as - Rust
Description: Cast between types, rename an import, or qualify paths to associated items.

## as

# Keyword as

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Cast between types, rename an import, or qualify paths to associated items.

## §Type casting

`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.

## §Renaming imports

`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`.

## §Qualifying paths

You’ll also find with `From` and `Into`, and indeed all traits, that `as` is used for the fully qualified path, a means of disambiguating associated items, i.e. functions, constants, and types. For example, if you have a type which implements two traits with identical method names (e.g. `Into::<u32>::into` and `Into::<u64>::into`), you can clarify which method you’ll use with `<MyThing as Into<u32>>::into(my_thing)`1. This is quite verbose, but fortunately, Rust’s type inference usually saves you from needing this, although it is occasionally necessary, especially with methods that return a generic type like `Into::into` or methods that don’t take `self`. It’s more common to use in macros where it can provide necessary hygiene.

## §Further reading

For more information on what `as` is capable of, see the Reference on type cast expressions, renaming imported entities, renaming `extern` crates and qualified paths.

1. You should probably never use this syntax with `Into` and instead write `T::from(my_thing)`. It just happens that there aren’t any great examples for this syntax in the standard library. Also, at time of writing, the compiler tends to suggest fully-qualified paths to fix ambiguous `Into::into` calls, so the example should hopefully be familiar. ↩