v.markused: support a const used as default value in struct init

vlib/v/markused/walker.v

@@ -33,6 +33,9 @@ pub fn (mut w Walker) mark_const_as_used(ckey string) {
 	$if trace_skip_unused_marked ? {
 		eprintln('    const > |$ckey|')
 	}
+	if w.used_consts[ckey] {
+		return
+	}
 	w.used_consts[ckey] = true
 	cfield := w.all_consts[ckey] or { return }
 	w.expr(cfield.expr)
@@ -42,6 +45,9 @@ pub fn (mut w Walker) mark_global_as_used(ckey string) {
 	$if trace_skip_unused_marked ? {
 		eprintln('  global > |$ckey|')
 	}
+	if w.used_globals[ckey] {
+		return
+	}
 	w.used_globals[ckey] = true
 	gfield := w.all_globals[ckey] or { return }
 	w.expr(gfield.expr)

vlib/v/tests/skip_unused/const_used_in_struct_default_init_recursive.run.out

@@ -0,0 +1,12 @@
+&Symbol{
+    name: 'void'
+    parent: &nil
+    return_type: &nil
+    is_top_level: true
+    generic_placeholder_len: 0
+    sumtype_children_len: 0
+    interface_children_len: 0
+    children: []
+    file_path: ''
+    file_version: 0
+}

vlib/v/tests/skip_unused/const_used_in_struct_default_init_recursive.skip_unused.run.out

@@ -0,0 +1,12 @@
+&Symbol{
+    name: 'void'
+    parent: &nil
+    return_type: &nil
+    is_top_level: true
+    generic_placeholder_len: 0
+    sumtype_children_len: 0
+    interface_children_len: 0
+    children: []
+    file_path: ''
+    file_version: 0
+}

vlib/v/tests/skip_unused/const_used_in_struct_default_init_recursive.vv

@@ -0,0 +1,27 @@
+module main
+
+pub const void_type = &Symbol{
+	name: 'void'
+	file_path: ''
+	file_version: 0
+	is_top_level: true
+}
+
+[heap]
+pub struct Symbol {
+pub mut:
+	name                    string
+	parent                  &Symbol = void_type // parent is for typedefs, aliases
+	return_type             &Symbol = void_type // return_type is for functions and variables
+	is_top_level            bool      [required]
+	generic_placeholder_len int
+	sumtype_children_len    int
+	interface_children_len  int
+	children                []&Symbol // methods, sum types, map types, optionals, struct fields, etc.
+	file_path               string    [required] // required in order to register the symbol at its appropriate directory.
+	file_version            int       [required] // file version when the symbol was registered
+}
+
+fn main() {
+	println(void_type)
+}