Say narrow_rel_base, and let a container pass None for it

`narrow` said what the argument did to a value it never named, so a reader
had to go and find out which value. It is `narrow_rel_base`, and the
resolved one stays the bare `rel_base` -- which is also the only one in
`Painter`, `Placing` and `LayoutHolds`, where there is nothing to tell it
apart from.

It takes `impl Into<Option<[Option<Len>; 2]>>`, so a container that does not
narrow anything writes `None` rather than `[None; 2]`, and `Span` builds the
one case that does with a `then` instead of a mutable array.

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

core/src/ui/active.rs

@@ -18,7 +18,7 @@ pub struct ActiveData {
     /// padding's rel base less its pixels -- as a length of the window. `None`
     /// forwards the parent's rel base. What it declared is kept separately in
     /// `declared` and is a fraction of whichever of the two reached it.
-    pub narrow: [Option<Len>; 2],
+    pub narrow_rel_base: [Option<Len>; 2],
     /// Where its drawing was put, and where it was asked, each as a part of
     /// its parent's box. The two differ where a container asks in one place
     /// and puts the answer in another -- a row measures from its cursor and

core/src/ui/painter.rs

@@ -174,14 +174,14 @@ impl<'a> Painter<'a> {
 
     /// Asks a child, saying what its fractions are of and where it is asked.
     ///
-    /// `narrow` is the child's rel base, per axis, as a length of the
+    /// `narrow_rel_base` is the child's rel base, per axis, as a length of
-    /// window: a resolved share, or a box a sibling's answer decided. `None`
+    /// the window: a resolved share, or a box a sibling's answer decided.
-    /// forwards this widget's own, which is what a container that only
+    /// `None`, whole or per axis, forwards this widget's own -- which is
-    /// divides room passes, so a fraction under it means the same wherever
+    /// what a container that only divides room passes, so a fraction under
-    /// it sits and however deeply it is nested. It only ever narrows -- a
+    /// it means the same wherever it sits and however deeply it is nested.
-    /// length the child declares narrows it again here whatever the caller
+    /// It only ever narrows: a length the child declares narrows it again
-    /// says -- and what comes of it is also the box the child is asked in,
+    /// here whatever the caller says, and what comes of it is also the box
-    /// placed in the part by the child's alignment.
+    /// the child is asked in, placed in the part by the child's alignment.
     ///
     /// `place` is where the child is asked, per axis, as a part of this
     /// widget's box: see [`Place`]. The child draws once, in that box, and
@@ -191,14 +191,21 @@ impl<'a> Painter<'a> {
     pub fn widget_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        narrow: [Option<Len>; 2],
+        narrow_rel_base: impl Into<Option<[Option<Len>; 2]>>,
         place: [Place; 2],
     ) -> DrawResult<'s, 'a, W> {
+        let narrow_rel_base = narrow_rel_base.into().unwrap_or([None; 2]);
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
         let align = self.rsc.widgets().alignment(id.id());
-        let (rel_base, region) =
+        let (rel_base, region) = rel_base_and_region(
-            rel_base_and_region(self.region, self.rel_base, place, narrow, declared, align);
+            self.region,
+            self.rel_base,
+            place,
+            narrow_rel_base,
+            declared,
+            align,
+        );
         #[cfg(feature = "layout-diagnostics")]
         if region_node {
             diag::bump(Counter::RegionNodeDraws);
@@ -223,15 +230,15 @@ impl<'a> Painter<'a> {
                 region,
                 placed: place,
                 asked: place,
-                narrow,
+                narrow_rel_base,
                 re_asked,
                 px,
             },
             None,
             self.rsc,
         );
-        let holds = self.in_parent(holds, region, place, narrow, declared);
+        let holds = self.in_parent(holds, region, place, narrow_rel_base, declared);
-        let answer_holds = self.in_parent(answer_holds, region, place, narrow, declared);
+        let answer_holds = self.in_parent(answer_holds, region, place, narrow_rel_base, declared);
         match self.under.iter_mut().find(|(child, _)| *child == id.id()) {
             Some((_, kept)) => *kept = holds,
             None => self.under.push((id.id(), holds)),
@@ -266,11 +273,12 @@ impl<'a> Painter<'a> {
     pub fn place_at<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        narrow: [Option<Len>; 2],
+        narrow_rel_base: impl Into<Option<[Option<Len>; 2]>>,
         place: [Place; 2],
     ) -> DrawResult<'s, 'a, W> {
-        if narrow.iter().any(Option::is_some) || !self.children.contains(&id.id()) {
+        let narrow_rel_base = narrow_rel_base.into().unwrap_or([None; 2]);
-            return self.widget_at(id, narrow, place);
+        if narrow_rel_base.iter().any(Option::is_some) || !self.children.contains(&id.id()) {
+            return self.widget_at(id, narrow_rel_base, place);
         }
         let at = self.placing();
         self.state.place_in(id.id(), &at, place, self.rsc);
@@ -612,7 +620,7 @@ impl Painter<'_> {
         holds: LayoutHolds,
         region: UiRegion,
         place: [Place; 2],
-        narrow: [Option<Len>; 2],
+        narrow_rel_base: [Option<Len>; 2],
         declared: [Option<LayoutLen>; 2],
     ) -> LayoutHolds {
         let mut result = LayoutHolds::ANY;
@@ -621,7 +629,7 @@ impl Painter<'_> {
             // Every read became pixels against the window, so a range on
             // it is already in this widget's terms.
             result.window[n] = holds.window[n];
-            let reaches = narrow[n].is_none()
+            let reaches = narrow_rel_base[n].is_none()
                 && !matches!(place[n].part(), Part::Sized(_))
                 && declared[n].is_none_or(|len| len.rel != Rel::ZERO);
             result.rel_base[n] = holds.rel_base[n].and(reaches.then(|| self.rel_base.axis(axis)));
@@ -729,7 +737,7 @@ pub(crate) fn placement(
 /// widget asking draws in.
 ///
 /// `own` is that widget's own box, and `place` what of it the child is
-/// given. `narrow` is a rel base the container decided for the child -- a row's
+/// given. `narrow_rel_base` is a rel base the container decided for the child -- a row's
 /// slot, or padding's rel base less its pixels -- and [`Part::Sized`] one a
 /// sibling's answer decided; both are window lengths, like every other
 /// length here, since a slot of a row is not a fraction of anything the row
@@ -740,7 +748,7 @@ pub(crate) fn rel_base_and_region(
     own: UiRegion,
     parent_rel_base: UiVec2,
     place: [Place; 2],
-    narrow: [Option<Len>; 2],
+    narrow_rel_base: [Option<Len>; 2],
     declared: [Option<LayoutLen>; 2],
     align: RegionAlign,
 ) -> (UiVec2, UiRegion) {
@@ -754,7 +762,7 @@ pub(crate) fn rel_base_and_region(
             _ => None,
         };
         let base = sized
-            .or(narrow[n])
+            .or(narrow_rel_base[n])
             .unwrap_or_else(|| parent_rel_base.axis(axis));
         let len = declared[n]
             .map(|len| Len::from_parts(len.rel, len.px).within_len(base))

