Help for my parallel sum

In Rust, std primarily contains functionality that requires special support in the compiler. Functionality that does not require special support typically is developed in other crates.

Because of Rust's guarantees of backward compatibility it is very difficult to improve functionality in std, whereas functionality in other crates can be improved through semver without breaking Rust's guarantees. That is one reason why Rust is not a "batteries included" language, because it's almost impossible to replace defective batteries if they are in std.