Module blog::daily_rfcs::trait_sys::_0387_higher_ranked_trait_bounds

Add the ability to have trait bounds that are polymorphic over lifetimes.

Note

syntax

for<lifetimes> Trait<T1, ..., Tn>
for<lifetimes> Trait(T1, ..., tn) -> Tr

• Legal: in where clauses and types.
• Illegal: impl and qualified path <T as Trait>

Implicit binders are introduced for omitted lifetimes when using parentheses notation(Trait(T1, ..., Tn)) and in fn types(for<'a> fn(&'a T)).

Early_Bound vs Late_Bound lifetime in impls

• Early:
  • the self type of the impl;
  • a where clause associated with the impl.
• Late:
  • all other lifetimes.

// Here 'late does not appear in any where clause nor in the self type,
// and hence it is late-bound. Thus this impl is considered to provide:
//
//     SomeType : for<'late> FnMut<(&'late Foo,),()>
impl<'late> FnMut(&'late Foo) -> Bar for SomeType { ... }

// Here 'early appears in the self type and hence it is early bound.
// This impl thus provides:
//
//     SomeOtherType<'early> : FnMut<(&'early Foo,),()>
impl<'early> FnMut(&'early Foo) -> Bar for SomeOtherType<'early> { ... }

Instantiating late-bound lifetimes in a trait reference

• Accessing an associated constant, once those are implemented.
• Accessing an associated type.
• Accessing an associated item from a trait reference.

RFC: 0387-higher-ranked-trait-bounds