Nonparametric dropck (tracking issue for RFC 1238)

[nikomatsakis]

Tracking issue for rust-lang/rfcs#1238.

• subtask: #[may_dangle] attribute #[may_dangle], a refined dropck escape hatch (tracking issue for RFC 1327) #34761
• can't close this until we have stable support for 3rd party collection types

cc @pnkfelix

[nikomatsakis]

triage: P-high

[nikomatsakis]

triage: P-medium

[alexcrichton]

I think this was since implemented, so closing (but please reopen if there's something left!)

[SimonSapin]

I think this should be reopened with the B-unstable label. The RFC is implemented, but the #[unsafe_destructor_blind_to_params] attribute is still behind the dropck_parametricity feature gate which points to this as its tracking issue.

#[unsafe_destructor_blind_to_params] is used for Vec and a number of other types defined in the standard library. It isn’t any less useful for types outside the standard library. I’ve needed it for an arena allocator.

[nikomatsakis]

I agree.