Direct cast to a polymorphic trait type is not supported directly

[ahmadsalim]

It seems that one is not able to cast to a polymorphic type directly e.g.

extern mod std;
fn main () {
  let table : std::map::HashMap<int, int> = std::map::HashMap::<int, int>();
  (table as std::map::Map<int,int>).insert(123, 1233);
  io::println(table.get(123).to_str());
}

does not work, but

extern mod std;
fn main () {
  let table : std::map::HashMap<int, int> = std::map::HashMap::<int, int>();
  (table as @std::map::Map<int,int>).insert(123, 1233);
  io::println(table.get(123).to_str());
}

does.

It might be a documentation bug if one is only allowed to cast to a trait pointer type (as it is shown in the tutorial that one can cast to Drawable trait directly), or it might be an implementation bug regarding traits with type parameters.

[brson]

This is more-or-less working as intended, although there are further changes coming to trait types. Previously, traits as types were implicitly boxed, but not written with @. The new, mostly unimplemented design, is that you can only cast between pointer types, so @table as @Map, ~table as ~Map, &table as &Map.

The current functionality is still very limited, and not often used.

[brson]

Oh, it isn't possible to directly cast to a value-type trait because they don't have a fixed size (similar to how it isn't possible to use a plain str type without a pointer).

[ahmadsalim]

Hmm, I still believe that section "15.6 Casting to an interface type" on the tutorial should be revised to reflect this (in the future, when it is working anyways).

[brson]

Here's the tutorial text for 0.4: https://github.com/mozilla/rust/blob/incoming/doc/tutorial.md#casting-to-a-trait-type-and-dynamic-method-dispatch