Adding new tests - Rust Compiler Development Guide (original) (raw)

Rust Compiler Development Guide

Adding new tests

In general, we expect every PR that fixes a bug in rustc to come accompanied by a regression test of some kind. This test should fail in master but pass after the PR. These tests are really useful for preventing us from repeating the mistakes of the past.

The first thing to decide is which kind of test to add. This will depend on the nature of the change and what you want to exercise. Here are some rough guidelines:

After deciding on which kind of test to add, see best practices for guidance on how to author tests that are easy to work with that stand the test of time (i.e. if a test fails or need to be modified several years later, how can we make it easier for them?).

UI test walkthrough

The following is a basic guide for creating a UI test, which is one of the most common compiler tests. For this tutorial, we'll be adding a test for an async error message.

Step 1: Add a test file

The first step is to create a Rust source file somewhere in the tests/uitree. When creating a test, do your best to find a good location and name (seeTest organization for more). Since naming is the hardest part of development, everything should be downhill from here!

Let's place our async test at tests/ui/async-await/await-without-async.rs:

// Provide diagnostics when the user writes `await` in a non-`async` function.
//@ edition:2018

async fn foo() {}

fn bar() {
    foo().await
}

fn main() {}

A few things to notice about our test:

Step 2: Generate the expected output

The next step is to create the expected output snapshots from the compiler. This can be done with the --bless option:

./x test tests/ui/async-await/await-without-async.rs --bless

This will build the compiler (if it hasn't already been built), compile the test, and place the output of the compiler in a file calledtests/ui/async-await/await-without-async.stderr.

However, this step will fail! You should see an error message, something like this:

error: /rust/tests/ui/async-await/await-without-async.rs:7: unexpected error: '7:10: 7:16: await is only allowed inside async functions and blocks E0728'

This is because the stderr contains errors which were not matched by error annotations in the source file.

Step 3: Add error annotations

Every error needs to be annotated with a comment in the source with the text of the error. In this case, we can add the following comment to our test file:

fn bar() {
    foo().await
    //~^ ERROR `await` is only allowed inside `async` functions and blocks
}

The //~^ squiggle caret comment tells compiletest that the error belongs to the previous line (more on this in the Error annotations section).

Save that, and run the test again:

./x test tests/ui/async-await/await-without-async.rs

It should now pass, yay!

Step 4: Review the output

Somewhat hand-in-hand with the previous step, you should inspect the .stderrfile that was created to see if it looks like how you expect. If you are adding a new diagnostic message, now would be a good time to also consider how readable the message looks overall, particularly for people new to Rust.

Our example tests/ui/async-await/await-without-async.stderr file should look like this:

error[E0728]: `await` is only allowed inside `async` functions and blocks
  --> $DIR/await-without-async.rs:7:10
   |
LL | fn bar() {
   |    --- this is not `async`
LL |     foo().await
   |          ^^^^^^ only allowed inside `async` functions and blocks

error: aborting due to previous error

For more information about this error, try `rustc --explain E0728`.

You may notice some things look a little different than the regular compiler output.

Around this stage, you may need to iterate over the last few steps a few times to tweak your test, re-bless the test, and re-review the output.

Step 5: Check other tests

Sometimes when adding or changing a diagnostic message, this will affect other tests in the test suite. The final step before posting a PR is to check if you have affected anything else. Running the UI suite is usually a good start:

./x test tests/ui

If other tests start failing, you may need to investigate what has changed and if the new output makes sense.

You may also need to re-bless the output with the --bless flag.

The first comment of a test file should summarize the point of the test, and highlight what is important about it. If there is an issue number associated with the test, include the issue number.

This comment doesn't have to be super extensive. Just something like "Regression test for #18060: match arms were matching in the wrong order." might already be enough.

These comments are very useful to others later on when your test breaks, since they often can highlight what the problem is. They are also useful if for some reason the tests need to be refactored, since they let others know which parts of the test were important. Often a test must be rewritten because it no longer tests what it was meant to test, and then it's useful to know what it _was_meant to test exactly.