subscriber: add lifetime parameter to MakeWriter (#781)

[hawkw]

This backports PR #781 from master.

Motivation

Currently, the tracing-subscriber crate has the MakeWriter trait for
customizing the io writer used by fmt. This trait is necessary (rather
than simply using a Write instance) because the default implementation
performs the IO on the thread where an event was recorded, meaning that
a separate writer needs to be acquired by each thread (either by calling
a function like io::stdout, by locking a shared Write instance,
etc).

Right now there is a blanket impl for Fn() -> T where T: Write. This
works fine with functions like io::stdout. However, the other common
case for this trait is locking a shared writer.

Therefore, it makes sense to see an implementation like this:

impl<'a, W: io::Write> MakeWriter for Mutex<W>
where
    W: io::Write,
{
    type Writer = MutexWriter<'a, W>;
    fn make_writer(&self) -> Self::Writer {
        MutexWriter(self.lock().unwrap())
    }
}

pub struct MutexWriter<'a, W>(MutexGuard<'a, W>);

impl<W: io::Write> io::Write for MutexWriter<'_, W> {
    // write to the shared writer in the `MutexGuard`...
}

Unfortunately, it's impossible to write this. Since MakeWriter always
takes an &self parameter and returns Self::Writer, the generic
parameter is unbounded:

    Checking tracing-subscriber v0.2.4 (/home/eliza/code/tracing/tracing-subscriber)
error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
  --> tracing-subscriber/src/fmt/writer.rs:61:6
   |
61 | impl<'a, W: io::Write> MakeWriter for Mutex<W>
   |      ^^ unconstrained lifetime parameter

error: aborting due to previous error

This essentially precludes any MakeWriter impl where the writer is
borrowed from the type implementing MakeWriter. This is a significant
blow to the usefulness of the trait. For example, it prevented the use
of MakeWriter in tracing-flame as suggested in
#631 (comment).

Proposal

This PR changes MakeWriter to be generic over a lifetime 'a:

pub trait MakeWriter<'a> {
    type Writer: io::Write;

    fn make_writer(&'a self) -> Self::Writer;
}

The self parameter is now borrowed for the &'a lifetime, so it is
okay to return a writer borrowed from self, such as in the Mutex
case.

I've also added an impl of MakeWriter for Mutex<T> where T: Writer.

Unfortunately, this is a breaking change and will need to wait until we
release tracing-subscriber 0.3.

Fixes #675.

Signed-off-by: Eliza Weisman eliza@buoyant.io

Motivation

Solution