Try using closures for Container::style

`Box` should not allocate for zero-sized types; so
we should not be incurring much overhead. Just a
bit of indirection.

widget/src/container.rs

@@ -36,7 +36,7 @@ pub struct Container<
     vertical_alignment: alignment::Vertical,
     clip: bool,
     content: Element<'a, Message, Theme, Renderer>,
-    style: Style<Theme>,
+    style: Style<'a, Theme>,
 }
 
 impl<'a, Message, Theme, Renderer> Container<'a, Message, Theme, Renderer>
@@ -48,15 +48,15 @@ where
         content: impl Into<Element<'a, Message, Theme, Renderer>>,
     ) -> Self
     where
-        Theme: DefaultStyle,
+        Theme: DefaultStyle + 'a,
     {
-        Self::with_style(content, Theme::default_style())
+        Self::with_style(content, Theme::default_style)
     }
 
     /// Creates a [`Container`] with the given content and style.
     pub fn with_style(
         content: impl Into<Element<'a, Message, Theme, Renderer>>,
-        style: fn(&Theme, Status) -> Appearance,
+        style: impl Fn(&Theme, Status) -> Appearance + 'a,
     ) -> Self {
         let content = content.into();
         let size = content.as_widget().size_hint();
@@ -71,8 +71,8 @@ where
             horizontal_alignment: alignment::Horizontal::Left,
             vertical_alignment: alignment::Vertical::Top,
             clip: false,
+            style: Box::new(style),
             content,
-            style,
         }
     }
 
@@ -137,8 +137,11 @@ where
     }
 
     /// Sets the style of the [`Container`].
-    pub fn style(mut self, style: fn(&Theme, Status) -> Appearance) -> Self {
-        self.style = style;
+    pub fn style(
+        mut self,
+        style: impl Fn(&Theme, Status) -> Appearance + 'a,
+    ) -> Self {
+        self.style = Box::new(style);
         self
     }
 
@@ -546,41 +549,41 @@ pub enum Status {
 }
 
 /// The style of a [`Container`].
-pub type Style<Theme> = fn(&Theme, Status) -> Appearance;
+pub type Style<'a, Theme> = Box<dyn Fn(&Theme, Status) -> Appearance + 'a>;
 
 /// The default style of a [`Container`].
 pub trait DefaultStyle {
     /// Returns the default style of a [`Container`].
-    fn default_style() -> Style<Self>;
+    fn default_style(&self, status: Status) -> Appearance;
 }
 
 impl DefaultStyle for Theme {
-    fn default_style() -> Style<Self> {
-        transparent
+    fn default_style(&self, status: Status) -> Appearance {
+        transparent(self, status)
     }
 }
 
 impl DefaultStyle for Appearance {
-    fn default_style() -> Style<Self> {
-        |appearance, _status| *appearance
+    fn default_style(&self, _status: Status) -> Appearance {
+        *self
     }
 }
 
 impl DefaultStyle for Color {
-    fn default_style() -> Style<Self> {
-        |color, _status| Appearance::default().with_background(*color)
+    fn default_style(&self, _status: Status) -> Appearance {
+        Appearance::default().with_background(*self)
     }
 }
 
 impl DefaultStyle for Gradient {
-    fn default_style() -> Style<Self> {
-        |gradient, _status| Appearance::default().with_background(*gradient)
+    fn default_style(&self, _status: Status) -> Appearance {
+        Appearance::default().with_background(*self)
     }
 }
 
 impl DefaultStyle for gradient::Linear {
-    fn default_style() -> Style<Self> {
-        |gradient, _status| Appearance::default().with_background(*gradient)
+    fn default_style(&self, _status: Status) -> Appearance {
+        Appearance::default().with_background(*self)
     }
 }
 

widget/src/helpers.rs

@@ -58,7 +58,7 @@ pub fn container<'a, Message, Theme, Renderer>(
     content: impl Into<Element<'a, Message, Theme, Renderer>>,
 ) -> Container<'a, Message, Theme, Renderer>
 where
-    Theme: container::DefaultStyle,
+    Theme: container::DefaultStyle + 'a,
     Renderer: core::Renderer,
 {
     Container::new(content)
@@ -134,7 +134,7 @@ pub fn tooltip<'a, Message, Theme, Renderer>(
     position: tooltip::Position,
 ) -> crate::Tooltip<'a, Message, Theme, Renderer>
 where
-    Theme: container::DefaultStyle,
+    Theme: container::DefaultStyle + 'a,
     Renderer: core::text::Renderer,
 {
     Tooltip::new(content, tooltip, position)

