refactor: move Classes into its own file

`widget_style.rs` becomes `widget_style/mod.rs`, with `Classes`,
`HasClasses`, `ClassName` and the class constants in `widget_style/classes.rs`.
All public paths are unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/egui/src/widget_style/classes.rs

@@ -0,0 +1,109 @@
+use std::{borrow::Cow, fmt};
+
+use smallvec::SmallVec;
+
+use crate::TextBuffer as _;
+
+/// The root class is a special class present on every top-level [`crate::Ui`].
+pub const ROOT_CLASS: &str = "root";
+
+/// The selected class is a special class present on selected [`crate::Button`].
+pub const SELECTED_CLASS: &str = "selected";
+
+/// A class is a static string identifier.
+pub type ClassName = Cow<'static, str>;
+
+/// Classes are string identifier that can be set on widget/Ui.
+///
+/// This can be used by styling engine to compute a different style
+/// based on the set of classes present on the widget/Ui.
+#[derive(Debug, Default, Clone, Hash)]
+pub struct Classes {
+    classes: SmallVec<[ClassName; 5]>,
+}
+
+impl Classes {
+    /// Add a class to the list if the condition is true
+    #[inline]
+    fn add_if(&mut self, class: impl Into<ClassName>, condition: bool) {
+        if condition {
+            self.classes.push(class.into());
+        }
+    }
+}
+
+impl HasClasses for Classes {
+    fn classes(&self) -> &Classes {
+        self
+    }
+
+    fn classes_mut(&mut self) -> &mut Classes {
+        self
+    }
+}
+
+impl std::fmt::Display for Classes {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.classes.iter().for_each(|class| {
+            let _ = f.write_str(class.as_str());
+        });
+        f.write_str("")
+    }
+}
+
+/// Any widgets supporting [`Classes`] must implement this trait
+pub trait HasClasses {
+    fn classes(&self) -> &Classes;
+
+    fn classes_mut(&mut self) -> &mut Classes;
+
+    /// Add the given class by consuming [`self`]
+    #[inline]
+    fn with_class(mut self, class: impl Into<ClassName>) -> Self
+    where
+        Self: Sized,
+    {
+        self.classes_mut().add_if(class.into(), true);
+        self
+    }
+
+    /// Add the given class by consuming [`self`] if the condition is true
+    #[inline]
+    fn with_class_if(mut self, class: impl Into<ClassName>, condition: bool) -> Self
+    where
+        Self: Sized,
+    {
+        self.classes_mut().add_if(class.into(), condition);
+        self
+    }
+
+    /// Add the given class in-place
+    #[inline]
+    fn add_class(&mut self, class: impl Into<ClassName>) -> &mut Self
+    where
+        Self: Sized,
+    {
+        self.classes_mut().add_if(class.into(), true);
+        self
+    }
+
+    /// Add the given class in-place if the condition is true
+    #[inline]
+    fn add_class_if(&mut self, class: impl Into<ClassName>, condition: bool) -> &mut Self
+    where
+        Self: Sized,
+    {
+        self.classes_mut().add_if(class.into(), condition);
+        self
+    }
+
+    /// True if the class is present
+    fn has(&self, class: impl Into<ClassName>) -> bool {
+        self.classes().classes.contains(&class.into())
+    }
+
+    /// The list of class
+    fn as_slice(&self) -> &[ClassName] {
+        &self.classes().classes
+    }
+}

crates/egui/src/widget_style/mod.rs

@@ -1,13 +1,13 @@
-use std::{
-    borrow::Cow,
-    fmt::{self, Debug},
-};
+mod classes;
+
+pub use self::classes::{ClassName, Classes, HasClasses, ROOT_CLASS, SELECTED_CLASS};
+
+use std::fmt::Debug;
 
 use epaint::{Color32, FontId, Stroke, text::TextWrapMode};
-use smallvec::SmallVec;
 
 use crate::{
-    Context, Frame, Response, Style, TextBuffer as _, UiStack,
+    Context, Frame, Response, Style, UiStack,
     style::{WidgetVisuals, Widgets},
 };
 
@@ -136,110 +136,6 @@ impl Response {
     }
 }
 
-/// The root class is a special class present on every top-level [`crate::Ui`].
-pub const ROOT_CLASS: &str = "root";
-
-/// The selected class is a special class present on selected [`crate::Button`].
-pub const SELECTED_CLASS: &str = "selected";
-
-/// A class is a static string identifier.
-pub type ClassName = Cow<'static, str>;
-
-/// Classes are string identifier that can be set on widget/Ui.
-///
-/// This can be used by styling engine to compute a different style
-/// based on the set of classes present on the widget/Ui.
-#[derive(Debug, Default, Clone, Hash)]
-pub struct Classes {
-    classes: SmallVec<[ClassName; 5]>,
-}
-
-impl Classes {
-    /// Add a class to the list if the condition is true
-    #[inline]
-    fn add_if(&mut self, class: impl Into<ClassName>, condition: bool) {
-        if condition {
-            self.classes.push(class.into());
-        }
-    }
-}
-
-impl HasClasses for Classes {
-    fn classes(&self) -> &Classes {
-        self
-    }
-
-    fn classes_mut(&mut self) -> &mut Classes {
-        self
-    }
-}
-
-impl std::fmt::Display for Classes {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.classes.iter().for_each(|class| {
-            let _ = f.write_str(class.as_str());
-        });
-        f.write_str("")
-    }
-}
-
-/// Any widgets supporting [`Classes`] must implement this trait
-pub trait HasClasses {
-    fn classes(&self) -> &Classes;
-
-    fn classes_mut(&mut self) -> &mut Classes;
-
-    /// Add the given class by consuming [`self`]
-    #[inline]
-    fn with_class(mut self, class: impl Into<ClassName>) -> Self
-    where
-        Self: Sized,
-    {
-        self.classes_mut().add_if(class.into(), true);
-        self
-    }
-
-    /// Add the given class by consuming [`self`] if the condition is true
-    #[inline]
-    fn with_class_if(mut self, class: impl Into<ClassName>, condition: bool) -> Self
-    where
-        Self: Sized,
-    {
-        self.classes_mut().add_if(class.into(), condition);
-        self
-    }
-
-    /// Add the given class in-place
-    #[inline]
-    fn add_class(&mut self, class: impl Into<ClassName>) -> &mut Self
-    where
-        Self: Sized,
-    {
-        self.classes_mut().add_if(class.into(), true);
-        self
-    }
-
-    /// Add the given class in-place if the condition is true
-    #[inline]
-    fn add_class_if(&mut self, class: impl Into<ClassName>, condition: bool) -> &mut Self
-    where
-        Self: Sized,
-    {
-        self.classes_mut().add_if(class.into(), condition);
-        self
-    }
-
-    /// True if the class is present
-    fn has(&self, class: impl Into<ClassName>) -> bool {
-        self.classes().classes.contains(&class.into())
-    }
-
-    /// The list of class
-    fn as_slice(&self) -> &[ClassName] {
-        &self.classes().classes
-    }
-}
-
 pub struct StyleArgs<'a> {
     pub classes: &'a Classes,
     pub state: WidgetState,