Add toOpenArray[T](ptr UncheckedArray[T]) for clarity. (nim-lang#9316)

* Add `toOpenArray[T](ptr UncheckedArray[T])` for clarity.  `ptr array[0,T]`
for some unchecked type already works but A) `UncheckedArray` seems to be
the intended future way for this kind of access, and B) essentially all use
cases will have a `ptr` for that kind of array source and this call signature
lets callers drop the trailing `[]` corresponding to that `ptr` deref.
This PR relates to issue https://github.com/nim-lang/Nim/issues/9001 .

* Add a test for toOpenArray() for UncheckedArray[T]s.

compiler/ccgcalls.nim

@@ -106,7 +106,7 @@ proc openArrayLoc(p: BProc, n: PNode): Rope =
         result = "($1)+($2), ($3)-($2)+1" % [rdLoc(a), rdLoc(b), rdLoc(c)]
       else:
         result = "($1)+(($2)-($4)), ($3)-($2)+1" % [rdLoc(a), rdLoc(b), rdLoc(c), intLiteral(first)]
-    of tyOpenArray, tyVarargs:
+    of tyOpenArray, tyVarargs, tyUncheckedArray:
       result = "($1)+($2), ($3)-($2)+1" % [rdLoc(a), rdLoc(b), rdLoc(c)]
     of tyString, tySequence:
       if skipTypes(n.typ, abstractInst).kind == tyVar and

lib/system.nim

@@ -4338,6 +4338,8 @@ when not defined(js):
     magic: "Slice".}
   proc toOpenArray*[T](x: openarray[T]; first, last: int): openarray[T] {.
     magic: "Slice".}
+  proc toOpenArray*[T](x: ptr UncheckedArray[T]; first, last: int): openarray[T] {.
+    magic: "Slice".}
   proc toOpenArray*[I, T](x: array[I, T]; first, last: I): openarray[T] {.
     magic: "Slice".}
   proc toOpenArray*(x: string; first, last: int): openarray[char] {.

tests/system/tsystem_misc.nim

@@ -15,6 +15,7 @@ discard """
 1
 2
 3
+2
 48
 49
 50
@@ -98,6 +99,10 @@ doAssertRaises(IndexError):
 doAssertRaises(IndexError):
   foo(toOpenArray(arrNeg, -1, -3))
 
+type seqqType = ptr UncheckedArray[int]
+let qData = cast[seqqType](addr seqq[0])
+oaFirstElm(toOpenArray(qData, 1, 3))
+
 proc foo(a: openArray[byte]) =
   for x in a: echo x