337 lines
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<h1 id="the-test-attribute"><a class="header" href="#the-test-attribute">The <code>#[test]</code> attribute</a></h1>
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<ul>
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<li><a href="#step-1-re-exporting">Step 1: Re-Exporting</a></li>
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<li><a href="#step-2-harness-generation">Step 2: Harness generation</a></li>
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<li><a href="#step-3-test-object-generation">Step 3: Test object generation</a></li>
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<li><a href="#inspecting-the-generated-code">Inspecting the generated code</a></li>
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</ul>
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<p>Many Rust programmers rely on a built-in attribute called <code>#[test]</code>. All
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you have to do is mark a function and include some asserts like so:</p>
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<pre><code class="language-rust ignore">#[test]
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fn my_test() {
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assert!(2+2 == 4);
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}</code></pre>
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<p>When this program is compiled using <code>rustc --test</code> or <code>cargo test</code>, it will
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produce an executable that can run this, and any other test function. This
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method of testing allows tests to live alongside code in an organic way. You
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can even put tests inside private modules:</p>
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<pre><code class="language-rust ignore">mod my_priv_mod {
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fn my_priv_func() -> bool {}
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#[test]
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fn test_priv_func() {
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assert!(my_priv_func());
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}
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}</code></pre>
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<p>Private items can thus be easily tested without worrying about how to expose
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them to any sort of external testing apparatus. This is key to the
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ergonomics of testing in Rust. Semantically, however, it's rather odd.
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How does any sort of <code>main</code> function invoke these tests if they're not visible?
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What exactly is <code>rustc --test</code> doing?</p>
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<p><code>#[test]</code> is implemented as a syntactic transformation inside the compiler's
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<a href="https://github.com/rust-lang/rust/tree/master/compiler/rustc_ast"><code>rustc_ast</code></a>. Essentially, it's a fancy <a href="./macro-expansion.html"><code>macro</code></a> that
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rewrites the crate in 3 steps:</p>
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<h2 id="step-1-re-exporting"><a class="header" href="#step-1-re-exporting">Step 1: Re-Exporting</a></h2>
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<p>As mentioned earlier, tests can exist inside private modules, so we need a
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way of exposing them to the main function, without breaking any existing
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code. To that end, <a href="https://github.com/rust-lang/rust/tree/master/compiler/rustc_ast"><code>rustc_ast</code></a> will create local modules called
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<code>__test_reexports</code> that recursively reexport tests. This expansion translates
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the above example into:</p>
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<pre><code class="language-rust ignore">mod my_priv_mod {
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fn my_priv_func() -> bool {}
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pub fn test_priv_func() {
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assert!(my_priv_func());
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}
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pub mod __test_reexports {
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pub use super::test_priv_func;
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}
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}</code></pre>
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<p>Now, our test can be accessed as
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<code>my_priv_mod::__test_reexports::test_priv_func</code>. For deeper module
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structures, <code>__test_reexports</code> will reexport modules that contain tests, so a
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test at <code>a::b::my_test</code> becomes
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<code>a::__test_reexports::b::__test_reexports::my_test</code>. While this process seems
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pretty safe, what happens if there is an existing <code>__test_reexports</code> module?
