),
+                .ui_mut()
-        }
+                .widgets
-    }
+                .set_size_rule(id, Axis::Y, SizeRule::Exact(len));
-
+            id
-    fn width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, SetSize> {
-        let len = len.into();
-        move |state| SetSize {
-            inner: self.add_strong(state),
-            x: Some(len),
-            y: None,
-        }
-    }
-
-    fn height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, SetSize> {
-        let len = len.into();
-        move |state| SetSize {
-            inner: self.add_strong(state),
-            x: None,
-            y: Some(len),
         }
     }
 

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]];
+    h.rsc
-    sized.x = lens[0];
+        .widgets_mut()
-    sized.y = lens[1];
+        .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 = rect(Color::RED).width(1).add_strong(&mut h.rsc);
-    let mark = SetSize {
-        inner,
-        x: Some(Len::px(1.0)),
-        y: None,
-    }
-    .add_strong(&mut h.rsc);
     marks.push(mark.id());
     mark
 }
 
 fn share(h: &mut Harness, inner: StrongWidget, ratio: f32) -> StrongWidget {
-    SetSize {
+    h.set_len(&inner, Axis::X, Len::leftover(ratio));
-        inner,
+    inner
-        x: Some(Len::leftover(ratio)),
-        y: None,
-    }
-    .add_strong(&mut h.rsc)
 }
 
 /// 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 {
+    let mixed = rect(Color::BLUE)
-        inner: rect(Color::BLUE).add_strong(&mut h.rsc),
+        .width(Len::px(20) + Len::LEFTOVER)
-        x: Some(Len::px(20) + Len::LEFTOVER),
+        .add(&mut h.rsc);
-        y: None,
-    }
-    .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 {
+                h.rsc.widgets_mut().set_size_rules(&inner, *x, *y);
-                    inner,
+                inner
-                    x: *x,
-                    y: *y,
-                }
-                .add_strong(&mut h.rsc)
             }
             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 {
+    let sized = wrapped.width(76).add(&mut h.rsc);
-        inner: wrapped.add_strong(&mut h.rsc),
-        x: Some(Len::px(76.0)),
-        y: None,
-    }
-    .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 {
+    let sized = inner.sized((189, 176)).add(&mut h.rsc);
-        inner: inner.add_strong(&mut h.rsc),
-        x: Some(Len::px(189.0)),
-        y: Some(Len::px(176.0)),
-    }
-    .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 {
+    let sized = wrapped.width(76).add(&mut h.rsc);
-        inner: wrapped.add_strong(&mut h.rsc),
-        x: Some(Len::px(76.0)),
-        y: None,
-    }
-    .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 {
+    let sized = inner.sized((189, 176)).add(&mut h.rsc);
-        inner: inner.add_strong(&mut h.rsc),
-        x: Some(Len::px(189.0)),
-        y: Some(Len::px(176.0)),
-    }
-    .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 {
+    let fixed = block.width(87).add(&mut h.rsc);
-        inner: block.add_strong(&mut h.rsc),
-        x: Some(Len::px(87.0)),
-        y: None,
-    }
-    .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 {
+    // Carried no rule even before rules were a property: it is here to be a
-        inner: outer.add_strong(&mut h.rsc),
+    // widget between the span and the scroll, not to declare anything.
-        x: None,
+    let through = (outer,).span(Dir::RIGHT).add(&mut h.rsc);
-        y: None,
-    }
-    .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));
     (