Make FrameCache::get return a reference instead of cloning the cached value (#7834)

This is a breaking change.

- Enables using `FrameCache` in cases where the cached value cannot be
cloned.
- Improves use cases where only a reference to the cached value is
needed.
- If the user needs an owned value, they can clone it themselves.

Adding a `get_ref` method instead of changing `get` would avoid the
breaking change, but I didn't want to do so because it is kind of
expected for `get` to return `&V` when querying a collection.

crates/egui/src/cache/cache_storage.rs

@@ -19,7 +19,7 @@ use super::CacheTrait;
 ///
 /// # let mut cache_storage = CacheStorage::default();
 /// let mut cache = cache_storage.cache::<CharCountCache<'_>>();
-/// assert_eq!(cache.get("hello"), 5);
+/// assert_eq!(*cache.get("hello"), 5);
 /// ```
 #[derive(Default)]
 pub struct CacheStorage {

crates/egui/src/cache/frame_cache.rs

@@ -46,10 +46,9 @@ impl<Value, Computer> FrameCache<Value, Computer> {
 impl<Value, Computer> FrameCache<Value, Computer> {
     /// Get from cache (if the same key was used last frame)
     /// or recompute and store in the cache.
-    pub fn get<Key>(&mut self, key: Key) -> Value
+    pub fn get<Key>(&mut self, key: Key) -> &Value
     where
         Key: Copy + std::hash::Hash,
-        Value: Clone,
         Computer: ComputerMut<Key, Value>,
     {
         let hash = crate::util::hash(key);
@@ -58,12 +57,12 @@ impl<Value, Computer> FrameCache<Value, Computer> {
             std::collections::hash_map::Entry::Occupied(entry) => {
                 let cached = entry.into_mut();
                 cached.0 = self.generation;
-                cached.1.clone()
+                &cached.1
             }
             std::collections::hash_map::Entry::Vacant(entry) => {
                 let value = self.computer.compute(key);
-                entry.insert((self.generation, value.clone()));
+                let inserted = entry.insert((self.generation, value));
-                value
+                &inserted.1
             }
         }
     }

crates/egui/src/memory/mod.rs

@@ -68,7 +68,7 @@ pub struct Memory {
     /// # let mut ctx = egui::Context::default();
     /// ctx.memory_mut(|mem| {
     ///     let cache = mem.caches.cache::<CharCountCache<'_>>();
-    ///     assert_eq!(cache.get("hello"), 5);
+    ///     assert_eq!(*cache.get("hello"), 5);
     /// });
     /// ```
     #[cfg_attr(feature = "persistence", serde(skip))]

crates/egui_extras/src/syntax_highlighting.rs

@@ -116,6 +116,7 @@ fn highlight_inner(
         mem.caches
             .cache::<HighlightCache>()
             .get((&font_id, theme, code, language, settings))
+            .clone()
     })
 }