How do you actually run cargo flamegraph on tests?

Question

I have a test I can run as:

cargo test tests::my_test -- --nocapture

I am trying to profile it without any luck, the docs claim that you can use one of the following:

# Profile unit tests.
# Note that a separating `--` is necessary if `--unit-test` is the last flag.
cargo flamegraph --unit-test -- test::in::package::with::single::crate
cargo flamegraph --unit-test crate_name -- test::in::package::with::multiple:crate
cargo flamegraph --unit-test --dev test::may::omit::separator::if::unit::test::flag::not::last::flag

# Profile integration tests.
cargo flamegraph --test test_name

I have tried them all:

cargo flamegraph --unit-test -- test::mytest
Error: crate has no automatically selectable target:

cargo flamegraph --unit-test my_crate -- test::mytest

error: unexpected argument 'tests::mytest' found

Usage: cargo flamegraph [OPTIONS] [-- <TRAILING_ARGUMENTS>...]

For more information, try '--help'.

cargo flamegraph --test tests::mytest -- --nocapture
error: no test target named `tests::mytest`.

Error: cargo build failed

The documentation is doing a very poor job of conveying what you are actually meant to do.

Answer 1

I ran into this same problem and after updating from flamegraph v0.5.1 to flamegraph v0.6.4 it worked as expected.

In my project (which has multiple bin and lib crates) the magic invocation was:

cargo flamegraph --root --dev --unit-test {the-crate} -- {the-test}