Casting to a trait doesn't auto-coerce to a &T type

[erickt]

Here's an example:

https://gist.github.com/3907509

It seems that even if I produce a proper boxed type @T, rust 0.4 cannot coerce that type to a &T.

---

Felix is putting the contents of the above linked gist so that people using ghi can see them readily:

test.rs:

trait T {
  fn print(&self);
}

struct S {
  s: int,
}

impl S: T {
  fn print(&self) {
    io::println(fmt!("%?", self));
  }
}

fn print_t(t: &T) {
  t.print();
}

fn print_s(s: &S) {
  s.print();
}

fn main() {
  let s: @S = @S { s: 5 };
  print_s(s);
  let t: @T = s as @T;
  print_t(t);
}

output:

test.rs:27:12: 27:13 error: mismatched types: expected `&/T` but found `@T` (trait storage differs: expected & but found @)
test.rs:27 print_t(t);
^
error: aborting due to previous error

[nikomatsakis]

Is this fixed?

[catamorphism]

Not yet (tested in d2ad028 )

[catamorphism]

I have a fix. Running tests.

[catamorphism]

Pending review: #4178

[nikomatsakis]

Not critical for 0.6; removing milestone

[catamorphism]

Nominating for milestone 5 (production-ready)

[metajack]

Still a problem, but here is the updated test case:

use std::io;

trait T {
  fn print(&self);
}

struct S {
  s: int,
}

impl T for S {
  fn print(&self) {
    io::println(fmt!("%?", self));
  }
}

fn print_t(t: &T) {
  t.print();
}

fn print_s(s: &S) {
  s.print();
}

fn main() {
  let s: @S = @S { s: 5 };
  print_s(s);
  let t: @T = s as @T;
  print_t(t);
}

which produces

issue3974.rs:29:10: 29:11 error: mismatched types: expected `&T<no-bounds>` but found `@T:'static` (trait storage differs: expected & but found @)
issue3974.rs:29   print_t(t);
                          ^

[graydon]

accepted for production-ready milestone