checker: fix -skip-unused with struct inits with default expressions

vlib/v/checker/mark_used.v

@@ -131,6 +131,7 @@ fn (mut c Checker) mark_used(ast_files []ast.File) {
 	}
 
 	mut walker := mark_used_walker.Walker{
+		table: c.table
 		files: ast_files
 		all_fns: all_fns
 		all_consts: all_consts

vlib/v/checker/mark_used_walker/walker.v

@@ -5,9 +5,11 @@ module mark_used_walker
 // This module walks the entire program starting at fn main and marks used (called) functions.
 // Unused functions can be safely skipped by the backends to save CPU time and space.
 import v.ast
+import v.table
 
 pub struct Walker {
 pub mut:
+	table       &table.Table
 	used_fns    map[string]bool // used_fns['println'] == true
 	used_consts map[string]bool // used_consts['os.args'] == true
 	n_maps      int
@@ -258,8 +260,19 @@ fn (mut w Walker) expr(node ast.Expr) {
 			w.expr(node.where_expr)
 		}
 		ast.StructInit {
-			// eprintln('>>>> ast.StructInit: $node')
-			w.expr(node.update_expr)
+			sym := w.table.get_type_symbol(node.typ)
+			if sym.kind == .struct_ {
+				info := sym.info as table.Struct
+				for ifield in info.fields {
+					if ifield.has_default_expr {
+						defex := ast.fe2ex(ifield.default_expr)
+						w.expr(defex)
+					}
+				}
+			}
+			if node.has_update_expr {
+				w.expr(node.update_expr)
+			}
 			for sif in node.fields {
 				w.expr(sif.expr)
 			}
@@ -321,13 +334,13 @@ pub fn (mut w Walker) fn_decl(mut node ast.FnDecl) {
 }
 
 pub fn (mut w Walker) call_expr(mut node ast.CallExpr) {
+	for arg in node.args {
+		w.expr(arg.expr)
+	}
 	if node.language == .c {
 		return
 	}
 	w.expr(node.left)
-	for arg in node.args {
-		w.expr(arg.expr)
-	}
 	w.or_block(node.or_block)
 	//
 	fn_name := if node.is_method { node.receiver_type.str() + '.' + node.name } else { node.name }