diff --git a/compiler/fn.v b/compiler/fn.v
index ea6250fa9e..7e761640f2 100644
--- a/compiler/fn.v
+++ b/compiler/fn.v
@@ -443,8 +443,8 @@ fn (p mut Parser) async_fn_call(f Fn, method_ph int, receiver_var, receiver_type
 	// Normal function => just its name, method => TYPE_FNNAME
 	mut fn_name := f.name
 	if f.is_method {
-		_receiver_type := receiver_type.replace('*', '')
-		fn_name = '${_receiver_type}_${f.name}'
+		receiver_type = receiver_type.replace('*', '')
+		fn_name = '${receiver_type}_${f.name}'
 	}
 	// Generate tmp struct with args
 	arg_struct_name := 'thread_arg_$fn_name'
diff --git a/compiler/jsgen.v b/compiler/jsgen.v
index bbe4972715..5b62c098ce 100644
--- a/compiler/jsgen.v
+++ b/compiler/jsgen.v
@@ -115,8 +115,8 @@ fn is_js_prim(typ string) bool {
 	typ == 'i8' || typ == 'i16' || typ == 'i32' || typ == 'i64'
 }
 
-fn (p mut Parser) decode_array(_typ string) string {
-	typ := _typ.replace('array_', '')
+fn (p mut Parser) decode_array(typ string) string {
+	typ = typ.replace('array_', '')
 	t := p.table.find_type(typ)
 	fn_name := js_dec_name(typ)
 	// If we have `[]Profile`, have to register a Profile en(de)coder first
@@ -149,8 +149,8 @@ fn js_dec_name(typ string) string {
 	return name
 }
 
-fn (p &Parser) encode_array(_typ string) string {
-	typ := _typ.replace('array_', '')
+fn (p &Parser) encode_array(typ string) string {
+	typ = typ.replace('array_', '')
 	fn_name := js_enc_name(typ)
 	return '
 o = cJSON_CreateArray();
diff --git a/compiler/main.v b/compiler/main.v
index dfd567025c..93a9df14ca 100644
--- a/compiler/main.v
+++ b/compiler/main.v
@@ -418,7 +418,7 @@ string _STR_TMP(const char *fmt, ...) {
 	}
 }
 
-fn (c mut V) cc_windows_cross() {
+fn (c &V) cc_windows_cross() {
        if !c.out_name.ends_with('.exe') {
                c.out_name = c.out_name + '.exe'
        }
diff --git a/compiler/parser.v b/compiler/parser.v
index 529a43d1c4..a6101d24e5 100644
--- a/compiler/parser.v
+++ b/compiler/parser.v
@@ -1044,8 +1044,7 @@ fn (p mut Parser) statement(add_semi bool) string {
 fn (p mut Parser) assign_statement(v Var, ph int, is_map bool) {
 	p.log('assign_statement() name=$v.name tok=')
 	tok := p.tok
-	//if !v.is_mut && !v.is_arg && !p.pref.translated && !v.is_global{
-	if !v.is_mut && !p.pref.translated && !v.is_global{
+	if !v.is_mut && !v.is_arg && !p.pref.translated && !v.is_global{
 		p.error('`$v.name` is immutable')
 	}
 	is_str := v.typ == 'string'
@@ -1455,7 +1454,7 @@ fn (p mut Parser) var_expr(v Var) string {
 	}
 	// a++ and a--
 	if p.tok == INC || p.tok == DEC {
-		if !v.is_mut && !p.pref.translated {
+		if !v.is_mut && !v.is_arg && !p.pref.translated {
 			p.error('`$v.name` is immutable')
 		}
 		if typ != 'int' {
@@ -1582,12 +1581,11 @@ fn (p mut Parser) dot(str_typ string, method_ph int) string {
 	return method.typ
 }
 
-fn (p mut Parser) index_expr(_typ string, fn_ph int) string {
+fn (p mut Parser) index_expr(typ string, fn_ph int) string {
 	//if p.fileis('main.v') {
 		//println('index expr typ=$typ')
 	//}
 	// a[0]
-	mut typ := _typ 
 	v := p.expr_var
 	is_map := typ.starts_with('map_')
 	is_str := typ == 'string'
@@ -2135,8 +2133,8 @@ fn (p mut Parser) typ_to_fmt(typ string) string {
 	return ''
 }
 
-fn format_str(_str string) string {
-	mut str := _str.replace('"', '\\"')
+fn format_str(str string) string {
+	str = str.replace('"', '\\"')
 	$if windows {
 		str = str.replace('\r\n', '\\n')
 	} 
@@ -3079,8 +3077,7 @@ fn (p mut Parser) go_statement() {
 	}
 }
 
