Function predicate 

pub fn predicate<T, F>(name: &'static str, pred: F) -> impl Matcher<T>
where
    T: Debug,
    F: Fn(&T) -> bool,

Matches a value for which pred returns true.

The escape hatch for when no standard matcher fits. name is what the failure reports as the expectation, so give it a readable one: a closure has no name of its own, and a failure that says expected: <closure> helps no one.

The matcher pairs naturally with Subject::satisfies, reading as “x satisfies the predicate <name>”.

use test_better_core::{OrFail, TestResult};
use test_better_matchers::{Matcher, check, eq, predicate};

fn main() -> TestResult {
    check!(4).satisfies(predicate("an even number", |n: &i32| n % 2 == 0))?;

    // The `name` is what a failure reports, not `<closure>`.
    let failure = predicate("an even number", |n: &i32| n % 2 == 0)
        .check(&3)
        .failure
        .or_fail_with("3 is not even")?;
    check!(failure.expected.to_string()).satisfies(eq("an even number".to_string()))?;
    Ok(())
}