parser: add an error for an invalid string prefix (#7019)

vlib/v/parser/parser.v

@@ -1037,8 +1037,13 @@ pub fn (mut p Parser) name_expr() ast.Expr {
 		}
 	}
 	// Raw string (`s := r'hello \n ')
-	if p.tok.lit in ['r', 'c', 'js'] && p.peek_tok.kind == .string && !p.inside_str_interp {
+	if p.peek_tok.kind == .string && !p.inside_str_interp && p.peek_tok2.kind != .colon {
-		return p.string_expr()
+		if p.tok.lit in ['r', 'c', 'js'] && p.tok.kind == .name {
+			return p.string_expr()
+		} else {
+			// don't allow any other string prefix except `r`, `js` and `c`
+			p.error('only `c`, `r`, `js` are recognized string prefixes, but you tried to use `$p.tok.lit`')
+		}
 	}
 	// don't allow r`byte` and c`byte`
 	if p.tok.lit in ['r', 'c'] && p.peek_tok.kind == .chartoken {

vlib/v/parser/tests/string_invalid_prefix_err.out

@@ -0,0 +1,6 @@
+vlib/v/parser/tests/string_invalid_prefix_err.vv:2:12: error: only `c`, `r`, `js` are recognized string prefixes, but you tried to use `w`
+    1 | fn main() {
+    2 |     why := w'why'
+      |            ^
+    3 |     println(why)
+    4 | }

vlib/v/parser/tests/string_invalid_prefix_err.vv

@@ -0,0 +1,4 @@
+fn main() {
+    why := w'why'
+    println(why)
+}