core/src/ui/render_state.rs

@@ -33,7 +33,7 @@ pub(super) struct DrawInfo {
     pub asked: [Place; 2],
     /// A rel base the parent decided for it on each axis, as a length of the
     /// window, which the widget's own declaration is a fraction of.
-    pub narrow: [Option<Len>; 2],
+    pub narrow_rel_base: [Option<Len>; 2],
     /// Whether the parent already asked about this widget in this draw.
     pub re_asked: bool,
     /// The rel base in pixels, resolved once against the window.
@@ -133,7 +133,7 @@ impl UiRenderState {
         }
     }
 
-    /// The root is asked about in the output. Its own rules narrow both its
+    /// The root is asked about in the output. Its own rules narrow_rel_base both its
     /// rel base and box; nothing above it chose a different one.
     fn root_info(&self, rel_base: UiVec2, region: UiRegion) -> DrawInfo {
         let px = rel_base.to_px(self.output_size);
@@ -148,7 +148,7 @@ impl UiRenderState {
             region,
             placed: [Place::Within(Part::WHOLE); 2],
             asked: [Place::Within(Part::WHOLE); 2],
-            narrow: [None; 2],
+            narrow_rel_base: [None; 2],
             re_asked: false,
             px,
         }
@@ -273,7 +273,7 @@ impl UiRenderState {
         // and what of its own box it asked in. A local redraw asks the same
         // question again from these.
         active.rel_base = info.rel_base;
-        active.narrow = info.narrow;
+        active.narrow_rel_base = info.narrow_rel_base;
         active.re_asked = info.re_asked;
         active.answer = Some(answer);
         active.asked = info.asked;
@@ -470,7 +470,7 @@ impl UiRenderState {
                         region: UiRegion::FULL,
                         placed: [Place::Within(Part::WHOLE); 2],
                         asked: [Place::Within(Part::WHOLE); 2],
-                        narrow: [None; 2],
+                        narrow_rel_base: [None; 2],
                         re_asked: false,
                         px,
                     },
@@ -484,7 +484,7 @@ impl UiRenderState {
             id,
             placement: region,
             rel_base: info.rel_base,
-            narrow: info.narrow,
+            narrow_rel_base: info.narrow_rel_base,
             placed: info.placed,
             asked: info.asked,
             region,
@@ -741,7 +741,7 @@ impl UiRenderState {
             region,
             placed: place,
             asked: active.asked,
-            narrow: active.narrow,
+            narrow_rel_base: active.narrow_rel_base,
             re_asked: active.re_asked,
             px: rel_base.to_px(at.window),
         };
@@ -757,7 +757,7 @@ impl UiRenderState {
             at.region,
             at.rel_base,
             place,
-            active.narrow,
+            active.narrow_rel_base,
             active.declared,
             active.own_align,
         )
@@ -886,7 +886,7 @@ impl UiRenderState {
                 id,
                 placement: UiRegion::FULL,
                 rel_base: UiVec2::FULL_SIZE,
-                narrow: [None; 2],
+                narrow_rel_base: [None; 2],
                 placed: [Place::Within(Part::WHOLE); 2],
                 asked: [Place::Within(Part::WHOLE); 2],
                 region: UiRegion::FULL,
@@ -1146,7 +1146,7 @@ impl UiRenderState {
             region,
             placed: active.asked,
             asked: active.asked,
-            narrow: active.narrow,
+            narrow_rel_base: active.narrow_rel_base,
             re_asked: false,
             px: rel_base.to_px(self.output_size),
         };
@@ -1171,7 +1171,7 @@ impl UiRenderState {
         }
         if active.answer != was_answer || active.holds != was_holds {
             // The parent retains both the answer and the drawing's validity;
-            // even an unchanged size can narrow the range safe for a resize.
+            // even an unchanged size can narrow_rel_base the range safe for a resize.
             #[cfg(feature = "layout-diagnostics")]
             {
                 diag::bump(Counter::SizeChanges);