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The answer: nothing.</p>
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<p>To explain, we need to understand how Rust's <a href="./ast-validation.html">Abstract Syntax Tree</a>
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represents <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Ident.html">identifiers</a>. The name of every function, variable, module,
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etc. is not stored as a string, but rather as an opaque <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html">Symbol</a> which
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is essentially an ID number for each identifier. The compiler keeps a separate
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hashtable that allows us to recover the human-readable name of a Symbol when
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necessary (such as when printing a syntax error). When the compiler generates
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the <code>__test_reexports</code> module, it generates a new <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html">Symbol</a> for the
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identifier, so while the compiler-generated <code>__test_reexports</code> may share a name
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with your hand-written one, it will not share a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html">Symbol</a>. This
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technique prevents name collision during code generation and is the foundation
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of Rust's <a href="./macro-expansion.html"><code>macro</code></a> hygiene.</p>
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<h2 id="step-2-harness-generation"><a class="header" href="#step-2-harness-generation">Step 2: Harness generation</a></h2>
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<p>Now that our tests are accessible from the root of our crate, we need to do
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something with them using <a href="./ast-validation.html"><code>rustc_ast</code></a> generates a module like so:</p>
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<pre><code class="language-rust ignore">#[main]
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pub fn main() {
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extern crate test;
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test::test_main_static(&[&path::to::test1, /*...*/]);
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}</code></pre>
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<p>Here <code>path::to::test1</code> is a constant of type <a href="https://doc.rust-lang.org/test/struct.TestDescAndFn.html"><code>test::TestDescAndFn</code></a>.</p>
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<p>While this transformation is simple, it gives us a lot of insight into how
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tests are actually run. The tests are aggregated into an array and passed to
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a test runner called <code>test_main_static</code>. We'll come back to exactly what
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<a href="https://doc.rust-lang.org/test/struct.TestDescAndFn.html"><code>TestDescAndFn</code></a> is, but for now, the key takeaway is that there is a crate
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called <a href="https://doc.rust-lang.org/test/index.html"><code>test</code></a> that is part of Rust core, that implements all of the
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runtime for testing. <a href="https://doc.rust-lang.org/test/index.html"><code>test</code></a>'s interface is unstable, so the only stable way
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to interact with it is through the <code>#[test]</code> macro.</p>
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<h2 id="step-3-test-object-generation"><a class="header" href="#step-3-test-object-generation">Step 3: Test object generation</a></h2>
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<p>If you've written tests in Rust before, you may be familiar with some of the
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optional attributes available on test functions. For example, a test can be
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annotated with <code>#[should_panic]</code> if we expect the test to cause a panic. It
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looks something like this:</p>
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<pre><code class="language-rust ignore">#[test]
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#[should_panic]
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fn foo() {
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panic!("intentional");
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}</code></pre>
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<p>This means our tests are more than just simple functions, they have
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configuration information as well. <code>test</code> encodes this configuration data into
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a <code>struct</code> called <a href="https://doc.rust-lang.org/test/struct.TestDesc.html"><code>TestDesc</code></a>. For each test function in a crate,
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<a href="https://github.com/rust-lang/rust/tree/master/compiler/rustc_ast"><code>rustc_ast</code></a> will parse its attributes and generate a <a href="https://doc.rust-lang.org/test/struct.TestDesc.html"><code>TestDesc</code></a>
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instance. It then combines the <a href="https://doc.rust-lang.org/test/struct.TestDesc.html"><code>TestDesc</code></a> and test function into the
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predictably named <a href="https://doc.rust-lang.org/test/struct.TestDescAndFn.html"><code>TestDescAndFn</code></a> <code>struct</code>, that <a href="https://doc.rust-lang.org/test/fn.test_main_static.html"><code>test_main_static</code></a>
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operates on.
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For a given test, the generated <a href="https://doc.rust-lang.org/test/struct.TestDescAndFn.html"><code>TestDescAndFn</code></a> instance looks like so:</p>
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<pre><code class="language-rust ignore">self::test::TestDescAndFn{
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desc: self::test::TestDesc{
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name: self::test::StaticTestName("foo"),
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ignore: false,
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should_panic: self::test::ShouldPanic::Yes,
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allow_fail: false,
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},
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testfn: self::test::StaticTestFn(||
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self::test::assert_test_result(::crate::__test_reexports::foo())),
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}</code></pre>
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<p>Once we've constructed an array of these test objects, they're passed to the
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test runner via the harness generated in Step 2.</p>
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<h2 id="inspecting-the-generated-code"><a class="header" href="#inspecting-the-generated-code">Inspecting the generated code</a></h2>
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<p>On <code>nightly</code> <code>rustc</code>, there's an unstable flag called <code>unpretty</code> that you can use
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to print out the module source after <a href="./macro-expansion.html"><code>macro</code></a> expansion:</p>
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<pre><code class="language-bash">$ rustc my_mod.rs -Z unpretty=hir
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</code></pre>
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