sugar: add var x syntax to => (#1492)

## Summary

Allow prefixing parameter names with `var`, to designate the parameter
as being a `var` parameter.

## Details

Type parameters cannot be inferred as `var`, and therefore it's
effectively not possible to use the `=>` macro for creating anonymous
procedures where the procedures needs `var` parameters. The `x: T`
syntax does exist, but it applies to all preceding parameter, limiting
its usefulness.

To address the aforementioned shortcoming, the `var x` syntax is now
supported (example: `(x, var y) => x`), assigning `var auto` as the
respective parameter's type. Due a NimSkull syntax/parser limitation,
`var x` cannot be used as the first parameter.

lib/pure/sugar.nim

@@ -53,7 +53,8 @@ proc createProcType(p, b: NimNode): NimNode =
   result.add prag
 
 macro `=>`*(p, b: untyped): untyped =
-  ## Syntax sugar for anonymous procedures. It also supports pragmas.
+  ## Syntax sugar for anonymous procedures. It also supports pragmas. Prefixing
+  ## a parameter name with ``var`` makes the parameter a var parameter.
   ##
   ## .. warning:: Semicolons can not be used to separate procedure arguments.
   runnableExamples:
@@ -109,6 +110,10 @@ macro `=>`*(p, b: untyped): untyped =
         identDefs.add(newIdentNode("auto"))
         identDefs.add(newEmptyNode())
         inc untypedBeforeColon
+      of nnkVarTy:
+        identDefs.add(c[0])
+        identDefs.add(newTree(nnkVarTy, newIdentNode("auto")))
+        identDefs.add(newEmptyNode())
       of nnkInfix:
         if c[0].kind == nnkIdent and c[0].eqIdent"->":
           var procTy = createProcType(c[1], c[2])

tests/lang_callable/macros/tsugar_closuremacro.nim

@@ -78,3 +78,24 @@ type
 var myBot = Bot()
 myBot.call = () {.noSideEffect.} => "I'm a bot."
 doAssert myBot.call() == "I'm a bot."
+
+proc singleVar(p: (var int) -> int): int =
+  var x = 0
+  discard p(x)
+  x
+
+doAssert singleVar((x: var auto) => (inc x; x)) == 1
+
+proc trailingVar(p: (int, var int) -> int): int =
+  var x = 0
+  discard p(1, x)
+  x
+
+doAssert trailingVar((x, var y) => (y += x; 0)) == 1
+
+proc multiVar(p: (int, var int, var int) -> int): int =
+  var x, y = 0
+  discard p(1, x, y)
+  x
+
+doAssert multiVar((x, var y, var z) => x) == 0