[Bug] A pre-installed cargo-llvm-cov does not work as expected

[niebayes]

Situation 1:
We embed cargo-llvm-cov into a docker image and use it as the base image for running CI. However, only invoked with cargo llvm-cov, the coverage is correct. If we instead build the binary with cargo build ... and then perform testing with ./target/debug/<binary> ..., the coverage is not correct, specifically almost all lines are missed. Note, we have checked the README and properly set the environment variables with source <(cargo llvm-cov show-env --export-prefix).

Situation 2:
We also use a docker image as the base image for running CI, however, with cargo-llvm-cov not pre-installed. Instead, each CI job installs cargo-llvm-cov on its own. This time, astonishingly, the generated coverage is correct even if some tests are not invoked with cargo llvm-cov.

Additional context:

• Many profraw files are indeed generated in situation 1.
• We embed cargo-llvm-cov into the docker image by installing it with cargo install cargo-llvm-cov and by installing llvm-tools-preview with rustup component add llvm-tools-preview.

[taiki-e]

Hmm, I have a feeling it is something related to the propagation of environment variables, but it doesn't seem to be an obvious problem that I can figure out just by reading the description. Could you provide a reproduction? As always, it is difficult to investigate an issue unless I can reproduce it on my end.

[niebayes]

@taiki-e Very appreciate for your reply! Since our project is commercial and not open-sourced, I cannot provide a direct reproduction. However, I think I could make a toy project while enough for reproduction. I will come back when the toy project is ready and I can reproduce the issue with the toy project.