scanner/parser: change error msgs for `\0` and `\x00` and add error for r`byte` and c`byte` (#6436)

vlib/builtin/string_test.v

@@ -729,6 +729,12 @@ fn test_raw() {
 	raw2 := r'Hello V\0'
 	assert raw2[7] == `\\`
 	assert raw2[8] == `0`
+
+	raw3 := r'Hello V\x00'
+	assert raw3[7] == `\\`
+	assert raw3[8] == `x`
+	assert raw3[9] == `0`
+	assert raw3[10] == `0`	
 }
 
 fn test_raw_with_quotes() {

vlib/v/checker/checker.v

@@ -1358,7 +1358,7 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 		return f.return_type
 	}
 	// println can print anything
-	if (fn_name == 'println' || fn_name == 'print') && call_expr.args.len > 0 {
+	if fn_name in ['println', 'print'] && call_expr.args.len > 0 {
 		c.expected_type = table.string_type
 		call_expr.args[0].typ = c.expr(call_expr.args[0].expr)
 		// check optional argument

vlib/v/checker/tests/string_char_null_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/string_char_null_err.vv:2:31: error: 0 character in a string literal
+vlib/v/checker/tests/string_char_null_err.vv:2:31: error: cannot use `\0` (NULL character) in the string literal
     1 | fn main() {
     2 |     println('Null character: \0')
       |                               ^

vlib/v/parser/parser.v

@@ -994,6 +994,11 @@ pub fn (mut p Parser) name_expr() ast.Expr {
 	if p.tok.lit in ['r', 'c', 'js'] && p.peek_tok.kind == .string && !p.inside_str_interp {
 		return p.string_expr()
 	}
+	// don't allow r`byte` and c`byte`
+	if p.tok.lit in ['r', 'c'] && p.peek_tok.kind == .chartoken {
+		opt := if p.tok.lit == 'r' { '`r` (raw string)' } else { '`c` (c string)' }
+		p.error('cannot use $opt with `byte` and `rune`')
+	}
 	known_var := p.mark_var_as_used(p.tok.lit)
 	mut is_mod_cast := false
 	if p.peek_tok.kind == .dot && !known_var &&

vlib/v/scanner/scanner.v

@@ -1218,14 +1218,14 @@ fn (mut s Scanner) ident_string() string {
 		if c == `0` && s.pos > 2 && s.text[s.pos - 1] == slash {
 			if (s.pos < s.text.len - 1 && s.text[s.pos + 1].is_digit()) || s.count_symbol_before(s.pos - 1, slash) % 2 == 0 {
 			} else if !is_cstr && !is_raw {
-				s.error('0 character in a string literal')
+				s.error(r'cannot use `\0` (NULL character) in the string literal')
 			}
 		}
 		// Don't allow \x00
 		if c == `0` && s.pos > 5 && s.expect('\\x0', s.pos - 3) {
 			if s.count_symbol_before(s.pos - 3, slash) % 2 == 0 {
 			} else if !is_cstr && !is_raw {
-				s.error('0 character in a string literal')
+				s.error(r'cannot use `\x00` (NULL character) in the string literal')
 			}
 		}
 		// ${var} (ignore in vfmt mode)