Switch to cargo-llvm-cov for code coverage

[Copilot]

Description

Replaces grcov with cargo-llvm-cov for coverage generation. grcov lacks branch coverage support and has other limitations.

Workflow changes:

• Use nightly toolchain with cargo-llvm-cov
• Set RUSTFLAGS="--cfg coverage_nightly" during coverage runs only
• Remove lcov tools and old coverage environment variables
• Enable branch coverage with --branch flag
• Add --quiet flag to reduce output verbosity

Code changes:

• Enable coverage_attribute feature via #[cfg_attr(coverage_nightly, feature(coverage_attribute))] in lib.rs
• Exclude test modules from coverage with #[cfg_attr(coverage_nightly, coverage(off))] on 13 test modules
• Add coverage_nightly to [lints.rust] in Cargo.toml to declare the custom cfg

Stable builds are unaffected. Coverage exclusions only activate when the cfg is set.

Notes to the reviewers

The coverage_nightly cfg is only set during coverage generation in CI. Regular builds and tests use stable toolchain without this cfg.

Branch coverage is now explicitly enabled, providing more detailed coverage metrics than the previous grcov setup.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[coverage] Consider using cargo-llvm-cov</issue_title>
<issue_description>Describe the enhancement

Switch to using llvm-cov in .github/workflows/code_coverage.yml for generating coverage data. For example see bitcoindevkit/bdk#1986.

Impact

• Blocking production usage
• Nice-to-have / UX improvement
• Developer experience / maintainability

Additional context

• grcov has some limitations including lacking support for branch coverage which makes it a less appealing option.
• Noise in coverage scores bdk#1984</issue_description>

<agent_instructions>In order to switch to using cargo-llvm-cov in code_coverage.yml I recommend using a recent version of the nightly compiler in the "Install Rust toolchain" step. Note also that it shouldn't be necessary to include the extra environment variables (RUSTFLAGS, etc) under env: in code_coverage.yml. Additionally, consider enabling the coverage_nightly config attribute globally in lib.rs so we can exclude test modules from coverage reports using #[cfg_attr(coverage_nightly, coverage(off))].</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [coverage] Consider using cargo-llvm-cov #366

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.85%. Comparing base (8f8a8e9) to head (5aaa70b).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   85.33%   77.85%   -7.49%     
==========================================
  Files          24       22       -2     
  Lines        8335     4927    -3408     
  Branches        0      233     +233     
==========================================
- Hits         7113     3836    -3277     
+ Misses       1222     1021     -201     
- Partials        0       70      +70     

Flag	Coverage Δ
rust	77.85% <ø> (-7.49%)	⬇️

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[ValuedMammal]

Concept ACK, haven't done an in-depth review.

[ValuedMammal]

@copilot When running the cargo llvm-cov command can you make the output quieter, and also enable --branch coverage, thanks.

[Copilot]

@copilot When running the cargo llvm-cov command can you make the output quieter, and also enable --branch coverage, thanks.

Done in commit 5ffaacc. Added --branch flag for branch coverage and --quiet flag to reduce output verbosity.

[ValuedMammal]

Looks good to me. I'd only suggest redoing the commits to follow the contribution guidelines before merging. The drop in coverage is expected, since we were previously reporting coverage for non-library test code.