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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><
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<main>
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<h2><a class="header" href="#controlling-how-tests-are-run" id="controlling-how-tests-are-run">Controlling How Tests Are Run</a></h2>
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<p>Just as <code>cargo run</code> compiles your code and then runs the resulting binary,
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<code>cargo test</code> compiles your code in test mode and runs the resulting test
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binary. You can specify command line options to change the default behavior of
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<code>cargo test</code>. For example, the default behavior of the binary produced by
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<code>cargo test</code> is to run all the tests in parallel and capture output generated
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during test runs, preventing the output from being displayed and making it
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easier to read the output related to the test results.</p>
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<p>Some command line options go to <code>cargo test</code>, and some go to the resulting test
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binary. To separate these two types of arguments, you list the arguments that
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go to <code>cargo test</code> followed by the separator <code>--</code> and then the ones that go to
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the test binary. Running <code>cargo test --help</code> displays the options you can use
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with <code>cargo test</code>, and running <code>cargo test -- --help</code> displays the options you
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can use after the separator <code>--</code>.</p>
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<h3><a class="header" href="#running-tests-in-parallel-or-consecutively" id="running-tests-in-parallel-or-consecutively">Running Tests in Parallel or Consecutively</a></h3>
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<p>When you run multiple tests, by default they run in parallel using threads.
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This means the tests will finish running faster so you can get feedback quicker
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on whether or not your code is working. Because the tests are running at the
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same time, make sure your tests don’t depend on each other or on any shared
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state, including a shared environment, such as the current working directory or
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environment variables.</p>
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<p>For example, say each of your tests runs some code that creates a file on disk
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named <em>test-output.txt</em> and writes some data to that file. Then each test reads
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the data in that file and asserts that the file contains a particular value,
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which is different in each test. Because the tests run at the same time, one
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test might overwrite the file between when another test writes and reads the
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file. The second test will then fail, not because the code is incorrect but
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because the tests have interfered with each other while running in parallel.
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One solution is to make sure each test writes to a different file; another
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solution is to run the tests one at a time.</p>
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<p>If you don’t want to run the tests in parallel or if you want more fine-grained
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control over the number of threads used, you can send the <code>--test-threads</code> flag
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and the number of threads you want to use to the test binary. Take a look at
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the following example:</p>
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<pre><code class="language-text">$ cargo test -- --test-threads=1
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</code></pre>
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<p>We set the number of test threads to <code>1</code>, telling the program not to use any
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parallelism. Running the tests using one thread will take longer than running
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them in parallel, but the tests won’t interfere with each other if they share
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state.</p>
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<h3><a class="header" href="#showing-function-output" id="showing-function-output">Showing Function Output</a></h3>
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<p>By default, if a test passes, Rust’s test library captures anything printed to
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standard output. For example, if we call <code>println!</code> in a test and the test
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passes, we won’t see the <code>println!</code> output in the terminal; we’ll see only the
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line that indicates the test passed. If a test fails, we’ll see whatever was
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printed to standard output with the rest of the failure message.</p>
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<p>As an example, Listing 11-10 has a silly function that prints the value of its
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parameter and returns 10, as well as a test that passes and a test that fails.</p>
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<p><span class="filename">Filename: src/lib.rs</span></p>
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<pre><pre class="playpen"><code class="language-rust panics">
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<span class="boring">#![allow(unused_variables)]
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</span><span class="boring">fn main() {
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</span>fn prints_and_returns_10(a: i32) -> i32 {
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println!("I got the value {}", a);
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10
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn this_test_will_pass() {
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let value = prints_and_returns_10(4);
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assert_eq!(10, value);
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}
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#[test]
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fn this_test_will_fail() {
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let value = prints_and_returns_10(8);
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assert_eq!(5, value);
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}
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p><span class="caption">Listing 11-10: Tests for a function that calls
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<code>println!</code></span></p>
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<p>When we run these tests with <code>cargo test</code>, we’ll see the following output:</p>
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<pre><code class="language-text">running 2 tests
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test tests::this_test_will_pass ... ok
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test tests::this_test_will_fail ... FAILED
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failures:
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---- tests::this_test_will_fail stdout ----
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I got the value 8
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thread 'tests::this_test_will_fail' panicked at 'assertion failed: `(left == right)`
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left: `5`,
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right: `10`', src/lib.rs:19:9
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note: Run with `RUST_BACKTRACE=1` for a backtrace.
