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​​dyn trait coherence design meeting

​​Background gist

• ​​https://gist.github.com/nikomatsakis/0cf84ac05ce7751b5759cbf335c4d327 

​​

​​trait Object<U> {

​​    type Output;

​​}

​​


​​impl<T: ?Sized, U> Object<U> for T {

​​// ^-- Here is the blanket impl; relies on `?Sized`.

​​    type Output = U;

​​}

​​


​​fn foo<T: ?Sized, U>(x: <T as Object<U>>::Output) -> U {

​​    x

​​}

​​


​​fn transmute<T, U>(x: T) -> U {

​​    foo::<dyn Object<U, Output = T>, U>(x)

​​}

​​Notes from meeting

• ​​https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=fe9210df7b9326d9dae8ad8a3be6eb56

​​

​​why have a vtable impl?

​​

​​// trait MyWrite: Write

​​impl<T: Write + ?Sized> MyWrite for T {

​​  fn my_write(&self) { self.write(); }

​​}

​​


​​fn foo<T: ?Sized + Any>(x: &T) {

​​    let foo = x.type_id();

​​}

​​

• ​​observation on Any​ trait

• ​​if we made it default​, that would allow other crates to specialize Any​, which we don’t want

• ​​why does dyn Any: Any​?

• ​​why not have virtual calls in MIR, in particular

​​

​​// Upstream crate

​​impl<A> Object for A { 

​​  default type Output = ();

​​}

​​


​​fn foo<T>() {

​​  let y: T::Output = ..;

​​  // before specialization: T::Output = ()

​​  // after specialization: T::Output kept as a placeholder

​​}

​​