error message when creating a closure and directly calling not very clear

[gnzlbg]

Creating a closure and directly calling it:

fn main() { let _ = ||{}(); }

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0618]: expected function, found `()`
 --> src/main.rs:1:23
  |
1 | fn main() { let _ = ||{}(); }
  |                       ^^^^ not a function

error: aborting due to previous error
For more information about this error, try `rustc --explain E0618`.

Which doesn't really hint at what the problem is. The problem is that the closure must be surrounded by () - playground:

fn main() { let _ = (||{})(); }

[estebank]

To make matters more fun, the following is valid rust code:

fn main() { let _ = ||{||{}}(); }

---

It'll be hard to deal with this as it's the parser creating a certain AST that the type checker then chokes on without enough context. We might be able to special case this, but would only feel comfortable if it were a very small amount of code to deal with it.