Generalized Into<AssetSourceId> and Into<AssetPath> Implementations over Lifetime (#10823)

# Objective

- Fixes #10478

## Solution

Generalised `From/Into` implementations over `&str` and `Option<&str>`
for `AssetSourceId` and `AssetPath` across all lifetimes, not just
static. To maintain access to the `'static`-only specialisation, these
types (and `CowArc`) now include an `as_static` method which will apply
the specialisation.

```rust
// Snipped from `AssetApp`
fn register_asset_source(
    &mut self,
    id: impl Into<AssetSourceId<'static>>,
    //                          ^^^^^^^
    //                          | as_static is only available for 'static lifetimes
    source: AssetSourceBuilder,
) -> &mut Self {
    let id = id.into().as_static();
    //          ^^^^^^ ^^^^^^^^^
    //          |      | Specialized (internally storing CowArc::Static)
    //          | Generic Into (internally storing CowArc::Borrowed)
    
    // ...
}
```

This post-fix specialisation is available here because the actual
specialisation performed is only a marker for if/when modification or
ownership is required, making the transform a very cheap operation. For
cleanliness, I've also added `from_static`, which wraps this behaviour
in a clean shorthand designed to replace `from` calls.

---

## Changelog

- Generalised the following implementations over a generic lifetime:
  - `From<&'static str> for AssetSourceId<'static>`
  - `From<Option<&'static str>> for AssetSourceId<'static>`
  - `From<&'static str> for AssetPath<'static>`
  - `From<&'static Path> for AssetPath<'static>`
- Added `as_static` specialisation to:
  - `CowArc`
  - `AssetSourceId`
  - `AssetPath`
- Added `from_static` specialised constructor to:
  - `AssetSourceId`
  - `AssetPath`

## Migration Guide

In areas where these implementations where being used, you can now add
`from_static` in order to get the original specialised implementation
which avoids creating an `Arc` internally.

```rust
// Before
let asset_path = AssetPath::from("my/path/to/an/asset.ext");

// After
let asset_path = AssetPath::from_static("my/path/to/an/asset.ext");
```

To be clear, this is only required if you wish to maintain the
performance benefit that came with the specialisation. Existing code is
_not_ broken by this change.

crates/bevy_asset/src/io/source.rs

@@ -70,9 +70,26 @@ impl<'a> AssetSourceId<'a> {
     }
 }
 
-impl From<&'static str> for AssetSourceId<'static> {
+impl AssetSourceId<'static> {
-    fn from(value: &'static str) -> Self {
+    /// Indicates this [`AssetSourceId`] should have a static lifetime.
-        AssetSourceId::Name(value.into())
+    #[inline]
+    pub fn as_static(self) -> Self {
+        match self {
+            Self::Default => Self::Default,
+            Self::Name(value) => Self::Name(value.as_static()),
+        }
+    }
+
+    /// Constructs an [`AssetSourceId`] with a static lifetime.
+    #[inline]
+    pub fn from_static(value: impl Into<Self>) -> Self {
+        value.into().as_static()
+    }
+}
+
+impl<'a> From<&'a str> for AssetSourceId<'a> {
+    fn from(value: &'a str) -> Self {
+        AssetSourceId::Name(CowArc::Borrowed(value))
     }
 }
 
@@ -82,10 +99,10 @@ impl<'a, 'b> From<&'a AssetSourceId<'b>> for AssetSourceId<'b> {
     }
 }
 
-impl From<Option<&'static str>> for AssetSourceId<'static> {
+impl<'a> From<Option<&'a str>> for AssetSourceId<'a> {
-    fn from(value: Option<&'static str>) -> Self {
+    fn from(value: Option<&'a str>) -> Self {
         match value {
-            Some(value) => AssetSourceId::Name(value.into()),
+            Some(value) => AssetSourceId::Name(CowArc::Borrowed(value)),
             None => AssetSourceId::Default,
         }
     }
@@ -302,7 +319,7 @@ pub struct AssetSourceBuilders {
 impl AssetSourceBuilders {
     /// Inserts a new builder with the given `id`
     pub fn insert(&mut self, id: impl Into<AssetSourceId<'static>>, source: AssetSourceBuilder) {
-        match id.into() {
+        match AssetSourceId::from_static(id) {
             AssetSourceId::Default => {
                 self.default = Some(source);
             }

crates/bevy_asset/src/lib.rs

@@ -341,7 +341,7 @@ impl AssetApp for App {
         id: impl Into<AssetSourceId<'static>>,
         source: AssetSourceBuilder,
     ) -> &mut Self {
-        let id = id.into();
+        let id = AssetSourceId::from_static(id);
         if self.world().get_resource::<AssetServer>().is_some() {
             error!("{} must be registered before `AssetPlugin` (typically added as part of `DefaultPlugins`)", id);
         }

crates/bevy_asset/src/path.rs

@@ -475,14 +475,43 @@ impl<'a> AssetPath<'a> {
     }
 }
 
-impl From<&'static str> for AssetPath<'static> {
+impl AssetPath<'static> {
+    /// Indicates this [`AssetPath`] should have a static lifetime.
     #[inline]
-    fn from(asset_path: &'static str) -> Self {
+    pub fn as_static(self) -> Self {
+        let Self {
+            source,
+            path,
+            label,
+        } = self;
+
+        let source = source.as_static();
+        let path = path.as_static();
+        let label = label.map(CowArc::as_static);
+
+        Self {
+            source,
+            path,
+            label,
+        }
+    }
+
+    /// Constructs an [`AssetPath`] with a static lifetime.
+    #[inline]
+    pub fn from_static(value: impl Into<Self>) -> Self {
+        value.into().as_static()
+    }
+}
+
+impl<'a> From<&'a str> for AssetPath<'a> {
+    #[inline]
+    fn from(asset_path: &'a str) -> Self {
         let (source, path, label) = Self::parse_internal(asset_path).unwrap();
+
         AssetPath {
             source: source.into(),
-            path: CowArc::Static(path),
+            path: CowArc::Borrowed(path),
-            label: label.map(CowArc::Static),
+            label: label.map(CowArc::Borrowed),
         }
     }
 }
@@ -501,12 +530,12 @@ impl From<String> for AssetPath<'static> {
     }
 }
 
-impl From<&'static Path> for AssetPath<'static> {
+impl<'a> From<&'a Path> for AssetPath<'a> {
     #[inline]
-    fn from(path: &'static Path) -> Self {
+    fn from(path: &'a Path) -> Self {
         Self {
             source: AssetSourceId::Default,
-            path: CowArc::Static(path),
+            path: CowArc::Borrowed(path),
             label: None,
         }
     }

crates/bevy_utils/src/cow_arc.rs

@@ -26,6 +26,18 @@ pub enum CowArc<'a, T: ?Sized + 'static> {
     Owned(Arc<T>),
 }
 
+impl<T: ?Sized> CowArc<'static, T> {
+    /// Indicates this [`CowArc`] should have a static lifetime.
+    /// This ensures if this was created with a value `Borrowed(&'static T)`, it is replaced with `Static(&'static T)`.
+    #[inline]
+    pub fn as_static(self) -> Self {
+        match self {
+            Self::Borrowed(value) | Self::Static(value) => Self::Static(value),
+            Self::Owned(value) => Self::Owned(value),
+        }
+    }
+}
+
 impl<'a, T: ?Sized> Deref for CowArc<'a, T> {
     type Target = T;