| Rust provides several mechanisms to change or define the type of primitive and user defined types. The following sections cover: - Casting between primitive types
- Specifying the desired type of literals
- Using type inference
- Aliasing types
CastingRust provides no implicit type conversion (coercion) between primitive types. But, explicit type conversion (casting) can be performed using the as keyword. Rules for converting between integral types follow C conventions generally, except in cases where C has undefined behavior. The behavior of all casts between integral types is well defined in Rust. // Suppress all warnings from casts which overflow.
#![allow(overflowing_literals)]
fn main() {
let decimal = 65.4321_f32;
// Error! No implicit conversion
let integer: u8 = decimal as u8;
// FIXME ^ Comment out this line
// Explicit conversion
let integer = decimal as u8;
let character = integer as char;
// Error! There are limitations in conversion rules.
// A float cannot be directly converted to a char.
let character = (decimal as u8) as char;
// FIXME ^ Comment out this line
println!("Casting: {} -> {} -> {}", decimal, integer, character);
// when casting any value to an unsigned type, T,
// T::MAX + 1 is added or subtracted until the value
// fits into the new type
// 1000 already fits in a u16
println!("1000 as a u16 is: {}", 1000 as u16);
// 1000 - 256 - 256 - 256 = 232
// Under the hood, the first 8 least significant bits (LSB) are kept,
// while the rest towards the most significant bit (MSB) get truncated.
println!("1000 as a u8 is : {}", 1000 as u8);
// -1 + 256 = 255
println!(" -1 as a u8 is : {}", (-1i8) as u8);
// For positive numbers, this is the same as the modulus
println!("1000 mod 256 is : {}", 1000 % 256);
// When casting to a signed type, the (bitwise) result is the same as
// first casting to the corresponding unsigned type. If the most significant
// bit of that value is 1, then the value is negative.
// Unless it already fits, of course.
println!(" 128 as a i16 is: {}", 128 as i16);
// In boundary case 128 value in 8-bit two's complement representation is -128
println!(" 128 as a i8 is : {}", 128 as i8);
// repeating the example above
// 1000 as u8 -> 232
println!("1000 as a u8 is : {}", 1000 as u8);
// and the value of 232 in 8-bit two's complement representation is -24
println!(" 232 as a i8 is : {}", 232 as i8);
// Since Rust 1.45, the `as` keyword performs a *saturating cast*
// when casting from float to int. If the floating point value exceeds
// the upper bound or is less than the lower bound, the returned value
// will be equal to the bound crossed.
// 300.0 as u8 is 255
println!(" 300.0 as u8 is : {}", 300.0_f32 as u8);
// -100.0 as u8 is 0
println!("-100.0 as u8 is : {}", -100.0_f32 as u8);
// nan as u8 is 0
println!(" nan as u8 is : {}", f32::NAN as u8);
// This behavior incurs a small runtime cost and can be avoided
// with unsafe methods, however the results might overflow and
// return **unsound values**. Use these methods wisely:
unsafe {
// 300.0 as u8 is 44
println!(" 300.0 as u8 is : {}", 300.0_f32.to_int_unchecked::<u8>());
// -100.0 as u8 is 156
println!("-100.0 as u8 is : {}", (-100.0_f32).to_int_unchecked::<u8>());
// nan as u8 is 0
println!(" nan as u8 is : {}", f32::NAN.to_int_unchecked::<u8>());
}
}
LiteralsNumeric literals can be type annotated by adding the type as a suffix. As an example, to specify that the literal 42 should have the type i32, write 42i32. The type of unsuffixed numeric literals will depend on how they are used. If no constraint exists, the compiler will use i32 for integers, and f64 for floating-point numbers. fn main() {
// Suffixed literals, their types are known at initialization
let x = 1u8;
let y = 2u32;
let z = 3f32;
// Unsuffixed literals, their types depend on how they are used
let i = 1;
let f = 1.0;
// `size_of_val` returns the size of a variable in bytes
println!("size of `x` in bytes: {}", std::mem::size_of_val(&x));
println!("size of `y` in bytes: {}", std::mem::size_of_val(&y));
println!("size of `z` in bytes: {}", std::mem::size_of_val(&z));
println!("size of `i` in bytes: {}", std::mem::size_of_val(&i));
println!("size of `f` in bytes: {}", std::mem::size_of_val(&f));
}
There are some concepts used in the previous code that haven't been explained yet, here's a brief explanation for the impatient readers: std::mem::size_of_val is a function, but called with its full path. Code can be split in logical units called modules. In this case, the size_of_val function is defined in the mem module, and the mem module is defined in the std crate. For more details, see modules and crates.
InferenceThe type inference engine is pretty smart. It does more than looking at the type of the value expression during an initialization. It also looks at how the variable is used afterwards to infer its type. Here's an advanced example of type inference: fn main() {
// Because of the annotation, the compiler knows that `elem` has type u8.
let elem = 5u8;
// Create an empty vector (a growable array).
let mut vec = Vec::new();
// At this point the compiler doesn't know the exact type of `vec`, it
// just knows that it's a vector of something (`Vec<_>`).
// Insert `elem` in the vector.
vec.push(elem);
// Aha! Now the compiler knows that `vec` is a vector of `u8`s (`Vec<u8>`)
// TODO ^ Try commenting out the `vec.push(elem)` line
println!("{:?}", vec);
}
No type annotation of variables was needed, the compiler is happy and so is the programmer! AliasingThe type statement can be used to give a new name to an existing type. Types must have UpperCamelCase names, or the compiler will raise a warning. The exception to this rule are the primitive types: usize, f32, etc. // `NanoSecond`, `Inch`, and `U64` are new names for `u64`.
type NanoSecond = u64;
type Inch = u64;
type U64 = u64;
fn main() {
// `NanoSecond` = `Inch` = `U64` = `u64`.
let nanoseconds: NanoSecond = 5 as u64;
let inches: Inch = 2 as U64;
// Note that type aliases *don't* provide any extra type safety, because
// aliases are *not* new types
println!("{} nanoseconds + {} inches = {} unit?",
nanoseconds,
inches,
nanoseconds + inches);
}
The main use of aliases is to reduce boilerplate; for example the io::Result<T> type is an alias for the Result<T, io::Error> type. | 
