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>

src/random.rs

@@ -118,9 +118,9 @@ pub struct Branch {
 impl Widget for Branch {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
-        let top = PlaceDescAxis::shifted(UiSpan::new(Len::ZERO, cut));
+        let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
         let measured = painter
-            .widget_at(&self.probe, PlaceDesc::new(PlaceDescAxis::WHOLE, top))
+            .widget_at(&self.probe, top.axis(Axis::Y))
             .len(Axis::X);
         let len = measured.apply_leftover();
         let px = painter.to_px(len, Axis::X);
@@ -134,8 +134,8 @@ impl Widget for Branch {
         };
         painter.window_holds(Axis::X, holds.through(len));
 
-        let below = PlaceDescAxis::shifted(UiSpan::new(cut, painter.region_len(Axis::Y)));
-        let place = PlaceDesc::new(PlaceDescAxis::WHOLE, below);
+        let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
+        let place = below.axis(Axis::Y);
         match px > threshold {
             true => painter.widget_at(&self.wide, place),
             false => painter.widget_at(&self.narrow, place),

src/widget/image.rs

@@ -12,7 +12,7 @@ impl Widget for Image {
     }
 
     fn size_hint(&self, axis: Axis) -> Option<LayoutLen> {
-        Some(LayoutLen::px(self.handle.size().axis(axis)))
+        Some(LayoutLen::px(self.handle.size()[axis]))
     }
 }
 

src/widget/position/scroll.rs

@@ -16,7 +16,7 @@ impl Widget for Scroll {
         let answer_len = painter
             .widget_at(&self.inner, PlaceDesc::WHOLE.fills())
             .len(self.axis);
-        let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
+        let fixed = painter.to_px(answer_len.without_leftover(), self.axis);
         self.container_len = container_len;
         self.content_len = fixed.max(container_len);
 
@@ -24,14 +24,14 @@ impl Widget for Scroll {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
-        let align = painter.alignment().axis(self.axis);
+        let align = painter.alignment()[self.axis];
         // Content of a fixed length that fits sits at the start of any box it
         // fits in -- but only anchored there. Anywhere else it is a part of
         // the room left over, so it moves with every length the box takes and
         // the drawing holds for that length alone. One scrolled part way sits
         // where it is until the box shrinks past what is left of it. Kept to
         // the end, it moves with every length.
-        let fixed_len = answer_len.rel == Rel::ZERO && answer_len.leftover == Weight::ZERO;
+        let fixed_len = answer_len.is_px();
         if fixed_len && self.content_len <= self.container_len && align == AxisAlign::NEG {
             painter.holds(self.axis, fixed..=Px::MAX);
         } else if fixed_len && !self.snap_end {
@@ -54,7 +54,7 @@ impl Widget for Scroll {
         let content = match moved || self.content_len != self.container_len {
             true => {
                 let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
-                PlaceDescAxis::shifted(UiSpan::new(start, start.offset(self.content_len)))
+                UiSpan::new(start, start.offset(self.content_len)).shifted_desc()
             }
             false => PlaceDescAxis::WHOLE,
         };
@@ -62,10 +62,7 @@ impl Widget for Scroll {
         // reports is a fraction of what is on screen rather than of the
         // content box its own answer decided. Where it goes is the content
         // box, scrolled: its drawing moved there, not made again there.
-        painter.place_at(
-            &self.inner,
-            PlaceDesc::from_axis(self.axis, content.fills(), PlaceDescAxis::WHOLE.fills()),
-        );
+        painter.place_at(&self.inner, content.axis(self.axis).fills());
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for
         // more. The content's length is what it scrolls through, not what it

src/widget/position/span.rs

@@ -18,10 +18,6 @@ impl Widget for Span {
             Sign::Pos => UiSpan::new(from, to),
             Sign::Neg => UiSpan::new(far - to, far - from),
         };
-        // Across itself the child sits where its own alignment says, in the
-        // whole of the row: a span is what contains its children there, and
-        // nothing divides that axis.
-        let across = PlaceDescAxis::WHOLE;
         // A length for every child before their final slots are chosen: from
         // a hint where one says, and from drawing otherwise. The rel base passes
         // through unchanged, so `rel(0.5)` is half the area this span was
@@ -34,10 +30,11 @@ impl Widget for Span {
             let len = match painter.size_hint(child, axis) {
                 Some(len) => len,
                 None => {
-                    let room = PlaceDescAxis::shifted(along(cursor, far));
-                    painter
-                        .widget_at(child, PlaceDesc::from_axis(axis, room, across))
-                        .len(axis)
+                    // Across itself the child sits where its own alignment
+                    // says, in the whole of the row: a span is what contains
+                    // its children there, and nothing divides that axis.
+                    let room = along(cursor, far).shifted_desc().axis(axis);
+                    painter.widget_at(child, room).len(axis)
                 }
             };
             cursor.px += len.px + self.gap;
@@ -58,7 +55,7 @@ impl Widget for Span {
 
