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