Polymorphically creating traits barely works

[alexcrichton]

I was confused by the following code:

fn main() {}

trait A {}
impl<T: 'static> A for T {}

fn owned1<T: 'static>(a: T) { ~a as ~A; }  // failed to find an implementation of trait ~A:Send for T
fn owned2<T: 'static>(a: ~T) { a as ~A; }  // failed to find an implementation of trait ~A:Send for T
fn owned3<T: 'static>(a: ~T) { ~a as ~A; } // cannot pack type `~~T`, which does not fulfill `Send`, as a trait bounded by Send

fn managed1<T: 'static>(a: T) { @a as @A; }  // failed to find an implementation of trait @A:'static for T
fn managed2<T: 'static>(a: @T) { a as @A; }  // failed to find an implementation of trait @A:'static for T
fn managed3<T: 'static>(a: @T) { @a as @A; } // only one that works

The only function which compiles is managed3. I was surprised that none of the others worked at all. For managed3, you're required to put a box in a box to get the trait out, while for all the others you just can't create the trait cast at all.

A very surprising result for me was that owned3 didn't work, even though its managed equivalent did work. The error message was different from owned1/2, but I still wasn't really able to decipher any of the errors or understand why they exist.

Are these all legitimate errors? Are some of these supposed to work but they aren't? If these are legitimate errors, I'm of the opinion that the error messages should be a bit more descriptive

[alexcrichton]

Nominating for the backwards-compatible milestone because it's blocking removing @ from TLS types (which is itself a backwards-compatible change)

[nikomatsakis]

owned1 and owned2 I don't understand and merit more investigation, same with managed1 and managed2. Odd.

owned3 is legit, the problem is that ~A is short for ~A:Send (that is, a ~Trait by default assumes sendable values). You probably want ~A: to skip the default.

[alexcrichton]

These all work now, flagging as needstest

[alexcrichton]

Note that to work the casts must be ~A:, not ~A