Add Extensions::get_or_insert[_with]() methods (#3561)

* add get_or_insert and get_or_insert_with for Extensions

* add docs

* fix doctest

* docs: update changelog

* chore: simplify get_or_insert

---------

Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-http/CHANGES.md

@@ -5,6 +5,7 @@
 ### Added
 
 - Add `header::CLEAR_SITE_DATA` constant.
+- Add `Extensions::get_or_insert[_with]()` methods.
 
 ### Changed
 

actix-http/src/extensions.rs

@@ -104,6 +104,46 @@ impl Extensions {
             .and_then(|boxed| boxed.downcast_mut())
     }
 
+    /// Inserts the given `value` into the extensions if it is not present, then returns a reference
+    /// to the value in the extensions.
+    ///
+    /// ```
+    /// # use actix_http::Extensions;
+    /// let mut map = Extensions::new();
+    /// assert_eq!(map.get::<Vec<u32>>(), None);
+    ///
+    /// map.get_or_insert(Vec::<u32>::new()).push(1);
+    /// assert_eq!(map.get::<Vec<u32>>(), Some(&vec![1]));
+    ///
+    /// map.get_or_insert(Vec::<u32>::new()).push(2);
+    /// assert_eq!(map.get::<Vec<u32>>(), Some(&vec![1,2]));
+    /// ```
+    pub fn get_or_insert<T: 'static>(&mut self, value: T) -> &mut T {
+        self.get_or_insert_with(|| value)
+    }
+
+    /// Inserts a value computed from `f` into the extensions if the given `value` is not present,
+    /// then returns a reference to the value in the extensions.
+    ///
+    /// ```
+    /// # use actix_http::Extensions;
+    /// let mut map = Extensions::new();
+    /// assert_eq!(map.get::<Vec<u32>>(), None);
+    ///
+    /// map.get_or_insert_with(Vec::<u32>::new).push(1);
+    /// assert_eq!(map.get::<Vec<u32>>(), Some(&vec![1]));
+    ///
+    /// map.get_or_insert_with(Vec::<u32>::new).push(2);
+    /// assert_eq!(map.get::<Vec<u32>>(), Some(&vec![1,2]));
+    /// ```
+    pub fn get_or_insert_with<T: 'static, F: FnOnce() -> T>(&mut self, default: F) -> &mut T {
+        self.map
+            .entry(TypeId::of::<T>())
+            .or_insert_with(|| Box::new(default()))
+            .downcast_mut()
+            .expect("extensions map should now contain a T value")
+    }
+
     /// Remove an item from the map of a given type.
     ///
     /// If an item of this type was already stored, it will be returned.