builtin: implement string_from_wide/1 and string_from_wide2/2 on *nix too (#18776)

vlib/builtin/utf8.c.v

@@ -1,5 +1,7 @@
 module builtin
 
+import strings
+
 const cp_utf8 = 65001
 
 // to_wide returns a pointer to an UTF-16 version of the string receiver.
@@ -34,7 +36,7 @@ pub fn (_str string) to_wide() &u16 {
 
 // string_from_wide creates a V string, encoded in UTF-8, given a windows
 // style string encoded in UTF-16.
-[unsafe]
+[manualfree; unsafe]
 pub fn string_from_wide(_wstr &u16) string {
 	$if windows {
 		unsafe {
@@ -42,7 +44,11 @@ pub fn string_from_wide(_wstr &u16) string {
 			return string_from_wide2(_wstr, wstr_len)
 		}
 	} $else {
-		return ''
+		mut i := 0
+		for unsafe { _wstr[i] } != 0 {
+			i++
+		}
+		return unsafe { string_from_wide2(_wstr, i) }
 	}
 }
 
@@ -50,7 +56,7 @@ pub fn string_from_wide(_wstr &u16) string {
 // style string, encoded in UTF-16. It is more efficient, compared to
 // string_from_wide, but it requires you to know the input string length,
 // and to pass it as the second argument.
-[unsafe]
+[manualfree; unsafe]
 pub fn string_from_wide2(_wstr &u16, len int) string {
 	$if windows {
 		unsafe {
@@ -64,6 +70,15 @@ pub fn string_from_wide2(_wstr &u16, len int) string {
 			return tos2(str_to)
 		}
 	} $else {
-		return ''
+		mut sb := strings.new_builder(len)
+		defer {
+			unsafe { sb.free() }
+		}
+		for i := 0; i < len; i++ {
+			u := unsafe { rune(_wstr[i]) }
+			sb.write_rune(u)
+		}
+		res := sb.str()
+		return res
 	}
 }

vlib/builtin/utf8_test.v

@@ -54,3 +54,24 @@ fn test_to_wide_cyrillic() {
 		assert w[5] == 0
 	}
 }
+
+const wide_serial_number = [u8(67), 0, 76, 0, 52, 0, 54, 0, 73, 0, 49, 0, 65, 0, 48, 0, 48, 0,
+	54, 0, 52, 0, 57, 0, 0, 0, 0]
+
+const swide_serial_number = 'CL46I1A00649'
+
+fn test_string_from_wide() {
+	z := unsafe { string_from_wide(wide_serial_number.data) }
+	assert z == swide_serial_number
+}
+
+fn test_string_from_wide2() {
+	z := unsafe { string_from_wide2(wide_serial_number.data, 12) }
+	assert z == swide_serial_number
+}
+
+fn test_reverse_cyrillic_with_string_from_wide() {
+	s := 'Проба'
+	z := unsafe { string_from_wide(s.to_wide()) }
+	assert z == s
+}