lenVarargs: number of varargs elements (#12907)

lib/system.nim

@@ -4198,6 +4198,11 @@ type
   NimNode* {.magic: "PNimrodNode".} = ref NimNodeObj
     ## Represents a Nim AST node. Macros operate on this type.
 
+macro lenVarargs*(x: varargs[untyped]): int {.since: (1, 1).} =
+  ## returns number of variadic arguments in `x`
+  proc lenVarargsImpl(x: NimNode): NimNode {.magic: "LengthOpenArray", noSideEffect.}
+  lenVarargsImpl(x)
+
 when false:
   template eval*(blk: typed): typed =
     ## Executes a block of code at compile time just as if it was a macro.

tests/system/tlenvarargs.nim

@@ -0,0 +1,59 @@
+discard """
+  output: '''
+tlenvarargs.nim:35:9 (1, 2)
+tlenvarargs.nim:36:9 12
+tlenvarargs.nim:37:9 1
+tlenvarargs.nim:38:8 
+done
+'''
+"""
+## line 10
+
+template myecho*(a: varargs[untyped]) =
+  ## shows a useful debugging echo-like proc that is dependency-free (no dependency
+  ## on macros.nim) so can be used in more contexts
+  const info = instantiationInfo(-1, false)
+  const loc = info.filename & ":" & $info.line & ":" & $info.column & " "
+  when lenVarargs(a) > 0:
+    echo(loc, a)
+  else:
+    echo(loc)
+
+template fun*(a: varargs[untyped]): untyped =
+  lenVarargs(a)
+
+template fun2*(a: varargs[typed]): untyped =
+  a.lenVarargs
+
+template fun3*(a: varargs[int]): untyped =
+  a.lenVarargs
+
+template fun4*(a: varargs[untyped]): untyped =
+  len(a)
+
+proc main()=
+  myecho (1, 2)
+  myecho 1, 2
+  myecho 1
+  myecho()
+
+  doAssert fun() == 0
+  doAssert fun('a') == 1
+  doAssert fun("asdf", 1) == 2
+
+  doAssert fun2() == 0
+  doAssert fun2('a') == 1
+  doAssert fun2("asdf", 1) == 2
+
+  doAssert fun3() == 0
+  doAssert fun3(10) == 1
+  doAssert fun3(10, 11) == 2
+
+  ## shows why `lenVarargs` can't be named `len`
+  doAssert fun4("abcdef") == len("abcdef")
+
+  ## workaround for BUG:D20191218T171447 whereby if testament expected output ends
+  ## in space, testament strips it from expected output but not actual output,
+  ## which leads to a mismatch when running test via megatest
+  echo "done"
+main()