Say layout's operations by name, and index a pair by its axis

Four rounds over the same idea: an expression that needed a comment to say
what it computed wanted to be a named operation.

The placement description is built by chaining off the value that says it.
`UiSpan::within_desc`/`shifted_desc` and `Len::as_desc` replace the
`PlaceDescAxis::` constructors, `PlaceDescAxis::axis` lifts one axis into a
pair with the whole box across it, and `PlaceDesc::per_axis` covers the case
where the two axes differ. `beside` is dropped: `from_axis` already said it.

Seven module-level functions become methods on the value each took first --
`Widgets::declared_lens`, `LayoutLen::fills`, `PlaceDesc::placement` and
`::rel_base_and_region`, `Size::within_box`, `UiRegion::at_origin` and
`::as_translation`.

`UiSpan::place` is the aligned-placement rule, which was written out three
times; `LayoutLen::without_leftover` is the sibling `apply_leftover` never
had, at six sites; `is_px` and `is_only_leftover` name field comparisons the
surrounding comments had to translate; `Holds::covers` was interval
containment spelled out by hand. A span's `shared` loses the two arguments
that did not vary across its loop.

`LayoutHolds` was four two-element arrays where every other pair here is a
struct of two per-axis values, so nothing it did could be written once.
It becomes `AxisHolds` on `x` and `y`, and `and`, `covers` and `contains`
lose their loops.

Every pair gets `Index<Axis>`/`IndexMut<Axis>` through one macro, and the
eighteen `axis`/`axis_mut` methods go. `const_index` keeps the accessors
usable in const context.

Cold layout is unchanged: `layout_dump` over 400 depth-5 trees is identical
to 58ce74d byte for byte, across all 34,492 boxes.

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

core/src/widget/size_rule.rs

@@ -1,4 +1,5 @@
-use crate::{Axis, LayoutLen, Weight};
+use crate::util::impl_axis_index;
+use crate::{LayoutLen, Weight};
 
 /// 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.
@@ -70,18 +71,4 @@ pub struct SizeRules {
     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,
-        }
-    }
-}
+impl_axis_index!(SizeRules => SizeRule);

core/src/widget/widgets.rs

@@ -130,10 +130,10 @@ impl Widgets {
     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 {
+        if data.size[axis] == rule {
             return;
         }
-        *data.size.axis_mut(axis) = rule;
+        data.size[axis] = rule;
         self.needs_redraw.insert(id);
     }
 
@@ -147,10 +147,10 @@ impl Widgets {
     pub fn set_alignment(&mut self, id: impl IdLike, axis: Axis, align: AxisAlign) {
         let id = id.id();
         let data = self.data_mut(id).unwrap();
-        if *data.align.axis_mut(axis) == align {
+        if data.align[axis] == align {
             return;
         }
-        *data.align.axis_mut(axis) = align;
+        data.align[axis] = align;
         self.needs_redraw.insert(id);
     }