Make a declared length one that cannot carry a share

`declared_lens` filtered `leftover` out of both its sources and every
consumer then re-dropped it, so the rule lived in two filters and a comment.
A declaration is a `Len`: `LayoutLen::declared` states the rule once and both
sources go through it, and `Declared` replaces the bare two-element array on
`ActiveData` and in four signatures.

The two sources stay one value deliberately. A rule decides the child's box;
a hint only promises what it will report -- but `size_hint` is by contract an
exact answer with no painter context, and `hints_agree` fails a widget that
draws something else, so narrowing the box to a hint cannot change what is
drawn. Every consumer asks about the length, never which said it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/orientation/len.rs

@@ -158,6 +158,13 @@ impl LayoutLen {
         self.leftover > Weight::ZERO && self.without_leftover() == Len::ZERO
     }
 
+    /// This as a length of a box, where it is one. `leftover` is not: a
+    /// share of what is left over 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> {
+        (self.leftover == Weight::ZERO).then(|| self.without_leftover())
+    }
+
     /// What this takes whatever is left over: the reading of a length for
     /// anyone not dividing a box between siblings, where a share is a claim
     /// on someone else's room rather than a length of its own.

core/src/ui/active.rs

@@ -1,5 +1,5 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
+    Declared, LayerId, LayoutHolds, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
     Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
@@ -59,7 +59,7 @@ pub struct ActiveData {
     /// The declared lengths whoever drew this widget resolved into its rel base.
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
-    pub declared: [Option<LayoutLen>; 2],
+    pub declared: Declared,
     /// Its alignment when it was last drawn, which a change to the property
     /// is found against.
     pub own_align: RegionAlign,

core/src/ui/painter.rs

@@ -1,7 +1,7 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc, Px, PxVec2, RegionAlign, Rel,
+    Axis, Declared, Holds, LayoutHolds, LayoutLen, Len, PlaceDesc, Px, PxVec2, RegionAlign, Rel,
     RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
     TextureHandle, UiRegion, UiRenderState, UiRsc, UiVec2, Weight, WidgetId, Widgets,
     render::{
@@ -292,7 +292,7 @@ impl<'a> Painter<'a> {
     /// it resolves into its rel base. Reading them depends on nothing -- the box
     /// that comes of them is kept on the child, and `redraw` compares it
     /// there.
-    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<LayoutLen>; 2] {
+    fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> Declared {
         self.rsc.widgets().declared_lens(id.id())
     }
 
@@ -599,11 +599,11 @@ impl Painter<'_> {
         holds: LayoutHolds,
         region: UiRegion,
         place: PlaceDesc,
-        declared: [Option<LayoutLen>; 2],
+        declared: Declared,
     ) -> LayoutHolds {
         let mut result = LayoutHolds::ANY;
         for axis in AXES {
-            let declared = declared[axis as usize];
+            let declared = declared[axis];
             let holds = holds[axis];
             let result = &mut result[axis];
             // Every read became pixels against the window, so a range on
@@ -644,13 +644,11 @@ impl Painter<'_> {
 }
 
