assertSoftly(foo) {
    shouldNotEndWith("b")
    length shouldBe 3
}
->
foo shouldBe customAssert

fun containFoo() = object : Matcher<String> {
  override fun test(value: String) = MatcherResult(value.contains("foo"), "String $value should include foo", "String $value should not include foo")
}

fun customAssert() = Matcher<String> { foo ->
   val error = try {
      assertSoftly(foo) {
         shouldNotEndWith("b")
         length shouldBe 3
      }
      null
   } catch (e: Throwable) {
      e
   }
   MatcherResult(error == null, "should not have failed", "should have failed")
}