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document new --show-output option in ch 11-2 (Rust >= 1.39) #2065

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document new --show-output option in ch 11-2 (Rust >= 1.39) #2065

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32 changes: 19 additions & 13 deletions src/ch11-02-running-tests.md
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Expand Up @@ -115,40 +115,46 @@ is printed when the test that passes runs. That output has been captured. The
output from the test that failed, `I got the value 8`, appears in the section
of the test summary output, which also shows the cause of the test failure.

If we want to see printed values for passing tests as well, we can disable the
output capture behavior by using the `--nocapture` flag:
If we want to see printed values for passing tests as well, we can tell Rust
to also show the output of successful tests at the end with `--show-output`.
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Seems like this should still document --nocapture since that remains a useful option.

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No, I don't think anyone should still use --nocapture.

At least the only case I can think of is if your test output is gigantic (as in larger than ram?) where buffering it would be a problem. And that's clearly not a good situation to be in the first place

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We're okay with not covering every possible option in the book, so I agree that we should just leave it out.


```text
$ cargo test -- --nocapture
$ cargo test -- --show-output
```

When we run the tests in Listing 11-10 again with the `--nocapture` flag, we
When we run the tests in Listing 11-10 again with the `--show-output` flag, we
see the following output:

```text
running 2 tests
test tests::this_test_will_pass ... ok
test tests::this_test_will_fail ... FAILED

successes:

---- tests::this_test_will_pass stdout ----
I got the value 4


successes:
tests::this_test_will_pass

failures:

---- tests::this_test_will_fail stdout ----
I got the value 8
test tests::this_test_will_pass ... ok
thread 'tests::this_test_will_fail' panicked at 'assertion failed: `(left == right)`
left: `5`,
right: `10`', src/lib.rs:19:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
test tests::this_test_will_fail ... FAILED
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

failures:

failures:
tests::this_test_will_fail

test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
```

Note that the output for the tests and the test results are interleaved; the
reason is that the tests are running in parallel, as we talked about in the
previous section. Try using the `--test-threads=1` option and the `--nocapture`
flag, and see what the output looks like then!

### Running a Subset of Tests by Name

Sometimes, running a full test suite can take a long time. If you’re working on
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