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.

To add a new test, the first thing you generally do is to create a file, typically a Rust source file. Test files have a particular structure:

Depending on the test suite, there may be some other details to be aware of:

What kind of test should I add?

It can be difficult to know what kind of test to use. Here are some rough heuristics:

  • Some tests have specialized needs:
    • need to run gdb or lldb? use the debuginfo test suite
    • need to inspect LLVM IR or MIR IR? use the codegen or mir-opt test suites
    • need to run rustdoc? Prefer a rustdoc test
    • need to inspect the resulting binary in some way? Then use run-make
  • For most other things, a ui (or ui-fulldeps) test is to be preferred:
    • ui tests subsume both run-pass, compile-fail, and parse-fail tests
    • in the case of warnings or errors, ui tests capture the full output, which makes it easier to review but also helps prevent "hidden" regressions in the output

Naming your test

We have not traditionally had a lot of structure in the names of tests. Moreover, for a long time, the rustc test runner did not support subdirectories (it now does), so test suites like src/test/ui have a huge mess of files in them. This is not considered an ideal setup.

For regression tests – basically, some random snippet of code that came in from the internet – we often name the test after the issue plus a short description. Ideally, the test should be added to a directory that helps identify what piece of code is being tested here (e.g., src/test/ui/borrowck/ If you've tried and cannot find a more relevant place, the test may be added to src/test/ui/issues/. Still, do include the issue number somewhere.

When writing a new feature, create a subdirectory to store your tests. For example, if you are implementing RFC 1234 ("Widgets"), then it might make sense to put the tests in a directory like src/test/ui/rfc1234-widgets/.

In other cases, there may already be a suitable directory. (The proper directory structure to use is actually an area of active debate.)

Comment explaining what the test is about

When you create a test file, include a comment summarizing the point of the test at the start of the file. This should highlight which parts of the test are more important, and what the bug was that the test is fixing. Citing an issue number is often very helpful.

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 is was meant to test, and then it's useful to know what it was meant to test exactly).

Header commands: configuring rustc

Header commands are special comments that the test runner knows how to interpret. They must appear before the Rust source in the test. They are normally put after the short comment that explains the point of this test. For example, this test uses the // compile-flags command to specify a custom flag to give to rustc when the test is compiled:

// Test the behavior of `0 - 1` when overflow checks are disabled.

// compile-flags: -Coverflow-checks=off

fn main() {
    let x = 0 - 1;

Ignoring tests

These are used to ignore the test in some situations, which means the test won't be compiled or run.

  • ignore-X where X is a target detail or stage will ignore the test accordingly (see below)
  • only-X is like ignore-X, but will only run the test on that target or stage
  • ignore-pretty will not compile the pretty-printed test (this is done to test the pretty-printer, but might not always work)
  • ignore-test always ignores the test
  • ignore-lldb and ignore-gdb will skip a debuginfo test on that debugger.
  • ignore-gdb-version can be used to ignore the test when certain gdb versions are used

Some examples of X in ignore-X:

  • Architecture: aarch64, arm, asmjs, mips, wasm32, x86_64, x86, ...
  • OS: android, emscripten, freebsd, ios, linux, macos, windows, ...
  • Environment (fourth word of the target triple): gnu, msvc, musl.
  • Pointer width: 32bit, 64bit.
  • Stage: stage0, stage1, stage2.
  • When cross compiling: cross-compile
  • When remote testing is used: remote
  • When debug-assertions are enabled: debug
  • When particular debuggers are being tested: cdb, gdb, lldb
  • Specific compare modes: compare-mode-nll, compare-mode-polonius

Other Header Commands

Here is a list of other header commands. This list is not exhaustive. Header commands can generally be found by browsing the TestProps structure found in from the compiletest source.

