rustc runs out of memory when compiling an if statement with no block, enclosed in parentheses

[kuretchi]

I tried this code:

fn main() {
    (if foobar)
}

$ ulimit -v 2000000  # 2 GB
$ rustc main.rs

rustc first outputs some error messages and then hangs up trying to allocate a large amount of memory.

error: expected `{`, found `)`
 --> main.rs:2:15
  |
2 |     (if foobar)
  |      --       ^ expected `{`
  |      |
  |      this `if` statement has a condition, but no block

error: expected expression, found `<eof>`
 --> main.rs:3:1
  |
3 | }
  | ^ expected expression

memory allocation of 1610612736 bytes failedAborted (core dumped)

Playground

Meta

rustc --version --verbose:

rustc 1.35.0 (3c235d560 2019-05-20)
binary: rustc
commit-hash: 3c235d5600393dfe6c36eeed34042efad8d4f26e
commit-date: 2019-05-20
host: x86_64-unknown-linux-gnu
release: 1.35.0
LLVM version: 8.0

[tesuji]

Rust 1.34.0 and before is OK: https://godbolt.org/z/dFMxwR

[pnkfelix]

triage: P-medium. Removing nomination. Assigning to self and @estebank for initial investigation.

[dwrensha]

I'm seeing the same behavior on the following programs:

pub fn m(<k<kn(m(nka[ nka[ (]k(

and

fn z() { loop { w& if ( if (r( loop { w if ( if (r

These were found with the help of libfuzzer-sys.

[dwrensha]

Another program that I also observe to make rustc run out of memory:

fn a() -> i32 { let c c2( } fn b(x:[

[estebank]

The original report is caused due to Parser::expected_tokens growing unbounded in a bad parser code path, in part due to it being a Vec and not a FxHashSet (which it can't be at the moment). Now that I have a clue at what causes the behavior, I'll dig deeper for the trigger.