inconsistent status of "not nil" in docs vs. compiler

[akavel]

The main Nim Manual mentions not nil as a regular feature, not in the "experimental features" appendix, and with no mention of pragma, but the compiler complains that it wants a pragma:

Example

type Foo = ref int not nil

Current Output

Error: enable the 'not nil' annotation with {.experimental: "notnil".}

Expected Output

Expected to compile just fine without "experimental" pragma.

Possible Solution

If "not nil" is still experimental, I'd expect its section to be moved from the main Nim Manual to the "experimental features" appendix (https://nim-lang.org/docs/manual_experimental.html)

$ nim -v
Nim Compiler Version 1.2.0 [Windows: amd64]
Compiled at 2020-04-03
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: 7e83adff84be5d0c401a213eccb61e321a3fb1ff
active boot switches: -d:release

[ghost]

Fixed by #14027