spec for view types (#15424)

* spec for view types
* spec additions
* refactoring; there are two different kinds of views
* refactorings and spec additions
* enforce that view types are initialized
* enforce borrowing from the first formal parameter
* enforce lifetimes for borrowing of locals
* typo in the manual
* clarify in the implementation what a borrow operation really is

tests/views/tcannot_borrow.nim

@@ -1,17 +1,22 @@
 discard """
   errormsg: "cannot borrow"
-  nimout: '''tcannot_borrow.nim(16, 7) Error: cannot borrow meh; what it borrows from is potentially mutated
-tcannot_borrow.nim(17, 3) the mutation is here
-tcannot_borrow.nim(16, 7) is the statement that connected the mutation to the parameter'''
-  line: 16
+  nimout: '''tcannot_borrow.nim(21, 7) Error: cannot borrow meh; what it borrows from is potentially mutated
+tcannot_borrow.nim(22, 3) the mutation is here'''
+  line: 21
 """
 
+
 {.experimental: "views".}
 
 type
   Foo = object
     field: string
 
+proc valid(s: var seq[Foo]) =
+  let v: lent Foo = s[0]  # begin of borrow
+  echo v.field            # end of borrow
+  s.setLen 0  # valid because 'v' isn't used afterwards
+
 proc dangerous(s: var seq[Foo]) =
   let meh: lent Foo = s[0]
   s.setLen 0

tests/views/tmust_borrow_from_first_parameter.nim

@@ -0,0 +1,9 @@
+discard """
+  errormsg: "'result' must borrow from the first parameter"
+  line: 9
+"""
+
+{.experimental: "views".}
+
+proc p(a, b: openArray[char]): openArray[char] =
+  result = b

tests/views/tsplit_into_seq.nim

@@ -23,9 +23,8 @@ proc split*(s: string, seps: set[char] = Whitespace,
     while last < len(s) and s[last] notin seps:
       inc(last)
     if splits == 0: last = len(s)
-    {.noSideEffect.}:
-      result.add toOpenArray(s, first, last-1)
-      result.add toOpenArray(s, first, last-1)
+    result.add toOpenArray(s, first, last-1)
+    result.add toOpenArray(s, first, last-1)
     if splits == 0: break
     dec(splits)
     inc(last)

tests/views/tviews1.nim

@@ -4,6 +4,7 @@ discard """
 33
 3
 2
+3
 3'''
   targets: "c cpp"
 """
@@ -22,6 +23,6 @@ proc main(s: seq[int]) =
   take(x.toOpenArray(0, 1))
   let y = x
   take y
-
+  take x
 
 main(@[11, 22, 33])