# assert2

All-purpose [`assert!(...)`](macro.assert.html) and [`check!(...)`](macro.check.html) macros, inspired by [Catch2](https://github.com/catchorg/Catch2).
There is also a [`debug_assert!(...)`](macro.debug_assert.html) macro that is disabled on optimized builds by default.
As cherry on top there is a [`let_assert!(...)`](macro.let_assert.html) macro that lets you test a pattern while capturing parts of it.

## Why these macros?

These macros offer some benefits over the assertions from the standard library:
  * The macros parse your expression to detect comparisons and adjust the error message accordingly.
    No more `assert_eq!(a, b)` or `assert_ne!(c, d)`, just write `assert!(1 + 1 == 2)`, or even `assert!(1 + 1 > 1)`!
  * You can test for pattern matches: `assert!(let Err(_) = File::open("/non/existing/file"))`.
  * You can capture parts of the pattern for further testing by using the `let_assert!(...)` macro.
  * The `check` macro can be used to perform multiple checks before panicking.
  * The macros provide more information than the standard `std::assert!()` when the assertion fails.
  * Colored failure messages with diffs!

The macros also accept additional arguments for a custom message, so it is fully compatible with `std::assert`.
This means that you can import the macro as a drop in replacement:
```rust
use assert2::assert;
```

## Examples

```rust
check!(6 + 1 <= 2 * 3);
```

![Output](https://raw.githubusercontent.com/de-vri-es/assert2-rs/ba98984a32d6381e6710e34eb1fb83e65e851236/binary-operator.png)

----------

```rust
check!(scrappy == coco);
```

![Output](https://raw.githubusercontent.com/de-vri-es/assert2-rs/54ee3141e9b23a0d9038697d34f29f25ef7fe810/multiline-diff.png)

----------

```rust
check!((3, Some(4)) == [1, 2, 3].iter().size_hint());
```

![Output](https://raw.githubusercontent.com/de-vri-es/assert2-rs/54ee3141e9b23a0d9038697d34f29f25ef7fe810/single-line-diff.png)

----------

```rust
check!(let Ok(_) = File::open("/non/existing/file"));
```

![Output](https://raw.githubusercontent.com/de-vri-es/assert2-rs/54ee3141e9b23a0d9038697d34f29f25ef7fe810/pattern-match.png)

----------

```rust
let_assert!(Err(e) = File::open("/non/existing/file"));
check!(e.kind() == ErrorKind::PermissionDenied);
```

![Output](https://github.com/de-vri-es/assert2-rs/blob/54ee3141e9b23a0d9038697d34f29f25ef7fe810/let-assert.png?raw=true)

## `assert` vs `check`
The crate provides two macros: `check!(...)` and `assert!(...)`.
The main difference is that `check` is really intended for test cases and doesn't immediately panic.
Instead, it will print the assertion error and fail the test.
This allows you to run multiple checks and can help to determine the reason of a test failure more easily.
The `assert` macro on the other hand simply prints the error and panics,
and can be used outside of tests just as well.

Currently, `check` uses a scope guard to delay the panic until the current scope ends.
Ideally, `check` doesn't panic at all, but only signals that a test case has failed.
If this becomes possible in the future, the `check` macro will change, so **you should not rely on `check` to panic**.

## Difference between stable and nightly.
If available, the crate uses the `proc_macro_span` feature to get the original source code.
On stable and beta, it falls back to stringifying the expression.
This makes the output a bit more readable on nightly.

## The `let_assert!()` macro
You can also use the [`let_assert!(...)`](macro.let_assert.html).
It is very similar to `assert!(let ...)`,
but all placeholders will be made available as variables in the calling scope.

This allows you to run additional checks on the captured variables.

For example:

```rust
let_assert!(Ok(foo) = Foo::try_new("bar"));
check!(foo.name() == "bar");

let_assert!(Err(Error::InvalidName(e)) = Foo::try_new("bogus name"));
check!(e.name() == "bogus name");
check!(e.to_string() == "invalid name: bogus name");
```

## Controlling the output format.

As an end-user, you can influence the way that `assert2` formats failed assertions by changing the `ASSERT2` environment variable.
You can specify any combination of options, separated by a comma.
The supported options are:
* `auto`: Automatically select the compact or pretty `Debug` format for an assertion based on the length (default).
* `pretty`: Always use the pretty `Debug` format for assertion messages (`{:#?}`).
* `compact`: Always use the compact `Debug` format for assertion messages (`{:?}`).
* `no-color`: Disable colored output, even when the output is going to a terminal.
* `color`: Enable colored output, even when the output is not going to a terminal.

For example, you can run the following command to force the use of the compact `Debug` format with colored output:
```shell
ASSERT2=compact,color cargo test
```

If neither the `color` or the `no-color` options are set,
then `assert2` follows the [clicolors specification](https://bixense.com/clicolors/):

 * `NO_COLOR != 0` or `CLICOLOR == 0`: Write plain output without color codes.
 * `CLICOLOR != 0`: Write colored output when the output is going to a terminal.
 * `CLICOLOR_FORCE != 0`:  Write colored output even when it is not going to a terminal.