Always instrument test targets for coverage

ctx.coverage_instrumented() returns False for test targets by design
(Bazel considers test sources not worth instrumenting). But Rust test
binaries statically link their dependencies, and the coverage runtime
only initializes if the binary itself is compiled with
-Cinstrument-coverage. Without this, test binaries produce no profraw
files and coverage collection fails with "no input files specified".

Always add -Cinstrument-coverage for test crates (crate_info.is_test)
when coverage is enabled, while still respecting the instrumentation
filter for library targets.

Author: Tamas Vajk

rust/private/rustc.bzl

@@ -1223,7 +1223,10 @@ def construct_arguments(
     # 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++).
-    if toolchain.coverage_supported and ctx.configuration.coverage_enabled and ctx.coverage_instrumented():
+    # Test targets always need instrumentation since the coverage runtime
+    # only initializes if the binary is compiled with -Cinstrument-coverage,
+    # even when the statically linked library code was instrumented.
+    if toolchain.coverage_supported and ctx.configuration.coverage_enabled and (crate_info.is_test or ctx.coverage_instrumented()):
         rustc_flags.add("--codegen=instrument-coverage")
 
     if toolchain._experimental_link_std_dylib: