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Mark defaulted `PartialEq`/`PartialOrd` methods as const by ecstatic-morse · Pul...

 3 years ago
source link: https://github.com/rust-lang/rust/pull/91439
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fee1-dead commented on Dec 2, 2021

cc @oli-obk per https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/default_method_body_is_const

Overview of #[default_method_body_is_const]

This attribute is currently a placeholder. When the const_trait_impl feature gets stabilized it should be changed to something else (preferably new syntax). When marked on a trait method with a default body, we perform checks on it as if it were a const function. This allows users to have traits that are easier to write const impls for.

trait PartialEq {
    fn eq(&self, other: &Self) -> bool;

    #[default_method_body_is_const]
    fn ne(&self, other: &Self) -> bool {
        !self.eq(other) // call to self.eq is allowed in const context because `eq` has a const implementation
    }
}

Note that we cannot use const fn as an indicator for default_method_body_is_const, because it can contradict with future extensions such as const trait fns which always require implementations to be const.


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