         // What is left for the shares to divide: the row less everything
         // fixed, as a length of the rel base rather than a number of pixels.
-        let room = far - Len::from_parts(total.rel, total.px);
+        let room = far - total.without_leftover();
         // Whether anything is left over is a question in pixels: `rel(0.5)`
         // beside 300 px is full at 600 and overfull at 400. Asked of `room`
         // itself, and answered back through the same expression, so the
@@ -94,24 +91,26 @@ impl Widget for Span {
         let mut taken = Weight::ZERO;
         let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
+        let shared = |fixed: Len, taken: Weight| match taken == Weight::ZERO {
+            true => fixed,
+            false => fixed + room.scale(Rel::ratio(taken, total.leftover)),
+        };
         for (child, &len) in self.children.iter().zip(&lens) {
             // A child asking for nothing but a part of what is left over,
             // when nothing is, is not drawn at all. One that also asked for
             // pixels or a fraction keeps those and overflows.
-            if len.leftover > Weight::ZERO && len.px == Px::ZERO && len.rel == Rel::ZERO && !shares
-            {
+            if len.is_only_leftover() && !shares {
                 painter.undraw(child);
                 fixed.px += self.gap;
-                start = shared(fixed, taken, total.leftover, room);
+                start = shared(fixed, taken);
                 continue;
             }
             let from = start;
             if len.leftover > Weight::ZERO && shares {
                 taken += len.leftover;
             }
-            fixed.px += len.px;
-            fixed.rel += len.rel;
-            start = shared(fixed, taken, total.leftover, room);
+            fixed += len.without_leftover();
+            start = shared(fixed, taken);
             // Along the row the span says where the child goes, and that slot
             // is the child's box outright rather than something to place an
             // answer inside again. A share is decided here and nowhere
@@ -120,8 +119,7 @@ impl Widget for Span {
             // fixed child's slot is its own answer, so a drawing made in the
             // room is put there as it is, and one not made yet is made here.
             let slot = along(from, start);
-            let slot_place = PlaceDescAxis::shifted(slot).fills();
-            let mut place = PlaceDesc::from_axis(axis, slot_place, across);
+            let mut place = slot.shifted_desc().fills().axis(axis);
             if len.leftover > Weight::ZERO && shares {
                 place = place.rel_base(axis, slot.len());
             }
@@ -131,14 +129,14 @@ impl Widget for Span {
                 // span's own eventual width admits multiple fixed points.
                 // A scalable child therefore makes Children scalable too;
                 // only fixed children are compared with one another.
-                if used.rel != Rel::ZERO || used.leftover != Weight::ZERO {
+                if !used.is_px() {
                     ortho = LayoutLen::LEFTOVER;
                 } else if ortho.leftover == Weight::ZERO {
                     ortho.px = ortho.px.max(used.px);
                 }
             }
             fixed.px += self.gap;
-            start = shared(fixed, taken, total.leftover, room);
+            start = shared(fixed, taken);
         }
 
         // Carried whole rather than collapsed to one share: a span that sizes
@@ -157,16 +155,6 @@ impl Widget for Span {
     }
 }
 
-/// Where a row has reached: everything fixed before this point, which is a
-/// sum and exact, plus the share of the room the weights so far are worth,
-/// which is one rounding wherever it is asked for.
-fn shared(fixed: Len, taken: Weight, weight: Weight, room: Len) -> Len {
-    if taken == Weight::ZERO {
-        return fixed;
-    }
-    fixed + room.scale(Rel::ratio(taken, weight))
-}
-
 impl Span {
     pub fn empty(dir: Dir) -> Self {
         Self {

src/widget/position/stack.rs

@@ -31,14 +31,13 @@ impl Widget for Stack {
         // fraction under them is a fraction of it. A share leaves the axis
         // to whoever gave the stack its box. Where a child sits in a box
         // bigger than itself is its own business.
-        let on = |axis| {
-            let len = size.axis(axis);
+        let place = PlaceDesc::per_axis(|axis| {
+            let len = size[axis];
             match len.leftover == Weight::ZERO {
-                true => PlaceDescAxis::sized(Len::from_parts(len.rel, len.px)).fills(),
+                true => len.without_leftover().as_desc().fills(),
                 false => PlaceDescAxis::WHOLE,
             }
-        };
-        let place = PlaceDesc::new(on(Axis::X), on(Axis::Y));
+        });
         for (i, child) in self.children.iter().enumerate() {
             if sizing == Some(i) {
                 continue;