 impl Widgets {
-    /// 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_lens(&self, id: WidgetId) -> [Option<LayoutLen>; 2] {
+    /// What a widget's box is where a rule or its own hint says so outright.
+    pub(crate) fn declared_lens(&self, id: WidgetId) -> Declared {
         let rules = self.size_rules(id);
         let widget = self.get_dyn(id);
-        AXES.map(|axis| {
+        Declared::per_axis(|axis| {
             rules[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
@@ -660,7 +658,7 @@ impl Widgets {
                 // the box it was offered.
                 widget
                     .and_then(|widget| widget.size_hint(axis))
-                    .filter(|len| len.leftover == Weight::ZERO)
+                    .and_then(LayoutLen::declared)
             })
         })
     }
@@ -674,7 +672,7 @@ impl LayoutLen {
     /// the rule already gave the region its length, and the rule's length is
     /// what the widget reports there. And an axis the parent decided from
     /// the answer is the answer already.
-    pub(crate) fn fills(self, declared: Option<LayoutLen>, decided: bool) -> bool {
+    pub(crate) fn fills(self, declared: Option<Len>, decided: bool) -> bool {
         self.leftover != Weight::ZERO || declared.is_some() || decided
     }
 }
@@ -693,14 +691,13 @@ impl PlaceDesc {
         self,
         region: UiRegion,
         size: Size,
-        declared: [Option<LayoutLen>; 2],
+        declared: Declared,
         align: RegionAlign,
     ) -> UiRegion {
         let mut placed = region;
         for axis in AXES {
-            let n = axis as usize;
             let reported = size[axis];
-            if reported.fills(declared[n], self[axis].does_fill()) {
+            if reported.fills(declared[axis], self[axis].does_fill()) {
                 continue;
             }
             placed[axis] = placed[axis].place(reported.without_leftover(), align[axis]);
@@ -722,22 +719,21 @@ impl PlaceDesc {
         self,
         own: UiRegion,
         parent_rel_base: UiVec2,
-        declared: [Option<LayoutLen>; 2],
+        declared: Declared,
         align: RegionAlign,
     ) -> (UiVec2, UiRegion) {
         let given = self.of(own, align);
         let mut rel_base = parent_rel_base;
         let mut region = given;
         for axis in AXES {
-            let n = axis as usize;
             let base = self[axis]
                 .stated_rel_base()
                 .unwrap_or_else(|| parent_rel_base[axis]);
-            let len = declared[n]
-                .map(|len| len.without_leftover().within_len(base))
+            let len = declared[axis]
+                .map(|len| len.within_len(base))
                 .unwrap_or(base);
             rel_base[axis] = len;
-            if declared[n].is_some() {
+            if declared[axis].is_some() {
                 region[axis] = given[axis].place(len, align[axis]);
             }
         }

core/src/ui/render_state.rs

@@ -1,9 +1,9 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::{
-    ActiveData, Axis, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Moves,
-    Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan,
-    UiVec2, Weight, WidgetId, Widgets,
+    ActiveData, Axis, Declared, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
+    Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc,
+    UiSpan, UiVec2, Weight, WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
@@ -868,7 +868,7 @@ impl UiRenderState {
                 children: Vec::new(),
                 size_deps: Vec::new(),
                 move_idx: info.parent_move,
-                declared: [None; 2],
+                declared: Declared::NONE,
                 own_align: rsc.widgets().alignment(id),
                 parent_move: info.parent_move,
                 mask: info.mask,

core/src/widget/size_rule.rs

@@ -1,5 +1,5 @@
 use crate::util::impl_axis_index;
-use crate::{LayoutLen, Weight};
+use crate::{Axis, LayoutLen, 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.
@@ -20,14 +20,9 @@ pub enum SizeRule {
 
 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<LayoutLen> {
-        match self {
-            Self::Exact(len) if len.leftover == Weight::ZERO => Some(*len),
-            _ => None,
-        }
+    /// give one.
+    pub fn declared(&self) -> Option<Len> {
+        self.exact().and_then(LayoutLen::declared)
     }
 
     /// The length this rule gives outright, whatever the widget reports --
@@ -72,3 +67,28 @@ pub struct SizeRules {
 }
 
 impl_axis_index!(SizeRules => SizeRule);
+
+/// What a widget's box is on each axis where something says so outright,
+/// before it is drawn: a rule beside it, or a hint it gives about itself.
+/// Whoever draws the widget resolves these against its rel base.
+///
+/// A [`Len`] rather than a [`LayoutLen`], because a share can never be one
+/// -- see [`LayoutLen::declared`].
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Declared {
+    pub x: Option<Len>,
+    pub y: Option<Len>,
+}
+
+impl Declared {
+    pub const NONE: Self = Self { x: None, y: None };
+
+    pub fn per_axis(f: impl Fn(Axis) -> Option<Len>) -> Self {
+        Self {
+            x: f(Axis::X),
+            y: f(Axis::Y),
+        }
+    }
+}
+
+impl_axis_index!(Declared => Option<Len>);