Request: IsZero trait and #[derive(IsZero)]

[dhm]

Having a IsDefault trait has already been discussed, and as a TL,DR: Default is a not a trait for null-ness or neutrality, but for having the ability to create some arbitrary inhabitant of the given type.

Source

@https://doc.rust-lang.org/1.44.0/std/default/trait.Default.html

Trait std::default::Default

pub trait Default {
    fn default() -> Self;
}

A trait for giving a type a useful default value.

Sometimes, you want to fall back to some kind of default value, and don't particularly care what it is.

• emphasis mine

Although concrete types may express additional invariants w.r.t. the actual instance created by Default::default(), there is no part of the contract specifying it. The values could change in between invocations, and the value may very well be "non-empty".

Having a IsDefault trait would thus contradict this purposely simple trait definition.

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That being said, nothing forbids you from having your own IsDefault trait within your crate, or to publish a dedicated crate for this very functionality.