`tracing_core::dispatcher::get_default()` takes a `FnMut` when it should take a `FnOnce`

[lilyball]

Bug Report

Version

tracing-core v0.1.21
Also current 0.2.0 as documented on tracing.rs

Crates

tracing-core

Description

tracing_core::dispatcher::get_default() (and consequently tracing::dispatcher::get_default()) takes a FnMut argument, which it executes exactly once. The use of FnMut here is unnecessarily restrictive.

Proposal

Change it to take an FnOnce. This is a backwards-compatible change (as every FnMut is also an FnOnce). The implementation can just wrap it in an Option and use take() to pull out the function; this is safe as the function will be called exactly once for every possible code path.

[hawkw]

I would happily accept a PR that makes this change, if anyone is interested in working on it.