let-chains parsing is not resilient to stray `{` between `let`s

[estebank]

Code

#![feature(let_chains)]
fn main() {
    if let () = ()
        && let () = () {
        && let () = ()
    {
    }
}

Current output

error: this file contains an unclosed delimiter
 --> src/main.rs:8:2
  |
2 | fn main() {
  |           - unclosed delimiter
3 |     if let () = ()
4 |         && let () = () {
  |                        - this delimiter might not be properly closed...
...
8 | }
  | -^
  | |
  | ...as it matches this but it has different indentation

Desired output

error: this file contains an unclosed delimiter
 --> src/main.rs:8:2
  |
2 | fn main() {
  |           - unclosed delimiter
3 |     if let () = ()
4 |         && let () = () {
  |                        - this delimiter might not be properly closed...
5 |         && let () = ()
  |         -------------- you likely meant to continue the let-chain
...
8 | }
  | -^
  | |
  | ...as it matches this but it has different indentation
help: remove the spurious block start
  |
4 -         && let () = () {
4 +         && let () = ()
  |

Rationale and extra context

Unmatched delimiters are hard to deal with by taking consideration of surrounding code because they actually occur during lexing. Still, it'd be nice if we detected some common sources of issues like this one.

Other cases

No response

Anything else?

No response

[sjwang05]

@rustbot claim