Only render subject description on failure

Use a lambda to lazily render the subject description only when the test fails. This significantly speeds up the test execution. In particular, this can really help property based tests where thousands of tests may be executed.

Constructing an exception is relatively fast and it captures the stack trace. Converting the stack trace from the internal representation to Java StackTraceElements is very costly (i.e. calling ex.getStackTrace). I had to recreate getCallerInfo from FilePeek to accept an exception rather than creating one internally. I submitted a PR to filepeek that adds a getCallerInfo that accepts an exception.

This super contrived test goes from 10.6 seconds to 1.1s (on my machine).

@Test
fun `get perf`() {
  class Person(val id: Int)
  val range = Int.MIN_VALUE..Int.MAX_VALUE

  repeat(10_000) {
    expectThat(Person(Random.nextInt())) {
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
      get { id } isIn range
    }
  }
}

robfletcher / strikt

Only render subject description on failure #292