Make widget::Tree mutable in Widget::layout

core/src/element.rs

@@ -306,7 +306,7 @@ where
 
     fn layout(
         &self,
-        tree: &Tree,
+        tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
@@ -492,7 +492,7 @@ where
 
     fn layout(
         &self,
-        tree: &Tree,
+        tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {

core/src/layout/flex.rs

@@ -67,7 +67,7 @@ pub fn resolve<Message, Renderer>(
     spacing: f32,
     align_items: Alignment,
     items: &[Element<'_, Message, Renderer>],
-    trees: &[widget::Tree],
+    trees: &mut [widget::Tree],
 ) -> Node
 where
     Renderer: crate::Renderer,
@@ -83,7 +83,7 @@ where
     let mut nodes: Vec<Node> = Vec::with_capacity(items.len());
     nodes.resize(items.len(), Node::default());
 
-    for (i, (child, tree)) in items.iter().zip(trees).enumerate() {
+    for (i, (child, tree)) in items.iter().zip(trees.iter_mut()).enumerate() {
         let fill_factor = match axis {
             Axis::Horizontal => child.as_widget().width(),
             Axis::Vertical => child.as_widget().height(),

core/src/overlay.rs

@@ -25,7 +25,7 @@ where
     ///
     /// [`Node`]: layout::Node
     fn layout(
-        &self,
+        &mut self,
         renderer: &Renderer,
         bounds: Size,
         position: Point,

core/src/overlay/element.rs

@@ -54,7 +54,7 @@ where
 
     /// Computes the layout of the [`Element`] in the given bounds.
     pub fn layout(
-        &self,
+        &mut self,
         renderer: &Renderer,
         bounds: Size,
         translation: Vector,
@@ -150,7 +150,7 @@ where
     Renderer: crate::Renderer,
 {
     fn layout(
-        &self,
+        &mut self,
         renderer: &Renderer,
         bounds: Size,
         position: Point,

core/src/overlay/group.rs

@@ -61,7 +61,7 @@ where
     Renderer: crate::Renderer,
 {
     fn layout(
-        &self,
+        &mut self,
         renderer: &Renderer,
         bounds: Size,
         position: Point,
@@ -71,7 +71,7 @@ where
         layout::Node::with_children(
             bounds,
             self.children
-                .iter()
+                .iter_mut()
                 .map(|child| child.layout(renderer, bounds, translation))
                 .collect(),
         )

core/src/widget.rs

@@ -55,7 +55,7 @@ where
     /// user interface.
     fn layout(
         &self,
-        tree: &Tree,
+        tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node;

core/src/widget/text.rs

@@ -3,12 +3,11 @@ use crate::alignment;
 use crate::layout;
 use crate::mouse;
 use crate::renderer;
-use crate::text::{self, Paragraph as _};
+use crate::text::{self, Paragraph};
 use crate::widget::tree::{self, Tree};
 use crate::{Color, Element, Layout, Length, Pixels, Point, Rectangle, Widget};
 
 use std::borrow::Cow;
-use std::cell::RefCell;
 
 pub use text::{LineHeight, Shaping};
 
@@ -120,7 +119,7 @@ where
 
 /// The internal state of a [`Text`] widget.
 #[derive(Debug, Default)]
-pub struct State<T>(RefCell<T>);
+pub struct State<P: Paragraph>(P);
 
 impl<'a, Message, Renderer> Widget<Message, Renderer> for Text<'a, Renderer>
 where
@@ -132,7 +131,7 @@ where
     }
 
     fn state(&self) -> tree::State {
-        tree::State::new(State(RefCell::new(Renderer::Paragraph::default())))
+        tree::State::new(State(Renderer::Paragraph::default()))
     }
 
     fn width(&self) -> Length {
@@ -145,12 +144,12 @@ where
 
     fn layout(
         &self,
-        tree: &Tree,
+        tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
         layout(
-            tree.state.downcast_ref::<State<Renderer::Paragraph>>(),
+            tree.state.downcast_mut::<State<Renderer::Paragraph>>(),
             renderer,
             limits,
             self.width,
@@ -189,7 +188,7 @@ where
 
 /// Produces the [`layout::Node`] of a [`Text`] widget.
 pub fn layout<Renderer>(
-    state: &State<Renderer::Paragraph>,
+    state: &mut State<Renderer::Paragraph>,
     renderer: &Renderer,
     limits: &layout::Limits,
     width: Length,
@@ -211,10 +210,10 @@ where
     let size = size.unwrap_or_else(|| renderer.default_size());
     let font = font.unwrap_or_else(|| renderer.default_font());
 
-    let mut paragraph = state.0.borrow_mut();
+    let State(ref mut paragraph) = state;
 
     renderer.update_paragraph(
-        &mut paragraph,
+        paragraph,
         text::Text {
             content,
             bounds,
@@ -251,7 +250,7 @@ pub fn draw<Renderer>(
 ) where
     Renderer: text::Renderer,
 {
-    let paragraph = state.0.borrow();
+    let State(ref paragraph) = state;
     let bounds = layout.bounds();
 
     let x = match paragraph.horizontal_alignment() {