Const value as target = value in instrument.

[dpc]

Feature Request

Crates

tracing

Motivation

We keep the set of tracing targets as a list of const LOG_SMTH : &str = "smth";
to avoid typos.

Proposal

I'd like to:

#[instrument(tracing = LOG_SMTH)]

[hawkw]

Currently, the target value is parsed using the StrArg helper:

tracing/tracing-attributes/src/attr.rs

Line 97 in 264a417

let target = input.parse::<StrArg<kw::target>>()?.value;

This parses the input as a keyword followed by a string literal:

tracing/tracing-attributes/src/attr.rs

Lines 206 to 216 in 264a417

	impl<T: Parse> Parse for StrArg<T> {
	fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
	let _ = input.parse::<T>()?;
	let _ = input.parse::<Token![=]>()?;
	let value = input.parse()?;
	Ok(Self {
	value,
	_p: std::marker::PhantomData,
	})
	}
	}

We could support constants in the attribute if we changed this code to accept a string literal or an identifier. We might want to do the same for other attribute arguments that are currently only able to accept string literals and not constants, besides the target.

This is probably a pretty straightforward change, if anyone is interested in submitting a PR. I'm happy to provide some guidance!