unchecked-std

Common Rust standard library methods with capacity and bounds checks removed to improve performance.

For safety, assertions are enabled in debug builds.

Most APIs are unchecked counterparts of standard methods, with an exception of push_many_unchecked, which doesn't have a corresponding safe std method.

Most implementations are manual, except for extend_from_slice_unchecked which calls extend_from_slice and uses unreachable_unchecked with a codegen test to confirm that the capacity check gets elided.

The crate is no_std, but requires alloc.

Feature flags

The heapless feature adds unchecked methods for heapless data structures.

Example

format! way:

fn hello_format(name: &str) -> String {
    format!("Hello, {name}!")
}

macro-free std way:

fn hello_checked(name: &str) -> String {
    let mut s = String::with_capacity("Hello, !".len() + name.len());
    s.push_str("Hello, ");
    s.push_str(name);
    s.push('!');
    s
}

unchecked-std way:

use unchecked_std::prelude::*;

fn hello_unchecked(name: &str) -> String {
    let mut s = String::with_capacity("Hello, !".len() + name.len());
    // SAFETY: `s` has been initialized with sufficient capacity
    unsafe {
        s.push_str_unchecked("Hello, ");
        s.push_str_unchecked(name);
        s.push_unchecked('!');
    }
    s
}

Example benchmark result:

test bench_hello_format    ... bench:          30.56 ns/iter (+/- 0.30)
test bench_hello_checked   ... bench:          14.57 ns/iter (+/- 0.44)
test bench_hello_unchecked ... bench:           9.98 ns/iter (+/- 0.22)