cgen: fix short struct init with `mut` (#8384)

vlib/v/checker/checker.v

@@ -1122,6 +1122,9 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) (string, token.Position) {
 		ast.ArrayInit {
 			return '', pos
 		}
+		ast.StructInit {
+			return '', pos
+		}
 		else {
 			c.error('unexpected expression `$expr.type_name()`', expr.position())
 		}

vlib/v/gen/cgen.v

@@ -4710,6 +4710,13 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 		} else {
 			g.write('($styp*)memdup(&($styp){')
 		}
+	} else if struct_init.typ.is_ptr() {
+		basetyp := g.typ(struct_init.typ.set_nr_muls(0))
+		if is_multiline {
+			g.writeln('&($basetyp){')
+		} else {
+			g.write('&($basetyp){')
+		}
 	} else {
 		if g.is_shared {
 			g.writeln('{.val = {')

vlib/v/tests/struct_test.v

@@ -231,6 +231,7 @@ fn test_fixed_field() {
 }
 */
 struct Config {
+mut:
 	n   int
 	def int = 10
 }
@@ -241,6 +242,11 @@ fn foo_config(def int, c Config) {
 fn bar_config(c Config, def int) {
 	assert c.def == def
 }
+fn mut_bar_config(mut c Config, def int) &Config {
+	c.n = c.def
+	assert c.n == def
+	return c
+}
 
 fn foo_user(u User) {}
 
@@ -256,6 +262,10 @@ fn test_struct_literal_args() {
 	bar_config({}, 10)
 	bar_config({def:4}, 4)
 
+	c := mut_bar_config(mut {def: 10}, 10)
+	assert c.n == 10
+	assert c.def == 10
+
 	foo_user({
 		name: 'Peter'
 	})