fix(rust): pass the configured llvm tools to rust_test; document coverage

.github/workflows/ci-rust.yml

@@ -66,3 +66,10 @@ jobs:
         run: ./pleasew --plain_output test //...
         env:
           VERITAS_NO_HOOK: "1"
+
+      - name: Coverage smoke test (lib_test)
+        run: |
+          ./pleasew --plain_output cover //test/rust/lib:lib_test 2>&1 | tee cover.log
+          grep -Eq '^\s+lib\.rs [1-9][0-9]*/' cover.log
+        env:
+          VERITAS_NO_HOOK: "1"

CHANGELOG.md

@@ -6,6 +6,23 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- `rust_test` ignored `LlvmProfdataTool`, `LlvmCovTool` and `Coverage` (plugin
+  config was read with `CONFIG.get`, which does not see it), so `llvm-profdata`
+  was taken from the host. With an older host LLVM the merge failed
+  (`unsupported instrumentation profile format version`) and `plz cover`
+  reported 0%. The configured tools are now passed to the test.
+- The coverage error now names the `llvm-profdata` that failed.
+
+### Documentation
+
+- Documented the Rust coverage report location, format and contents, and the
+  silent `No data` result when `[cover] fileextension = .rs` is missing.
+- Added a CI coverage smoke test.
+
 ## [0.5.0] - 2026-09-19
 
 ### Added

build_defs/rust/rust.build_defs

@@ -61,13 +61,13 @@ def _is_coverage_enabled(rule_coverage=None):
     """Determines whether coverage instrumentation is enabled for a target."""
     if rule_coverage is not None:
         return rule_coverage
-    return CONFIG.get("RUST.COVERAGE", "true").lower() in ("true", "1", "yes", "on")
+    return (CONFIG.RUST.COVERAGE or "true").lower() in ("true", "1", "yes", "on")
 
 def _test_tools(profdata_override=None, cov_override=None):
     """Returns test_tools dictionary for rust_test."""
     tools = {"TOOL": [_rust_tool()]}
-    profdata = profdata_override or CONFIG.get("RUST.LLVM_PROFDATA_TOOL", "")
-    cov = cov_override or CONFIG.get("RUST.LLVM_COV_TOOL", "")
+    profdata = profdata_override or CONFIG.RUST.LLVM_PROFDATA_TOOL
+    cov = cov_override or CONFIG.RUST.LLVM_COV_TOOL
     if profdata and (profdata.startswith("//") or profdata.startswith("///") or profdata.startswith(":")):
         tools["LLVM_PROFDATA"] = [profdata]
     if cov and (cov.startswith("//") or cov.startswith("///") or cov.startswith(":")):

docs/rust/usage.md

@@ -308,7 +308,18 @@ Display detailed, line-by-line covered source listings:
 ./pleasew cover -l //test/lib:lib_test
 ```
 
-### 4. Per-Rule Coverage Control
+### 4. Report Output
+
+- **Location**: Please merges the per-test results into
+  `plz-out/log/coverage.xml` (Cobertura XML) and `plz-out/log/coverage.json`
+  (per-file line markers plus per-file and per-directory percentages).
+- **Contents**: line coverage only. Branch coverage and per-function data are
+  not emitted, so consumers cannot join coverage with per-function complexity
+  from this report.
+- **Missing `[cover]` setting**: without `fileextension = .rs` the report is
+  silently `Total coverage: No data`.
+
+### 5. Per-Rule Coverage Control
 
 You can enable or disable coverage instrumentation on individual rules:
 

tools/please_rust/testrunner/runner.go

@@ -133,7 +133,8 @@ func mergeProfdata(profdataPath, tmpDir string, profrawFiles []string) (string,
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	if err := cmd.Run(); err != nil {
-		return "", fmt.Errorf("llvm-profdata merge failed: %w", err)
+		version, _ := exec.Command(resolved, "--version").CombinedOutput()
+		return "", fmt.Errorf("llvm-profdata merge failed using %s (%s): %w", resolved, strings.Join(strings.Fields(string(version)), " "), err)
 	}
 	return mergedPath, nil
 }