Prevent crash on integer overflow... by removing LALRPOP match

[roguh]

Replicate

Try to parse an overflowing integer (also works with any input that makes from_str fail).

nitro>>> 11111111111111111111111
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: Overflow }'...

Fix

The action code must be able to return a ParseError when i64::from_str(<>) fails.
This is possible using LALRPOP; however, it is impossible to return a ParseError::User with a useful error message AND use the match keyword. This is because "match and extern definitions are mutually exclusive," as reported by LALRPOP.

With match, but without extern and a custom ParseError::User type:

match {...}

pub Int: i64 = {
    INT =>? match i64::from_str(<>) {
        Ok(i) => Ok(i),

        // Don't panic on overflow or other parse error
        Err(e) => Err(ParseError::User { error: () })
      }
};

With extern, but without match:

extern {
  Error = useful ParseError::User type
}

pub Int: i64 = {
    INT =>? match i64::from_str(<>) {
        Ok(i) => Ok(i),

        // Don't panic on overflow or other parse error
        Err(e) => Err(ParseError::User { error: /*useful info (your integer's too big!)*/ })
      }
};

See how LALRPOP parses itself:
https://github.com/nikomatsakis/lalrpop/blob/f934229ed76f123a1d4c190edd4d05cf5cc4ff2d/lalrpop/src/p

Simple example of using extern { Error = /*custom parse error type*/ }
https://github.com/nikomatsakis/lalrpop/blob/master/lalrpop-test/src/error.lalrpop