cbuilder: implement designated initializers, finish default value braces (#24312)

follows up #24259

This is the remaining missing use of `StructInitializer` in
`getDefaultValue` after #24259 and #24302. The only remaining direct C
code in getDefaultValue is [this
line](922f7dfd71/compiler/ccgexprs.nim (L3525))
which creates a global array variable, which isn't implemented yet. Next
steps would be all remaining variable and `typedef` declarations, then
hopefully we can move on to general statements and expressions.

compiler/cbuilderdecls.nim

@@ -77,9 +77,7 @@ type
     needsComma: bool
 
 proc initStructInitializer(builder: var Builder, kind: StructInitializerKind): StructInitializer =
-  ## starts building a struct initializer, `orderCompliant = true` means
-  ## built fields must be ordered correctly
-  assert kind != siNamedStruct, "named struct constructors unimplemented"
+  ## starts building a struct initializer, i.e. braced initializer list
   result = StructInitializer(kind: kind, needsComma: false)
   if kind != siWrapper:
     builder.add("{")
@@ -100,7 +98,11 @@ template addField(builder: var Builder, constr: var StructInitializer, name: str
     assert name.len != 0, "name has to be given for struct initializer field"
     valueBody
   of siNamedStruct:
-    assert false, "named struct constructors unimplemented"
+    assert name.len != 0, "name has to be given for struct initializer field"
+    builder.add(".")
+    builder.add(name)
+    builder.add(" = ")
+    valueBody
 
 proc finishStructInitializer(builder: var Builder, constr: StructInitializer) =
   ## finishes building a struct initializer

compiler/ccgexprs.nim

@@ -3289,26 +3289,35 @@ proc getNullValueAux(p: BProc; t: PType; obj, constOrNil: PNode,
           break
 
     let selectedBranch = caseObjDefaultBranch(obj, branch)
-    # XXX siNamedStruct needs to be implemented to replace `res` here
-    var res = "{"
-    var branchInit: StructInitializer
     let b = lastSon(obj[selectedBranch])
     # designated initilization is the only way to init non first element of unions
     # branches are allowed to have no members (b.len == 0), in this case they don't need initializer
+    var fieldName: string = ""
     if b.kind == nkRecList and not isEmptyCaseObjectBranch(b):
-      res.add "._" & mangleRecFieldName(p.module, obj[0].sym) & "_" & $selectedBranch & " = "
-      res.addStructInitializer(branchInit, kind = siOrderedStruct):
-        getNullValueAux(p, t, b, constOrNil, res, branchInit, isConst, info)
+      fieldName = "_" & mangleRecFieldName(p.module, obj[0].sym) & "_" & $selectedBranch
+      result.addField(init, name = "<anonymous union>"):
+        # XXX figure out name for the union, see use of `addAnonUnion`
+        var branchInit: StructInitializer
+        result.addStructInitializer(branchInit, kind = siNamedStruct):
+          result.addField(branchInit, name = fieldName):
+            var branchObjInit: StructInitializer
+            result.addStructInitializer(branchObjInit, kind = siOrderedStruct):
+              getNullValueAux(p, t, b, constOrNil, result, branchObjInit, isConst, info)
     elif b.kind == nkSym:
-      res.add "." & mangleRecFieldName(p.module, b.sym) & " = "
-      res.addStructInitializer(branchInit, kind = siWrapper):
-        getNullValueAux(p, t, b, constOrNil, res, branchInit, isConst, info)
+      fieldName = mangleRecFieldName(p.module, b.sym)
+      result.addField(init, name = "<anonymous union>"):
+        # XXX figure out name for the union, see use of `addAnonUnion`
+        var branchInit: StructInitializer
+        result.addStructInitializer(branchInit, kind = siNamedStruct):
+          result.addField(branchInit, name = fieldName):
+            # we need to generate the default value of the single sym,
+            # to do this create a dummy wrapper initializer and recurse
+            var branchFieldInit: StructInitializer
+            result.addStructInitializer(branchFieldInit, kind = siWrapper):
+              getNullValueAux(p, t, b, constOrNil, result, branchFieldInit, isConst, info)
     else:
+      # no fields, don't initialize
       return
-    result.addField(init, name = "<anonymous union>"):
-      # XXX figure out name for the union, see use of `addAnonUnion`
-      result.add res
-      result.add "}"
 
   of nkSym:
     let field = obj.sym