all: add string range OrExpr (#13189)

vlib/builtin/string.v

@@ -818,6 +818,32 @@ pub fn (s string) substr(start int, end int) string {
 	return res
 }
 
+// version of `substr()` that is used in `a[start..end] or {`
+// return an error when the index is out of range
+[direct_array_access]
+pub fn (s string) substr_with_check(start int, end int) ?string {
+	if start > end || start > s.len || end > s.len || start < 0 || end < 0 {
+		return error('substr($start, $end) out of bounds (len=$s.len)')
+	}
+	len := end - start
+	if len == s.len {
+		return s.clone()
+	}
+	mut res := string{
+		str: unsafe { malloc_noscan(len + 1) }
+		len: len
+	}
+	for i in 0 .. len {
+		unsafe {
+			res.str[i] = s.str[start + i]
+		}
+	}
+	unsafe {
+		res.str[len] = 0
+	}
+	return res
+}
+
 // substr_ni returns the string between index positions `start` and `end` allowing negative indexes
 // This function always return a valid string.
 [direct_array_access]

vlib/builtin/string_test.v

@@ -120,6 +120,34 @@ fn test_sort_reverse() {
 	assert vals[3] == 'arr'
 }
 
+fn test_ranges() {
+	s := 'test'
+	s1 := s[0..20] or { 'both' }
+	s2 := s[..20] or { 'last' }
+	s3 := s[10..] or { 'first' }
+	s4 := ranges_propagate_both(s) or { 'both' }
+	s5 := ranges_propagate_last(s) or { 'last' }
+	s6 := ranges_propagate_first(s) or { 'first' }
+	assert s1 == 'both'
+	assert s2 == 'last'
+	assert s3 == 'first'
+	assert s4 == 'both'
+	assert s5 == 'last'
+	assert s6 == 'first'
+}
+
+fn ranges_propagate_first(s string) ?string {
+	return s[10..] ?
+}
+
+fn ranges_propagate_last(s string) ?string {
+	return s[..20] ?
+}
+
+fn ranges_propagate_both(s string) ?string {
+	return s[1..20] ?
+}
+
 fn test_split_nth() {
 	a := '1,2,3'
 	assert a.split(',').len == 3