Can bypass `repr(packed)` checking by using --cap-lints

[taiki-e]

NOTE: pin-project (not lite) is not affected by this issue. see taiki-e/pin-project#34 (comment) for more.
EDIT: see taiki-e/pin-project#342 for pin-project (not lite)

Currently, pin-project-lite uses safe_packed_borrow lint to check that the struct is not repr(packed). Unfortunately, safe_packed_borrow is lint and you can use the rustc's --cap-lints flag to lower the lint cap level: https://godbolt.org/z/jEf1fG

The current pin-project-lite generates almost the same code as pin-project, but does not have the equivalent of the repr(packed) checking that proc-macro(pin-project) does during expansion. (Since the proc-macro-attribute used on the struct expand later than declarative macro used for the struct, a complete fix for the problem is probably impossible until safe_packed_borrow gets a hard error.)

[RalfJung]

FWIW, safe_packed_borrow should become a hard error eventually. However, that cannot happen before we stabilize rust-lang/rust#73394 (and then wait some time).

[taiki-e]

unaligned_references is now a hard error 🎉: rust-lang/rust#102513