Casting Between Types

Rust, with its focus on safety, provides two different ways of casting different types between each other. The first, as, is for safe casts. In contrast, transmute allows for arbitrary casting, and is one of the most dangerous features of Rust!

Coercion

Coercion between types is implicit and has no syntax of its own, but can be spelled out with as.

Coercion occurs in let, const, and static statements; in function call arguments; in field values in struct initialization; and in a function result.

The most common case of coercion is removing mutability from a reference:

• &mut T to &T

An analogous conversion is to remove mutability from a raw pointer:

• *mut T to *const T

References can also be coerced to raw pointers:

• &T to *const T

• &mut T to *mut T

Custom coercions may be defined using Deref.

Coercion is transitive.

as

The as keyword does safe casting:

let x: i32 = 5;

let y = x as i64;

There are three major categories of safe cast: explicit coercions, casts between numeric types, and pointer casts.

Casting is not transitive: even if e as U1 as U2 is a valid expression, e as U2 is not necessarily so (in fact it will only be valid if U1 coerces to U2).

Explicit coercions

A cast e as U is valid if e has type T and T coerces to U.

Numeric casts

A cast e as U is also valid in any of the following cases:

• e has type T and T and U are any numeric types; numeric-cast
• e is a C-like enum (with no data attached to the variants), and U is an integer type; enum-cast
• e has type bool or char and U is an integer type; prim-int-cast
• e has type u8 and U is char; u8-char-cast

For example

let one = true as u8;
let at_sign = 64 as char;
let two_hundred = -56i8 as u8;

The semantics of numeric casts are:

• Casting between two integers of the same size (e.g. i32 -> u32) is a no-op
• Casting from a larger integer to a smaller integer (e.g. u32 -> u8) will truncate
• Casting from a smaller integer to a larger integer (e.g. u8 -> u32) will
  • zero-extend if the source is unsigned
  • sign-extend if the source is signed
• Casting from a float to an integer will round the float towards zero
  • NOTE: currently this will cause Undefined Behavior if the rounded value cannot be represented by the target integer type. This includes Inf and NaN. This is a bug and will be fixed.
• Casting from an integer to float will produce the floating point representation of the integer, rounded if necessary (rounding strategy unspecified)
• Casting from an f32 to an f64 is perfect and lossless
• Casting from an f64 to an f32 will produce the closest possible value (rounding strategy unspecified)
  • NOTE: currently this will cause Undefined Behavior if the value is finite but larger or smaller than the largest or smallest finite value representable by f32. This is a bug and will be fixed.

Pointer casts

Perhaps surprisingly, it is safe to cast raw pointers to and from integers, and to cast between pointers to different types subject to some constraints. It is only unsafe to dereference the pointer:

let a = 300 as *const char; // `a` is a pointer to location 300.
let b = a as u32;

e as U is a valid pointer cast in any of the following cases:

• e has type *T, U has type *U_0, and either U_0: Sized or unsize_kind(T) == unsize_kind(U_0); a ptr-ptr-cast

• e has type *T and U is a numeric type, while T: Sized; ptr-addr-cast

• e is an integer and U is *U_0, while U_0: Sized; addr-ptr-cast

• e has type &[T; n] and U is *const T; array-ptr-cast

• e is a function pointer type and U has type *T, while T: Sized; fptr-ptr-cast

• e is a function pointer type and U is an integer; fptr-addr-cast

transmute

as only allows safe casting, and will for example reject an attempt to cast four bytes into a u32:

let a = [0u8, 0u8, 0u8, 0u8];

let b = a as u32; // Four u8s makes a u32.

This errors with:

error: non-scalar cast: `[u8; 4]` as `u32`
let b = a as u32; // Four u8s makes a u32.
        ^~~~~~~~

This is a ‘non-scalar cast’ because we have multiple values here: the four elements of the array. These kinds of casts are very dangerous, because they make assumptions about the way that multiple underlying structures are implemented. For this, we need something more dangerous.

The transmute function is provided by a compiler intrinsic, and what it does is very simple, but very scary. It tells Rust to treat a value of one type as though it were another type. It does this regardless of the typechecking system, and completely trusts you.

In our previous example, we know that an array of four u8s represents a u32 properly, and so we want to do the cast. Using transmute instead of as, Rust lets us:

use std::mem;

fn main() {
    unsafe {
        let a = [0u8, 1u8, 0u8, 0u8];
        let b = mem::transmute::<[u8; 4], u32>(a);
        println!("{}", b); // 256
        // Or, more concisely:
        let c: u32 = mem::transmute(a);
        println!("{}", c); // 256
    }
}

We have to wrap the operation in an unsafe block for this to compile successfully. Technically, only the mem::transmute call itself needs to be in the block, but it's nice in this case to enclose everything related, so you know where to look. In this case, the details about a are also important, and so they're in the block. You'll see code in either style, sometimes the context is too far away, and wrapping all of the code in unsafe isn't a great idea.

While transmute does very little checking, it will at least make sure that the types are the same size. This errors:

use std::mem;

unsafe {
    let a = [0u8, 0u8, 0u8, 0u8];

    let b = mem::transmute::<[u8; 4], u64>(a);
}

with:

error: transmute called with differently sized types: [u8; 4] (32 bits) to u64
(64 bits)

Other than that, you're on your own!