Compiler hangs and eats memory on malformed macro extension

[caryoscelus]

macro_rules! failed {
    () => {{
        let x = 5 "";
    }}
}

fn main() {
    failed!();
//    let x = 5 ""; //fails normally
}

rustc successfully detects error, but then never returns and starts eating memory

error: expected one of `.`, `;`, `?`, or an operator, found `""`
 --> fail.rs:3:19
  |
3 |         let x = 5 "";
  |                   ^^

Version info:

rustc 1.12.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.12.0

I'm using version from Debian (unstable) repository.

[TimNN]

Likely related to #37113 and #33136.

[Mark-Simulacrum]

Closing.

error: expected one of `.`, `;`, `?`, or an operator, found `""`
 --> test.rs:3:19
  |
3 |         let x = 5 "";
  |                  -^^ unexpected token
  |                  |
  |                  expected one of `.`, `;`, `?`, or an operator here

error: macro expansion ignores token `}` and any following
 --> test.rs:4:5
  |
4 |     }}
  |     ^
  |
note: caused by the macro expansion here; the usage of `failed!` is likely invalid in statement context
 --> test.rs:8:5
  |
8 |     failed!();
  |     ^^^^^^^^^^

error: aborting due to 2 previous errors