Explain editions when trying to use async block in 2015 edition

[estebank]

Trying to use an async block in 2015 edition doesn't mention the concept of editions nor pushes the user in the right direction:

error: expected one of `,` or `}`, found `(`
  --> file12.rs:11:18
   |
10 |     let fut = async {
   |               ----- while parsing this struct
11 |         make_unit()?;
   |                  ^ expected one of `,` or `}`

error[E0422]: cannot find struct, variant or union type `async` in this scope
  --> file12.rs:10:15
   |
10 |     let fut = async {
   |               ^^^^^ not found in this scope

[Centril]

Can you provide the full input used to produce ^--?

[csmoe]

fn foo() {
    let x = async {};
}

error[E0422]: cannot find struct, variant or union type `async` in this scope
 --> src/main.rs:3:12
  |
3 |    let x = async {}; 
  |            ^^^^^ not found in this scope

error: aborting due to previous error

For more information about this error, try `rustc --explain E0422`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

playground

[Centril]

That doesn't include:

error: expected one of , or }, found (

What does the parser error come from?

[estebank]

fn main() {}
fn bar() -> Result<(), ()> {
    panic!()
}

fn foo() -> Result<(), ()> {
   let x = async {
       bar()?;
   }; 
   Ok(())
}

[estebank]

For what is worth, I think the parser should use the snapshot approach here when in 2015 mode and try both parses, if one succeeds and the other one fails, give a targeted error.

[Centril]

I think we could just do this in resolve instead of the parser as it seems like the parser is able to recover and we reach resolve.

[estebank]

We can, but if we also handle it in the parser we can make the output be

error: `async` blocks are only available in 2018 edition
 --> src/main.rs:7:11
  |
7 |     let x = async {
  | ____________^
8 ||        bar()?;
9 ||    }; 
  ||____^
  |
  = note: edit your `Cargo.toml` file and add `edition = 2018` to the `[package]` section if you wish to use async/await in your crate
  = note: for more information, visit <APPROPRIATE BOOK SECTION>

error: aborting due the previous errors

The reason the parser recovers is because when it finds a parse error it consumes the rest of the block.