Improving mapping of generic types #316
Description
Currently, we're mapping generic types roughly as:
extern_class!(
#[derive(PartialEq, Eq, Hash)]
pub struct NSArray<T: Message = Object, O: Ownership = Shared> {
p: PhantomData<Id<T, O>>,
}
unsafe impl<T: Message, O: Ownership> ClassType for NSArray<T, O> {
type Super = NSObject;
}
);
impl<T: Message, O: Ownership> NSArray<T, O> {
fn get(&self, index: usize) -> Option<&T>;
}
impl<T: Message> NSArray<T, Owned> {
fn get_mut(&mut self, index: usize) -> Option<&mut T>;
}But this can be a bit limiting, since there's no easy way to carry any lifetime information in the type. Also, it is IMO a bit confusing to see Id<NSArray<NSObject, Shared>, Shared>, and this only gets worse on types with multiple generics (like NSDictionary).
So perhaps we could instead introduce a helper trait and do it like this:
trait Foo: Sealed {
type Item: Message;
type Ownership: Ownership;
}
impl<T: Message> Foo for &T {
type Item = T;
type Ownership = Shared;
}
impl<T: Message> Foo for &mut T {
type Item = T;
type Ownership = Owned;
}
impl<T: Message, O: Ownership> Foo for Id<T, Owned> {
type Item = T;
type Ownership = O;
}
// Usage
extern_class!(
#[derive(PartialEq, Eq, Hash)]
pub struct NSArray<T: Foo> {
p: PhantomData<T>,
}
unsafe impl<T: Foo> ClassType for NSArray<T> {
type Super = NSObject;
}
);
impl<T: Foo> NSArray<T> {
fn get(&self, index: usize) -> Option<&T>;
}
impl<T: Foo<Owneship = Owned>> NSArray<T> {
fn get_mut(&mut self, index: usize) -> Option<&mut T>;
}Where usage would then be Id<NSArray<&NSObject>, Shared> or Id<NSArray<Id<NSObject, Shared>>, Shared>.
This will also help with #304, since -[NSEnumerator nextObject] by itself could now be safe by default (though -[NSArray objectEnumerator] would still be unsafe).