Crate uncon [] [src]

Traits for unchecked conversions between types.

The purpose of this crate is to provide FromUnchecked and IntoUnchecked, which can be used across different crates to allow for fast conversions between types when speed is necessary. These traits allow for code to be much more expressive than when using mem::transmute. They are the unchecked/unsafe equivalents of From and Into, respectively.

Some types already implement FromUnchecked out-of-the-box.


This crate is available on and can be used by adding the following to your project's Cargo.toml:

uncon = "1.1.0"

and this to your crate root:

extern crate uncon;


A type may wrap around another but must maintain certain invariants that aren't met by the inner type. An example of this is str in relation to [u8] where a string is just a UTF-8 encoded byte slice.

In this example, U4 is a simple wrapper around u8 where valid instances must only ever have 4 bits set.

struct U4(u8);

impl From<u8> for U4 {
    fn from(byte: u8) -> U4 {
        U4(byte & 0b1111)

impl FromUnchecked<u8> for U4 {
    unsafe fn from_unchecked(byte: u8) -> U4 {

If a type T implements FromUnchecked for some type U, then U automatically implements IntoUnchecked for T.

let b = [b'h', b'i'];
let s: &str = unsafe { b.as_ref().into_unchecked() };

assert_eq!(s, "hi");

Deriving Traits

See the docs of uncon_derive for info on deriving this crate's traits.

struct Flags(u8);

let f = unsafe { Flags::from_unchecked(0b1100) };


  • Vec<U> to Vec<T>, Box<U> to Box<T>, &U to &T, and other conversions are similar to mem::transmute except without the undefined behavior. There are absolutely no safety measures.
    • These conversions are extremely unsafe and should only be done in cases such as turning Vec<i8> into Vec<u8> or something similarly trivial.
    • If T implements Drop in the case of Vec<T>, consider mapping from_unchecked and collecting the results.



Unchecked and potentially unsafe conversions from T into Self.


Unchecked and potentially unsafe conversions from Self into T.