Slower than serde_json in --release, much slower than serde_json in --debug

[woody77]

I found that json5 was taking many minutes to parse a single 180KB json file, which caused me to write up a benchmark-ish app which deserializes the following struct:

#[derive(Deserialize)]
struct MyStruct {
    items: Vec<String>,
}

with the items being 100-char strings, varying in number from 10 to 1,000,000, using both serde_json and json5.

With cargo run --release, I get the following results:

Final results (MiB/second):
        size    serde_json      json5 difference
  ==========    ========== ========== ==========
          10 =>    116.835      7.806     14.967x
         100 =>    499.105     13.894     35.923x
        1000 =>    520.992     12.013     43.367x
       10000 =>    472.506     11.575     40.822x
      100000 =>    490.463     12.185     40.251x
     1000000 =>    408.434     11.058     36.935x

But when running in --debug, it's much worse:

Final results (MiB/second):
        size    serde_json      json5 difference
  ==========    ========== ========== ==========
          10 =>     20.340      0.355     57.347x
         100 =>     57.123      0.340    167.898x
        1000 =>     68.129      0.343    198.836x
       10000 =>     63.127      0.368    171.572x
      100000 =>     67.684      0.371    182.643x
      500000 =>     67.894      0.371    183.127x

And with more complicated structures like the following, the disparity in --debug is much, much greater (such as an hour vs. seconds to parse the same file):

struct MyStruct {
  items_a: Vec<String>,
  items_b: Vec<ItemB>,
  field_c: String,
  field_d: String,
  field_e: String,
}

struct ItemB {
  field_1: String,
  field_2: u64,
  field_3: u64
}

times are from a 2015 MacBook Pro, running rustc 1.52.1 (9bc8c42bb 2021-05-09)