`break` inside `gen` blocks misidentifies gen block as a closure

[dtolnay]

Code

#![feature(gen_blocks)]

fn main() {
    let _ = || break;
    let _ = async { break };
    let _ = gen { break };
}

Current output

error[E0267]: `break` inside of a closure
 --> src/main.rs:4:16
  |
4 |     let _ = || break;
  |             -- ^^^^^ cannot `break` inside of a closure
  |             |
  |             enclosing closure

error[E0267]: `break` inside of an `async` block
 --> src/main.rs:5:21
  |
5 |     let _ = async { break };
  |             --------^^^^^--
  |             |       |
  |             |       cannot `break` inside of an `async` block
  |             enclosing `async` block

error[E0267]: `break` inside of a closure
 --> src/main.rs:6:19
  |
6 |     let _ = gen { break };
  |             ------^^^^^--
  |             |     |
  |             |     cannot `break` inside of a closure
  |             enclosing closure

Desired output

...

error[E0267]: `break` inside of a `gen` block
 --> src/main.rs:6:19
  |
6 |     let _ = gen { break };
  |             ------^^^^^--
  |             |     |
  |             |     cannot `break` inside of a `gen` block
  |             enclosing `gen` block

Rationale and extra context

Same issue as #63391 which was for async blocks.

Rust Version

rustc 1.79.0-nightly (aed2187d5 2024-04-27)
binary: rustc
commit-hash: aed2187d53b8789e3a37f50ae36f894a2a679077
commit-date: 2024-04-27
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.4

[veera-sivarajan]

@rustbot claim