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/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]);
             }
         }