widget/src/pane_grid/content.rs

@@ -24,7 +24,7 @@ pub struct Content<
 {
     title_bar: Option<TitleBar<'a, Message, Theme, Renderer>>,
     body: Element<'a, Message, Theme, Renderer>,
-    style: container::Style<Theme>,
+    style: container::Style<'a, Theme>,
 }
 
 impl<'a, Message, Theme, Renderer> Content<'a, Message, Theme, Renderer>
@@ -34,12 +34,12 @@ where
     /// Creates a new [`Content`] with the provided body.
     pub fn new(body: impl Into<Element<'a, Message, Theme, Renderer>>) -> Self
     where
-        Theme: container::DefaultStyle,
+        Theme: container::DefaultStyle + 'a,
     {
         Self {
             title_bar: None,
             body: body.into(),
-            style: Theme::default_style(),
+            style: Box::new(Theme::default_style),
         }
     }
 
@@ -55,9 +55,9 @@ where
     /// Sets the style of the [`Content`].
     pub fn style(
         mut self,
-        style: fn(&Theme, container::Status) -> container::Appearance,
+        style: impl Fn(&Theme, container::Status) -> container::Appearance + 'a,
     ) -> Self {
-        self.style = style.into();
+        self.style = Box::new(style);
         self
     }
 }
@@ -403,7 +403,7 @@ impl<'a, T, Message, Theme, Renderer> From<T>
     for Content<'a, Message, Theme, Renderer>
 where
     T: Into<Element<'a, Message, Theme, Renderer>>,
-    Theme: container::DefaultStyle,
+    Theme: container::DefaultStyle + 'a,
     Renderer: crate::core::Renderer,
 {
     fn from(element: T) -> Self {

widget/src/pane_grid/title_bar.rs

@@ -25,7 +25,7 @@ pub struct TitleBar<
     controls: Option<Element<'a, Message, Theme, Renderer>>,
     padding: Padding,
     always_show_controls: bool,
-    style: container::Style<Theme>,
+    style: container::Style<'a, Theme>,
 }
 
 impl<'a, Message, Theme, Renderer> TitleBar<'a, Message, Theme, Renderer>
@@ -37,14 +37,14 @@ where
         content: impl Into<Element<'a, Message, Theme, Renderer>>,
     ) -> Self
     where
-        Theme: container::DefaultStyle,
+        Theme: container::DefaultStyle + 'a,
     {
         Self {
             content: content.into(),
             controls: None,
             padding: Padding::ZERO,
             always_show_controls: false,
-            style: Theme::default_style(),
+            style: Box::new(Theme::default_style),
         }
     }
 
@@ -66,9 +66,9 @@ where
     /// Sets the style of the [`TitleBar`].
     pub fn style(
         mut self,
-        style: fn(&Theme, container::Status) -> container::Appearance,
+        style: impl Fn(&Theme, container::Status) -> container::Appearance + 'a,
     ) -> Self {
-        self.style = style.into();
+        self.style = Box::new(style);
         self
     }
 

widget/src/tooltip.rs

@@ -28,7 +28,7 @@ pub struct Tooltip<
     gap: f32,
     padding: f32,
     snap_within_viewport: bool,
-    style: container::Style<Theme>,
+    style: container::Style<'a, Theme>,
 }
 
 impl<'a, Message, Theme, Renderer> Tooltip<'a, Message, Theme, Renderer>
@@ -47,7 +47,7 @@ where
         position: Position,
     ) -> Self
     where
-        Theme: container::DefaultStyle,
+        Theme: container::DefaultStyle + 'a,
     {
         Tooltip {
             content: content.into(),
@@ -56,7 +56,7 @@ where
             gap: 0.0,
             padding: Self::DEFAULT_PADDING,
             snap_within_viewport: true,
-            style: Theme::default_style(),
+            style: Box::new(Theme::default_style),
         }
     }
 
@@ -81,9 +81,9 @@ where
     /// Sets the style of the [`Tooltip`].
     pub fn style(
         mut self,
-        style: fn(&Theme, container::Status) -> container::Appearance,
+        style: impl Fn(&Theme, container::Status) -> container::Appearance + 'a,
     ) -> Self {
-        self.style = style.into();
+        self.style = Box::new(style);
         self
     }
 }
@@ -239,7 +239,7 @@ where
                 positioning: self.position,
                 gap: self.gap,
                 padding: self.padding,
-                style: self.style,
+                style: &self.style,
             })))
         } else {
             None
@@ -309,7 +309,8 @@ where
     positioning: Position,
     gap: f32,
     padding: f32,
-    style: container::Style<Theme>,
+    style:
+        &'b (dyn Fn(&Theme, container::Status) -> container::Appearance + 'a),
 }
 
 impl<'a, 'b, Message, Theme, Renderer>