BigUint::from(24u32) 有什么作用？

Question

BigUint::from(24u32)

I understand that it is related to a big unsigned integer since it is part of a cryptography module to factor big numbers. I don't understand what the part from(24u32) does. For example, if it is used in this context

let b: BigUint = (BigUint::from(24u32) * &n).sqrt() + &one;

Answer 1

Conversions between types in Rust are defined by the From trait which defines a function from , so that Foo::from (bar) converts the value bar to the type Foo . In your case, you are therefore converting 24u32 to type BigUint , but what's this 24u32 ?

Integer literals in Rust can use a suffix to specify their actual type. If you wrote just 24 , it could be for example an i32 , a u8 or a u32 . Most of the time the compiler is able to infer the actual type from the way the value is used, and when it can't it defaults to i32 . But in your case that won't work: there is no BigUint::from<i32> but there are conversion functions for all the regular unsigned types: u8 , u16 , u32 , u64 and u128 , so the compiler doesn't know which one to use. Adding the u32 suffix clarifies the type, so 24u32 is the value 24 with the type u32 , allowing the compiler to understand that you want BigUint::from<u32> (24) .

Answer 2

The BigUint struct implements the From<u32> trait , which means that it will implement a from(u32) function.

Implementations of the From<_> trait are used to perform a value conversion that will consume the original input. In your example the BigUint struct is constructed from the 24u32 number.