Keyword as

Cast between types, or rename an import.

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);

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.

as can be seen as the primitive for From and Into: as only works with primitives (u8, bool, str, pointers, …) whereas From and Into also works with types like String or Vec.

as can also be used with the _ placeholder when the destination type can be inferred. Note that this can cause inference breakage and usually such code should use an explicit type for both clarity and stability. This is most useful when converting pointers using as *const _ or as *mut _ though the cast method is recommended over as *const _ and it is the same for as *mut _: those methods make the intent clearer.

as is also used to rename imports in use and extern crate statements:

use std::{mem as memory, net as network};
// Now you can use the names `memory` and `network` to refer to `std::mem` and `std::net`.

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