Generalized Into<AssetSourceId> and Into<AssetPath> Implementations over Lifetime

[bushrat011899]

Objective

• Fixes Restrictive AssetPath From impls cause hard-to-diagnose compile errors #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.

// 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.

// 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.

[alice-i-cecile]

Merging once merge conflicts are resolved :)