regression: target_feature no longer permitted in some places

[Mark-Simulacrum]

• https://crater-reports.s3.amazonaws.com/beta-1.48-1/beta-2020-10-09/gh/atkurtul.xvmath/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.48-1/beta-2020-10-09/reg/xvmath-0.1.4/log.txt

Likely caused by #71205

[Mark-Simulacrum]

I suspect we'll call this won't fix but I have nominated it for T-lang to get it on the agenda for next triage meeting.

[nikomatsakis]

Summary:

• unused #[target_feature] attributes appearing in surprising places because of rustc: fix check_attr() for methods, closures and foreign functions #71205, places that have no effect
• another case where we have to think about our regression policy
• seems clear that the "detect if attributes are used" part of the compiler winds up incorrectly thinking that things are used fairly regularly
• given limited impact and the fact that it's clearly a bug, we're inclined to let this change go forward as a bug fix (we can revisit if/when we receive more complaints)

[pnkfelix]

assigning to mark to ensure that release notes include the note about this being a bug fix with limited impact.

[Mark-Simulacrum]

https://github.com/rust-lang/rust/pull/78364/files#diff-4a00447b47c36ebd3ded77321deff288e424f4605496ee1199fc9953a20f4473R80 (though that link may go stale). Closing.