Trait that automatically implies several From implementations?

help
6

Here is part of my real code (actually example code of the documentation I'm working on). But this might actually be more confusing, and I don't need help in fixing the particular code yet. But since you asked, here it is:

use std::rc::Rc;
use std::cell::RefCell;
use my_sandboxing_system::prelude::*;
use my_sandboxing_system::{Callback, Compile, Machine};

fn collect_output<'a, 'b, M, C>(
    machine: &'b M,
    setup: C,
    run: C,
) -> Result<String, MachineError>
where
    M: Machine<'a> + Compile<'a, C> + Callback<'a>,
    for<'c> <M as Machine<'a>>::Datum<'b, 'c>: MaybeString<'c>,
    // I want to avoid this:
    for<'c, 'd> &'d <M as Machine<'a>>::Datum<'b, 'c>: TryInto<&'d str, Error = MachineError>,
{
    let output_cell = RefCell::new(String::new());
    let output_rc = Rc::new(output_cell);
    let output_weak = Rc::downgrade(&output_rc);

    let my_print = machine
        .callback_1arg(move |s| {
            output_weak
                .upgrade()
                .ok_or("expired")?
                .borrow_mut()
                // but use `try_into`,
                // as I think that's most idiomatic, isn't it?
                .push_str((&s).try_into()?);
            Ok(vec![])
        })?;

    machine.compile(None, setup)?.call([my_print])?;
    machine.compile(None, run)?.call([])?;

    Ok(Rc::try_unwrap(output_rc).unwrap().into_inner())
}

Also note my Problem with HRTBs and associated type bounds in that matter.

I feel like I should remember the wisdom I found once.

Thank you a lot for showing me that workaround, I'll consider using that, but I'm currently tempted to simply do the following:

/// Types that can be an UTF-8 text string
pub trait MaybeString<'c>
where
    Self: From<String>,
    Self: From<&'c str>,
    Self: TryInto<String, Error = DatumConversionError<Self>>,
{
    /// Try to interpret datum as Unicode string
    fn try_as_str(&self) -> Result<&str, DatumViewError>;
}

Using TryInto here, because of what @kpreid said here:

And I plan to not use for<'d> &'d Self: TryInto<&'d str, Error = DatumViewError> either, due to the error I described here:

P.S.: In either case, there must be a compiler bug. Those hints aren't right!