  • run-rustfix for UI tests, indicates that the test produces structured suggestions. The test writer should create a .fixed file, which contains the source with the suggestions applied. When the test is run, compiletest first checks that the correct lint/warning is generated. Then, it applies the suggestion and compares against .fixed (they must match). Finally, the fixed source is compiled, and this compilation is required to succeed. The .fixed file can also be generated automatically with the --bless option, described in this section.
  • min-gdb-version specifies the minimum gdb version required for this test; see also ignore-gdb-version
  • min-lldb-version specifies the minimum lldb version required for this test
  • rust-lldb causes the lldb part of the test to only be run if the lldb in use contains the Rust plugin
  • no-system-llvm causes the test to be ignored if the system llvm is used
  • min-llvm-version specifies the minimum llvm version required for this test
  • min-system-llvm-version specifies the minimum system llvm version required for this test; the test is ignored if the system llvm is in use and it doesn't meet the minimum version. This is useful when an llvm feature has been backported to rust-llvm
  • ignore-llvm-version can be used to skip the test when certain LLVM versions are used. This takes one or two arguments; the first argument is the first version to ignore. If no second argument is given, all subsequent versions are ignored; otherwise, the second argument is the last version to ignore.
  • build-pass for UI tests, indicates that the test is supposed to successfully compile and link, as opposed to the default where the test is supposed to error out.
  • compile-flags passes extra command-line args to the compiler, e.g. compile-flags -g which forces debuginfo to be enabled.
  • should-fail indicates that the test should fail; used for "meta testing", where we test the compiletest program itself to check that it will generate errors in appropriate scenarios. This header is ignored for pretty-printer tests.
  • gate-test-X where X is a feature marks the test as "gate test" for feature X. Such tests are supposed to ensure that the compiler errors when usage of a gated feature is attempted without the proper #![feature(X)] tag. Each unstable lang feature is required to have a gate test.
  • needs-profiler-support - a profiler runtime is required, i.e., profiler = true in rustc's config.toml.
  • needs-sanitizer-support - a sanitizer runtime is required, i.e., sanitizers = true in rustc's config.toml.
  • needs-sanitizer-{address,leak,memory,thread} - indicates that test requires a target with a support for AddressSanitizer, LeakSanitizer, MemorySanitizer or ThreadSanitizer respectively.

Error annotations

Error annotations specify the errors that the compiler is expected to emit. They are "attached" to the line in source where the error is located. Error annotations are considered during tidy lints of line length and should be formatted according to tidy requirements. You may use an error message prefix sub-string if necessary to meet line length requirements. Make sure that the text is long enough for the error message to be self-documenting.

The error annotation definition and source line definition association is defined with the following set of idioms:

  • ~: Associates the following error level and message with the current line
  • ~|: Associates the following error level and message with the same line as the previous comment
  • ~^: Associates the following error level and message with the previous error annotation line. Each caret (^) that you add adds a line to this, so ~^^^ is three lines above the error annotation line.

Error annotation examples

Here are examples of error annotations on different lines of UI test source.

Positioned on error line

Use the //~ ERROR idiom:

fn main() {
    let x = (1, 2, 3);
    match x {
        (_a, _x @ ..) => {} //~ ERROR `_x @` is not allowed in a tuple
        _ => {}

Positioned below error line

Use the //~^ idiom with number of carets in the string to indicate the number of lines above. In the example below, the error line is four lines above the error annotation line so four carets are included in the annotation.

fn main() {
    let x = (1, 2, 3);
    match x {
        (_a, _x @ ..) => {}  // <- the error is on this line
        _ => {}
//~^^^^ ERROR `_x @` is not allowed in a tuple

Use same error line as defined on error annotation line above

Use the //~| idiom to define the same error line as the error annotation line above:

struct Binder(i32, i32, i32);

fn main() {
    let x = Binder(1, 2, 3);
    match x {
        Binder(_a, _x @ ..) => {}  // <- the error is on this line
        _ => {}
//~^^^^ ERROR `_x @` is not allowed in a tuple struct
//~| ERROR this pattern has 1 field, but the corresponding tuple struct has 3 fields [E0023]

The error levels that you can have are:

  1. ERROR
  3. NOTE

* Note: SUGGESTION must follow immediately after HELP.


Certain classes of tests support "revisions" (as of the time of this writing, this includes compile-fail, run-fail, and incremental, though incremental tests are somewhat different). Revisions allow a single test file to be used for multiple tests. This is done by adding a special header at the top of the file:

fn main() {
// revisions: foo bar baz

This will result in the test being compiled (and tested) three times, once with --cfg foo, once with --cfg bar, and once with --cfg baz. You can therefore use #[cfg(foo)] etc within the test to tweak each of these results.

