We've gotten a number of requests to support coverage. Here's an email I wrote in response to one of those requests, which I'm reposting here as a feature request:

Ideally, we'd want blaze coverage to exist and work in Bazel, just as it does in Blaze. The code is mostly there, but not quite. The coverage command itself doesn't actually do a lot of work; it sets --collect_code_coverage and a default --instrumentation_filter, and it optionally runs a tool to generate an HTML report (which is almost never used internally, where we push to a service instead). Otherwise it's identical to the test command, and the two flags can be manually set.

Internally, we have a wrapper script (collect_coverage.sh) that wraps an individual test invocation and uses a tool (lcov_merger.par) to merge the coverage data from that invocation, which both aren't in Bazel, and Bazel also doesn't setup the environment correctly to tell the wrapper script about output locations (StandaloneTestStrategy.java).

If you actually tried, you'd get an error like this one:
ERROR: /usr/local/google/home/ulfjack/Projects/os-bazel/src/test/cpp/BUILD:5:1: output 'src/test/cpp/blaze_util_test/coverage.dat' was not created.

It doesn't seem very hard to make the necessary Bazel changes, but we can't export the scripts as-is.

From my reading of the lcov documentation, lcov --capture looks a lot like what lcov_merger.par does, though I can't be completely certain (the docs are a bit scarce). It may make sense for us to integrate with lcov rather than export lcov_merger.par (or rewrite from scratch).