I do not know in what part the problem begins, so if rustc has nothing to do with it, please tell me how to check that. I'm a maintainer of quick-xml and I setup a coverage for it and collect coverage, using RUSTFLAGS: -C instrument-coverage.

However, results on codecov.io looks very strange:

• many lines are marked as not covered while they contains simple doc comments even without doctests. The most striking example of a file in which there is no executable code at all, but it is all marked as not covered: https://app.codecov.io/gh/tafia/quick-xml/blob/master/src/lib.rs
• non-executable code, such as uses or mod declarations marked as not covered (in the same place)
• sometimes coverage looks like it started and ended at random lines: some examples in https://app.codecov.io/gh/tafia/quick-xml/blob/master/src/de/mod.rs
• code of module tests (i.e. in #[cfg(test)] modules) are also included in coverage which is unexpected, for example: https://app.codecov.io/gh/tafia/quick-xml/blob/master/src/de/mod.rs. That code should be used to produce coverage, not for coverage. This seriously biases the results, especially when you have more lines with tests than with code itself.

Meta

The coverage is collected by GitHub ubuntu-latest runner, currently it reports Rust 1.64.0, but the issue was there since coverage was added to quick-xml, i.e. at least since 2022-05-30. The corresponding GitHub Actions file: https://github.com/tafia/quick-xml/blob/240f2049aec50876b6e13e632120b20cd60a37af/.github/workflows/rust.yml