Function apple_usage 

pub fn apple_usage(capabilities: &Capabilities<'_>) -> String

The Apple usage descriptions the declaration becomes, as a property list to merge into an Info.plist.

It is a property list of its own, so an Apple build merges it with one line and no tool beyond the ones macOS ships:

/usr/libexec/PlistBuddy -c "Merge target/cranpose/my-app-usage.plist" MyApp.app/Info.plist