Non-parametric dropck; instead trust an unsafe attribute (RFC 1238).
Non-parametric dropck; instead trust an unsafe attribute (RFC 1238).

Implement cannot-assume-parametricity (CAP) from RFC 1238, and add the
UGEH attribute.

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Note that we check for the attribute attached to the dtor method, not
the Drop impl.

(This is just to match the specification of RFC and the tests; I am
not wedded to this approach.)

rust-lang/reference,rust-lang/reference