Reject specialized Drop impls. #23638

one of the helper method impls.

This was necessary to avoid specialized `Drop` impls for the two structs.

…Drop` impl.

See Issue 8142 for discussion. This makes it illegal for a Drop impl to be more specialized than the original item. So for example, all of the following are now rejected (when they would have been blindly accepted before): ```rust struct S<A> { ... }; impl Drop for S<i8> { ... } // error: specialized to concrete type struct T<'a> { ... }; impl Drop for T<'static> { ... } // error: specialized to concrete region struct U<A> { ... }; impl<A:Clone> Drop for U<A> { ... } // error: added extra type requirement struct V<'a,'b>; impl<'a,'b:a> Drop for V<'a,'b> { ... } // error: added extra region requirement ``` Due to examples like the above, this is a [breaking-change]. (The fix is to either remove the specialization from the `Drop` impl, or to transcribe the requirements into the struct/enum definition; examples of both are shown in the PR's fixed to `libstd`.) ---- This is likely to be the last thing blocking the removal of the `#[unsafe_destructor]` attribute. Includes two new error codes for the new dropck check. Update run-pass tests to accommodate new dropck pass. Update tests and docs to reflect new destructor restriction. ---- Implementation notes: We identify Drop impl specialization by not being as parametric as the struct/enum definition via unification. More specifically: 1. Attempt unification of a skolemized instance of the struct/enum with an instance of the Drop impl's type expression where all of the impl's generics (i.e. the free variables of the type expression) have been replaced with unification variables. 2. If unification fails, then reject Drop impl as specialized. 3. If unification succeeds, check if any of the skolemized variables "leaked" into the constraint set for the inference context; if so, then reject Drop impl as specialized. 4. Otherwise, unification succeeded without leaking skolemized variables: accept the Drop impl. We identify whether a Drop impl is injecting new predicates by simply looking whether the predicate, after an appropriate substitution, appears on the struct/enum definition.

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Reject specialized Drop impls. #23638

Reject specialized Drop impls. #23638

Commits on Mar 24, 2015