prokopyl / selfie Goto Github PK

Salut Adrien,

Thank you for selfie.

This, I believe, is not an issue with selfie, but rather my understanding of Pin, or a difficulty with Rust types/lifetimes. Instead of having a selfie-based type only used for a local variable (and hence inferred), I'd like to use it as a part of a larger struct (or enum with data field(s)). Is that possible, and how?

I understand that the whole struct/enum could not be moved. Would the whole struct/enum have to be used inside Pin, or could it be an ordinary struct (or enum) but not a Copy and with custom clone()?

use selfie::{SelfieMut, refs::Ref, refs::RefType};

type Inner<'a> where Ref<&'a str>: for<'this> RefType<'this> = SelfieMut<'a, [u8; 8], Ref<&'a str>>;

// same error when I add this bound - probably redundant, based on https://github.com/rust-lang/rust/issues/70263 (which is not Pin-specific, however)
// type Inner<'a> where Ref<&'a str>: for<'this> RefType<'this> = SelfieMut<'a, [u8; 8], Ref<&'a str>>;

pub struct Outer<'a> {
    inner: Inner<'a>,
}

which fails with: implementation of RefType is not general enough

So in particular, your function signatures for referential, with_referential, and with_referential_mut are all unsound.

The latter two can be fixed, look at the kinds of signatures that ouroboros or self_cell generate to see what approach can work soundly. On the other hand, the function referential is entirely impossible to write soundly, as far as I’m aware, without resorting to macros or unsafe traits, in order to check for covariance so that the lifetime of <R as RefType<'s>>::Ref is legal to change.

“But where did any lifetimes change?” – the change happens by the fact that the callbacks or return value in this API receives a reference to <R as RefType<'s>>::Ref where 's is taken from the lifetime of &'s self. Since every borrow of self in subsequent calls is a new borrow with a different lifetime, you just give inconsistently typed views of <R as RefType<'s>>::Ref with different short-lived lifetimes, which will be unsound if 's is not covariant in <R as RefType<'s>>::Ref (with_referential_mut is unsound regardless of variance).

For example, if you use <R as RefType<'s>>::Ref = Cell<&'a …>, then one call to with_referential/with_referential_mut allows placing something way-too-shortlived into that cell, and the next call can access it after free:

use selfie::{refs::RefType, Selfie};
use std::cell::Cell;

struct CellRefStr;
impl<'a> RefType<'a> for CellRefStr {
    type Ref = Cell<&'a str>;
}

fn main() {
    let x = Selfie::<_, CellRefStr>::new(Box::pin(()), |_| Cell::new(""));
    let s = String::from("Hello World");
    x.with_referential(|r| r.set(&s));
    let r: &str = x.with_referential(|r| r.get());
    println!("String exists: {r}");
    drop(s);
    println!("String used after free: {r}");
}

Of course, the above works just as well with with_referential_mut.

For referential, we need a Copy type; but for instance &'a Cell<&'a …> will work fine (with a little bit of leaking in order to construct it).

use selfie::{refs::RefType, Selfie};
use std::cell::Cell;

struct RefCellRefStr;
impl<'a> RefType<'a> for RefCellRefStr {
    type Ref = &'a Cell<&'a str>;
}

fn main() {
    let x = Selfie::<_, RefCellRefStr>::new(Box::pin(()), |_| Box::leak(Box::new(Cell::new(""))));
    let s = String::from("Hello World");
    x.referential().set(&s);
    let r: &str = x.referential().get();
    println!("String exists: {r}");
    drop(s);
    println!("String used after free: {r}");
}