fix(stdlib): use first-element flag in $ for collections (#18583) (#25832)

Fixes #18583.

## Problem

Several stdlib collection types compute the separator for `$` using
`result.len > 1`, where `result` starts as the opening bracket (`"["` or
`"{"`). This breaks when a collection element type has a `$` that
returns an empty string: `result.len` stays at 1 after the first item
contributes nothing, so the separator is never inserted for subsequent
items.

```nim
import std/deques

type Test = object
proc `$`(x: Test): string = ""

echo [Test(), Test()].toDeque  # prints [] — expected [, ]
```

## Fix

Replace the length check with an explicit `first` flag in all affected
modules: `deques`, `heapqueue`, `lists`, `critbits`, and `strtabs`.

## Tests

Regression tests added to `tdeques`, `theapqueue`, and `tlists` using a
local type whose `$` returns `""`. All three test files pass with `nim c
-r`.

## Notes

I work with Claude as a co-processor. I'm 56, came to programming late,
and this is genuinely how I learn and contribute. I understand what I'm
submitting, but I didn't write it alone. If your project prefers
human-only contributions, just say so and I'll close without friction.

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/pure/collections/critbits.nim

@@ -495,13 +495,16 @@ func `$`*[T](c: CritBitTree[T]): string =
       const avgItemLen = 16
     result = newStringOfCap(c.count * avgItemLen)
     result.add("{")
+    var first = true
     when T is void:
       for key in keys(c):
-        if result.len > 1: result.add(", ")
+        if first: first = false
+        else: result.add(", ")
         result.addQuoted(key)
     else:
       for key, val in pairs(c):
-        if result.len > 1: result.add(", ")
+        if first: first = false
+        else: result.add(", ")
         result.addQuoted(key)
         result.add(": ")
         result.addQuoted(val)

lib/pure/collections/deques.nim

@@ -454,8 +454,10 @@ proc `$`*[T](deq: Deque[T]): string =
     assert $a == "[10, 20, 30]"
 
   result = "["
+  var first = true
   for x in deq:
-    if result.len > 1: result.add(", ")
+    if first: first = false
+    else: result.add(", ")
     result.addQuoted(x)
   result.add("]")
 

lib/pure/collections/heapqueue.nim

@@ -260,7 +260,9 @@ proc `$`*[T](heap: HeapQueue[T]): string =
     assert $heap == "[1, 2]"
 
   result = "["
+  var first = true
   for x in heap.data:
-    if result.len > 1: result.add(", ")
+    if first: first = false
+    else: result.add(", ")
     result.addQuoted(x)
   result.add("]")

lib/pure/collections/lists.nim

@@ -304,8 +304,10 @@ proc `$`*[T](L: SomeLinkedCollection[T]): string =
     assert $a == "[1, 2, 3, 4]"
 
   result = "["
+  var first = true
   for x in nodes(L):
-    if result.len > 1: result.add(", ")
+    if first: first = false
+    else: result.add(", ")
     result.addQuoted(x.value)
   result.add("]")
 

lib/pure/strtabs.nim

@@ -380,8 +380,10 @@ proc `$`*(t: StringTableRef): string {.rtlFunc, extern: "nstDollar".} =
     result = "{:}"
   else:
     result = "{"
+    var first = true
     for key, val in pairs(t):
-      if result.len > 1: result.add(", ")
+      if first: first = false
+      else: result.add(", ")
       result.add(key)
       result.add(": ")
       result.add(val)

tests/stdlib/tdeques.nim

@@ -241,3 +241,12 @@ proc main() =
 
 static: main()
 main()
+
+# https://github.com/nim-lang/Nim/issues/18583
+# $ separator must be emitted even when the item's string repr is empty
+type EmptyStr18583 = object
+proc `$`(x: EmptyStr18583): string = ""
+
+block:
+  var d = [EmptyStr18583(), EmptyStr18583()].toDeque
+  doAssert $d == "[, ]", "got: " & $d

tests/stdlib/theapqueue.nim

@@ -104,3 +104,15 @@ template main() =
 
 static: main()
 main()
+
+# https://github.com/nim-lang/Nim/issues/18583
+type EmptyStr18583HeapQ = object
+proc `$`(x: EmptyStr18583HeapQ): string = ""
+proc `<`(a, b: EmptyStr18583HeapQ): bool = false
+
+block:
+  var h = initHeapQueue[EmptyStr18583HeapQ]()
+  push(h, EmptyStr18583HeapQ())
+  push(h, EmptyStr18583HeapQ())
+  let s = $h
+  doAssert s == "[, ]", "got: " & s

tests/stdlib/tlists.nim

@@ -287,3 +287,14 @@ template main =
 
 static: main()
 main()
+
+# https://github.com/nim-lang/Nim/issues/18583
+type EmptyStr18583List = object
+proc `$`(x: EmptyStr18583List): string = ""
+
+block:
+  var L: SinglyLinkedList[EmptyStr18583List]
+  L.prepend(EmptyStr18583List())
+  L.prepend(EmptyStr18583List())
+  let s = $L
+  doAssert s == "[, ]", "got: " & s