Marking a return value as inappropriate for _

[kornel]

There isn't such attribute yet.

Pre-RFC: `#[must_bind]`

Confusing let _ = ...; and let _a = ...; seems like a common beginner mistake that leads to a buggy program instead of a compile-time error. (example mention). It is natural to think that once I bound it do something, it will live up to the end of the block and do its work. #[must_use] does its job well for Result-returning functions, but when the main point of return type is its Drop implementation, #[must_use] may be not enough. I propose an enhanced version of must_use: #[must_bind] attrib…