result type inference for defer, to help with a very useful debugging / contracts idiom

[timotheecour]

the following is a very useful idiom for debugging, where adding defer: echo("result:", result) at top of body of a function allows printing its result, regardless of whether the function can return in many places:

• use case1: debugging

proc foo(a:Arg): auto=
  defer: echo("result:", result)
  # restOfCode which may return in multiple places

• use case1: contracts

proc foo(a:Arg): auto=
  defer: doAssert result.isSomeProperty
  # restOfCode which may return in multiple places

it's yet another advantage of defer over try/finally as all it does is add a single line of code instead of having to wrap lots of code into a try/finally (and then reverting back when we're done debugging).
However it has 1 limitation: it requires either an explicit type (see case3) or an explicit dummy initialization before defer (see case4); otherwise it gives Error: type mismatch: got <untyped>

Could that limitation be lifted? since the defer unwraps to try/finally, it would seem like it could, and be smarter about type inference in this case

when defined(case2):
  proc foo(a:int):auto=
    # Error: type mismatch: got <untyped>
    defer: echo("result:", result)
    if a==0:
      return 42
    if a==1:
      result=123
    elif a==2:
      return 101

  echo foo(1)

when defined(case3):
  # explicit return type: ok
  proc foo(a:int):int=
    defer: echo("result:", result)
    if a==0:
      return 42
    if a==1:
      result=123
    elif a==2:
      return 101

  let a = foo(1)

when defined(case4):
  # initialization of result before defer: ok
  proc foo(a:int):auto=
    if false: result = 0
    defer: echo("result:", result)
    if a==0:
      return 42
    if a==1:
      result=123
    elif a==2:
      return 101

  let a = foo(1)

links

• https://forum.nim-lang.org/t/4022#25111

[mratsim]

Nice trick, though I'm surprised return rewrites implicitly to result.

You might be relying on implementation details of return that is leaking and that may change.

[metagn]

@mratsim not leaked