-fn (p mut Parser) register_var(var Var) {
-	mut v := var 
+fn (p mut Parser) register_var(v Var) {
 	if v.line_nr == 0 {
 		v.line_nr = p.scanner.line_nr
 	}
@@ -3144,8 +3141,8 @@ fn (p mut Parser) js_decode() string {
 	return ''
 }
 
-fn is_compile_time_const(_s string) bool {
-	s := _s.trim_space()
+fn is_compile_time_const(s string) bool {
+	s = s.trim_space()
 	if s == '' {
 		return false
 	}
@@ -3169,17 +3166,15 @@ fn (p &Parser) building_v() bool {
 ///////////////////////////////////////////////////////////////////////////////
 
 // fmt helpers
-fn (scanner mut Scanner) fgen(_s string) {
-	mut s := _s 
+fn (scanner mut Scanner) fgen(s string) {
 	if scanner.fmt_line_empty {
-		s = strings.repeat(`\t`, scanner.fmt_indent) + s 
+		s = strings.repeat(`\t`, scanner.fmt_indent) + s
 	}
 	scanner.fmt_out.write(s)
 	scanner.fmt_line_empty = false
 }
 
-fn (scanner mut Scanner) fgenln(_s string) {
-	mut s := _s 
+fn (scanner mut Scanner) fgenln(s string) {
 	if scanner.fmt_line_empty {
 		s = strings.repeat(`\t`, scanner.fmt_indent) + s
 	}
diff --git a/compiler/table.v b/compiler/table.v
index 209d9c8a8e..1de6cbcfb5 100644
--- a/compiler/table.v
+++ b/compiler/table.v
@@ -184,8 +184,7 @@ fn (t mut Table) register_fn(new_fn Fn) {
 	t.fns[new_fn.name] = new_fn 
 }
 
-fn (table &Table) known_type(_typ string) bool {
-	mut typ := _typ 
+fn (table &Table) known_type(typ string) bool {
 	// 'byte*' => look up 'byte', but don't mess up fns
 	if typ.ends_with('*') && !typ.contains(' ') {
 		typ = typ.left(typ.len - 1)
@@ -381,8 +380,7 @@ fn (p &Parser) find_type(name string) *Type {
 	return typ
 }
 
-fn (t &Table) find_type(_name string) *Type {
-	mut name := _name 
+fn (t &Table) find_type(name string) *Type {
 	if name.ends_with('*') && !name.contains(' ') {
 		name = name.left(name.len - 1)
 	}
@@ -395,9 +393,7 @@ fn (t &Table) find_type(_name string) *Type {
 	return &Type{}
 }
 
-fn (p mut Parser) _check_types(_got, _expected string, throw bool) bool {
-	mut expected := _expected 
-	mut got := _got 
+fn (p mut Parser) _check_types(got, expected string, throw bool) bool {
 	p.log('check types got="$got" exp="$expected"  ')
 	if p.pref.translated {
 		return true
@@ -522,8 +518,8 @@ fn (p mut Parser) satisfies_interface(interface_name, _typ string, throw bool) b
 
 fn type_default(typ string) string {
 	if typ.starts_with('array_') {
-		elm_type := typ.right(6) 
-		return 'new_array(0, 1, sizeof($elm_type))'
+		typ = typ.right(6)
+		return 'new_array(0, 1, sizeof($typ))'
 	}
 	// Always set pointers to 0
 	if typ.ends_with('*') {
diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index add140a477..eade49afe7 100644
--- a/vlib/builtin/string.v
+++ b/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]
-	_end := if end >= u.runes.len {
-		u.s.len
+	start = u.runes[start]
+	if end >= u.runes.len {
+		end = u.s.len
 	}
 	else {
-		u.runes[end]
+		end = u.runes[end]
 	}
-	return u.s.substr(_start, _end)
+	return u.s.substr(start, end)
 }
 
 fn (u ustring) left(pos int) string {
diff --git a/vlib/os/os.v b/vlib/os/os.v
index 92a919ffac..83c1ffe043 100644
--- a/vlib/os/os.v
+++ b/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
diff --git a/vlib/strings/builder.v b/vlib/strings/builder.v
index ecc55c1cf5..11f34c6127 100644
--- a/vlib/strings/builder.v
+++ b/vlib/strings/builder.v
@@ -33,7 +33,7 @@ pub fn (b Builder) str() string {
 	return tos(b.buf.data, b.len)
 }
 
-pub fn (b mut Builder) cut(n int) {
+pub fn (b Builder) cut(n int) {
 	b.len -= n
 }