make function arguments immutable

vlib/builtin/string.v

@@ -642,14 +642,14 @@ pub fn (s string) ustring_tmp() ustring {
 }
 
 fn (u ustring) substr(start, end int) string {
-	start = u.runes[start]
-	if end >= u.runes.len {
-		end = u.s.len
+	_start := u.runes[start]
+	_end := if end >= u.runes.len {
+		u.s.len
 	}
 	else {
-		end = u.runes[end]
+		u.runes[end]
 	}
-	return u.s.substr(start, end)
+	return u.s.substr(_start, _end)
 }
 
 fn (u ustring) left(pos int) string {

vlib/os/os.v

@@ -262,8 +262,8 @@ fn popen(path string) *FILE {
 }
 
 // exec starts the specified command, waits for it to complete, and returns its output.
-pub fn exec(cmd string) string {
-	cmd = '$cmd 2>&1'
+pub fn exec(_cmd string) string {
+	cmd := '$_cmd 2>&1'
 	f := popen(cmd) 
 	if isnil(f) {
 		// TODO optional or error code 
@@ -331,8 +331,8 @@ pub fn dir_exists(path string) bool {
 // mkdir creates a new directory with the specified path.
 pub fn mkdir(path string) {
 	$if windows {
-		path = path.replace('/', '\\')
-		C.CreateDirectory(path.cstr(), 0)
+		_path := path.replace('/', '\\')
+		C.CreateDirectory(_path.cstr(), 0)
 	}
 	$else {
 		C.mkdir(path.cstr(), 511)// S_IRWXU | S_IRWXG | S_IRWXO

vlib/strings/builder.v

@@ -33,7 +33,7 @@ pub fn (b Builder) str() string {
 	return tos(b.buf.data, b.len)
 }
 
-pub fn (b Builder) cut(n int) {
+pub fn (b mut Builder) cut(n int) {
 	b.len -= n
 }