parser: require ++/-- to be on the same line as the previous token (#8621)

vlib/v/parser/pratt.v

@@ -377,6 +377,10 @@ pub fn (mut p Parser) expr_with_left(left ast.Expr, precedence int, is_stmt_iden
 				p.warn_with_pos('`$p.tok.kind` operator can only be used as a statement',
 					p.peek_tok.position())
 			}
+			if p.tok.kind in [.inc, .dec] && p.prev_tok.line_nr != p.tok.line_nr {
+				p.error_with_pos('$p.tok must be on the same line as the previous token',
+					p.tok.position())
+			}
 			node = ast.PostfixExpr{
 				op: p.tok.kind
 				expr: node

vlib/v/parser/tests/postfix_inc.out

@@ -0,0 +1,5 @@
+vlib/v/parser/tests/postfix_inc.vv:3:1: error: `++` must be on the same line as the previous token
+    1 | mut v := 4
+    2 | _ = v
+    3 | ++v
+      | ~~

vlib/v/parser/tests/postfix_inc.vv

@@ -0,0 +1,3 @@
+mut v := 4
+_ = v
+++v