chore: show agents how to view test code coverage

.github/workflows/rust.yml

@@ -68,10 +68,6 @@ jobs:
   linux-build:
     runs-on: "warp-ubuntu-latest-x64-4x"
     timeout-minutes: 60
-    strategy:
-      matrix:
-        toolchain:
-          - stable
     env:
       # Need up-to-date compilers for kernels
       CC: clang
@@ -81,8 +77,8 @@ jobs:
       # pin the toolchain version to avoid surprises
       - name: Setup rust toolchain
         run: |
-          rustup toolchain install ${{ matrix.toolchain }}
-          rustup default ${{ matrix.toolchain }}
+          rustup toolchain install nightly
+          rustup default nightly
       - uses: rui314/setup-mold@v1
       - uses: Swatinem/rust-cache@v2
       - name: Install dependencies
@@ -94,12 +90,10 @@ jobs:
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov
       - name: Run tests
-        if: ${{ matrix.toolchain == 'stable' }}
         run: |
           ALL_FEATURES=`cargo metadata --format-version=1 --no-deps | jq -r '.packages[] | .features | keys | .[]' | grep -v protoc | sort | uniq | paste -s -d "," -`
-          cargo llvm-cov --profile ci --locked --workspace --codecov --output-path coverage.codecov --features ${ALL_FEATURES}
+          cargo +nightly llvm-cov --profile ci --locked --workspace --codecov --output-path coverage.codecov --features ${ALL_FEATURES}
       - name: Upload coverage to Codecov
-        if: ${{ matrix.toolchain == 'stable' }}
         uses: codecov/codecov-action@v4
         with:
           token: ${{ secrets.CODECOV_TOKEN }}

AGENTS.md

@@ -58,6 +58,10 @@ The project is organized as a Rust workspace with Python and Java bindings. Rust
 * Run specific test: `cargo test -p <package> <test_name>`
 * Lint: `cargo clippy --all --tests --benches -- -D warnings`
 * Format: `cargo fmt --all`
+* Output code coverage report for a crate: `cargo +nightly llvm-cov -q -p lance-core --branch`
+* Create HTML coverage report for a crate: `cargo +nightly llvm-cov -q -p lance-core --branch --html`
+* Print lines in file missing coverage: `cargo +nightly llvm-cov -q -p lance-core --show-missing-lines | grep rust/lance-core/src/datatypes/schema.rs`
+* Show detailed coverage for a file: `python ci/coverage.py -p lance-core -f rust/lance-core/src/datatypes/schema.rs`
 
 ### Python Development
 
@@ -135,6 +139,8 @@ Tests:
   /// # }
   /// ```
   ```
+* Code coverage can be skipped for test utilities and non-critical paths using
+  `#[cfg_attr(coverage, coverage(off))]`.
 
 ## Review Guidelines
 

Cargo.toml

@@ -213,6 +213,9 @@ debug-assertions = false
 strip = "debuginfo"
 incremental = false
 
+[workspace.lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(coverage,coverage_nightly)'] }
+
 [workspace.lints.clippy]
 all = { level = "deny", priority = -1 }
 style = { level = "deny", priority = -1 }

ci/coverage.py

@@ -0,0 +1,38 @@
+import argparse
+import subprocess
+
+parser = argparse.ArgumentParser(description="Run code coverage analysis.")
+parser.add_argument("-p", "--package", type=str, help="The Rust crate to analyze.")
+parser.add_argument(
+    "-f", "--file", type=str, help="The specific file to show coverage for."
+)
+args = parser.parse_args()
+
+cmd = ["cargo", "+nightly", "llvm-cov", "-q", "--branch", "--text", "--color", "always"]
+if args.package:
+    cmd += ["-p", args.package]
+
+result = subprocess.run(cmd, capture_output=True)
+if result.returncode != 0:
+    print("Error running coverage analysis:")
+    print(result.stderr.decode())
+elif args.file:
+    # Look for the specific file's coverage details
+    # Section headers look like: /path/to/file.rs:
+    lines = result.stdout.splitlines()
+    in_file_section = False
+    file_bytes = args.file.encode()
+    for line in lines:
+        # Check if this is a section header (path ending with colon)
+        stripped = line.rstrip()
+        is_section_header = stripped.endswith(b":") and b"|" not in line
+        if is_section_header:
+            if file_bytes in line:
+                in_file_section = True
+            elif in_file_section:
+                # Hit a new section, stop
+                break
+        if in_file_section:
+            print(line.decode())
+else:
+    print(result.stdout.decode())

rust/lance-core/src/lib.rs

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 // SPDX-FileCopyrightText: Copyright The Lance Authors
+#![cfg_attr(coverage, feature(coverage_attribute))]
 
 use arrow_schema::{DataType, Field as ArrowField};
 use std::sync::LazyLock;

rust/lance-core/src/utils/backoff.rs

@@ -164,6 +164,7 @@ mod tests {
 
     #[test]
     fn test_slot_backoff() {
+        #[cfg_attr(coverage, coverage(off))]
         fn assert_in(value: u128, expected: &[u128]) {
             assert!(
                 expected.contains(&value),