let_underscore_drop warns even when drop order is not affected

Lint name: clippy::let_underscore_drop

In this context, the semantics of _ do not alter drop order (which is the reason the lint exists, I think?) but the lint fires anyway:

struct FlushOnDrop<T: std::io::Write>(T);

impl<T: std::io::Write> Drop for FlushOnDrop<T> {
    fn drop(&mut self) {
        let _ = self.0.flush();
    }
}

Meta

• cargo clippy -V: clippy 0.1.52 (476acbf 2021-03-03)
• rustc -Vv:

  rustc 1.52.0-nightly (476acbf1e 2021-03-03)
  binary: rustc
  commit-hash: 476acbf1e9965b5e95c90f0d7d658709812b7003
  commit-date: 2021-03-03
  host: x86_64-unknown-linux-gnu
  release: 1.52.0-nightly
  LLVM version: 11.0.1
  

Start with the clippy::let_underscore_drop lint entry point and reproduce the supplied FlushOnDrop example using the reported Rust and Clippy versions. Add or update a regression case for this context; done means the example no longer warns when _ does not affect drop order.