implements EnumToStrEntry for nim ic (#25614)

```nim
type
  NodeKind = enum
    nkInt, nkStr, nkAdd

  Node = object
    case kind: NodeKind
    of nkInt: intVal: int
    of nkStr: strVal: string
    of nkAdd: left, right: ref Node

proc newInt(v: int): ref Node =
  new(result)
  result[] = Node(kind: nkInt, intVal: v)

let n = newInt(42)
echo n.intVal
```

gives `unhandled exception: key not found: (module: 68, item: 3)
[KeyError]`

compiler/ast2nif.nim

@@ -700,7 +700,10 @@ proc writeOp(w: var Writer; content: var TokenBuf; op: LogEntry) =
   of MethodEntry:
     discard "to implement"
   of EnumToStrEntry:
-    discard "to implement"
+    content.addParLe repEnumToStrTag, NoLineInfo
+    content.add strToken(pool.strings.getOrIncl(op.key), NoLineInfo)
+    content.add symToken(pool.syms.getOrIncl(w.toNifSymName(op.sym)), NoLineInfo)
+    content.addParRi()
   of GenericInstEntry:
     discard "will only be written later to ensure it is materialized"
 

compiler/modulegraphs.nim

@@ -66,6 +66,7 @@ type
     memberProcsPerType*: Table[ItemId, seq[PSym]] # Type ID, attached member procs (only c++, virtual,member and ctor so far).
     initializersPerType*: Table[ItemId, PNode] # Type ID, AST call to the default ctor (c++ only)
     enumToStringProcs*: Table[ItemId, PSym]
+    loadedEnumToStringProcs: Table[string, PSym]
     emittedTypeInfo*: Table[string, FileIndex]
 
     packageSyms*: TStrTable
@@ -147,6 +148,7 @@ proc resetForBackend*(g: ModuleGraph) =
     a.clear()
   g.methodsPerGenericType.clear()
   g.enumToStringProcs.clear()
+  g.loadedEnumToStringProcs.clear()
   g.dispatchers.setLen(0)
   g.methodsPerType.clear()
   for a in mitems(g.loadedOps):
@@ -332,7 +334,10 @@ iterator getMethodsPerType*(g: ModuleGraph; t: PType): PSym =
       yield it
 
 proc getToStringProc*(g: ModuleGraph; t: PType): PSym =
-  result = g.enumToStringProcs[t.itemId]
+  result = g.enumToStringProcs.getOrDefault(t.itemId)
+  if result == nil and g.config.cmd in {cmdNifC, cmdM}:
+    let key = typeKey(t, g.config, loadTypeCallback, loadSymCallback)
+    result = g.loadedEnumToStringProcs.getOrDefault(key)
   assert result != nil
 
 proc setToStringProc*(g: ModuleGraph; t: PType; value: PSym) =
@@ -692,7 +697,7 @@ when not defined(nimKochBootstrap):
       of MethodEntry:
         discard "todo"
       of EnumToStrEntry:
-        discard "todo"
+        g.loadedEnumToStringProcs[x.key] = x.sym
       of GenericInstEntry:
         raiseAssert "GenericInstEntry should not be in the NIF index"
     # Register methods per type from NIF index

tests/ic/tenum.nim

@@ -0,0 +1,22 @@
+discard """
+  disabled: "linux"
+  output: "42"
+"""
+
+# Object variant / case object
+type
+  NodeKind = enum
+    nkInt, nkStr, nkAdd
+
+  Node = object
+    case kind: NodeKind
+    of nkInt: intVal: int
+    of nkStr: strVal: string
+    of nkAdd: left, right: ref Node
+
+proc newInt(v: int): ref Node =
+  new(result)
+  result[] = Node(kind: nkInt, intVal: v)
+
+let n = newInt(42)
+echo n.intVal