Merge bitcoindevkit#371: Switch to cargo-llvm-cov for code coverage

527037c ci(coverage): Switch to cargo-llvm-cov for code coverage (copilot-swe-agent[bot])

Pull request description:

  ### 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

  - ci: Switch to `cargo-llvm-cov` for code coverage

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  <!-- START COPILOT ORIGINAL PROMPT -->

  <details>

  <summary>Original prompt</summary>

  >
  > ----
  >
  > *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`](https://crates.io/crates/cargo-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
  > - [x] Developer experience / maintainability
  >
  > **Additional context**
  >
  > - `grcov` has some limitations including lacking support for branch coverage which makes it a less appealing option.
  > - bitcoindevkit/bdk#1984</issue_description>
  >
  > ## Comments on the Issue (you are @copilot in this section)
  >
  > <comments>
  > </comments>
  >

  </details>

  <!-- START COPILOT CODING AGENT SUFFIX -->

  - Fixes bitcoindevkit#366

  <!-- START COPILOT CODING AGENT TIPS -->
  ---

  💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more [Copilot coding agent tips](https://gh.io/copilot-coding-agent-tips) in the docs.

ACKs for top commit:
  ValuedMammal:
    ACK 527037c
  notmandatory:
    ACK 527037c
  luisschwab:
    ACK 527037c

Tree-SHA512: c5dc740c3e5b90b12b92ccff0581f9ca21ae45f3b08b486b58d6f9bdcc54865999b48a1eeef1349a0fd81727bdba18b5de615e60cc9e0e41cdda6d96bc50cc13

Author: ValuedMammal

.github/workflows/code_coverage.yml

@@ -1,6 +1,6 @@
 name: Code Coverage
 
-# Generates code coverage reports using grcov and uploads results to Codecov.
+# Generates code coverage reports using cargo-llvm-cov and uploads results to Codecov.
 # Runs on every push and pull request to track test coverage metrics.
 # Uploads coverage data to Codecov for tracking and produces an HTML report artifact for download.
 
@@ -14,38 +14,32 @@ jobs:
   Coverage:
     name: Code Coverage
     runs-on: ubuntu-latest
-    env:
-      RUSTFLAGS: "-Cinstrument-coverage"
-      RUSTDOCFLAGS: "-Cinstrument-coverage"
-      LLVM_PROFILE_FILE: "./target/coverage/%p-%m.profraw"
 
     steps:
       - name: Checkout
         uses: actions/checkout@v6
         with:
           persist-credentials: false
-      - name: Install lcov tools
-        run: sudo apt-get install lcov -y
-      # This action automatically reads and applies rust-toolchain.toml
       - name: Install Rust toolchain
         uses: actions-rust-lang/setup-rust-toolchain@v1
         with:
+          toolchain: nightly
           components: llvm-tools-preview
           cache: true
-      - name: Install grcov
-        run: if [[ ! -e ~/.cargo/bin/grcov ]]; then cargo install grcov; fi
-      - name: Test
-        run: cargo test --all-features
-      - name: Make coverage directory
-        run: mkdir coverage
-      - name: Run grcov
-        run: grcov . --binary-path ./target/debug/ -s . -t lcov --branch --ignore-not-existing --keep-only 'src/**' --ignore 'tests/**' --ignore 'examples/**' -o ./coverage/lcov.info
+      - name: Install cargo-llvm-cov
+        run: cargo install cargo-llvm-cov
+      - name: Generate coverage data
+        run: cargo llvm-cov --all-features --branch --quiet --ignore-filename-regex "test_utils" --lcov --output-path lcov.info
+        env:
+          RUSTFLAGS: "--cfg coverage_nightly"
       - name: Generate HTML coverage report
-        run: genhtml -o coverage-report.html --ignore-errors unmapped ./coverage/lcov.info
+        run: cargo llvm-cov --all-features --branch --quiet --ignore-filename-regex "test_utils" --html
+        env:
+          RUSTFLAGS: "--cfg coverage_nightly"
       - name: Codecov upload
         uses: codecov/codecov-action@671740ac38dd9b0130fbe1cec585b89eea48d3de # v5.5.2
         with:
-          files: ./coverage/lcov.info
+          files: ./lcov.info
           flags: rust
           name: codecov-bdk-wallet
           token: ${{ secrets.CODECOV_TOKEN }}
@@ -54,4 +48,4 @@ jobs:
         uses: actions/upload-artifact@v6
         with:
           name: coverage-report
-          path: coverage-report.html
+          path: target/llvm-cov/html

.gitignore

@@ -9,3 +9,6 @@ Cargo.lock
 *.db
 *.sqlite*
 examples/test_data
+
+# Coverage reports
+lcov.info

Cargo.toml

@@ -16,6 +16,9 @@ rust-version = "1.85.0"
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(coverage_nightly)'] }
+
 [dependencies]
 bdk_chain = { version = "0.23.1", features = ["miniscript", "serde"], default-features = false }
 bitcoin = { version = "0.32.7", features = ["serde", "base64"], default-features = false }

src/descriptor/checksum.rs

@@ -34,6 +34,7 @@ pub fn calc_checksum(desc: &str) -> Result<String, DescriptorError> {
     }
 }
 
+#[cfg_attr(coverage_nightly, coverage(off))]
 #[cfg(test)]
 mod test {
     use super::*;

src/descriptor/dsl.rs

@@ -823,6 +823,7 @@ macro_rules! fragment {
     });
 }
 
+#[cfg_attr(coverage_nightly, coverage(off))]
 #[cfg(test)]
 mod test {
     use alloc::string::ToString;

src/descriptor/mod.rs

@@ -609,6 +609,7 @@ impl DescriptorMeta for ExtendedDescriptor {
     }
 }
 
+#[cfg_attr(coverage_nightly, coverage(off))]
 #[cfg(test)]
 mod test {
     use alloc::string::ToString;

src/descriptor/policy.rs

@@ -1173,6 +1173,7 @@ impl ExtractPolicy for Descriptor<DescriptorPublicKey> {
     }
 }
 
+#[cfg_attr(coverage_nightly, coverage(off))]
 #[cfg(test)]
 mod test {
     use super::*;

src/descriptor/template.rs

@@ -632,6 +632,7 @@ expand_make_bipxx!(legacy, Legacy);
 expand_make_bipxx!(segwit_v0, Segwitv0);
 expand_make_bipxx!(segwit_v1, Tap);
 
+#[cfg_attr(coverage_nightly, coverage(off))]
 #[cfg(test)]
 mod test {
     // Test existing descriptor templates to make sure they are expanded to the right descriptors.

src/keys/bip39.rs

@@ -149,6 +149,7 @@ impl<Ctx: ScriptContext> GeneratableKey<Ctx> for Mnemonic {
     }
 }
 
+#[cfg_attr(coverage_nightly, coverage(off))]
 #[cfg(test)]
 mod test {
     use super::WordCount;

src/keys/mod.rs

@@ -1041,6 +1041,7 @@ impl fmt::Display for KeyError {
 #[cfg(feature = "std")]
 impl std::error::Error for KeyError {}
 
+#[cfg_attr(coverage_nightly, coverage(off))]
 #[cfg(test)]
 mod test {
     use super::*;