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| <h1 id="best-practices-for-writing-tests"><a class="header" href="#best-practices-for-writing-tests">Best practices for writing tests</a></h1> |
| <p>This chapter describes best practices related to authoring and modifying tests. |
| We want to make sure the tests we author are easy to understand and modify, even |
| several years later, without needing to consult the original author and perform |
| a bunch of git archeology.</p> |
| <p>It's good practice to review the test that you authored by pretending that you |
| are a different contributor who is looking at the test that failed several years |
| later without much context (this also helps yourself even a few days or months |
| later!). Then ask yourself: how can I make my life and their lives easier?</p> |
| <p>To help put this into perspective, let's start with an aside on how to write a |
| test that makes the life of another contributor as hard as possible.</p> |
| <blockquote> |
| <p><strong>Aside: Simple Test Sabotage Field Manual</strong></p> |
| <p>To make the life of another contributor as hard as possible, one might:</p> |
| <ul> |
| <li>Name the test after an issue number alone without any other context, e.g. |
| <code>issue-123456.rs</code>.</li> |
| <li>Have no comments at all on what the test is trying to exercise, no links to |
| relevant context.</li> |
| <li>Include a test that is massive (that can otherwise be minimized) and |
| contains non-essential pieces which distracts from the core thing the test |
| is actually trying to test.</li> |
| <li>Include a bunch of unrelated syntax errors and other errors which are not |
| critical to what the test is trying to check.</li> |
| <li>Weirdly format the snippets.</li> |
| <li>Include a bunch of unused and unrelated features.</li> |
| <li>Have e.g. <code>ignore-windows</code> <a href="./directives.html">compiletest directives</a> but don't offer any |
| explanation as to <em>why</em> they are needed.</li> |
| </ul> |
| </blockquote> |
| <h2 id="test-naming"><a class="header" href="#test-naming">Test naming</a></h2> |
| <p>Make it easy for the reader to immediately understand what the test is |
| exercising, instead of having to type in the issue number and dig through github |
| search for what the test is trying to exercise. This has an additional benefit |
| of making the test possible to be filtered via <code>--test-args</code> as a collection of |
| related tests.</p> |
| <ul> |
| <li>Name the test after what it's trying to exercise or prevent regressions of.</li> |
| <li>Keep it concise.</li> |
| <li>Avoid using issue numbers alone as test names.</li> |
| <li>Avoid starting the test name with <code>issue-xxxxx</code> prefix as it degrades |
| auto-completion.</li> |
| </ul> |
| <blockquote> |
| <p><strong>Avoid using only issue numbers as test names</strong></p> |
| <p>Prefer including them as links or <code>#123456</code> in test comments instead. Or if it |
| makes sense to include the issue number, also include brief keywords like |
| <code>macro-external-span-ice-123956.rs</code>.</p> |
| <pre><code class="language-text">tests/ui/typeck/issue-123456.rs // bad |
| tests/ui/typeck/issue-123456-asm-macro-external-span-ice.rs // bad (for tab completion) |
| tests/ui/typeck/asm-macro-external-span-ice-123456.rs // good |
| tests/ui/typeck/asm-macro-external-span-ice.rs // good |
| </code></pre> |
| <p><code>issue-123456.rs</code> does not tell you immediately anything about what the test |
| is actually exercising meaning you need to do additional searching. Including |
| the issue number in the test name as a prefix makes tab completion less useful |
| (if you <code>ls</code> a test directory and get a bunch of <code>issue-xxxxx</code> prefixes). We |
| can link to the issue in a test comment.</p> |
| <pre><code class="language-rs">//! Check that `asm!` macro including nested macros that come from external |
| //! crates do not lead to a codepoint boundary assertion ICE. |
| //! |
| //! Regression test for <https://github.com/rust-lang/rust/issues/123456>. |
| </code></pre> |
| <p>One exception to this rule is <a href="./compiletest.html#crash-tests">crash tests</a>: there it is canonical that |
| tests are named only after issue numbers because its purpose is to track |
| snippets from which issues no longer ICE/crash, and they would either be |
| removed or converted into proper ui/other tests in the fix PRs.</p> |
| </blockquote> |
| <h2 id="test-organization"><a class="header" href="#test-organization">Test organization</a></h2> |
| <ul> |
| <li>For most test suites, try to find a semantically meaningful subdirectory to |
| home the test. |
| <ul> |
| <li>E.g. for an implementation of RFC 2093 specifically, we can group a |
| collection of tests under <code>tests/ui/rfc-2093-infer-outlives/</code>. For the |
| directory name, include what the RFC is about.</li> |
| </ul> |
| </li> |
| <li>For the <a href="./compiletest.html#run-make-tests"><code>run-make</code></a>/<code>run-make-support</code> test suites, each <code>rmake.rs</code> must |
| be contained within an immediate subdirectory under <code>tests/run-make/</code> or |
| <code>tests/run-make-cargo/</code> respectively. Further nesting is not presently |
| supported. Avoid using <em>only</em> an issue number for the test name as well.</li> |
| </ul> |
| <h2 id="test-descriptions"><a class="header" href="#test-descriptions">Test descriptions</a></h2> |
