6cc6c47354
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: ``` rust 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 https://github.com/tokio-rs/tracing/pull/631#discussion_r391138233. ## Proposal This PR changes `MakeWriter` to be generic over a lifetime `'a`: ```rust 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>
tracing-subscriber
Utilities for implementing and composing tracing subscribers.
Compiler support: requires rustc 1.42+
Supported Rust Versions
Tracing is built against the latest stable release. The minimum supported version is 1.42. The current Tracing version is not guaranteed to build on Rust versions earlier than the minimum supported version.
Tracing follows the same compiler support policies as the rest of the Tokio project. The current stable Rust compiler and the three most recent minor versions before it will always be supported. For example, if the current stable compiler version is 1.45, the minimum supported version will not be increased past 1.42, three minor versions prior. Increasing the minimum supported compiler version is not considered a semver breaking change as long as doing so complies with this policy.
License
This project is licensed under the MIT license.
Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tracing by you, shall be licensed as MIT, without any additional terms or conditions.