You can also customize headers and expected error messages to a particular revision. To do this, add [foo] (or bar, baz, etc) after the // comment, like so:

fn main() {
// A flag to pass in only for cfg `foo`:
//[foo]compile-flags: -Z verbose

fn test_foo() {
    let x: usize = 32_u32; //[foo]~ ERROR mismatched types

Note that not all headers have meaning when customized to a revision. For example, the ignore-test header (and all "ignore" headers) currently only apply to the test as a whole, not to particular revisions. The only headers that are intended to really work when customized to a revision are error patterns and compiler flags.

Guide to the UI tests

The UI tests are intended to capture the compiler's complete output, so that we can test all aspects of the presentation. They work by compiling a file (e.g., ui/hello_world/, capturing the output, and then applying some normalization (see below). This normalized result is then compared against reference files named ui/hello_world/main.stderr and ui/hello_world/main.stdout. If either of those files doesn't exist, the output must be empty (that is actually the case for this particular test). If the test run fails, we will print out the current output, but it is also saved in build/<target-triple>/test/ui/hello_world/main.stdout (this path is printed as part of the test failure message), so you can run diff and so forth.

Tests that do not result in compile errors

By default, a UI test is expected not to compile (in which case, it should contain at least one //~ ERROR annotation). However, you can also make UI tests where compilation is expected to succeed, and you can even run the resulting program. Just add one of the following header commands:

  • // check-pass - compilation should succeed but skip codegen (which is expensive and isn't supposed to fail in most cases)
  • // build-pass – compilation and linking should succeed but do not run the resulting binary
  • // run-pass – compilation should succeed and we should run the resulting binary


The compiler output is normalized to eliminate output difference between platforms, mainly about filenames.

The following strings replace their corresponding values:

  • $DIR: The directory where the test is defined.
    • Example: /path/to/rust/src/test/ui/error-codes
  • $SRC_DIR: The root source directory.
    • Example: /path/to/rust/src
  • $TEST_BUILD_DIR: The base directory where the test's output goes.
    • Example: /path/to/rust/build/x86_64-unknown-linux-gnu/test/ui

Additionally, the following changes are made:

  • Line and column numbers for paths in $SRC_DIR are replaced with LL:CC. For example, /path/to/rust/src/libcore/ is replaced with $SRC_DIR/libcore/

    Note: The line and column numbers for --> lines pointing to the test are not normalized, and left as-is. This ensures that the compiler continues to point to the correct location, and keeps the stderr files readable. Ideally all line/column information would be retained, but small changes to the source causes large diffs, and more frequent merge conflicts and test errors. See also -Z ui-testing below which applies additional line number normalization.

  • \t is replaced with an actual tab character.

  • Error line annotations like // ~ERROR some message are removed.

  • Backslashes (\) are converted to forward slashes (/) within paths (using a heuristic). This helps normalize differences with Windows-style paths.

  • CRLF newlines are converted to LF.

Additionally, the compiler is run with the -Z ui-testing flag which causes the compiler itself to apply some changes to the diagnostic output to make it more suitable for UI testing. For example, it will anonymize line numbers in the output (line numbers prefixing each source line are replaced with LL). In extremely rare situations, this mode can be disabled with the header command // compile-flags: -Z ui-testing=no.

Sometimes these built-in normalizations are not enough. In such cases, you may provide custom normalization rules using the header commands, e.g.

fn main() {
// normalize-stdout-test: "foo" -> "bar"
// normalize-stderr-32bit: "fn\(\) \(32 bits\)" -> "fn\(\) \($$PTR bits\)"
// normalize-stderr-64bit: "fn\(\) \(64 bits\)" -> "fn\(\) \($$PTR bits\)"

This tells the test, on 32-bit platforms, whenever the compiler writes fn() (32 bits) to stderr, it should be normalized to read fn() ($PTR bits) instead. Similar for 64-bit. The replacement is performed by regexes using default regex flavor provided by regex crate.

The corresponding reference file will use the normalized output to test both 32-bit and 64-bit platforms:

   = note: source type: fn() ($PTR bits)
   = note: target type: u16 (16 bits)

Please see ui/transmute/ and main.stderr for a concrete usage example.

Besides normalize-stderr-32bit and -64bit, one may use any target information or stage supported by ignore-X here as well (e.g. normalize-stderr-windows or simply normalize-stderr-test for unconditional replacement).