Explicitly null check the stdlib

[hamzaremmal]

No description provided.

[hamzaremmal]

@noti0na1 As agreed to, please push the changes to explicitly null check the stdlib here.

[sjrd]

I wonder if we could pull something off with a universal match type and its mapNull helper.

scala> type MapNull[X, Y] <: Y | Null = X match { case Null => Null; case _ => Y }

scala> inline def mapNull[A, B](a: A | Null, inline f: A => B): MapNull[a.type, B] = (if a == null then null else f(a)).asInstanceOf[MapNull[a.type, B]]
def mapNull[A, B](a: A | Null, f: A => B): MapNull[a.type, B]

scala> def foo(x: String | Null): MapNull[x.type, Array[Char]] = mapNull(x, _.toCharArray())
def foo(x: String | Null): MapNull[x.type, Array[Char]]

scala> foo("hello"): Array[Char]
val res7: Array[Char] = Array(h, e, l, l, o)

scala> foo(null): Null
val res8: Null = null

scala> var s: String | Null = "hello"
var s: String | Null = hello

scala> foo(s): Array[Char]
-- [E007] Type Mismatch Error: -------------------------------------------------
1 |foo(s): Array[Char]
  |^^^^^^
  |Found:    MapNull[(?1 : String | Null), Array[Char]]
  |Required: Array[Char]
  |
  |where:    ?1 is an unknown value of type String | Null
  |
  |
  |Note: a match type could not be fully reduced:
  |
  |  trying to reduce  MapNull[(?1 : String | Null), Array[Char]]
  |  failed since selector (?1 : String | Null)
  |  does not match  case Null => Null
  |  and cannot be shown to be disjoint from it either.
  |  Therefore, reduction cannot advance to the remaining case
  |
  |    case _ => Array[Char]
  |
  | longer explanation available when compiling with `-explain`
1 error found

scala> foo(s): Array[Char] | Null
val res9: Array[Char] | Null = Array(h, e, l, l, o)

Note that the signature of foo is specially crafted so that its TASTy Signature and its binary descriptor are the same as if it were defined def foo(s: String | Null): Array[Char] | Null.

It's not bullet-proof. Sometimes, use site inference gets in the way, as in:

scala> foo(s): Array[Char] | Null
val res9: Array[Char] | Null = Array(h, e, l, l, o)

scala> Array(foo(s))
-- [E172] Type Error: ----------------------------------------------------------
1 |Array(foo(s))
  |             ^
  |      No ClassTag available for MapNull[(?1 : String | Null), Array[Char]]
  |
  |      where:    ?1 is an unknown value of type String | Null
  |
  |
  |      Note: a match type could not be fully reduced:
  |
  |        trying to reduce  MapNull[(?1 : String | Null), Array[Char]]
  |        failed since selector (?1 : String | Null)
  |        does not match  case Null => Null
  |        and cannot be shown to be disjoint from it either.
  |        Therefore, reduction cannot advance to the remaining case
  |
  |          case _ => Array[Char]
1 error found

scala> Array(foo("hello"))
val res10: Array[Array[Char]] = Array(Array(h, e, l, l, o))

However, that's only for the nullable argument case, and in some use site scenarios. It might be worth exploring.

[noti0na1]

This is an interesting idea. I will try this and see how it looks.

[noti0na1]

@hamzaremmal Do you know how the stdlib is compiled at the first step? When I do scalac, it seems compiling my new code using the old library.

[noti0na1]

.nn like this can be removed after #23864