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

…1537)

## 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).

.github/workflows/CI.yml

@@ -217,6 +217,9 @@ jobs:
 
   features-stable:
     # Feature flag tests that run on stable Rust.
+    # TODO(david): once tracing's MSRV goes up to Rust 1.51, we should be able to switch to 
+    # using cargo's V2 feature resolver (https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions)
+    # and avoid cd'ing into each crate's directory.
     needs: check
     runs-on: ubuntu-latest
     steps:
@@ -234,6 +237,10 @@ jobs:
       run: (cd tracing-core && cargo test --no-default-features)
     - name: "Test tracing no-std support"
       run: (cd tracing && cargo test --no-default-features)
+      # this skips running doctests under the `--no-default-features` flag,
+      # as rustdoc isn't aware of cargo's feature flags.
+    - name: "Test tracing-subscriber with all features disabled"
+      run: (cd tracing-subscriber && cargo test --lib --tests --no-default-features)
 
   style:
     # Check style.