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Allow using generic trait methods in `const fn` by jonas-schievink · Pull Reques...

 3 years ago
source link: https://github.com/rust-lang/rust/pull/79287
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Next step for #67792, this now also allows code like the following:

struct S;

impl const PartialEq for S {
    fn eq(&self, _: &S) -> bool {
        true
    }
}

const fn equals_self<T: PartialEq>(t: &T) -> bool {
    *t == *t
}

pub const EQ: bool = equals_self(&S);

This works by threading const-ness of trait predicates through trait selection, in particular through ParamCandidate, and exposing it in the resulting ImplSource.

Since this change makes two bounds T: Trait and T: ?const Trait that only differ in their const-ness be treated like different bounds, candidate winnowing has been changed to drop the ?const candidate in favor of the const candidate, to avoid ambiguities when both a const and a non-const bound is present.


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