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failures:
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tests::this_test_will_fail
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test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
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</code></pre>
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<p>Note that nowhere in this output do we see <code>I got the value 4</code>, which is what
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is printed when the test that passes runs. That output has been captured. The
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output from the test that failed, <code>I got the value 8</code>, appears in the section
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of the test summary output, which also shows the cause of the test failure.</p>
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|||
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<p>If we want to see printed values for passing tests as well, we can disable the
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output capture behavior by using the <code>--nocapture</code> flag:</p>
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|||
|
|
<pre><code class="language-text">$ cargo test -- --nocapture
|
|||
|
|
</code></pre>
|
|||
|
|
<p>When we run the tests in Listing 11-10 again with the <code>--nocapture</code> flag, we
|
|||
|
|
see the following output:</p>
|
|||
|
|
<pre><code class="language-text">running 2 tests
|
|||
|
|
I got the value 4
|
|||
|
|
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
|
|||
|
|
|
|||
|
|
failures:
|
|||
|
|
|
|||
|
|
failures:
|
|||
|
|
tests::this_test_will_fail
|
|||
|
|
|
|||
|
|
test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
|
|||
|
|
</code></pre>
|
|||
|
|
<p>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 <code>--test-threads=1</code> option and the <code>--nocapture</code>
|
|||
|
|
flag, and see what the output looks like then!</p>
|
|||
|
|
<h3><a class="header" href="#running-a-subset-of-tests-by-name" id="running-a-subset-of-tests-by-name">Running a Subset of Tests by Name</a></h3>
|
|||
|
|
<p>Sometimes, running a full test suite can take a long time. If you’re working on
|
|||
|
|
code in a particular area, you might want to run only the tests pertaining to
|
|||
|
|
that code. You can choose which tests to run by passing <code>cargo test</code> the name
|
|||
|
|
or names of the test(s) you want to run as an argument.</p>
|
|||
|
|
<p>To demonstrate how to run a subset of tests, we’ll create three tests for our
|
|||
|
|
<code>add_two</code> function, as shown in Listing 11-11, and choose which ones to run.</p>
|
|||
|
|
<p><span class="filename">Filename: src/lib.rs</span></p>
|
|||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
|
</span><span class="boring">fn main() {
|
|||
|
|
</span>pub fn add_two(a: i32) -> i32 {
|
|||
|
|
a + 2
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
#[cfg(test)]
|
|||
|
|
mod tests {
|
|||
|
|
use super::*;
|
|||
|
|
|
|||
|
|
#[test]
|
|||
|
|
fn add_two_and_two() {
|
|||
|
|
assert_eq!(4, add_two(2));
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
#[test]
|
|||
|
|
fn add_three_and_two() {
|
|||
|
|
assert_eq!(5, add_two(3));
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
#[test]
|
|||
|
|
fn one_hundred() {
|
|||
|
|
assert_eq!(102, add_two(100));
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
<span class="boring">}
|
|||
|
|
</span></code></pre></pre>
|
|||
|
|
<p><span class="caption">Listing 11-11: Three tests with three different
|
|||
|
|
names</span></p>
|
|||
|
|
<p>If we run the tests without passing any arguments, as we saw earlier, all the
|
|||
|
|
tests will run in parallel:</p>
|
|||
|
|
<pre><code class="language-text">running 3 tests
|
|||
|
|
test tests::add_two_and_two ... ok
|
|||
|
|
test tests::add_three_and_two ... ok
|
|||
|
|
test tests::one_hundred ... ok
|
|||
|
|
|
|||
|
|
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
|
|||
|
|
</code></pre>
|
|||
|
|
<h4><a class="header" href="#running-single-tests" id="running-single-tests">Running Single Tests</a></h4>
|
|||
|
|
<p>We can pass the name of any test function to <code>cargo test</code> to run only that test:</p>
|
|||
|
|
<pre><code class="language-text">$ cargo test one_hundred
|
|||
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
|
|||
|
|
Running target/debug/deps/adder-06a75b4a1f2515e9
|
|||
|
|
|
|||
|
|
running 1 test
|
|||
|
