destructor failing should not lead to leaks

[alaviss]

Example

var counter = 0

type
  Lease = object
    initialized: bool

proc `=destroy`(v: var Lease) =
  if v.initialized:
    dec counter

proc lease(): Lease =
  inc counter
  Lease(initialized: true)

type
  Fail = object
    initialized: bool

  FailError = object of CatchableError

proc `=destroy`(v: var Fail) =
  if v.initialized:
    raise newException(FailError, "operation failed")

proc initFail(): Fail =
  Fail(initialized: true)

proc leaky() =
  let a = lease()
  let b = initFail()

proc main() =
  doAssert counter == 0, "counter is not zero at start"

  try:
    leaky()
  except FailError:
    discard "not an issue"

  doAssert counter == 0, "the lease should be released"

main()

Actual Output

fatal.nim(48) sysFatal
Error: unhandled exception: test.nim(40, 12) `counter == 0` the lease should be released [AssertionDefect]

Expected Output

No output

Possible Solution

• Force Defect to be the only allowed exception type raised by destructors
• Generates code to continue cleanup even after exception, then raises the exception stack afterwards

[zerbina]

With #1236, hooks can no longer raise exceptions of any kind, so it's not possible for leaks to happen anymore.