Double Dashes in Cargo Commands

Before --

These are flags that are directly passed to the cargo binary.

After --

Test

cargo test [options] [testname] [-- _test-options_]

The test filtering argument TESTNAME and all the arguments following the two dashes (—) are passed to the test binaries and thus to libtest (rustc’s built-in unit-test and micro-benchmarking framework).

There are no separate docs for libtest, but it’s in the Tests section of the rustc book.

Run

All the arguments following the two dashes (—) are passed to the binary to run. If you’re passing arguments to both Cargo and the binary, the ones after — go to the binary, the ones before go to Cargo.