|
test tests::one_hundred ... ok
|
|||
|
|
|
|||
|
|
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 2 filtered out
|
|||
|
|
</code></pre>
|
|||
|
|
<p>Only the test with the name <code>one_hundred</code> ran; the other two tests didn’t match
|
|||
|
|
that name. The test output lets us know we had more tests than what this
|
|||
|
|
command ran by displaying <code>2 filtered out</code> at the end of the summary line.</p>
|
|||
|
|
<p>We can’t specify the names of multiple tests in this way; only the first value
|
|||
|
|
given to <code>cargo test</code> will be used. But there is a way to run multiple tests.</p>
|
|||
|
|
<h4><a class="header" href="#filtering-to-run-multiple-tests" id="filtering-to-run-multiple-tests">Filtering to Run Multiple Tests</a></h4>
|
|||
|
|
<p>We can specify part of a test name, and any test whose name matches that value
|
|||
|
|
will be run. For example, because two of our tests’ names contain <code>add</code>, we can
|
|||
|
|
run those two by running <code>cargo test add</code>:</p>
|
|||
|
|
<pre><code class="language-text">$ cargo test add
|
|||
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
|
|||
|
|
Running target/debug/deps/adder-06a75b4a1f2515e9
|
|||
|
|
|
|||
|
|
running 2 tests
|
|||
|
|
test tests::add_two_and_two ... ok
|
|||
|
|
test tests::add_three_and_two ... ok
|
|||
|
|
|
|||
|
|
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 1 filtered out
|
|||
|
|
</code></pre>
|
|||
|
|
<p>This command ran all tests with <code>add</code> in the name and filtered out the test
|
|||
|
|
named <code>one_hundred</code>. Also note that the module in which a test appears becomes
|
|||
|
|
part of the test’s name, so we can run all the tests in a module by filtering
|
|||
|
|
on the module’s name.</p>
|
|||
|
|
<h3><a class="header" href="#ignoring-some-tests-unless-specifically-requested" id="ignoring-some-tests-unless-specifically-requested">Ignoring Some Tests Unless Specifically Requested</a></h3>
|
|||
|
|
<p>Sometimes a few specific tests can be very time-consuming to execute, so you
|
|||
|
|
might want to exclude them during most runs of <code>cargo test</code>. Rather than
|
|||
|
|
listing as arguments all tests you do want to run, you can instead annotate the
|
|||
|
|
time-consuming tests using the <code>ignore</code> attribute to exclude them, as shown
|
|||
|
|
here:</p>
|
|||
|
|
<p><span class="filename">Filename: src/lib.rs</span></p>
|
|||
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
|
</span><span class="boring">fn main() {
|
|||
|
|
</span>#[test]
|
|||
|
|
fn it_works() {
|
|||
|
|
assert_eq!(2 + 2, 4);
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
#[test]
|
|||
|
|
#[ignore]
|
|||
|
|
fn expensive_test() {
|
|||
|
|
// code that takes an hour to run
|
|||
|
|
}
|
|||
|
|
<span class="boring">}
|
|||
|
|
</span></code></pre></pre>
|
|||
|
|
<p>After <code>#[test]</code> we add the <code>#[ignore]</code> line to the test we want to exclude. Now
|
|||
|
|
when we run our tests, <code>it_works</code> runs, but <code>expensive_test</code> doesn’t:</p>
|
|||
|
|
<pre><code class="language-text">$ cargo test
|
|||
|
|
Compiling adder v0.1.0 (file:///projects/adder)
|
|||
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.24 secs
|
|||
|
|
Running target/debug/deps/adder-ce99bcc2479f4607
|
|||
|
|
|
|||
|
|
running 2 tests
|
|||
|
|
test expensive_test ... ignored
|
|||
|
|
test it_works ... ok
|
|||
|
|
|
|||
|
|
test result: ok. 1 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out
|
|||
|
|
</code></pre>
|
|||
|
|
<p>The <code>expensive_test</code> function is listed as <code>ignored</code>. If we want to run only
|
|||
|
|
the ignored tests, we can use <code>cargo test -- --ignored</code>:</p>
|
|||
|
|
<pre><code class="language-text">$ cargo test -- --ignored
|
|||
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
|
|||
|
|
Running target/debug/deps/adder-ce99bcc2479f4607
|
|||
|
|
|
|||
|
|
running 1 test
|
|||
|
|
test expensive_test ... ok
|
|||
|
|
|
|||
|
|
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 1 filtered out
|
|||
|
|
</code></pre>
|
|||
|
|
<p>By controlling which tests run, you can make sure your <code>cargo test</code> results
|
|||
|
|
will be fast. When you’re at a point where it makes sense to check the results
|
|||
|
|
of the <code>ignored</code> tests and you have time to wait for the results, you can run
|
|||
|
|
<code>cargo test -- --ignored</code> instead.</p>
|
|||
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