uncover reports on `#[cfg_attr]`

We are measuring coverage using grcov.

We provide tracing as an optional feature, so we use #[cfg_attr(feature="trace", instrument)] to instrument when the trace feature is enabled.

When we collect coverage information using grcov, the cfg_attr line is reported as unexecuted lines.

Similarly, specifying traced_test for trace using #[cfg_attr(feature = "trace", traced_test)] is also reported as unexecuted.

We want to avoid reporting attribute settings as unexecuted.

Here are the test environment and compile options:

Environment:

kazuk@kazuk-ryzen5800x:~/parse-it$ rustup show
Default host: x86_64-unknown-linux-gnu
rustup home:  /home/kazuk/.rustup

installed toolchains
--------------------

stable-aarch64-unknown-linux-gnu
stable-x86_64-unknown-linux-gnu (default)
nightly-x86_64-unknown-linux-gnu

installed targets for active toolchain
--------------------------------------

aarch64-unknown-linux-gnu
aarch64-unknown-linux-musl
wasm32-unknown-unknown
x86_64-unknown-linux-gnu

active toolchain
----------------

stable-x86_64-unknown-linux-gnu (default)
rustc 1.68.2 (9eb3afe9e 2023-03-27)

kazuk@kazuk-ryzen5800x:~/parse-it$ grcov --version
grcov 0.8.16

Compiler options:

  "RUSTFLAGS"= "-Cinstrument-coverage -Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code -Coverflow-checks=off"

grcov options:

grcov ${COVERAGE_PROF_OUTPUT} \
  -b ${COVERAGE_BINARIES} -s ${CARGO_MAKE_WORKSPACE_WORKING_DIRECTORY} \
  -t lcov --llvm --branch --ignore-not-existing --ignore "/*" -o ${CARGO_MAKE_WORKSPACE_WORKING_DIRECTORY}/target/lcov.info

Start by reproducing the report with grcov 0.8.16, Rust 1.68.2, the shown RUSTFLAGS, and a minimal #[cfg_attr] example. Inspect how grcov processes LLVM coverage and source lines; done means lcov output no longer marks cfg_attr attribute lines as unexecuted, including the instrument and traced_test cases.