# 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:

- They should have some kind of
  [comment explaining what the test is about](#explanatory_comment);
- next, they can have one or more [header commands](#header_commands), which
  are special comments that the test interpreter knows how to interpret.
- finally, they have the Rust source. This may have various [error
  annotations](#error_annotations) which indicate expected compilation errors or
  warnings.

Depending on the test suite, there may be some other details to be aware of:
  - For [the `ui` test suite](#ui), you need to generate reference output files.

## 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` or `rustdoc-ui` test.
    Occasionally you'll need `rustdoc-js` as well.
  - need to inspect the resulting binary in some way? Then use `run-make`
- Library tests should go in `library/${crate}/tests` (where `${crate}` is
  usually `core`, `alloc`, or `std`). Library tests include:
  - tests that an API behaves properly, including accepting various types or
    having some runtime behavior
  - tests where any compiler warnings are not relevant to the test
  - tests that a use of an API gives a compile error, where the exact error
    message is not relevant to the test. These should have an
    [error number] (`E0XXX`) in the code block to make sure it's the correct error.
- For most other things, [a `ui` (or `ui-fulldeps`) test](#ui) is to be preferred:
  - 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

[error number]: https://doc.rust-lang.org/rustdoc/unstable-features.html#error-numbers-for-compile-fail-doctests

## 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.

[`src/test/ui`]: https://github.com/rust-lang/rust/tree/master/src/test/ui/

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/issue-54597-reject-move-out-of-borrow-via-pat.rs`)
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**.
But please avoid putting your test there as possible since that
directory has too many tests and it causes poor semantic organization.

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:

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

// compile-flags: -C overflow-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 [`header.rs`] 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][bless].
* `rustfix-only-machine-applicable` is equivalent to `run-rustfix` except it
  will only apply [`MachineApplicable`](../diagnostics.md#suggestions)
  suggestions. `run-rustfix` will apply *all* suggestions. This should be used
  if there is a mixture of different suggestion levels, and some of the
  non-machine-applicable ones do not apply cleanly.
* `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.
* `edition` controls the edition the test should be compiled with
  (defaults to 2015). Example usage: `// edition:2018`.
* `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,hwaddress,leak,memory,thread}` - indicates that
  test requires a target with a support for AddressSanitizer, hardware-assisted
  AddressSanitizer, LeakSanitizer, MemorySanitizer or ThreadSanitizer
  respectively.
* `error-pattern` checks the diagnostics just like the `ERROR` annotation
  without specifying error line. This is useful when the error doesn't give
  any span.
* `incremental` runs the test with the `-C incremental` flag and an empty
  incremental directory. This should be avoided when possible; you should use
  an *incremental mode* test instead. Incremental mode tests support running
  the compiler multiple times and verifying that it can load the generated
  incremental cache. This flag is for specialized circumstances, like checking
  the interaction of codegen unit partitioning with generating an incremental
  cache.
* `aux-build` is used to compile additional crates to link. Just pass it the
  name of the source file. The source file should be in a directory called
  `auxiliary` beside the test file. The aux crate will be built as a dylib if
  possible (unless on a platform that does not support them, or
  `no-prefer-dynamic` is specified in the aux file). The `-L` flag is used to
  find the extern crates.
* `aux-crate` is very similar to `aux-build`; however, it uses the `--extern`
  flag to link to the extern crate. That allows you to specify the additional
  syntax of the `--extern` flag, such as renaming a dependency. For example,
  `// aux-crate:foo=bar.rs` will compile `auxiliary/bar.rs` and make it
  available under then name `foo` within the test. This is similar to how
  Cargo does dependency renaming.
* `no-prefer-dynamic` will force an auxiliary crate to be built as an rlib
  instead of a dylib. When specified in a test, it will remove the use of `-C
  prefer-dynamic`. This can be useful in a variety of circumstances. For
  example, it can prevent a proc-macro from being built with the wrong crate
  type. Or if your test is specifically targeting behavior of other crate
  types, it can be used to prevent building with the wrong crate type.
* `force-host` will force the test to build for the host platform instead of
  the target. This is useful primarily for auxiliary proc-macros, which need
  to be loaded by the host compiler.
* `pretty-mode` specifies the mode pretty-print tests should run in.
  The default is `normal` if not specified.
* `pretty-compare-only` causes a pretty test to only compare the
  pretty-printed output. It will not try to compile the expanded output to
  typecheck it. This is needed for a pretty-mode that does not expand to valid
  Rust, or for other situations where the expanded output cannot be compiled.
* `pretty-expanded` allows a pretty test to also run with
  `-Zunpretty=expanded` as a final step. It will also try to compile the
  resulting output (without codegen). This is needed because not all code can
  be compiled after being expanded. Pretty tests should specify this if they
  can. An example where this cannot be used is if the test includes
  `println!`. That macro expands to reference private internal functions of
  the standard library that cannot be called directly without the
  `fmt_internals` feature gate.

