`#[may_dangle]`, a refined dropck escape hatch (tracking issue for RFC 1327)

[pnkfelix]

Tracking issue for rust-lang/rfcs#1327

• Allow attributes on lifetime/type formal parameters (generic_param_attrs feature): Attributes on generic formals #34764
• Add #[may_dangle] attribute for lifetime/type formal parameters on unsafe impl<...> Drop (dropck_eyepatch feature)
• Replace uses of #[unsafe_destructor_blind_to_params] attribute in libstd with #[may_dangle] Replace uses of #[unsafe_destructor_blind_to_params] with #[may_dangle] #38664
• Update nomicon to use #[may_dangle] instead of #[unsafe_destructor_blind_to_params] Update nomicon to describe #[may_dangle] #39196
• Remove support for #[unsafe_destructor_blind_to_params] attribute (with warning cycle before removal).
  • Deprecated in Deprecate #[unsafe_destructor_blind_to_params] #38970
  • need to replace remaining references (e.g. in test suite) to #[unsafe_destructor_blind_to_params] with [may_dangle] before we can actually remove the former.

For docs explaining the feature, also see the forge.

[pnkfelix]

see also #28498

[pnkfelix]

Some people (myself included) have noted that may_dangle is not an ideal name. (Some have gone so far as to call it "terrible".)

Perhaps we could return to calling it an eyepatch: