cargo test -- --nocapture: show println! output of passing tests

By default the Rust test harness captures everything a test prints with println! or eprintln! and shows it only when that test fails. cargo test -- --nocapture turns capturing off, so the output of passing tests appears in the terminal as it happens. The flag belongs to the test binary, which is why it goes after the -- separator; --show-output is the alternative that keeps capturing but prints the output after each successful test.

Default, --nocapture and --show-output

cargo new -q --vcs none app && cd app
cat > src/main.rs <<'EOF'
fn main() {}

#[test]
fn prints_something() {
    println!("hello from the test");
    assert_eq!(2 + 2, 4);
}
EOF
cargo build -q --tests 2>/dev/null
echo "--- cargo test"
cargo test -q 2>/dev/null | sed -E 's/finished in [0-9.]+s/finished in Xs/'
echo "--- cargo test -- --nocapture"
cargo test -q -- --nocapture 2>/dev/null | sed -E 's/finished in [0-9.]+s/finished in Xs/'
echo "--- cargo test -- --show-output"
cargo test -q -- --show-output 2>/dev/null | sed -E 's/finished in [0-9.]+s/finished in Xs/'
echo "--- wrong side of --"
cargo test -q --nocapture 2>&1 | head -1

--- cargo test

running 1 test
.
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in Xs

--- cargo test -- --nocapture

running 1 test
hello from the test
.
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in Xs

--- cargo test -- --show-output

running 1 test
.
successes:

---- prints_something stdout ----
hello from the test

successes:
    prints_something

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in Xs

--- wrong side of --
error: unexpected argument '--nocapture' found

-q makes the progress compact (one dot per test) and sed removes the run time. With --nocapture the line appears in the middle of the run; with --show-output it is listed under successes: at the end. Before the --, Cargo itself rejects the option.

Related harness options

Selecting single tests by name is covered in run a single test with cargo.

Readable output with parallel tests

Tests run in parallel threads, so with --nocapture the lines of different tests can interleave. Combine both options when you debug a group of tests:

cargo test parser -- --nocapture --test-threads=1

Pitfalls

• Output written by spawned child processes or directly to the file descriptor (std::io::stdout().write_all on a raw handle, C libraries) is not captured by the harness at all, with or without the flag.
• dbg! and eprintln! write to stderr; they are captured the same way as println!.
• Integration tests in tests/ and doc tests accept the same options: cargo test --doc -- --nocapture.
• Options before -- belong to Cargo (--release, -p crate, --lib, --test name); the same split applies to cargo run -- args.

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