Eliza Weisman 06daa9512b

subscriber: add an abstraction for building visitors (#241) ...

## Motivation

The `tracing-fmt` crate crate currently uses a `NewVisitor` trait to
abstract over constructing field visitors of different types or with
different configurations:
d51d4d69d8/tracing-fmt/src/lib.rs (L192-L196)

There have been other use-cases where having this abstraction seems
valuable. For example, issue #73 is a proposal for adding utilities for
wrapping a visitor to skip a set of excluded field names. A `MakeVisitor`
abstraction would allow us to provide much more ergonomic APIs for
composing this kind of visitor behaviour.

Additionally, the current trait in `tracing-fmt` is tied pretty closely
to `tracing-fmt- specific behaviour. A generalized version of this trait
should be generic over the input type used to build the visitor.

## Solution

This PR adds a new `MakeVisitor` trait in `tracing-subscriber`.
`MakeVisitor` generalizes the `NewVisitor` trait in `tracing-fmt` to
represent constructing visitors for arbitrary targets. In addition, I've
added some additional traits representing common patterns for visitors,
such as those that format to a `fmt::Write` instance or write to an IO
stream, or those that produce output once a set of fields have been
visited. I've also added some extension traits & combinators to make
composing visitors easier, and to demonstrate that the trait in this
branch can be used for implementing this kind of wrapper.

I've also rewritten the `tracing-fmt` crate to use the new
`tracing-subscriber::MakeVisitor` trait rather than its homegrown
`fmt`-specifc version.

Closes: #240 

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

2019-10-08 14:06:37 -07:00

..

custom_visitor.rs

subscriber: add an abstraction for building visitors (#241)

2019-10-08 14:06:37 -07:00