markused: Fix markused behavior on struct field's default expression

vlib/v/ast/str.v

@@ -51,6 +51,12 @@ pub fn (node &FnDecl) fkey() string {
 	return node.name
 }
 
+// skey returns a unique name of the struct field.
+// it is used in v.markused.
+pub fn (node &StructField) sfkey() string {
+	return '${int(node.container_typ)}.${node.name}}'
+}
+
 pub fn (node &Fn) fkey() string {
 	if node.is_method {
 		return '${int(node.receiver_type)}.${node.name}'

vlib/v/markused/markused.v

@@ -8,7 +8,7 @@ import v.pref
 
 // mark_used walks the AST, starting at main() and marks all used fns transitively
 pub fn mark_used(mut table ast.Table, mut pref_ pref.Preferences, ast_files []&ast.File) {
-	mut all_fns, all_consts, all_globals := all_fn_const_and_global(ast_files)
+	mut all_fns, all_consts, all_globals, all_fields := all_global_decl(ast_files)
 	util.timing_start('MARKUSED')
 	defer {
 		util.timing_measure('MARKUSED')
@@ -464,11 +464,13 @@ pub fn mark_used(mut table ast.Table, mut pref_ pref.Preferences, ast_files []&a
 		all_fns:     all_fns
 		all_consts:  all_consts
 		all_globals: all_globals
+		all_fields:  all_fields
 		pref:        pref_
 	)
 	walker.mark_markused_consts() // tagged with `@[markused]`
 	walker.mark_markused_globals() // tagged with `@[markused]`
 	walker.mark_markused_fns() // tagged with `@[markused]`, `@[export]` and veb actions
+	walker.mark_struct_field_default_expr()
 
 	for k, _ in table.used_features.comptime_calls {
 		walker.fn_by_name(k)
@@ -553,14 +555,15 @@ pub fn mark_used(mut table ast.Table, mut pref_ pref.Preferences, ast_files []&a
 	}
 }
 
-fn all_fn_const_and_global(ast_files []&ast.File) (map[string]ast.FnDecl, map[string]ast.ConstField, map[string]ast.GlobalField) {
+fn all_global_decl(ast_files []&ast.File) (map[string]ast.FnDecl, map[string]ast.ConstField, map[string]ast.GlobalField, map[string]ast.StructField) {
 	util.timing_start(@METHOD)
 	defer {
 		util.timing_measure(@METHOD)
 	}
 	mut all_fns := map[string]ast.FnDecl{}
 	mut all_consts := map[string]ast.ConstField{}
 	mut all_globals := map[string]ast.GlobalField{}
+	mut all_fields := map[string]ast.StructField{}
 	for i in 0 .. ast_files.len {
 		for node in ast_files[i].stmts {
 			match node {
@@ -582,11 +585,17 @@ fn all_fn_const_and_global(ast_files []&ast.File) (map[string]ast.FnDecl, map[st
 						all_globals[gkey] = gfield
 					}
 				}
+				ast.StructDecl {
+					for sfield in node.fields {
+						sfkey := sfield.sfkey()
+						all_fields[sfkey] = sfield
+					}
+				}
 				else {}
 			}
 		}
 	}
-	return all_fns, all_consts, all_globals
+	return all_fns, all_consts, all_globals, all_fields
 }
 
 fn mark_all_methods_used(mut table ast.Table, mut all_fn_root_names []string, typ ast.Type) {

vlib/v/markused/walker.v

@@ -22,6 +22,7 @@ mut:
 	all_fns     map[string]ast.FnDecl
 	all_consts  map[string]ast.ConstField
 	all_globals map[string]ast.GlobalField
+	all_fields  map[string]ast.StructField
 }
 
 pub fn Walker.new(params Walker) &Walker {
@@ -149,6 +150,17 @@ pub fn (mut w Walker) mark_markused_globals() {
 	}
 }
 
+pub fn (mut w Walker) mark_struct_field_default_expr() {
+	for sfkey, mut structfield in w.all_fields {
+		$if trace_skip_unused_roots ? {
+			println('>>>> walking struct field: ${sfkey} ...')
+		}
+		if structfield.has_default_expr {
+			w.expr(structfield.default_expr)
+		}
+	}
+}
+
 pub fn (mut w Walker) stmt(node_ ast.Stmt) {
 	mut node := unsafe { node_ }
 	match mut node {

vlib/v/tests/skip_unused/struct_field_default_expr.vv

@@ -0,0 +1,15 @@
+module main
+
+import flag
+
+const c_default_port = u16(12345)
+
+struct Options {
+	headless bool
+	port     u16 = c_default_port @[short: p]
+}
+
+fn main() {
+	args := arguments()
+	_, _ := flag.to_struct[Options](args, style: .v, skip: 1)!
+}