Converting between `tracing` key values and `log` key values

tracing_log.rs

use std::fmt;

use log::kv::{self, Source, value::{self, Fill}};

use tracing::{Value, Field, field::{self, Visit}};

#[doc(hidden)]
pub struct LogField<'kvs>(&'kvs Field, &'kvs dyn Value);

impl fmt::Debug for LogField<'_> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let mut visitor = f.debug_tuple("");
        self.visit(&mut visitor)?;
        visitor.finish()
    }
}

impl Source for LogField<'_> {
    fn visit<'kvs>(&'kvs self, visitor: &mut dyn kv::Visitor<'kvs>) -> Result<(), kv::Error> {
        visitor.visit_pair(kv::Key::from_str(self.0.as_ref()), kv::Value::from_fill(self))
    }
}

/*
This `Fill` trait lets us capture a value that might not be able to produce
its concrete representation for the lifetime `log` requires of its values.

This is an issue here because `tracing`'s record API essentially 'forgets'
the lifetime of the value being recorded. As an example, to record some
`Debug`, we go from:

```
&'kvs tracing::Value -> tracing::Visit::record_debug -> &'_ Debug
```

while in `log` we need to go from:

```
&'kvs log::kv::Source -> log::kv::Visitor<'kvs>::visit_pair -> &'kvs Debug -> log::kv::Value<'kvs>;
```

so that values can be tied to the lifetime of their source and outlive
a single call to `log::kv::Source::visit`. The lifetime of the `&Debug`
we can pull from `tracing` simply doesn't match up with the lifetime we
need in `log`.

So this trait gives us a place to host the code that translates a `tracing::Value`
into a `log::Value`, and doesn't require the lifetime of the concrete value
materialized by `tracing` to satisfy the lifetime needed by `log`.
*/
impl<'kvs> Fill for LogField<'kvs> {
    fn fill(&self, slot: value::Slot) -> Result<(), value::Error> {
        struct VisitValue<'slot> {
            slot: value::Slot<'slot>,
            err: Option<value::Error>,
        }

        impl VisitValue<'_> {
            fn record(&mut self, value: kv::Value) {
                if let Err(err) = self.slot.fill(value) {
                    self.err = Some(err);
                }
            }
        }

        impl Visit for VisitValue<'_> {
            fn record_i64(&mut self, _field: &Field, value: i64) {
                self.record(value.into());
            }

            fn record_u64(&mut self, _field: &Field, value: u64) {
                self.record(value.into());
            }

            fn record_bool(&mut self, _field: &Field, value: bool) {
                self.record(value.into());
            }

            fn record_str(&mut self, _field: &Field, value: &str) {
                self.record(value.into());
            }

            fn record_debug(&mut self, _field: &Field, value: &dyn fmt::Debug) {
                self.record(kv::Value::from_debug(&value));
            }
        }

        let mut visit = VisitValue {
            slot,
            err: None,
        };

        self.1.record(self.0, &mut visit);

        if let Some(err) = visit.err {
            Err(err)
        } else {
            Ok(())
        }
    }
}

fn example_record(mut iter: field::Iter) {
    log::Record::builder()
        .key_values(&(&[
            LogField(
                &iter
                    .next()
                    .expect("FieldSet corrupted (this is a bug)"),
                &42 as &dyn Value,
            ),
            LogField(
                &iter
                    .next()
                    .expect("FieldSet corrupted (this is a bug)"),
                &false as &dyn Value,
            ),
        ] as &[_]))
        .build();
}