[]Keyword as

The keyword for casting a value to a type.

as is most commonly used to turn primitive types into other primitive types, but it has other uses that include turning pointers into addresses, addresses into pointers, and pointers into other pointers.

let thing1: u8 = 89.0 as u8;
assert_eq!('B' as u32, 66);
assert_eq!(thing1 as char, 'Y');
let thing2: f32 = thing1 as f32 + 10.5;
assert_eq!(true as u8 + thing2 as u8, 100);Run

In general, any cast that can be performed via ascribing the type can also be done using as, so instead of writing let x: u32 = 123, you can write let x = 123 as u32 (Note: let x: u32 = 123 would be best in that situation). The same is not true in the other direction, however, explicitly using as allows a few more coercions that aren't allowed implicitly, such as changing the type of a raw pointer or turning closures into raw pointers.

Other places as is used include as extra syntax for crate and use, to change the name something is imported as.

For more information on what as is capable of, see the Reference