parser: provide better suggestions and errors on closures with braces missing

[ohno418]

We currently provide wrong suggestions and unhelpful errors on closure bodies with braces missing.

For example, given the following code:

fn main() {
    let _x = Box::new(|x|x+1;);
}

the current output is:

error: expected expression, found `)`
 --> ./main.rs:2:30
  |
2 |     let _x = Box::new(|x|x+1;);
  |                              ^ expected expression

error: closure bodies that contain statements must be surrounded by braces
 --> ./main.rs:2:25
  |
2 |     let _x = Box::new(|x|x+1;);
  |                         ^
3 | }
  | ^
  |
note: statement found outside of a block
 --> ./main.rs:2:29
  |
2 |     let _x = Box::new(|x|x+1;);
  |                          ---^ this `;` turns the preceding closure into a statement
  |                          |
  |                          this expression is a statement because of the trailing semicolon
note: the closure body may be incorrectly delimited
 --> ./main.rs:2:23
  |
2 |     let _x = Box::new(|x|x+1;);
  |                       ^^^^^^ this is the parsed closure...
3 | }
  | - ...but likely you meant the closure to end here
help: try adding braces
  |
2 ~     let _x = Box::new(|x| {x+1;);
3 ~ }}
  |

error: expected `;`, found `}`
 --> ./main.rs:2:32
  |
2 |     let _x = Box::new(|x|x+1;);
  |                                ^ help: add `;` here
3 | }
  | - unexpected token

error: aborting due to 3 previous errors

We got 3 errors, but all but the second are unnecessary or just wrong.

This commit allows outputting correct suggestions and errors. The above code would output like this:

error: closure bodies that contain statements must be surrounded by braces
 --> ./main.rs:2:25
  |
2 |     let _x = Box::new(|x|x+1;);
  |                         ^    ^
  |
note: statement found outside of a block
 --> ./main.rs:2:29
  |
2 |     let _x = Box::new(|x|x+1;);
  |                          ---^ this `;` turns the preceding closure into a statement
  |                          |
  |                          this expression is a statement because of the trailing semicolon
note: the closure body may be incorrectly delimited
 --> ./main.rs:2:23
  |
2 |     let _x = Box::new(|x|x+1;);
  |                       ^^^^^^ - ...but likely you meant the closure to end here
  |                       |
  |                       this is the parsed closure...
help: try adding braces
  |
2 |     let _x = Box::new(|x| {x+1;});
  |                           +    +

error: aborting due to previous error

Fixes #107959.

r? diagnostics

[ohno418]

Recovery within parse_stmt was doing something wrong. So changed it not to do.

[davidtwco]

@bors r+

[bors]

📌 Commit 0e42298 has been approved by davidtwco

It is now in the queue for this repository.