Support pretty-printing fields `tracing::instrument` (i.e., `{:#?}` instead of `{:?}`).

[BGR360]

Feature Request

My request is that tracing::instrument supports using the pretty-print debug specifier (i.e., {:#?}) to print fields instead of the regular debug specifier (i.e.,{:?}).

Related to / possibly overlapping with:

• Can #[instrument] macro offer default format specifier #1312
• Introduce a formatter sigil for "alternate" Display #1311
• Add ability to print error Result using err via Debug instead of only Display #1630
• Alternative field wrappers in tracing-attributes #1089

Crates

tracing-attributes

Motivation

I use tracing::instrument liberally when hacking on the Rust compiler. Oftentimes, the logs get really hard to read when a function has many parameters and each parameter has many fields and nested structs. I very frequently find myself wanting the ability to pretty-print fields so I can more easily see what's going on.

Proposal

No strong feelings on exactly how it should work. Here's one idea:

Two new options, pretty and pretty_all:

#[tracing::instrument(skip(self), pretty(bing, bang))]
fn foo(self, bing: BigNestedStruct, bong: u32, bang: BigNestedStruct) {}

#[tracing::instrument(pretty_all)]
fn bar(sneeb: BigNestedStruct, snob: BigNestedStruct) {}

Perhaps this could be generalized so that users could write their own field formatter and easily inject that into the instrument invocation:

#[tracing::instrument(fmt = tracing::field::debug)]  // default behavior
fn foo(bing: &str, bong: u32) {}

#[tracing::instrument(fmt = path::to::my_formatter)]
fn bar(sneeb: BigNestedStruct, snob: BigNestedStruct) {}

EDIT: I think this is precisely the idea of #1312

Alternatives

Quoting from #1089 (comment):

Note, though, that you can also do this by skipping all the automatically-generated fields, and adding custom fields using your own wrapper type. For example:

#[tracing::instrument(
    skip(foo, bar),
    fields(foo = ?my_wrapper(foo), bar = ?my_wrapper(bar))
)]
fn my_function(foo: Foo, bar: Bar) {
   /// ...
}

Too much work :P

[vladh]

Something like this is already supported, but it's extremely non-obvious and it took me around an hour to find. The format using # is called the alternate format, and you can tell tracing_subscriber to use it:

use tracing_subscriber::field::MakeExt;

tracing_subscriber::fmt()
    .map_fmt_fields(|f| f.debug_alt())
    .init();