cgen: fix enumval str() call on stringinterliteral (fix #24702) (#24705)

vlib/v/gen/c/str.v

@@ -98,12 +98,15 @@ fn (mut g Gen) gen_expr_to_string(expr ast.Expr, etype ast.Type) {
 		}
 		g.write('_S("<none>")')
 	} else if sym.kind == .enum {
-		if expr !is ast.EnumVal {
+		if expr !is ast.EnumVal || sym.has_method('str') {
 			str_fn_name := g.get_str_fn(typ)
 			g.write('${str_fn_name}(')
 			if typ.nr_muls() > 0 {
 				g.write('*'.repeat(typ.nr_muls()))
 			}
+			if expr is ast.EnumVal {
+				g.write2(sym.cname, '__')
+			}
 			g.enum_expr(expr)
 			g.write(')')
 		} else {

vlib/v/tests/enums/enum_val_test.v

@@ -0,0 +1,26 @@
+enum Traffic {
+	rx_bytes
+	tx_bytes
+}
+
+fn (t Traffic) str() string {
+	return match t {
+		.rx_bytes { 'rx-bytes' }
+		.tx_bytes { 'tx-bytes' }
+	}
+}
+
+fn Traffic.from_string(s string) ?Traffic {
+	return match s {
+		'rx-bytes' { .rx_bytes }
+		'tx-bytes' { .tx_bytes }
+		else { none }
+	}
+}
+
+fn test_main() {
+	traffic := Traffic.from_string('rx-bytes') or { return }
+	assert Traffic.rx_bytes.str() == 'rx-bytes'
+	assert '${traffic}' == 'rx-bytes'
+	assert '${Traffic.rx_bytes}' == 'rx-bytes'
+}