add isNamedTuple; make $(1, 2) be (1, 2) instead of (Field0: 1, Field1: 2) which leaked implementation detail (#10070)

* add `isNamedTuple`; make $(1, 2) be (1, 2) instead of leaking implementation detail (Field0: 1, Field1: 2)
  fixes this: #8670 (comment) /cc @alehander42 @Vindaar @mratsim

* Note: isNamedTuple is useful in other places, eg #10010 (comment)

* move isNamedTuple to helpers.nim to avoid exposing new symbol to system.nim

* remove workaround in tests/vm/tissues.nim failing test now that #10218 was makes it work

lib/pure/typetraits.nim

@@ -10,7 +10,10 @@
 ## This module defines compile-time reflection procs for
 ## working with types
 
+include "system/helpers" # for `isNamedTuple`
+
 export system.`$`
+export isNamedTuple
 
 proc name*(t: typedesc): string {.magic: "TypeTrait".}
   ## Alias for system.`$`(t) since Nim v0.20.0.

lib/system.nim

@@ -2669,19 +2669,28 @@ proc compiles*(x: untyped): bool {.magic: "Compiles", noSideEffect, compileTime.
   ##     echo "'+' for integers is available"
   discard
 
+include "system/helpers" # for `lineInfoToString`, `isNamedTuple`
+
 proc `$`*[T: tuple|object](x: T): string =
   ## generic ``$`` operator for tuples that is lifted from the components
   ## of `x`. Example:
   ##
   ## .. code-block:: nim
-  ##   $(23, 45) == "(Field0: 23, Field1: 45)"
+  ##   $(23, 45) == "(23, 45)"
+  ##   $(a: 23, b: 45) == "(a: 23, b: 45)"
   ##   $() == "()"
   result = "("
   var firstElement = true
+  const isNamed = T is object or isNamedTuple(T)
+  when not isNamed:
+    var count = 0
   for name, value in fieldPairs(x):
     if not firstElement: result.add(", ")
-    result.add(name)
-    result.add(": ")
+    when isNamed:
+      result.add(name)
+      result.add(": ")
+    else:
+      count.inc
     when compiles($value):
       when value isnot string and value isnot seq and compiles(value.isNil):
         if value.isNil: result.add "nil"
@@ -2691,6 +2700,10 @@ proc `$`*[T: tuple|object](x: T): string =
       firstElement = false
     else:
       result.add("...")
+  when not isNamed:
+    if count == 1:
+      result.add(",") # $(1,) should print as the semantically legal (1,)
+
   result.add(")")
 
 proc collectionToString[T](x: T, prefix, separator, suffix: string): string =
@@ -3958,8 +3971,6 @@ proc failedAssertImpl*(msg: string) {.raises: [], tags: [].} =
                                     tags: [].}
   Hide(raiseAssert)(msg)
 
-include "system/helpers" # for `lineInfoToString`
-
 template assertImpl(cond: bool, msg: string, expr: string, enabled: static[bool]) =
   const loc = $instantiationInfo(-1, true)
   bind instantiationInfo

lib/system/helpers.nim

@@ -6,6 +6,23 @@
 proc lineInfoToString(file: string, line, column: int): string =
   file & "(" & $line & ", " & $column & ")"
 
-proc `$`(info: type(instantiationInfo(0))): string =
+type InstantiationInfo = tuple[filename: string, line: int, column: int]
+
+proc `$`(info: InstantiationInfo): string =
   # The +1 is needed here
+  # instead of overriding `$` (and changing its meaning), consider explicit name.
   lineInfoToString(info.fileName, info.line, info.column+1)
+
+proc isNamedTuple(T: type): bool =
+  ## return true for named tuples, false for any other type.
+  when T isnot tuple: result = false
+  else:
+    var t: T
+    for name, _ in t.fieldPairs:
+      when name == "Field0":
+        return compiles(t.Field0)
+      else:
+        return true
+    # empty tuple should be un-named,
+    # see https://github.com/nim-lang/Nim/issues/8861#issue-356631191
+    return false

lib/system/helpers2.nim

@@ -1,3 +1,5 @@
+# imported by other modules, unlike helpers.nim which is included
+
 template formatErrorIndexBound*[T](i, a, b: T): string =
   "index out of bounds: (a:" & $a & ") <= (i:" & $i & ") <= (b:" & $b & ") "