Why is My Nim Code Faster when Doing This

[Isofruit]

Very much seems like a compiler edge case. Here a benchy benchmark:
    
    
    import benchy
    proc reverse(s: string): string =
      result = s
      
      let size = s.len
      for (i, c) in s.pairs():
        result[(size - i - 1)] = c
    
    let s = "Hello world"
    timeIt "With Discard":
      discard reverse(s)
      
      for i in 1..200_000:
        let x = s.reverse()
        keep(x)
    
    timeIt "No Discard":
      for i in 1..200_000:
        let x = s.reverse()
        keep(x)
    
    
    Run
    
    
       min time    avg time  std dv   runs name
      44.975 ms   47.967 ms  ±1.599   x104 With Discard
      45.317 ms   48.030 ms  ±1.566   x104 No Discard
    
    
    Run