scanner: add error for invalid newline rune literal, make errors more informative (#19091)

vlib/v/scanner/scanner.v

@@ -1472,17 +1472,23 @@ fn (mut s Scanner) ident_char() string {
 		u := c.runes()
 		if u.len != 1 {
 			if escaped_hex || escaped_unicode {
-				s.error('invalid character literal `${orig}` => `${c}` (${u}) (escape sequence did not refer to a singular rune)')
+				s.error_with_pos('invalid character literal `${orig}` => `${c}` (${u}) (escape sequence did not refer to a singular rune)',
+					lspos)
 			} else if u.len == 0 {
 				s.add_error_detail_with_pos('use quotes for strings, backticks for characters',
 					lspos)
-				s.error('invalid empty character literal `${orig}`')
+				s.error_with_pos('invalid empty character literal `${orig}`', lspos)
 			} else {
 				s.add_error_detail_with_pos('use quotes for strings, backticks for characters',
 					lspos)
-				s.error('invalid character literal `${orig}` => `${c}` (${u}) (more than one character)')
+				s.error_with_pos('invalid character literal `${orig}` => `${c}` (${u}) (more than one character)',
+					lspos)
 			}
 		}
+	} else if c == '\n' {
+		s.add_error_detail_with_pos('use quotes for strings, backticks for characters',
+			lspos)
+		s.error_with_pos('invalid character literal, use \`\\n\` instead', lspos)
 	}
 	// Escapes a `'` character
 	if c == "'" {
@@ -1566,15 +1572,19 @@ fn (mut s Scanner) eat_details() string {
 }
 
 pub fn (mut s Scanner) warn(msg string) {
-	if s.pref.warns_are_errors {
-		s.error(msg)
-		return
-	}
 	pos := token.Pos{
 		line_nr: s.line_nr
 		pos: s.pos
 		col: s.current_column() - 1
 	}
+	s.warn_with_pos(msg, pos)
+}
+
+pub fn (mut s Scanner) warn_with_pos(msg string, pos token.Pos) {
+	if s.pref.warns_are_errors {
+		s.error_with_pos(msg, pos)
+		return
+	}
 	details := s.eat_details()
 	if s.pref.output_mode == .stdout && !s.pref.check_only {
 		util.show_compiler_message('warning:',
@@ -1604,6 +1614,10 @@ pub fn (mut s Scanner) error(msg string) {
 		pos: s.pos
 		col: s.current_column() - 1
 	}
+	s.error_with_pos(msg, pos)
+}
+
+pub fn (mut s Scanner) error_with_pos(msg string, pos token.Pos) {
 	details := s.eat_details()
 	if s.pref.output_mode == .stdout && !s.pref.check_only {
 		util.show_compiler_message('error:',

vlib/v/scanner/tests/empty_character_literal_err.out

@@ -1,4 +1,4 @@
-vlib/v/scanner/tests/empty_character_literal_err.vv:2:8: error: invalid empty character literal ``
+vlib/v/scanner/tests/empty_character_literal_err.vv:2:7: error: invalid empty character literal ``
     1 | fn main() {
     2 |     a := ``
       |          ^

vlib/v/scanner/tests/newline_character_literal_err.out

@@ -0,0 +1,13 @@
+vlib/v/scanner/tests/newline_character_literal_err.vv:2:7: error: invalid character literal, use `\n` instead
+    1 | fn main() {
+    2 |     a := `
+      |          ^
+    3 | `
+    4 |     println(a)
+Details:
+vlib/v/scanner/tests/newline_character_literal_err.vv:2:7: details: use quotes for strings, backticks for characters
+    1 | fn main() {
+    2 |     a := `
+      |          ^
+    3 | `
+    4 |     println(a)

vlib/v/scanner/tests/newline_character_literal_err.vv

@@ -0,0 +1,5 @@
+fn main() {
+	a := `
+`
+	println(a)
+}