Better document running benchmarks outside of cargo bench

[Swatinem]

Running the benchmarks using cargo bench works just fine as expected.
It also prints the typical cargo Running benches/functions.rs (target/release/deps/functions-4227941eb3ee4115) line.

However running that target directly only lists the included benchmarks, it does not run them.
This is a bit confusing if you want to run the benchmark directly in a profiler like samply.

Actually running the benchmarks needs the --bench flag:

divan/src/divan.rs

Lines 367 to 376 in 0ff8585

	self.action = if matches.get_flag("list") {
	Action::List
	} else if matches.get_flag("test") || !matches.get_flag("bench") {
	// Either of:
	// `cargo bench -- --test`
	// `cargo test --benches`
	Action::Test
	} else {
	Action::Bench
	};

However that flag is not documented at all in the command line flags:

divan/src/cli.rs

Lines 128 to 129 in 0ff8585

	// ignored:
	.args([ignored_flag("bench"), ignored_flag("nocapture"), ignored_flag("show-output")])

It would be nice to actually document that flag, and maybe even provide an example how to run the benchmarks in a profiler like samply, which for me is as simple as running samply record target/release/deps/functions-4227941eb3ee4115 --bench

[vlovich]

FWIW this is pretty standard for all Rust benchmarking using a custom harness (e.g. Criterion has the same requirement).

[nvzqz]

Here's my suggestion for running Divan outside of the benchmark harness:

fn main() {
    divan::Divan::from_args().run_benches();
}