Value implementation for dyn Error + 'static prints first cause only

[mbr]

Feature Request

When logging a &std::error::Error directly, only the first .source() is printed, as seen in the example. This should be changed to log all of them instead.

Crates

Likely tracing, if changing the tracing::Value implementation.

Motivation

It's pretty common to have errors that are fairly nondescript until one follows more than step up the source chain, so it would be good to have the underlying causes logged.

Proposal

Alternatives

Implement your own newtype wrapper with a Display impl.

Somewhat related: #1308

[hawkw]

The issue here is in the default field formatter implementation for tracing_subscriber::fmt. If we look at these two lines:

tracing/tracing-subscriber/src/fmt/format/mod.rs

Lines 747 to 748 in 1e09e50

	if let Some(source) = value.source() {
	self.record_debug(field, &format_args!("{}, {}: {}", value, field, source))

we see that only the immediate source is formatted, and any subsequent sources are skipped.

We could fix this pretty easily by changing that code to format the whole source chain.

We should probably also change the delimiters for the source so that they match the way other fields are formatted. Right now, a macro invocation like

tracing::info!(foo = "some field", err = error.as_ref());

will be formatted like

foo="some field" err=<error's display impl> err: <error's source's display impl>

which is not consistent with how other fields are formatted. We probably want something more like

foo="some field" err=<error's display impl> err.source=<error's source's display impl>

or, when there are multiple sources, maybe

foo="some field" err=<error's display impl> err.sources=[<error's source's display impl>, ...]

If we also want to change the behavior of the JSON formatter to emit error sources as a JSON list, we could add a record_error impl here:

tracing/tracing-subscriber/src/fmt/format/json.rs

Line 417 in 1e09e50

impl<'a> field::Visit for JsonVisitor<'a> {