  More history about this may be found in [#23616].
* `pp-exact` is used to ensure a pretty-print test results in specific output.
  If specified without a value, then it means the pretty-print output should
  match the original source. If specified with a value, as in `//
  pp-exact:foo.pp`, it will ensure that the pretty-printed output matches the
  contents of the given file. Otherwise, if `pp-exact` is not specified, then
  the pretty-printed output will be pretty-printed one more time, and the
  output of the two pretty-printing rounds will be compared to ensure that the
  pretty-printed output converges to a steady state.

[`header.rs`]: https://github.com/rust-lang/rust/tree/master/src/tools/compiletest/src/header.rs
[bless]: ./running.md#editing-and-updating-the-reference-files
[#23616]: https://github.com/rust-lang/rust/issues/23616#issuecomment-484999901

## 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:

```rust,ignore
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.

```rust,ignore
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:

```rust,ignore
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]
```

#### When error line cannot be specified

Let's think about this test:

```rust,ignore
fn main() {
    let a: *const [_] = &[1, 2, 3];
    unsafe {
        let _b = (*a)[3];
    }
}
```

We want to ensure this shows "index out of bounds" but we cannot use the `ERROR` annotation
since the error doesn't have any span. Then it's time to use the `error-pattern`:

```rust,ignore
// error-pattern: index out of bounds
fn main() {
    let a: *const [_] = &[1, 2, 3];
    unsafe {
        let _b = (*a)[3];
    }
}
```

But for strict testing, try to use the `ERROR` annotation as much as possible.

#### Error levels

The error levels that you can have are:

1. `ERROR`
2. `WARNING`
3. `NOTE`
4. `HELP` and `SUGGESTION`[^sugg-placement]

[^sugg-placement]: **Note**: `SUGGESTION` must follow immediately after `HELP`.

## Revisions

Certain classes of tests support "revisions" (as of  February 2021,
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:

```rust
// 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:

```rust
// A flag to pass in only for cfg `foo`:
//[foo]compile-flags: -Z verbose

#[cfg(foo)]
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/main.rs`][hw-main]),
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][hw]). 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.

[hw-main]: https://github.com/rust-lang/rust/blob/master/src/test/ui/hello_world/main.rs
[hw]: https://github.com/rust-lang/rust/blob/master/src/test/ui/hello_world/

We now have a ton of UI tests and some directories have too many entries.
This is a problem because it isn't editor/IDE friendly and GitHub UI won't
show more than 1000 entries. To resolve it and organize semantic structure,
we have a tidy check to ensure the number of entries is less than 1000.
However, since `src/test/ui` (UI test root directory) and
`src/test/ui/issues` directories have more than 1000 entries,
we set a different limit for each directories. So, please
avoid putting a new test there and try to find a more relevant place.
For example, if your test is related to closures, you should put it in
`src/test/ui/closures`. If you're not sure where is the best place,
it's still okay to add to `src/test/ui/issues/`. When you reach the limit,
you could increase it by tweaking [here][ui test tidy].

[ui test tidy]: https://github.com/rust-lang/rust/blob/master/src/tools/tidy/src/ui_tests.rs

### 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](#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

### Output Normalization

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 `$SRC_DIR` are replaced with `LL:CC`.
  For example, `/path/to/rust/library/core/src/clone.rs:122:8` is replaced with
  `$SRC_DIR/core/src/clone.rs:LL:COL`.

  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.

```rust
// 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:

```text
...
   |
   = note: source type: fn() ($PTR bits)
   = note: target type: u16 (16 bits)
...
```

Please see [`ui/transmute/main.rs`][mrs] and [`main.stderr`][] for a
concrete usage example.

[mrs]: https://github.com/rust-lang/rust/blob/master/src/test/ui/transmute/main.rs
[`main.stderr`]: https://github.com/rust-lang/rust/blob/master/src/test/ui/transmute/main.stderr

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

## Input Normalization

Sometimes, you want to normalize the inputs to a test. For example, you may
want to pass `// compile-flags: --x=y.rs`, where y.rs is some file in the test
directory. In this case you can use input normalization. The following strings
are replaced in header inputs:

- {{cwd}}: The directory where compiletest is run from. This may not be the
  root of the checkout, so you should avoid using it where possible.
  - Examples: `/path/to/rust`, `/path/to/build/root`
- {{src-base}}: The directory where the test is defined. This is equivalent to
  `$DIR` for output normalization.
  - Example: `/path/to/rust/src/test/ui/error-codes`
- {{build-base}}: The base directory where the test's output goes. This is
  equivalent to `$TEST_BUILD_DIR` for output normalization.
  - Example: `/path/to/rust/build/x86_64-unknown-linux-gnu/test/ui`

See [`src/test/ui/commandline-argfile.rs`](https://github.com/rust-lang/rust/blob/a5029ac0ab372aec515db2e718da6d7787f3d122/src/test/ui/commandline-argfile.rs)
for an example of a test that uses input normalization.
