Update default error handler to use Severity.

[AlephCubed]

Followup to #22201. Includes the changes in #23408, which is currently in the queue.

Adds a new default error handler that defers to an error's Severity.

[kfc35]

The first two commits in this PR are the same as #23408 it looks like
Since that was merged just a bit ago, maybe a merge from main into this branch would help just to make sure the “Files Changed" actually reflects whats new

Edit: Oh I’m dumb, just read the description that it includes those changes 😅

[Freyja-moth]

Might be worth having a match_error_up_to method that crashes on errors on and above the level specified.

[AlephCubed]

That might be useful, but I think it would require something like:

match_severity_up_to::<{ Severity::Warning as u8 }>

since we don't have adt_const_params.

Alternatively we could have a separate handler for each case (match_severity_up_to_warning, match_severity_up_to_debug, etc).

[Freyja-moth]

couldn't you just do

#[track_caller]
#[inline]
pub fn match_severity_up_to(severity: Severity) -> impl Fn(BevyError, ErrorContext) {
    move |err, ctx| {
        ...
    }
}

[AlephCubed]

I can't seem to get that to work (at least without changing the signature of DefaultErrorHandler). It always complains "expected fn pointer, found opaque type" or similar.

I'm not very experienced with function types, so I could totaly be missing something.

[Freyja-moth]

at least without changing the signature of DefaultErrorHandler

I feel like that's an acceptable trade off. The error handler is the cold path so the extra bit of indirection from a function pointer is worth it I'd say (not certain but I'm fairly sure the cost of the indirection is out-shadowed by the cost of io from logging).

It's not a massive need so ultimately up to you, but it would be nice to have.