src/random.rs

@@ -120,7 +120,7 @@ impl Widget for Branch {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
         let measured = painter
-            .widget_at(&self.probe, [None; 2], [Place::Within(Part::WHOLE), top])
+            .widget_at(&self.probe, None, [Place::Within(Part::WHOLE), top])
             .len(Axis::X);
         let len = measured.apply_leftover();
         let px = painter.to_px(len, Axis::X);
@@ -137,8 +137,8 @@ impl Widget for Branch {
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.region_len(Axis::Y))));
         let place = [Place::Within(Part::WHOLE), below];
         match px > threshold {
-            true => painter.widget_at(&self.wide, [None; 2], place),
+            true => painter.widget_at(&self.wide, None, place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+            false => painter.widget_at(&self.narrow, None, place),
         };
         Size::LEFTOVER
     }

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, [None; 2], [Place::Fill(Part::WHOLE); 2])
+            .widget_at(&self.inner, None, [Place::Fill(Part::WHOLE); 2])
             .len(self.axis);
         let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
         self.container_len = container_len;
@@ -64,7 +64,7 @@ impl Widget for Scroll {
         // box, scrolled: its drawing moved there, not made again there.
         painter.place_at(
             &self.inner,
-            [None; 2],
+            None,
             self.axis
                 .pair(Place::Fill(content), Place::Fill(Part::WHOLE)),
         );

src/widget/position/span.rs

@@ -36,7 +36,7 @@ impl Widget for Span {
                 None => {
                     let room = Place::Within(Part::From(along(cursor, far)));
                     painter
-                        .widget_at(child, [None; 2], axis.pair(room, across))
+                        .widget_at(child, None, axis.pair(room, across))
                         .len(axis)
                 }
             };
@@ -121,10 +121,8 @@ impl Widget for Span {
             // 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 mut narrow = [None; 2];
+            let narrow =
-            if len.leftover > Weight::ZERO && shares {
+                (len.leftover > Weight::ZERO && shares).then(|| axis.pair(Some(slot.len()), None));
-                narrow[axis as usize] = Some(slot.len());
-            }
             let used = painter.place_at(child, narrow, place).len(!axis);
             if shrinks {
                 // Choosing between a fixed and a relative length from the

src/widget/position/stack.rs

@@ -23,7 +23,7 @@ impl Widget for Stack {
             Some((i, child)) => {
                 painter.child_layer_at(i);
                 painter
-                    .widget_at(child, [None; 2], [Place::Fill(Part::WHOLE); 2])
+                    .widget_at(child, None, [Place::Fill(Part::WHOLE); 2])
                     .size()
             }
             None => Size::LEFTOVER,
@@ -45,7 +45,7 @@ impl Widget for Stack {
                 continue;
             }
             painter.child_layer_at(i);
-            painter.widget_at(child, [None; 2], place);
+            painter.widget_at(child, None, place);
         }
         size
     }

tests/cases/determinism.rs

@@ -24,15 +24,15 @@ impl Widget for BranchesOnMeasurement {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
         let measured = painter
-            .widget_at(&self.probe, [None; 2], [Place::Within(Part::WHOLE), top])
+            .widget_at(&self.probe, None, [Place::Within(Part::WHOLE), top])
             .len(Axis::X);
         let px = painter.to_px(measured.apply_leftover(), Axis::X);
 
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.region_len(Axis::Y))));
         let place = [Place::Within(Part::WHOLE), below];
         match px > Px::from_f32(self.threshold) {
-            true => painter.widget_at(&self.wide, [None; 2], place),
+            true => painter.widget_at(&self.wide, None, place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+            false => painter.widget_at(&self.narrow, None, place),
         };
         Size::LEFTOVER
     }