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§Minimal Specialization

This module contains the checks for sound specialization used when the min_specialization feature is enabled. This requires that the impl is always applicable.

If impl1 specializes impl2 then impl1 is always applicable if we know that all the bounds of impl2 are satisfied, and all of the bounds of impl1 are satisfied for some choice of lifetimes then we know that impl1 applies for any choice of lifetimes.

§Basic approach

To enforce this requirement on specializations we take the following approach:

  1. Match up the args for impl2 so that the implemented trait and self-type match those for impl1.
  2. Check for any direct use of 'static in the args of impl2.
  3. Check that all of the generic parameters of impl1 occur at most once in the unconstrained args for impl2. A parameter is constrained if its value is completely determined by an associated type projection predicate.
  4. Check that all predicates on impl1 either exist on impl2 (after matching args), or are well-formed predicates for the trait’s type arguments.


Suppose we have the following always applicable impl:

impl<T> SpecExtend<T> for std::vec::IntoIter<T> { /* specialized impl */ }
impl<T, I: Iterator<Item=T>> SpecExtend<T> for I { /* default impl */ }

We get that the generic pamameters for impl2 are [T, std::vec::IntoIter<T>]. T is constrained to be <I as Iterator>::Item, so we check only std::vec::IntoIter<T> for repeated parameters, which it doesn’t have. The predicates of impl1 are only T: Sized, which is also a predicate of impl2. So this specialization is sound.


Unfortunately not all specializations in the standard library are allowed by this. So there are two extensions to these rules that allow specializing on some traits: that is, using them as bounds on the specializing impl, even when they don’t occur in the base impl.


If a trait is always applicable, then it’s sound to specialize on it. We check trait is always applicable in the same way as impls, except that step 4 is now “all predicates on impl1 are always applicable”. We require that specialization or min_specialization is enabled to implement these traits.


There are also some specialization on traits with no methods, including the stable FusedIterator trait. We allow marking marker traits with an unstable attribute that means we ignore them in point 3 of the checks above. This is unsound, in the sense that the specialized impl may be used when it doesn’t apply, but we allow it in the short term since it can’t cause use after frees with purely safe code in the same way as specializing on traits with methods can.