Respect --instrumentation_filter for Rust coverage

Add ctx.coverage_instrumented() check so that only targets matching
--instrumentation_filter get -Cinstrument-coverage, consistent with
Bazel's recommended approach for rules.

For rust_test targets with a crate attribute, also check if the
underlying crate should be instrumented. Rust compiles the crate
sources directly into the test binary, so the test must be built
with -Cinstrument-coverage for the crate's code to produce coverage.

Add --instrumentation_filter=^// and --instrument_test_targets to CI
coverage tasks so all workspace targets (including tests) are
instrumented while excluding external dependencies.

Author: Tamas Vajk

.bazelci/presubmit.yml

@@ -74,14 +74,20 @@ split_coverage_postprocessing_shell_commands: &split_coverage_postprocessing_she
   - echo "coverage --experimental_fetch_all_coverage_outputs" >> user.bazelrc
   - echo "coverage --experimental_split_coverage_postprocessing" >> user.bazelrc
   - echo "build --//rust/settings:experimental_use_coverage_metadata_files" >> user.bazelrc
+coverage_flags: &coverage_flags
+  - "--instrumentation_filter=^//"
+  - "--instrument_test_targets"
 rbe_coverage_flags: &rbe_coverage_flags
   # https://github.com/bazelbuild/bazel/issues/20578
   - "--strategy=CoverageReport=local"
+  - "--instrumentation_filter=^//"
+  - "--instrument_test_targets"
 tasks:
   ubuntu2204:
     build_targets: *default_linux_targets
     test_targets: *default_linux_targets
     coverage_targets: *default_linux_targets
+    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
     run_targets:
       - //test:query_test_binary
@@ -99,6 +105,7 @@ tasks:
     build_targets: *default_macos_targets
     test_targets: *default_macos_targets
     coverage_targets: *default_macos_targets
+    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
   windows:
     build_targets: *default_windows_targets
@@ -116,12 +123,14 @@ tasks:
     platform: ubuntu2204
     shell_commands: *split_coverage_postprocessing_shell_commands
     coverage_targets: *default_linux_targets
+    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
   macos_split_coverage_postprocessing:
     name: Split Coverage Postprocessing
     platform: macos_arm64
     shell_commands: *split_coverage_postprocessing_shell_commands
     coverage_targets: *default_macos_targets
+    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
   ubuntu2204_opt:
     name: Opt Mode
@@ -157,6 +166,7 @@ tasks:
     build_targets: *default_linux_targets
     test_targets: *default_linux_targets
     coverage_targets: *default_linux_targets
+    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
   rbe_ubuntu2204_with_aspects:
     name: With Aspects
@@ -191,6 +201,7 @@ tasks:
     build_targets: *default_macos_targets
     test_targets: *default_macos_targets
     coverage_targets: *default_macos_targets
+    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
   macos_rolling_with_aspects:
     name: "Macos Rolling Bazel Version With Aspects"
@@ -199,6 +210,7 @@ tasks:
     build_targets: *default_macos_targets
     test_targets: *default_macos_targets
     coverage_targets: *default_macos_targets
+    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
     soft_fail: yes
     bazel: "rolling"
@@ -358,6 +370,7 @@ tasks:
     build_targets: *default_linux_targets
     test_targets: *default_linux_targets
     coverage_targets: *default_linux_targets
+    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
   linux_docs:
     name: Docs

docs/src/SUMMARY.md

@@ -19,6 +19,7 @@
 - [Bzlmod](./rust_bzlmod.md)
 - [External Crates](./external_crates.md)
     - [crate_universe](crate_universe_bzlmod.md)
+- [Code Coverage](./coverage.md)
 - [Upstream Tooling](./upstream_tooling.md)
 - [IDE Integrations](./ide_integrations.md)
 - [Extensions](./extensions.md)

docs/src/coverage.md

@@ -0,0 +1,40 @@
+# Code Coverage
+
+Rules Rust supports collecting code coverage data using `bazel coverage`, leveraging LLVM's source-based coverage instrumentation.
+
+## Basic Usage
+
+```sh
+bazel coverage //my_project/...
+```
+
+This instruments all targets matching the default `--instrumentation_filter` and produces an LCOV report at `bazel-out/_coverage/_coverage_report.dat`.
+
+## Controlling Instrumentation
+
+Rules Rust respects Bazel's standard [`--instrumentation_filter`](https://bazel.build/reference/command-line-reference#flag--instrumentation_filter) flag to control which targets get compiled with `-Cinstrument-coverage`. Only targets whose label matches the filter are instrumented, consistent with how coverage works for other languages (C++, Java).
+
+### Recommended `.bazelrc` Settings
+
+For projects with vendored or third-party dependencies, restrict instrumentation to workspace targets to avoid unnecessary recompilation:
+
+```text
+coverage --instrumentation_filter=^//
+coverage --instrument_test_targets
+```
+
+The `--instrument_test_targets` flag is recommended because Rust test binaries (`rust_test`) often compile library source code directly into the test binary (e.g., when using the `crate` attribute). Without this flag, test targets are excluded from instrumentation by default, which means library code compiled into the test binary would not produce coverage data.
+
+To further exclude specific directories:
+
+```text
+coverage --instrumentation_filter=^//,-^//third_party
+```
+
+### Flags Summary
+
+| Flag | Purpose |
+|------|---------|
+| `--instrumentation_filter=<regex>` | Controls which targets are instrumented. Default: `-/javatests[/:],-/test/java[/:]` |
+| `--instrument_test_targets` | Also instrument test targets. Recommended for Rust. |
+| `--combined_report=lcov` | Produce a combined LCOV report (set by default with `bazel coverage`). |

rust/private/rustc.bzl

@@ -1219,8 +1219,17 @@ def construct_arguments(
         rustc_flags.add("--extern")
         rustc_flags.add("proc_macro")
 
-    if toolchain.coverage_supported and ctx.configuration.coverage_enabled:
-        # https://doc.rust-lang.org/rustc/instrument-coverage.html
+    # Use Bazel's standard instrumentation filter (--instrumentation_filter)
+    # so that only targets matching the filter get instrumented, consistent
+    # with how coverage works for other languages (Java, C++).
+    # For rust_test targets with a `crate` attribute, also check if the
+    # underlying crate should be instrumented. Rust compiles the crate
+    # sources directly into the test binary, so the test must be built
+    # with -Cinstrument-coverage for the crate's code to produce coverage.
+    _coverage_instrumented = ctx.coverage_instrumented()
+    if not _coverage_instrumented and crate_info.is_test and hasattr(ctx.attr, "crate") and ctx.attr.crate:
+        _coverage_instrumented = ctx.coverage_instrumented(ctx.attr.crate)
+    if toolchain.coverage_supported and ctx.configuration.coverage_enabled and _coverage_instrumented:
         rustc_flags.add("--codegen=instrument-coverage")
 
     if toolchain._experimental_link_std_dylib: