Add StyleEdition enum and StyleEditionDefault trait

**Note** This does not add the `style_edition` config option to rustfmt.
The `StyleEdition` enum will eventually be used to allow users to
configure the `style_edition`, but for now it's added so we can
introduce the the `StyleEditionDefault` trait.

src/config/mod.rs

@@ -27,6 +27,7 @@ pub(crate) mod file_lines;
 #[allow(unreachable_pub)]
 pub(crate) mod lists;
 pub(crate) mod macro_names;
+pub(crate) mod style_edition;
 
 // This macro defines configuration options used in rustfmt. Each option
 // is defined as follows:

src/config/options.rs

@@ -468,3 +468,27 @@ pub enum MatchArmLeadingPipe {
     /// Preserve any existing leading pipes
     Preserve,
 }
+
+/// Defines the default values for each config according to [the style guide].
+/// rustfmt output may differ between style editions.
+///
+/// [the style guide]: https://doc.rust-lang.org/nightly/style-guide/
+#[config_type]
+pub enum StyleEdition {
+    #[value = "2015"]
+    #[doc_hint = "2015"]
+    /// [Edition 2015]()
+    Edition2015,
+    #[value = "2018"]
+    #[doc_hint = "2018"]
+    /// [Edition 2018]()
+    Edition2018,
+    #[value = "2021"]
+    #[doc_hint = "2021"]
+    /// [Edition 2021]()
+    Edition2021,
+    #[value = "2024"]
+    #[doc_hint = "2024"]
+    /// [Edition 2024]().
+    Edition2024,
+}

src/config/style_edition.rs

@@ -0,0 +1,69 @@
+use crate::config::StyleEdition;
+
+/// Defines the default value for the given style edition
+pub(crate) trait StyleEditionDefault {
+    type ConfigType;
+    fn style_edition_default(style_edition: StyleEdition) -> Self::ConfigType;
+}
+
+/// macro to help implement `StyleEditionDefault` for config options
+#[macro_export]
+macro_rules! style_edition_default {
+    ($ty:ident, $config_ty:ty, _ => $default:expr) => {
+        impl $crate::config::style_edition::StyleEditionDefault for $ty {
+            type ConfigType = $config_ty;
+
+            fn style_edition_default(_: $crate::config::StyleEdition) -> Self::ConfigType {
+                $default
+            }
+        }
+    };
+    ($ty:ident, $config_ty:ty, Edition2024 => $default_2024:expr, _ => $default_2015:expr) => {
+        impl $crate::config::style_edition::StyleEditionDefault for $ty {
+            type ConfigType = $config_ty;
+
+            fn style_edition_default(
+                style_edition: $crate::config::StyleEdition,
+            ) -> Self::ConfigType {
+                match style_edition {
+                    $crate::config::StyleEdition::Edition2015
+                    | $crate::config::StyleEdition::Edition2018
+                    | $crate::config::StyleEdition::Edition2021 => $default_2015,
+                    $crate::config::StyleEdition::Edition2024 => $default_2024,
+                }
+            }
+        }
+    };
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::config::StyleEdition;
+
+    #[test]
+    fn test_impl_default_style_edition_struct_for_all_editions() {
+        struct Unit;
+        style_edition_default!(Unit, usize, _ => 100);
+
+        // regardless of the style edition used the value will always return 100
+        assert_eq!(Unit::style_edition_default(StyleEdition::Edition2015), 100);
+        assert_eq!(Unit::style_edition_default(StyleEdition::Edition2018), 100);
+        assert_eq!(Unit::style_edition_default(StyleEdition::Edition2021), 100);
+        assert_eq!(Unit::style_edition_default(StyleEdition::Edition2024), 100);
+    }
+
+    #[test]
+    fn test_impl_default_style_edition_for_old_and_new_editions() {
+        struct Unit;
+        style_edition_default!(Unit, usize, Edition2024 => 50, _ => 100);
+
+        // style edition 2015-2021 are all the same
+        assert_eq!(Unit::style_edition_default(StyleEdition::Edition2015), 100);
+        assert_eq!(Unit::style_edition_default(StyleEdition::Edition2018), 100);
+        assert_eq!(Unit::style_edition_default(StyleEdition::Edition2021), 100);
+
+        // style edition 2024
+        assert_eq!(Unit::style_edition_default(StyleEdition::Edition2024), 50);
+    }
+}