tests/ui/cat/
    meow.rs                 # main test file
    auxiliary/whiskers.rs   # auxiliary
```

```rs
// tests/ui/cat/meow.rs

//@ proc-macro: whiskers.rs

extern crate whiskers; // needed as ui test defaults to edition 2015

fn main() {
  whiskers::identity!();
}
```

```rs
// tests/ui/cat/auxiliary/whiskers.rs

extern crate proc_macro;
use proc_macro::*;

#[proc_macro]
pub fn identity(ts: TokenStream) -> TokenStream {
    ts
}
```

> **Note**: The `proc-macro` header currently does not work with the
> `build-aux-doc` header for rustdoc tests. In that case, you will need to use
> the `aux-build` header, and use `#![crate_type="proc_macro"]`, and `//@
> force-host` and `//@ no-prefer-dynamic` headers in the proc-macro.

## Revisions

Revisions allow a single test file to be used for multiple tests. This is done
by adding a special directive at the top of the file:

```rust,ignore
//@ 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 directives and expected error messages to a particular
revision. To do this, add `[revision-name]` after the `//@` for directives, and
after `//` for UI error annotations, like so:

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

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

Multiple revisions can be specified in a comma-separated list, such as
`//[foo,bar,baz]~^`.

In test suites that use the LLVM [FileCheck] tool, the current revision name is
also registered as an additional prefix for FileCheck directives:

```rust,ignore
//@ revisions: NORMAL COVERAGE
//@[COVERAGE] compile-flags: -Cinstrument-coverage
//@[COVERAGE] needs-profiler-runtime

// COVERAGE:   @__llvm_coverage_mapping
// NORMAL-NOT: @__llvm_coverage_mapping

// CHECK: main
fn main() {}
```

Note that not all directives have meaning when customized to a revision. For
example, the `ignore-test` directives (and all "ignore" directives) currently
only apply to the test as a whole, not to particular revisions. The only