carbon-lang/testing/file_test/README.md

# file_test

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## BUILD

A typical BUILD target will look like:

```starlark
load("rules.bzl", "file_test")

file_test(
    name = "my_file_test",
    srcs = ["my_file_test.cpp"],
    tests = glob(["testdata/**"]),
    deps = [
        ":my_lib",
        "//testing/file_test:file_test_base",
        "@googletest//:gtest",
        "@llvm-project//llvm:Support",
    ],
)
```

## Implementation

A typical implementation will look like:

```cpp
#include "my_library.h"

#include "testing/file_test/file_test_base.h"

namespace Carbon::Testing {
namespace {

class MyFileTest : public FileTestBase {
 public:
  using FileTestBase::FileTestBase;

  // Called as part of individual test executions.
  auto Run(const llvm::SmallVector<llvm::StringRef>& test_args,
           const llvm::SmallVector<TestFile>& test_files,
           llvm::raw_pwrite_stream& stdout, llvm::raw_pwrite_stream& stderr)
      -> ErrorOr<RunResult> override {
    return MyFunctionality(test_args, stdout, stderr);
  }

  // Provides arguments which are used in tests that don't provide ARGS.
  auto GetDefaultArgs() -> llvm::SmallVector<std::string> override {
    return {"default_args", "%s"};
  }
};

}  // namespace

// Registers for the framework to construct the tests.
CARBON_FILE_TEST_FACTORY(MyFileTest);

}  // namespace Carbon::Testing
```

## Filename `fail_` prefixes

When a run fails, information about what pieces failed are returned on
`RunResult`. This affects whether a `fail_` prefix on the file is required,
including in combination with split-file tests (using the `// --- <filename>`
comment marker).

The main test file and any split-files must have a `fail_` prefix if and only if
they have an associated error. An exception is that the main test file may omit
`fail_` when it contains split-files that have a `fail_` prefix.

## Content replacement

Some keywords can be inserted for content:

-   ```
    [[@TEST_NAME]]
    ```

    Replaces with the test name, which is the filename with the extension and
    any `fail_` or `todo_` prefixes removed. For split files, this is based on
    the split filename.

The `[[@` string is reserved for future replacements, but `[[` is allowed in
content (comment markers don't allow `[[`).

## Comment markers

Settings in files are provided in comments, similar to `FileCheck` syntax.
`bazel run :file_test -- --autoupdate` automatically constructs compatible
CHECK:STDOUT: and CHECK:STDERR: lines.

Supported comment markers are:

-   ```
    // AUTOUDPATE
    // NOAUTOUPDATE
    ```

    Controls whether the checks in the file will be autoupdated if --autoupdate
    is passed. Exactly one of these markers must be present. If the file uses
    splits, the marker must currently be before any splits.

    When autoupdating, CHECKs will be inserted starting below AUTOUPDATE. When a
    CHECK has line information, autoupdate will try to insert the CHECK
    immediately next to the line it's associated with, with stderr CHECKs
    preceding the line and stdout CHECKs following the line. When that happens,
    any subsequent CHECK lines without line information, or that refer to lines
    appearing earlier, will immediately follow. As an exception, if no STDOUT
    check line refers to any line in the test, all STDOUT check lines are placed
    at the end of the file instead of immediately after AUTOUPDATE.

    When using split files, if the last split file is named
    `// --- AUTOUPDATE-SPLIT`, all CHECKs will be added there; no line
    associations occur.

-   ```
    // ARGS: <arguments>
    ```

    Provides a space-separated list of arguments, which will be passed to
    RunWithFiles as test_args. These are intended for use by the command as
    arguments.

    Supported replacements within arguments are:

    -   `%s`

        Replaced with the list of files. Currently only allowed as a standalone
        argument, not a substring.

    -   `%t`

        Replaced with `${TEST_TMPDIR}/temp_file`.

    -   `%{identifier}`

        Replaces some implementation-specific identifier with a value. (Mappings
        provided by way of an optional `MyFileTest::GetArgReplacements`)

    ARGS can be specified at most once. If not provided, the FileTestBase child
    is responsible for providing default arguments.

-   ```
    // EXTRA-ARGS: <arguments>
    ```

    Same as `ARGS`, including substitution behavior, but appends to the default
    argument list instead of replacing it.

    `EXTRA-ARGS` can be specified at most once, and a test cannot specify both
    `ARGS` and `EXTRA-ARGS`.

-   ```
    // SET-CAPTURE-CONSOLE-OUTPUT
    ```

    By default, stderr and stdout are expected to be piped through provided
    streams. Adding this causes the test's own stderr and stdout to be captured
    and added as well.

    This should be avoided because we are partly ensuring that streams are an
    API, but is helpful when wrapping Clang, where stderr is used directly.

    SET-CAPTURE-CONSOLE-OUTPUT can be specified at most once.

-   ```
    // SET-CHECK-SUBSET
    ```

    By default, all lines of output must have a CHECK match. Adding this as a
    option sets it so that non-matching lines are ignored. All provided
    CHECK:STDOUT: and CHECK:STDERR: lines must still have a match in output.

    SET-CHECK-SUBSET can be specified at most once.

-   ```
    // --- <filename>
    ```

    By default, all file content is provided to the test as a single file in
    test_files. Using this marker allows the file to be split into multiple
    files which will all be passed to test_files.

    Files are not created on disk; it's expected the child will create an
    InMemoryFilesystem if needed.

-   ```
    // CHECK:STDOUT: <output line>
    // CHECK:STDERR: <output line>
    ```

    These provide a match for output from the command. See `SET-CHECK-SUBSET`
    for how to change from full to subset matching of output.

    Output line matchers may contain `[[@LINE+offset]` and `{{regex}}` syntaxes,
    similar to `FileCheck`.

-   ```
    // TIP: <tip>
    ```

    Tips like this are added by autoupdate, for example providing commands to
    run the test directly. Tips have no impact on validation; the marker informs
    autoupdate that it can update or remove them as needed.

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