log: avoid using string__plus memory leak (#11128)

vlib/term/colors.v

@@ -4,12 +4,11 @@
 module term
 
 pub fn format(msg string, open string, close string) string {
-	return '\x1b[' + open + 'm' + msg + '\x1b[' + close + 'm'
+	return '\x1b[${open}m$msg\x1b[${close}m'
 }
 
 pub fn format_rgb(r int, g int, b int, msg string, open string, close string) string {
-	return '\x1b[' + open + ';2;' + r.str() + ';' + g.str() + ';' + b.str() + 'm' + msg + '\x1b[' +
-		close + 'm'
+	return '\x1b[$open;2;$r;$g;${b}m$msg\x1b[${close}m'
 }
 
 pub fn rgb(r int, g int, b int, msg string) string {

vlib/time/format.v

@@ -117,7 +117,7 @@ pub fn (t Time) get_fmt_date_str(fmt_dlmtr FormatDelimiter, fmt_date FormatDate)
 	if fmt_date == .no_date {
 		return ''
 	}
-	month := '$t.smonth()'
+	month := t.smonth()
 	year := '${(t.year % 100):02d}'
 	mut res := match fmt_date {
 		.ddmmyy { '${t.day:02d}|${t.month:02d}|$year' }
@@ -156,7 +156,9 @@ pub fn (t Time) get_fmt_str(fmt_dlmtr FormatDelimiter, fmt_time FormatTime, fmt_
 		}
 	} else {
 		if fmt_time != .no_time {
-			return t.get_fmt_date_str(fmt_dlmtr, fmt_date) + ' ' + t.get_fmt_time_str(fmt_time)
+			dstr := t.get_fmt_date_str(fmt_dlmtr, fmt_date)
+			tstr := t.get_fmt_time_str(fmt_time)
+			return '$dstr $tstr'
 		} else {
 			return t.get_fmt_date_str(fmt_dlmtr, fmt_date)
 		}

vlib/v/tests/valgrind/logging.v

@@ -0,0 +1,18 @@
+import log
+import time
+
+fn main() {
+	mut l := log.Log{}
+	defer {
+		l.close()
+	}
+	l.set_level(.info)
+	l.info('info')
+	l.warn('warn')
+	l.error('an error')
+	l.debug('some debug info')
+	for i := 0; i < 100; i++ {
+		l.info('123456')
+		time.sleep(1 * time.millisecond)
+	}
+}