Can't wrap 2 structs in Ouroboros

[culebron]

Struct FgbReader takes mutable reference to opened file [definition of the struct], so after I open an FgbReader, I must keep both vars within one scope (may pass FgbReader down the stack, but not the File).

	let mut myfile = File::open("local.fgb")?;
	let myreader: FgbReader<'a, File, FeaturesSelectedSeek> =
            FgbReader::open(&mut myfile)?.select_all()?;
    while let Ok(...) = myreader.next() { ... }

I'm trying to pack both myfile and myreader into a self-ref struct (like what's been discussed in this thread) with Ouroboros. After looking at examples, here's what I came up with:

#[self_referencing]
struct FgbDriver<'a> {
	fp: File,
	#[borrows(mut fp)]
	features_selected: FgbReader<'a, File, FeaturesSelectedSeek>,
	_p: PhantomData<&'a bool>

}

fn make_fgb<'a>(path: &str) -> Result<FgbDriver<'a>, Box<dyn Error>> {
	let mut fp = File::open(path)?;
	Ok(FgbDriverBuilder {
		fp,
		features_selected_builder: |fp| FgbReader::open(fp).unwrap().select_all().unwrap()
	}.build())
}

Compiler tells me that 'this is unused, and I should put it in PhantomData:

5 | #[self_referencing]
  | ^^^^^^^^^^^^^^^^^^^ unused parameter
  |
  = help: consider removing `'this`, referring to it in a field, or using a marker such as `std::marker::PhantomData`

Switching _p to 'this results in lifetime errors:

_p: PhantomData<&'this bool>

Errors:

10 |     _p: PhantomData<&'this bool>
   |         ^^^^^^^^^^^^^^^^^^^^^^^^

error[E0261]: use of undeclared lifetime name `'this`
  --> src/main.rs:10:19
   |
5  | #[self_referencing]
   | ------------------- lifetime `'this` is missing in item created through this procedural macro
...
10 |     _p: PhantomData<&'this bool>
   |                      ^^^^^ undeclared lifetime

error: lifetime may not live long enough
  --> src/main.rs:10:2
   |
6  | struct FgbDriver<'a> {
   |                  -- lifetime `'a` defined here
...
10 |     _p: PhantomData<&'this bool>
   |     ^^ assignment requires that `'a` must outlive `'static`

So, if I put 'this somewhere in the struct, compiler says it's undeclared. If I put it into struct definition, like FgbDriver<'a, 'this>, compiler tells me it shadows 'this in macro.

What should I do?

[steffahn]

The ouroboros macro is defined such that 'this is only usable in fields marked as #[borrows(…)] which is why putting 'this into the phantom data field failed. You don’t want either of the <'a> nor the marker field though. Simply

#[self_referencing]
struct FgbDriver {
    fp: File,
    #[borrows(mut fp)]
    #[covariant]
    features_selected: FgbReader<'this, File, FeaturesSelectedSeek>,
}

---

The compilation error about 'this being unused didn’t talk of the FgbDriver struct, but one of the helper struct generated by the self_referencing macro, so the compiler suggestions of adding a PhantomData do not apply, since you wouldn’t be directly modifying the struct in question; due to how the macro is implemented, only using 'this in one of the marked-#[borrows(…)] struct can fix that issue.

[culebron]

Great, this worked!

Ouroboros that you suggested in that thread was the only solution to my struggles with FlatGeobuf crate.