document or improve behavior of varargsLen in procs (differs from behavior in templates)

[timotheecour]

in procs, varargs is a RT construct, whereas in templates it's a CT construct. This leads to surprising behavior for varargsLen for users unaware of this subtlety.

refs #18418 (comment) /cc @quantimnot

I just found system.varargsLen but it seems to be broken.

Example

proc a0(b: varargs[string]) =
  let s2 = len(b) # len not available at CT for good reasons
  echo s2
proc a1(b: varargs[string]) =
  const s1 = varargsLen(b)
  let s2 = varargsLen(b)
  echo (s1, s2)
template a2(b: varargs[string]) =
  const s1 = varargsLen(b)
  let s2 = varargsLen(b)
  echo (s1, s2)
a0()
a0("x1")
a0("x1", "x2")

a1()
a1("x1")
a1("x1", "x2")

a2()
a2("x1")
a2("x1", "x2")

Current Output

0
1
2
(1, 1)
(1, 1)
(1, 1)
(0, 0)
(1, 1)
(2, 2)

Expected Output

either:

• give CT error for varargsLen inside a proc (a1)
• give CT error for varargsLen inside a a proc (a1) but only if used in a static context, otherwise make it equal to len

and document this with a runnableExamples

Additional Information

1.5.1 0d74f60
note: also need to consider macros, besides proc and template