no_effect lint should also lint calls to const_fn where the result is unused

[oli-obk]

Vec::new();

is an example (soon at least)

[clarfonthey]

This is rust-lang/rust#50805.

Well, was, anyway. I think this should be done upstream.

[est31]

I too think it should be done in rustc. But in order to be in rustc, it may not have false positives. I closed the PR because I couldn't figure out a watertight check.

Thinking about it a bit more we could maybe only lint in non-generic code, aka where all the generic input types are known.