chore: run tracing-subscriber's tests under --no-default-features

[davidbarsky]

Motivation

When reviewing #1523, I suggested that the examples use fmt::Layer directly to allow readers to copy-and-paste examples into their own code. However, @hawkw pointed out that rustdoc doesn't receive feature flags from Cargo, which means that running tracing-subscriber's tests under --no-default-features would fail.

Solution

It turned out that that we're only running cargo check --no-default-features for tracing-subscriber. Running cargo test --no-default-features resulted in compilation failures, but it seems like nobody noticed. Building on #1532, this branch argues that running tracing-subscriber's tests under cargo test --lib --tests --no-default-features is probably fine if we're able to skip the doctests (which are not aware of Cargo's feature flags).

[hawkw]

LGTM!

In the long run, we may want to do this for other crates that have a large number of feature flags, as well. However, I think tracing-subscriber, tracing-core, and tracing are the main crates that expose a significant amount of flags...

[hawkw]

note: i believe that if we switched the workspace to use resolver = "2", we wouldn't have to cd into each crate's directory. but, this only works on Rust 1.51+, so we would have to wait until our MSRV is 1.51 (e.g., we can't make that change today).

it might be worth leaving a TODO comment for that, though...

[hawkw]

oh, another thought. while we're changing this, we may want to change this to use the github actions working-directory key, rather than (cd tracing-subscriber && ...). this is what the cargo hack checks already do:

tracing/.github/workflows/CI.yml

Line 72 in 005b97a

working-directory: ${{ matrix.subcrate }}

Suggested change

	run: (cd tracing-subscriber && cargo test --lib --tests --no-default-features)
	run: cargo test --lib --tests --no-default-features
	working-directory: tracing-subscriber

We may want to change the other runs in this CI job as well. However, we can also do this in a separate branch.

[davidbarsky]

Added a todo assigned to myself!

[hawkw]

actually...while looking at the CI config, I noticed we have a separate job for tracing-subscriber feature combinations, which does include no-default-features. However, it only runs cargo check:

tracing/.github/workflows/CI.yml

Lines 96 to 119 in 005b97a

	cargo-check-subscriber:
	runs-on: ubuntu-latest
	strategy:
	matrix:
	featureset:
	- ""
	- fmt
	- fmt ansi
	- fmt json
	- fmt json ansi
	- fmt registry
	- fmt env-filter
	- registry
	- env-filter
	steps:
	- uses: actions/checkout@main
	- uses: actions-rs/toolchain@v1
	with:
	toolchain: stable
	profile: minimal
	override: true
	- name: cargo check
	working-directory: tracing-subscriber
	run: cargo check --no-default-features --features "${{ matrix.featureset }}"

perhaps what we really should do, rather than adding a new run to the tracing features run, is change that job to use cargo test rather than cargo check?

[davidbarsky]

For posterity: I reverted my changes as suggested in #1537 (review) because that uncovered some feature combinations that, under test, would not compile, and I was not interested in resolving them at the time.