fn get<Q>(&self, k: &Q) -> Option<&V>
where
    K: Borrow<Q>,
    Q: Hash + Eq + ?Sized,

trait Borrow<Borrowed>
where
    Borrowed: ?Sized,
{
    fn borrow(&self) -> &Borrowed;
}

trait Equivalent<K>
where
    K: ?Sized,
{
    fn equivalent(&self, key: &K) -> bool;
}

Overall, I reduced the typical runtime on very large crates down to ~2s from ~8s - nearly an 80% speedup - without compromising performance on smaller crates. Along the way I reduced test time from ~7min to ~1min.

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#[allow(dead_code)]
trait Foo {
    const FOO: u32;
}

impl Foo for u32 {
    const FOO: u32 = roundtrip(0);
}

// нет предупреждения о мёртвом коде
const fn roundtrip(x: u32) -> u32 {
    x
}

For example, part of one 2016 Advent of Code puzzle is looking for ABBA patterns: "two different characters followed by the reverse of that pair, such as xyyx or abba." If we assume only ASCII characters, that could be written by sweeping windows of the byte slice like this:

fn has_abba(s: &str) -> bool {
    s.as_bytes()
        .array_windows()
        .any(|[a1, b1, b2, a2]|
            (a1 != b1)
            && (a1 == a2)
            && (b1 == b2)
        )
}

let mut iter = "125 GOTO 10".chars().peekable();
let mut line_num = 0_u32;
while let Some(digit) = iter.next_if_map(|c| c.to_digit(10).ok_or(c)) {
    line_num = line_num * 10 + digit;
}
assert_eq!(line_num, 125);
assert_eq!(iter.collect::<String>(), " GOTO 10");

// \p{utf8} expands to:
// ([\x00-\x7F]
// | [\xC0-\xDF][\x80-\xBF]
// | [\xE0-\xEF][\x80-\xBF]{2}
// | [\xF0-\xF7][\x80-\xBF]{3})*

use resharp::Regex;

// basic matching
let re = Regex::new(r"hello.*world").unwrap();
assert!(re.is_match("hello beautiful world"));

// intersection: contains both "cat" and "dog", 5-15 chars
let re = Regex::new(r"_*cat_*&_*dog_*&_{5,15}").unwrap();

// complement: does not contain "1"
let re = Regex::new(r"~(_*1_*)").unwrap();