wrapnils: add ??. which returns an Option (#16931)

* wrapnils: add option-like API with ??., isSome, get
* fix test after rebase
* cleanups
* fix changelog
* address comments regarding get vs unsafeGet

tests/stdlib/twrapnils.nim

@@ -1,15 +1,11 @@
 import std/wrapnils
 
-const wrapnilExtendedExports = declared(wrapnil)
-  # for now, wrapnil, isValid, unwrap are not exported
-
 proc checkNotZero(x: float): float =
   doAssert x != 0
   x
 
-var witness = 0
-
 proc main() =
+  var witness = 0
   type Bar = object
     b1: int
     b2: ptr string
@@ -31,8 +27,12 @@ proc main() =
   proc fun(a: Bar): auto = a.b2
 
   var a: Foo
-  var x6 = create(Bar)
-  x6.b1 = 42
+
+  var x6: ptr Bar
+  when nimvm: discard # pending https://github.com/timotheecour/Nim/issues/568
+  else:
+    x6 = create(Bar)
+    x6.b1 = 42
   var a2 = Foo(x1: 1.0, x5: @[10, 11], x6: x6)
   var a3 = Foo(x1: 1.2, x3: "abc")
   a3.x2 = a3
@@ -50,26 +50,23 @@ proc main() =
   doAssert ?.a3.x2.x2.x5.len == 0
   doAssert a3.x2.x2.x3.len == 3
 
-  when wrapnilExtendedExports:
-    # example calling wrapnil directly, with and without unwrap
-    doAssert a3.wrapnil.x2.x2.x3.len == wrapnil(3)
-    doAssert a3.wrapnil.x2.x2.x3.len.unwrap == 3
-    doAssert a2.wrapnil.x4.isValid
-    doAssert not a.wrapnil.x4.isValid
-
   doAssert ?.a.x2.x2.x3[1] == default(char)
   # here we only apply wrapnil around gook.foo, not gook (and assume gook is not nil)
   doAssert ?.(gook.foo).x2.x2.x1 == 0.0
 
-  doAssert ?.a2.x6[] == Bar(b1: 42) # deref for ptr Bar
+  when nimvm: discard
+  else:
+    doAssert ?.a2.x6[] == Bar(b1: 42) # deref for ptr Bar
 
   doAssert ?.a2.x1.checkNotZero == 1.0
   doAssert a == nil
   # shows that checkNotZero won't be called if a nil is found earlier in chain
   doAssert ?.a.x1.checkNotZero == 0.0
 
-  # checks that a chain without nil but with an empty seq still throws IndexDefect
-  doAssertRaises(IndexDefect): discard ?.a2.x8[3]
+  when nimvm: discard
+  else:
+    # checks that a chain without nil but with an empty seq still raises
+    doAssertRaises(IndexDefect): discard ?.a2.x8[3]
 
   # make sure no double evaluation bug
   doAssert witness == 0
@@ -79,4 +76,10 @@ proc main() =
   # here, it's used twice, to deref `ref Bar` and then `ptr string`
   doAssert ?.a.x9[].fun[] == ""
 
+  block: # `??.`
+    doAssert (??.a3.x2.x2.x3.len).get == 3
+    doAssert (??.a2.x4).isSome
+    doAssert not (??.a.x4).isSome
+
 main()
+static: main()