On devel a new experimental mode has arrived called "strictFuncs". See also
[https://github.com/nim-lang/RFCs/issues/234](https://github.com/nim-lang/RFCs/issues/234)
For example:
{.experimental: "strictFuncs".}
type
Node = ref object
le, ri: Node
data: string
func len(n: Node): int =
# valid: len does not have side effects
var it = n
while it != nil:
inc result
it = it.ri
func mut(n: Node) =
let m = n # is the statement that connected the mutation to the
parameter
m.data = "yeah" # the mutation is here
# Error: 'mut' can have side effects
# an object reachable from 'n' is potentially mutated
Run
