One argument says where a child goes and what its fractions are of

`Place` was a product written as a sum -- a `Part` and a fill flag -- and
`Part` named three operations the geometry already had, under words that did
not match them. `Of` was `UiSpan::within`, `From` was `UiSpan::shift`, and
`Sized` was `placement`'s own body with the length given rather than
reported. Both enums are gone.

`PlaceDescAxis` says one axis, named after the operation it performs:
`within`, `shifted`, `sized`, and `WHOLE`. What is optional is a builder --
`fills` and `rel_base` -- so a caller writes only what it decided, and the
rel base it does not write follows the constructor: a span composed into the
caller's box narrows it, a span along a cursor does not, a decided length is
it. That was the one rule a caller could get wrong with nothing failing.

`PlaceDesc` says both axes with named fields, so `axis`, `axis_mut` and
`from_axis` work the way they do on every other pair here, and the joint
work -- resolving a region, reading the fill flags -- is written once rather
than per axis. `widget_at` and `place_at` take `impl Into<PlaceDesc>`, so a
wrapper passes a `UiRegion` and says nothing else. `widget_within` and
`ActiveData::narrow_rel_base` are deleted; `asked` and `placed` carry the
rel base their ask stated.

Cold layout is byte-identical to `84dad21`.

src/widget/position/offset.rs

@@ -8,7 +8,7 @@ pub struct Offset {
 impl Widget for Offset {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         painter
-            .widget_within(&self.inner, UiRegion::FULL.offset(self.amt))
+            .widget_at(&self.inner, UiRegion::FULL.offset(self.amt))
             .size()
     }
 }

src/widget/position/pad.rs

@@ -19,9 +19,7 @@ impl Widget for Pad {
         // padding, and it comes off the box, so what is drawn sits inside.
         // The two stay distinct -- the box can be narrower still, where a row
         // asked this widget in the room left, and a text wraps at that.
-        let inner = painter
-            .widget_within(&self.inner, self.padding.region())
-            .size();
+        let inner = painter.widget_at(&self.inner, self.padding.region()).size();
         Size {
             x: LayoutLen {
                 px: inner.x.px + self.padding.left + self.padding.right,

src/widget/position/scroll.rs

@@ -14,7 +14,7 @@ impl Widget for Scroll {
         let container_len = painter.px_len(self.axis);
         // Asked in the whole viewport, then put at the scrolled offset.
         let answer_len = painter
-            .widget_at(&self.inner, [Place::Fill(Part::WHOLE); 2], None)
+            .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);
         self.container_len = container_len;
@@ -54,9 +54,9 @@ 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);
-                Part::From(UiSpan::new(start, start.offset(self.content_len)))
+                PlaceDescAxis::shifted(UiSpan::new(start, start.offset(self.content_len)))
             }
-            false => Part::WHOLE,
+            false => PlaceDescAxis::WHOLE,
         };
         // The viewport is the inner's rel base, so a fraction it declares or
         // reports is a fraction of what is on screen rather than of the
@@ -64,9 +64,7 @@ impl Widget for Scroll {
         // box, scrolled: its drawing moved there, not made again there.
         painter.place_at(
             &self.inner,
-            self.axis
-                .pair(Place::Fill(content), Place::Fill(Part::WHOLE)),
-            None,
+            PlaceDesc::from_axis(self.axis, content.fills(), PlaceDescAxis::WHOLE.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

src/widget/position/span.rs

@@ -21,7 +21,7 @@ impl Widget for Span {
         // 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 = Place::Within(Part::WHOLE);
+        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
@@ -34,9 +34,9 @@ impl Widget for Span {
             let len = match painter.size_hint(child, axis) {
                 Some(len) => len,
                 None => {
-                    let room = Place::Within(Part::From(along(cursor, far)));
+                    let room = PlaceDescAxis::shifted(along(cursor, far));
                     painter
-                        .widget_at(child, axis.pair(room, across), None)
+                        .widget_at(child, PlaceDesc::from_axis(axis, room, across))
                         .len(axis)
                 }
             };
@@ -120,10 +120,12 @@ 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 place = axis.pair(Place::Fill(Part::From(slot)), across);
-            let narrow =
-                (len.leftover > Weight::ZERO && shares).then(|| axis.pair(Some(slot.len()), None));
-            let used = painter.place_at(child, place, narrow).len(!axis);
+            let slot_place = PlaceDescAxis::shifted(slot).fills();
+            let mut place = PlaceDesc::from_axis(axis, slot_place, across);
+            if len.leftover > Weight::ZERO && shares {
+                place = place.rel_base(axis, slot.len());
+            }
+            let used = painter.place_at(child, place).len(!axis);
             if shrinks {
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.

src/widget/position/stack.rs

@@ -22,9 +22,7 @@ impl Widget for Stack {
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter
-                    .widget_at(child, [Place::Fill(Part::WHOLE); 2], None)
-                    .size()
+                painter.widget_at(child, PlaceDesc::WHOLE.fills()).size()
             }
             None => Size::LEFTOVER,
         };
@@ -33,19 +31,20 @@ 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 place = [Axis::X, Axis::Y].map(|axis| {
+        let on = |axis| {
             let len = size.axis(axis);
             match len.leftover == Weight::ZERO {
-                true => Place::Fill(Part::Sized(Len::from_parts(len.rel, len.px))),
-                false => Place::Within(Part::WHOLE),
+                true => PlaceDescAxis::sized(Len::from_parts(len.rel, len.px)).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;
             }
             painter.child_layer_at(i);
-            painter.widget_at(child, place, None);
+            painter.widget_at(child, place);
         }
         size
     }