Matchers are an ergonomic and performant way to make type-specific assertions, outperformingassert!by 100x.

expect(true).to_be_true();

All matchers can be negated by callingnot().

expect("foobar").not().to_contain("bazz");

Built-in Matchers

expect("foobar").to_start_with("foo");
expect(my_result).to_be_ok();
expect(2).to_be_greater_than(1);

Extending Matchers

Matchers are easy to extend, particulary using theextendcrate.

use anyhow::Result;
use extend::ext;
use sweet::prelude::*;

#[derive(Debug)]
struct Awesomeness(u32);

#[ext]
pub impl Matcher<Awesomeness>{
	fn to_be_more_awesome_than(&self, other:Awesomeness) {
		let outcome = self.0 > other.0;
		let expected = format!("to be more awesome than {:?}", other);
		self.assert_correct(outcome, &expected);
	}
}

Note that here we are callingself.assert_correct()which does two things:

• Handles negation:not()
• Unwinds the backtrace output to one level up by emitting a specially formatted panic.

Its important that assert_correct is called in the matcher and not an extra layer deeper. This would result in a backtrace showingself.sub_func()instead of theexpect()location.

fn to_show_a_correct_backtrace(&self){
-	self.sub_func();
}fn sub_func(&self){
-	self.assert_correct(false, "");
}