| <p>To help other contributors understand what the test is about if their changes |
| lead to the test failing, we should make sure a test has sufficient docs about |
| its intent/purpose, links to relevant context (incl. issue numbers or other |
| discussions) and possibly relevant resources (e.g. can be helpful to link to |
| Win32 APIs for specific behavior).</p> |
| <p><strong>Synopsis of a test with good comments</strong></p> |
| <pre><code class="language-rust ignore">//! Brief summary of what the test is exercising. |
| //! Example: Regression test for #123456: make sure coverage attribute don't ICE |
| //! when applied to non-items. |
| //! |
| //! Optional: Remarks on related tests/issues, external APIs/tools, crash |
| //! mechanism, how it's fixed, FIXMEs, limitations, etc. |
| //! Example: This test is like `tests/attrs/linkage.rs`, but this test is |
| //! specifically checking `#[coverage]` which exercises a different code |
| //! path. The ICE was triggered during attribute validation when we tried |
| //! to construct a `def_path_str` but only emitted the diagnostic when the |
| //! platform is windows, causing an ICE on unix. |
| //! |
| //! Links to relevant issues and discussions. Examples below: |
| //! Regression test for <https://github.com/rust-lang/rust/issues/123456>. |
| //! See also <https://github.com/rust-lang/rust/issues/101345>. |
| //! See discussion at <https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/123456-example-topic>. |
| //! See [`clone(2)`]. |
| //! |
| //! [`clone(2)`]: https://man7.org/linux/man-pages/man2/clone.2.html |
| |
| //@ ignore-windows |
| // Reason: (why is this test ignored for windows? why not specifically |
| // windows-gnu or windows-msvc?) |
| |
| // Optional: Summary of test cases: What positive cases are checked? |
| // What negative cases are checked? Any specific quirks? |
| |
| fn main() { |
| #[coverage] |
| //~^ ERROR coverage attribute can only be applied to function items. |
| let _ = { |
| // Comment highlighting something that deserves reader attention. |
| fn foo() {} |
| }; |
| }</code></pre> |
| <p>For how much context/explanation is needed, it is up to the author and |
| reviewer's discretion. A good rule of thumb is non-trivial things exercised in |
| the test deserves some explanation to help other contributors to understand. |
| This may include remarks on:</p> |
| <ul> |
| <li>How an ICE can get triggered if it's quite elaborate.</li> |
| <li>Related issues and tests (e.g. this test is like another test but is kept |
| separate because...).</li> |
| <li>Platform-specific behaviors.</li> |
| <li>Behavior of external dependencies and APIs: syscalls, linkers, tools, |
| environments and the likes.</li> |
| </ul> |
| <h2 id="test-content"><a class="header" href="#test-content">Test content</a></h2> |
| <ul> |
| <li>Try to make sure the test is as minimal as possible.</li> |
| <li>Minimize non-critical code and especially minimize unnecessary syntax and type |
| errors which can clutter stderr snapshots.</li> |
| <li>Where possible, use semantically meaningful names (e.g. <code>fn bare_coverage_attributes() {}</code>).</li> |
| </ul> |
| <h2 id="flaky-tests"><a class="header" href="#flaky-tests">Flaky tests</a></h2> |
| <p>All tests need to strive to be reproducible and reliable. Flaky tests are the |
| worst kind of tests, arguably even worse than not having the test in the first |
| place.</p> |
| <ul> |
| <li>Flaky tests can fail in completely unrelated PRs which can confuse other |
| contributors and waste their time trying to figure out if test failure is |
| related.</li> |
| <li>Flaky tests provide no useful information from its test results other than |
| it's flaky and not reliable: if a test passed but it's flakey, did I just get |
| lucky? if a test is flakey but it failed, was it just spurious?</li> |
| <li>Flaky tests degrade confidence in the whole test suite. If a test suite can |
| randomly spuriously fail due to flaky tests, did the whole test suite pass or |
| did I just get lucky/unlucky?</li> |
| <li>Flaky tests can randomly fail in full CI, wasting previous full CI resources.</li> |
| </ul> |
| <h2 id="compiletest-directives"><a class="header" href="#compiletest-directives">Compiletest directives</a></h2> |
| <p>See <a href="./directives.html">compiletest directives</a> for a listing of directives.</p> |
| <ul> |
| <li>For <code>ignore-*</code>/<code>needs-*</code>/<code>only-*</code> directives, unless extremely obvious, |
| provide a brief remark on why the directive is needed. E.g. <code>"//@ ignore-wasi (wasi codegens the main symbol differently)"</code>.</li> |
| <li>When using <code>//@ ignore-auxiliary</code>, specify the corresponding main test files, |
| e.g. <code>//@ ignore-auxiliary (used by `./foo.rs`)</code>.</li> |
| </ul> |
| <h2 id="filecheck-best-practices"><a class="header" href="#filecheck-best-practices">FileCheck best practices</a></h2> |
| <p>See <a href="https://llvm.org/docs/CommandGuide/FileCheck.html">LLVM FileCheck guide</a> for details.</p> |
| <ul> |
| <li>Avoid matching on specific register numbers or basic block numbers unless |
| they're special or critical for the test. Consider using patterns to match |
| them where suitable.</li> |
| </ul> |
| <blockquote> |
| <p><strong>TODO</strong></p> |
| <p>Pending concrete advice.</p> |
| </blockquote> |
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