v.util: add Surrounder.builder_write_befores and Surrounder.builder_write_afters methods for writing to an existing strings.Builder

vlib/v/gen/c/auto_str_methods.v

@@ -802,9 +802,9 @@ fn (mut g Gen) gen_str_for_struct(info ast.Struct, styp string, str_fn_name stri
 	mut fn_body_surrounder := util.new_surrounder(info.fields.len)
 	mut fn_body := strings.new_builder(info.fields.len * 256)
 	defer {
-		fn_builder.write_string(fn_body_surrounder.before())
+		fn_body_surrounder.builder_write_befores(mut fn_builder)
 		fn_builder << fn_body
-		fn_builder.write_string(fn_body_surrounder.after())
+		fn_body_surrounder.builder_write_afters(mut fn_builder)
 		fn_builder.writeln('\tstring_free(&indents);')
 		fn_builder.writeln('\treturn res;')
 		fn_builder.writeln('}')

vlib/v/gen/c/dumpexpr.v

@@ -57,7 +57,7 @@ eprint(res);
 string_free(&res);
 strings__Builder_free(&sb);
 ')
-		dump_fns.writeln(surrounder.before())
+		surrounder.builder_write_befores(mut dump_fns)
 		dump_fns.writeln("\tstrings__Builder_write_rune(&sb, '[');")
 		dump_fns.writeln('\tstrings__Builder_write_string(&sb, fpath);')
 		dump_fns.writeln("\tstrings__Builder_write_rune(&sb, ':');")
@@ -72,7 +72,7 @@ strings__Builder_free(&sb);
 		}
 		dump_fns.writeln('\tstrings__Builder_write_string(&sb, value);')
 		dump_fns.writeln("\tstrings__Builder_write_rune(&sb, '\\n');")
-		dump_fns.writeln(surrounder.after())
+		surrounder.builder_write_afters(mut dump_fns)
 		dump_fns.writeln('\treturn dump_arg;')
 		dump_fns.writeln('}')
 	}

vlib/v/util/surrounder.v

@@ -61,6 +61,30 @@ pub fn (s &Surrounder) after() string {
 	return ''
 }
 
+pub fn (s &Surrounder) builder_write_befores(mut sb strings.Builder) {
+	len := s.befores.len
+	if len > 0 {
+		for i := 0; i < len; i++ {
+			x := &s.befores[i]
+			if x.len > 0 {
+				sb.writeln(x)
+			}
+		}
+	}
+}
+
+pub fn (s &Surrounder) builder_write_afters(mut sb strings.Builder) {
+	len := s.afters.len
+	if len > 0 {
+		for i := len - 1; i >= 0; i-- {
+			x := &s.afters[i]
+			if x.len > 0 {
+				sb.writeln(x)
+			}
+		}
+	}
+}
+
 [unsafe]
 pub fn (mut s Surrounder) free() {
 	unsafe {