Lifetime issues when trying to return a reference to an element

FeldrinH commented 3 months ago

Since all the frame and element types have an explicit lifetime parameter, I was hoping that I could write a function that takes a reference to bytes and return a reference to an element. Something like this:

pub fn borrow_element<'a>(packet: &'a [u8]) -> Option<VendorSpecificElement<'a>> {
    let frame = packet.pread::<IEEE80211Frame>(0).unwrap();

    if let IEEE80211Frame::Management(mgmt) = frame {
        if let ManagementFrameBody::Beacon(beacon) = mgmt.body {
            if let Some(vendor_specific) = beacon.elements.get_first_element::<VendorSpecificRepr>() {
                return Some(vendor_specific);
            }
        }
    }

    None
}

Unfortunately this does not compile, because the lifetime of the element is for some reason tied to the lifetime of beacon.elements itselt, not the bytes contained within. Compiler error:

error[E0515]: cannot return value referencing local data `beacon.elements`
   --> src\read_frame.rs:174:24
    |
173 |             if let Some(vendor_specific) = beacon.elements.get_first_element::<VendorSpecificRepr>() {
    |                                            --------------- `beacon.elements` is borrowed here
174 |                 return Some(vendor_specific);
    |                        ^^^^^^^^^^^^^^^^^^^^^ returns a value referencing data owned by the current function

I believe the problem starts with this signature:

pub fn get_first_element<ElementType: ElementTypeRepr>(
    &'a self,
) -> Option<<<ElementType as ElementTypeRepr>::ElementType<'a> as Element>::ReadType<'a>>

For my snippet to work the lifetime of &self should not be tied to 'a:

pub fn get_first_element<ElementType: ElementTypeRepr>(
    &self,
) -> Option<<<ElementType as ElementTypeRepr>::ElementType<'a> as Element>::ReadType<'a>>

Taking a quick look at the code, it seems that the problem is relatively pervasive. A number of other functions also have the same issue or something similar. It would be nice if this could be fixed, to make the library more flexible.

PS: Taking a quick look at the code, it seems to me that this should be fixable simply by tweaking some lifetime parameters, with no other changes necessary.