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/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
-    /// window: a resolved share, or a box a sibling's answer decided. `None`
-    /// forwards this widget's own, which is what a container that only
-    /// divides room passes, so a fraction under it means the same wherever
-    /// it sits and however deeply it is nested. It only ever narrows -- a
-    /// length the child declares narrows it again here whatever the caller
-    /// says -- and what comes of it is also the box the child is asked in,
-    /// placed in the part by the child's alignment.
+    /// `narrow_rel_base` is the child's rel base, per axis, as a length of
+    /// the window: a resolved share, or a box a sibling's answer decided.
+    /// `None`, whole or per axis, forwards this widget's own -- which is
+    /// what a container that only divides room passes, so a fraction under
+    /// it means the same wherever it sits and however deeply it is nested.
+    /// It only ever narrows: a length the child declares narrows it again
+    /// here whatever the caller says, and what comes of it is also the box
+    /// 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) =
-            rel_base_and_region(self.region, self.rel_base, place, narrow, declared, align);
+        let (rel_base, region) = rel_base_and_region(
+            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 answer_holds = self.in_parent(answer_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_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()) {
-            return self.widget_at(id, narrow, place);
+        let narrow_rel_base = narrow_rel_base.into().unwrap_or([None; 2]);
+        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))