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# Casting
Rust 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.
```rust,editable,ignore,mdbook-runnable
// 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;
// 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 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);
// 128 as i8 -> -128, whose two's complement in eight bits is:
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 two's complement of 232 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 is 255
println!("300.0 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 is 44
println!("300.0 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>());
}
}
```
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