i by chance saw James Munns's suggestion [here](https://github.com/jamesmunns/postcard-rpc/pull/97/files#r2021202132) that the serial number of the USB device should come from the  device itself. i now tried combining this with [this example for grabbing a unique ID from an RP2040](https://github.com/jamesmunns/postcard-rpc/blob/5d9a58d7e7ce084f9ad4a2f1b03279288efb2353/example/firmware/src/lib.rs#L33-L46).
this gives me a `u64`, but embassy expects an `Option<&'a str>` - where `'a` will have to be `'static` due to the prpc APIs (`postcard_rpc::embassy_usb_0_4::dispatch_impl::init`).

i can convert `Option<u64>` to `Option<heapless::String>` with no issues:
```rust
    let serial_number = get_unique_id(&mut p.FLASH)
        .map(String::try_from)
        .map(|x| x.ok())
        .flatten();
```

but how do i get from here to an `Option<&'static str>`?

using `serial_number.map(|x| x.as_str())` does not work as i don't get a static lifetime.

i'm not sure if this is (a) a prpc question (and would maybe require the prpc API to change to take `&'a` rather than `&'static`), (b) an embassy question or (c) a general rust (maybe no-alloc rust?) question