Add a method to Targets to get the default level

[connec]

Motivation

This makes it possible to fully "override" some base Targets filter with another (e.g. user-supplied) filter. Without some way to obtain the default, only explicit targets can be overridden (via IntoIter and friends).

Solution

We can add a method to Targets that filters the underlying DirectiveSet for the default directive. This works because DirectiveSet::add will replace directives with the same target/field_names, which is always None/vec![] for the directive added by with_default (and in fact we are only concerned with target, since no other Targets API allows adding directives with a None target).

Ideally the method would be named default, corresponding to with_default, however this conflicts with Default::default and so would be a breaking change (and harm ergonomics). default_level seemed a better name than get_default, since "getters" of this style are generally considered unidiomatic^{[citation needed]}.

Example usage:

let mut filter = Targets::new().with_target("some_module", LevelFilter::INFO);

// imagine this came from `RUST_LOG` or similar
let override: Targets = "trace".parse().unwrap();

// merge the override
if let Some(default) = override.default_level() {
    filter = filter.with_default(default);
}
for (target, level) in override.iter() {
    filter = filter.with_target(target, level);
}

[hawkw]

This definitely seems worth having, and I think the default_level naming is fine. I had some very minor suggestions about the API docs, but once those are addressed I'll be very happy to merge this!

Thank you!

[connec]

Thanks for the fast review! I think I've responded to all your comments.

I did end up second-guessing the naming a little bit, since what we really return is a LevelFilter rather than a Level (and LevelFilter is already an Option<Level> under the hood). It does match up with the level argument for with_default though so perhaps it's fine (I'm not sure default_level_filter() is much better?).

[hawkw]

I did end up second-guessing the naming a little bit, since what we really return is a LevelFilter rather than a Level (and LevelFilter is already an Option<Level> under the hood). It does match up with the level argument for with_default though so perhaps it's fine (I'm not sure default_level_filter() is much better?).

Personally, I think default_level is fine --- the return type should be pretty obvious just from looking at the function signature; I wouldn't worry about it too much. :)

[hawkw]

this looks good to me, thank you again!

[hawkw]

I believe this also closes #1790, as it implements the solution discussed in #1790 (comment).