A struct may hold references, and then it needs a lifetime parameter:

struct Frame<'a> {
    method: &'a str,
    params: &'a str,
}

The parameter is a promise: an instance of Frame may not outlive the buffer it points into. The compiler enforces it, so the struct can never be left holding a dangling pointer into a freed allocation.

This is the shape of every zero-copy parser. Instead of allocating a String per field, you hand out slices of a buffer someone else owns.