Bring back suggestion for splitting <- into < -

Closes #62632

src/libsyntax/parse/parser/expr.rs

@@ -224,6 +224,10 @@ impl<'a> Parser<'a> {
                 self.err_dotdotdot_syntax(self.token.span);
             }
 
+            if self.token == token::LArrow {
+                self.err_larrow_operator(self.token.span);
+            }
+
             self.bump();
             if op.is_comparison() {
                 self.check_no_chained_comparison(&lhs, &op);
@@ -1702,6 +1706,19 @@ impl<'a> Parser<'a> {
             .emit();
     }
 
+    fn err_larrow_operator(&self, span: Span) {
+        self.struct_span_err(
+            span,
+            "unexpected token: `<-`"
+        ).span_suggestion(
+            span,
+            "if you meant to write a comparison against a negative value, add a \
+             space in between `<` and `-`",
+            "< -".to_string(),
+            Applicability::MaybeIncorrect
+        ).emit();
+    }
+
     fn mk_assign_op(&self, binop: BinOp, lhs: P<Expr>, rhs: P<Expr>) -> ExprKind {
         ExprKind::AssignOp(binop, lhs, rhs)
     }