async function with default implementation won't compile

[rodrigorc]

I have a trait such as this:

#[trait_variant::make(Test: Send)]
trait LocalTest {
    async fn foo() -> i32 { 0 }
}

but if fails with an error:

  |
3 | #[trait_variant::make(Test: Send)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `{integer}` is not a future
  |

It looks like the body of the default impl is translated to the Test trait, unchanged.

If I instead I try:

    async fn foo() -> i32 { std::future::ready(0) }

Then it looks like the Test works, but LocalTest fails with a somewhat opposite error:

5 |     async fn foo() -> i32 { std::future::ready(0) }
  |                             ^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found `Ready<{integer}>`

I'm not sure if this is a limitation of this crate, if so I think it should be documented somewhere; or if it is an oversight in the implementation. Or maybe I'm missing something?

Then, just to see what would happen, I tried:

async fn foo() -> i32 { async move { 0 } }

And I got an ICE!

5 |     async fn foo() -> i32 { async move { 0 } }
  |                             ^^^^^^^^^^^^^^^^ expected `i32`, found `async` block
  |
  = note:       expected type `i32`
          found `async` block `{async block@src/main.rs:5:29: 5:45}`

error: internal compiler error: compiler/rustc_mir_build/src/build/mod.rs:655:72: impossible case reached
...